Latex Allergy

An increased incidence of allergy to latex, as found in surgical gloves, balloons, and urinary catheters, has been well documented in children with spinal dysraphism.³⁵ Allergy to latex is a type I, immediate, IgE-mediated reaction that can lead to anaphylaxis and death. Sensitization presumably arises from repeated exposure, at home and in the operating room. In many centers, all children with spina bifida are managed in a latex-free environment, and surgical procedures are preceded by prophylactic medications.

Late Deterioration

Myelomeningocele is a birth defect; although there are often severe neurologic sequelae, they should remain static. Any sign of worsening in a child or adult with spina bifida should provoke an intense search for a cause, which is usually treatable. Possible reasons for neurologic deterioration include uncontrolled hydrocephalus, Chiari malformation, hydromyelia, tethered cord syndrome, epidermoid or dermoid inclusion cyst, basilar impression, cervical instability, kyphoscoliosis, obesity, and reactive depression. Even with modern diagnostic techniques, it may prove difficult to determine which of the above-listed factors is responsible for worsening in a particular patient. This is true partly because the anatomic changes (e.g., Chiari malformation or tethered cord) are present in most children with myelodysplasia, whether or not they are symptomatic.

Hydromyelia

The routine availability of MRI has heightened awareness of this entity, which may be found in 68% of myelomeningocele patients.³⁶ Hydromyelia presumably is the result of hydrodynamic forces arising from persistent fetal or untreated hydrocephalus that forces CSF down the central canal of the spinal cord.⁴ This explanation accounts for the frequency of symptomatic hydromyelia in patients with unshunted ventriculomegaly and patients considered to have “compensated” hydrocephalus, whose nonfunctional shunts were not revised because they lacked overt signs or symptoms of intracranial hypertension.⁴¹ An alternative explanation has been proposed by Oldfield and colleagues,⁴² who used dynamic MRI to study CSF flow. They suggested that a systolic pressure wave in the cranial compartment is transmitted in a pistonlike manner to the cerebellar tonsils, causing a systolic spinal CSF pressure wave that pushes CSF into the cord through its outer surface.

Symptoms of hydromyelia in spina bifida differ from symptoms in classic syringomyelia. The latter entity typically produces a dissociated sensory loss (relative loss of pain and temperature modalities with preservation of light touch), atrophy, and fasciculations primarily affecting the shoulder girdle and hands. When found in association with myelomeningocele, hydromyelia typically results in progressive bladder dysfunction, spasticity of upper and lower extremities, quadriparesis, and generally preserved sensation. The myelomeningocele repair may become swollen and tender. Symptoms may be due in part to the direct effects of the hydrocephalus stretching the cortical motor fibers. Hydromyelia may also be seen with progressive scoliosis. Spinal cord tethering may be the primary underlying etiology, however, of scoliosis and hydromyelia in these cases.⁴³

Workup consists of MRI of the brain, followed by MRI of the entire spine to visualize the extent of the hydromyelia and the associated Chiari malformation and tethered spinal cord (Fig. 31–5). The hydromyelia cavity may be localized to a few spinal segments or extend throughout the length of the spinal cord. There may be multiple cavities and septations.

FIGURE 31–5 T1-weighted mid-sagittal MRI of a patient with Chiari malformation and large septated hydromyelia (H).

Treatment of symptomatic hydromyelia begins with a ventriculoperitoneal shunt or shunt revision if the ventricles are enlarged. If the ventricles are small or if the hydromyelia persists without improvement in symptoms after this procedure, a Chiari decompression is performed. MRI of the spine is repeated 3 to 6 months after decompression, and a hydromyelia shunt is considered at that time if the syrinx has not improved. The shunt is usually placed from the hydromyelia to the pleural cavity; this is ideally done in the thoracic area but should be at the spinal level where the cavity is largest. The shunt is made to drain at a very low pressure. Follow-up MRI is done when the patient is stable. Shunt failure is common.

Other procedures have been described. Terminal ventriculostomy was advocated by Gardner and colleagues⁴⁴ for classic syringomyelia, but the myelotomy tends to scar closed with time. In addition, the negative pressure provided by the pleural end of a shunt provides more decompression than simply allowing the hydromyelia to communicate with the subarachnoid space. Park and colleagues⁴⁵ advocated posterior fossa decompression and plugging of the obex to prevent CSF fluid from the fourth ventricle from entering the central canal in patients in whom ventriculoperitoneal shunt revisions do not alleviate symptoms. This is a formidable procedure, however, and recurrent vomiting may occur from irritation or compression of the brainstem at the obex.

Summary

Signs or symptoms of worsening in a patient with myelodysplasia should provoke a thorough evaluation. The importance of careful history taking and physical examination cannot be overemphasized because radiologic studies often reveal a plethora of abnormalities, and only an astute clinician can discern which is responsible for a particular complaint. If symptoms involve only the lower extremities and sphincters, possible causes include decompensated hydrocephalus, hydromyelia, and tethered cord. If the upper extremities or brainstem is involved, possible causes include hydrocephalus, hydromyelia or hydrobulbia, cervical instability, basilar impression, kyphoscoliosis, and Chiari malformation. Sometimes the clinician is forced simply to treat the various lesions serially until symptoms improve.

Cutaneous Syndrome

Cutaneous abnormalities indicative of an underlying spinal dysraphism are situated on or near the midline, usually in the lumbrosacral region, but occasionally in the thoracic region or the neck.⁶⁶ A wide variety of abnormalities are seen.

A striking finding is the hairy patch (hypertrichosis), also known as “faun’s tail,” which always occurs in the midline. It may be small but more commonly is a wide, diamond-shaped patch in the lumbar or lower thoracic area (Fig. 31–6). Frequently, the patient’s mother has trimmed the hair for years before presentation to the physician. If the dysraphism is in the cervical or upper thoracic region, there is usually a smaller patch of silky hair. The underlying abnormality may not be confined to the level of the hairy patch, and it is important to image the entire spine. Hypertrichosis may occur with any of the types of dysraphism but is particularly associated with split cord malformations.⁶⁷

FIGURE 31–6 Hypertrichosis in a young woman, indicating underlying tethered cord.

Subcutaneous lipomas at or near the midline of the lumbosacral spine may indicate an underlying intradural lipoma. These are nontender, poorly circumscribed soft masses of fat that are continuous with the normal subcutaneous tissue and with the intraspinal portion of a lipomyelomeningocele. The overlying skin is normal, hairy, or dimpled, and there may be an associated angioma or skin tag (Fig. 31–7). Other conditions may manifest as skin-covered masses overlying the caudal spine (Table 31–3). Often, an older patient gives a history of having had the superficial mass removed for cosmetic reasons.

FIGURE 31–7 Lumbosacral hemangioma with congenital dermal sinus tract below.

TABLE 31–3 Differential Diagnosis of Skin-Covered Lesions Overlying the Spine

Pigmented nevi may be red to brown with mottling. Cutaneous port-wine angiomas in the suboccipital area (“stork bites”) are usually not significant. Hemangiomas in the lumbosacral area are frequently associated with an underlying dysraphism.

Atretic meningoceles may also be seen, consisting of a central area of thin, pearly skin surrounded by a halo of red, pink, or brown (spinal aplasia cutis). These represent myelomeningoceles or meningoceles that have partially healed spontaneously. Dimples at the tip of the coccyx (coccygeal pits) are frequent findings in normal newborns and usually have no significance.⁶⁸ Deep dimplelike depressions higher in the spine may be the external stigma of an epithelialized tract that connects with the filum terminale, spinal cord, or intraspinal dermoid cyst and require further investigation. The meningocele manqué is a skin blemish sometimes referred to as a “cigarette burn” because it resembles a small scar with loss of skin in the midline. When this blemish is obvious at birth, it may cause concern of CSF leakage, but this seldom occurs. Other cutaneous lesions that may be associated with a tethered cord include rudimentary caudal appendages (“tails”) and asymmetrical gluteal folds.

Neurologic Syndrome

Muscle weakness and gait disturbance are usually apparent at 2 years of age when a child begins to walk. On examination, there may be striking muscular atrophy and leg-length asymmetry. The deep tendon reflexes may be normal, increased, or absent, giving the pattern of a mixed upper and lower motor neuron lesion. Patchy sensory loss is present, particularly in the distal leg and perineum. Pain in the back with a radicular component is frequent in older children or adults.⁶⁹

Orthopaedic Syndrome

The most frequent orthopaedic finding is unilateral or bilateral cavovarus deformity of the foot, with or without leg-length discrepancy (Fig. 31–8). There may also be “claw toes.” The abnormal gait is the result of orthopaedic deformity and muscular weakness. It is presumed that the foot deformity is due to lack of, or weak innervation of, antagonistic muscles of the lower extremity.⁷⁰ Scoliosis, especially if found in a young child or a boy or if noted to be rapidly progressive or accompanied by pain, also suggests an underlying spinal cause.

FIGURE 31–8 Foot deformity with tethered cord. The high arch is typical and may be accompanied by hammer toes and leg-length inequality.

Urologic Syndrome

Occult spinal dysraphism should always be considered when one encounters infants with an abnormal voiding pattern, new-onset urinary or fecal incontinence in a previously toilet-trained child, or urinary tract infection in a child of any age. As opposed to older children, most infants with occult spinal dysraphism have normal results on urodynamic testing.⁷¹ This fact underscores the importance of recognizing the syndrome as early as possible so that prophylactic surgery can be performed.

Anorectal Anomalies

Anorectal anomalies, including vesicointestinal fissures, cloacal extrophy, rectovesical fistulas, and imperforate anus, are frequently associated with an underlying tethered cord.⁷² The overall incidence of tethered cord with imperforate anus is 35% and may be increased in boys.⁷³ In many cases, no cutaneous abnormality is seen, and radiographic screening is indicated in these patients.

Lipomyelomeningocele

Lipomyelomeningocele occurs at a rate of 1 in 4000 live births and in contemporary series is the most common cause of tethered cord syndrome.⁸⁰ The term lipomyelomeningocele is misleading, in that it suggests herniation of neural elements through a spina bifida defect into a meningeal sac. The neural elements remain within the spinal canal, and only the lipoma itself may protrude through the spina bifida defect to manifest as a subcutaneous mass. Some authors have used the term lipoma of the cauda equina, but the fatty mass is invariably attached to the conus medullaris or filum terminale rather than to the cauda equina. Chapman⁸¹ classified lipomyelomeningoceles as lesions that attach to the caudal end of the conus, lesions that insert dorsally, and lesions that are transitional.

If the lipoma inserts onto the dorsal surface of the conus, there is usually a substantial subcutaneous mass, which may be asymmetrical. Along the lateral interface of the attachment of the lipoma to the spinal cord, the dura and pia are also fused (Fig. 31–9). Sensory roots emerge just anterior to this lateral line of fusion. As a result, neither the sensory roots nor the motor roots are actually within the lipoma. Alternatively, the lipoma may join the conus at its caudal end. The remaining mass may lie entirely within the spinal canal or extend dorsally through the spina bifida defect (Fig. 31–10). The fatty tumor may replace the filum terminale, or there may be a separate filum that lies anteriorly. The nerve roots usually lie ventral to the fatty mass but may lie within the fibrous ventral portion of the mass itself. Transitional forms may occur, which are often the most difficult to manage surgically. Probably the rarest subtype is the chaotic lipomyelomeningocele, described by Pang and colleagues,⁸² which has a prominent ventral component. MRI often defines the type of lipomyelomeningocele, which is helpful in understanding the anatomy that is encountered at operation.

FIGURE 31–9 Cross-sectional anatomy of dorsally inserting lipomyelomeningocele. There is a broad lateral attachment between lipoma and lateral dura. Nerve roots emerge anterior to fatty mass.

FIGURE 31–10 Caudally inserting lipomyelomeningocele. Left, Lipoma inserts on low-lying conus. Right, Cross section through lipoma, below level of cord. Nerve roots of cauda equina usually pass ventrally and may be adherent.

Lipomyelomeningocele is only one form of occult spinal dysraphism, and elements of other forms may coexist. Some patients may have associated anorectal or urogenital anomalies,⁸³ and the pathologic anatomy may include elements of myelocystocele, diastematomyelia, or thickened filum.⁸⁰ Despite the superficial resemblance of lipomyelomeningocele to open dysraphism (myelomeningocele), it is rare for patients to have associated hydrocephalus, gyral anomalies, or Chiari malformations. The relationship of Chiari type I malformation found in association with lipomyelomeningocele has been controversial, but a more recent study found a 13% incidence of Chiari I malformations in patients with lipomyelomeningocele. There was no difference, however, in hindbrain herniation or posterior fossa volumes of lipomyelomeningocele patients with or without Chiari I malformation.⁸⁴ It is unclear that CSF overdrainage occurring during untethering procedures has any relationship to tonsillar herniation.⁸⁵ Cadaver experimentation has not shown downward tonsillar descent with conus medullaris traction.⁸⁶ As with myelomeningocele, there is an association with hydromyelia, particularly in the terminal spinal segments.⁸⁷ The cognitive impairments characteristic of children with myelomeningocele seem to be absent, however, in children with spinal lipomas.⁸⁸

The specific cause of lipomyelomeningocele is unknown. Familial occurrence has been reported in two siblings but seems to be extraordinarily rare.⁸⁹ Other neural tube defects such as myelomeningocele may be found in 2% to 5% of siblings, suggesting a possible genetic predisposition to open and closed neural tube defects.⁹⁰ An environmental cause has not been established, although a case of maternal use of valproic acid and a child with lipomyelomeningocele has been reported.⁹¹

The usual presentation of lipomyelomeningocele varies according to the age of the patient. Newborns and infants up to age 1 year typically present with a skin-covered lumbosacral mass (Fig. 31–11). There may be associated hair, sinuses, tags, or nevi. Children 1 year of age or older present with progressive signs or symptoms of tethered spinal cord, as previously described. Adults may present with back pain and sciatica or acute neurologic deterioration associated with lifting, falls, exercise, or assuming positions of spinal flexion. It has been suggested that the lipoma may enlarge or shrink with overall body fat.⁹²

FIGURE 31–11 Typical presentation of lipomyelomeningocele in a newborn as skin-covered mass.

With frequent use of in utero screening, the diagnosis of lipomyelomeningocele is increasingly made prenatally with ultrasonography. The finding of an echogenic mass in the lumbosacral area without accompanying hydrocephalus may suggest the diagnosis, but it may be impossible to exclude myelomeningocele using this technique. High-resolution ultrasonography has also been used as a screening test for newborns with suspected tethered cords, but MRI is the study of choice.

MRI should be performed in sagittal and axial planes. Fatty tumors are readily visualized on T1-weighted images as areas of increased signal intensity because of their short relaxation times. The conus is low, extending to the caudal portion of the thecal sac. The lipoma may insert dorsally (Fig. 31–12) or caudally (Fig. 31–13) and may or may not include the filum. In some instances, the conus may be at the normal level (at or above the L1-2 disc space), but the filum may be thickened and of fatty density. If these patients have other clinical signs or symptoms of tethered cord syndrome, they should be considered to have a tethered cord, and exploration should be carried out.⁷⁹ An associated syrinx or hydromyelic cavity frequently may be seen within the terminal spinal cord, corresponding to a dilated terminal ventricle (Fig. 31–14). A large neurogenic bladder may also be seen. Lipomyelomeningocele must be distinguished from epidural lipomatosis, which is a rare condition in which fat accumulates in the dorsal epidural space, usually in the thoracic region, as the result of the use of exogenous corticosteroids or from corticosteroid-secreting tumors.

FIGURE 31–12 T1-weighted sagittal MRI of dorsally inserting lipomyelomeningocele. Cord is low lying, and lipoma attaches broadly along its dorsal aspect, tethering conus to spinal canal (arrows).

FIGURE 31–13 T1-weighted sagittal MRI of caudally inserting lipomyelomeningocele. Lipoma tethers conus to caudal end of thecal sac.

FIGURE 31–14 T1-weighted sagittal MRI of large terminal hydrosyringomyelia. This patient had previously undergone resection of a lipomyelomeningocele and presented with new-onset incontinence. Lesion was successfully treated with syringosubarachnoid shunt.

Urologic evaluation is recommended and should include ultrasonography of the bladder and urodynamic studies. More recent work has indicated that the presence of lipomyelomeningocele is predictive of a worse urologic outcome, in contrast to tethered cord syndrome without dysraphism, in which abnormal urodynamic studies alone are not predictive of poor urologic outcome.⁹³

Most neurosurgeons recommend that all children with skin-covered lumbosacral masses, with or without associated neurologic, urologic, or orthopaedic abnormalities, should undergo exploration, preferably in infancy. The surgeon must be prepared to manage a lipomyelomeningocele properly before undertaking the exploration of any midline spinal mass, to avoid damaging neural structures. The first operative procedure stands the best chance of success because scar formation and distorted anatomy found at reoperation compound the difficulties inherent in the procedure.⁹⁴ Although some authors have questioned the value of prophylactic surgery,⁹⁵ most believe that infants and children with tethered cord syndrome should have prophylactic surgery at the time the lesion is discovered, preferably in the first 6 months of life.^95–98 Early surgery is most likely to untether the cord effectively and, because tethered cord syndrome is progressive, is most likely to maximize functional outcome. An adult patient with a lipomyelomeningocele and nonprogressive symptoms may be followed nonoperatively, although this situation is uncommon. Adult patients who become symptomatic should be offered surgery.⁹⁹ The patient should be warned of the possibility that permanent worsening may occur.

Goals of surgery are (1) to untether the spinal cord from the lipoma itself, the filum terminale, or both and (2) to remove the lipomatous mass insofar as this is possible to prevent retethering and to relieve direct compression. It is also desirable to decrease the size of the subcutaneous mass for cosmetic reasons and (3) to reconstruct the dural canal and prevent CSF leakage and to allow sufficient room for the neural elements. This last goal often requires a dural graft. The surgeon must provide sufficient room to allow the conus to float freely within the dural sac and prevent retethering owing to surgical scarring.

General anesthesia is used. Muscle relaxants are avoided for portions of the procedure during which nerve stimulation may be employed. Several authors have described the use of SSEP monitoring, bladder manometry, or rectal sphincter and lower extremity electromyography (EMG) as adjuncts (see section on neurophysiologic monitoring).

The pediatric patient is placed in the prone position with towel rolls underneath the chest and iliac crest to allow the abdomen to hang freely. Adult patients are placed in the kneeling position with a binder to support the buttocks. The operative site is scrubbed with preparation solution, and after this is damp dried, a transparent adhesive plastic drape impregnated with iodine is applied over the skin. A separate plastic “apron” excludes the anus from the operative field.

Magnification with loupes is helpful, and the operative microscope may be required for portions of the dissection. An elliptical skin incision surrounding the subcutaneous mass is made along a vertical axis, with hemostasis being obtained by applying straight mosquito hemostats. The subcutaneous tissue is incised in a circumferential fashion down to the lumbodorsal fascia (Fig. 31–15). The ellipse must be narrow enough to allow skin closure at the end of the procedure but broad enough to resect most of the subcutaneous mass. The subcutaneous fat is incised in a circumferential fashion down to the lumbodorsal fascia using the monopolar cautery. When an anatomic cleavage plane becomes evident, the lipoma is mobilized medially by blunt dissection with the use of a sponge and periosteal elevator so that the stalk can be visualized as it traverses the fascial defect. A self-retaining retractor is inserted, and the lowest intact laminar arch above the bony defect is palpated. The fascia over this spinous process is opened with the cutting cautery, and the laminae are exposed.

FIGURE 31–15 Initial operative exposure of lipomyelomeningocele. A vertically oriented elliptical skin incision has been made, and subcutaneous lipoma has been followed to fascial defect where it enters spinal canal.

A laminectomy of this segment is done, exposing the underlying normal dura. It may be helpful at this point to amputate the large subcutaneous mass with the skin attached at the level of its stalk. Starting at the level of normal dura cephalad to the mass, the epidural fat is melted with the bipolar cautery until the dural defect with fatty tissue extruding through it is encountered. A midline dural opening is made above the defect, exposing the spinal cord (Fig. 31–16). As the dural opening is carried inferiorly toward the defect, a transverse band of thick fibrous tissue is noted at the rostral end of the lipoma stalk, which acts to kink the spinal cord. This is opened widely along with the dura. The dural opening is extended caudad on either side of the exiting lipoma circumferentially.

FIGURE 31–16 One-level laminectomy has been performed superior to spina bifida defect (right), and dura has been opened. The lipoma can be seen inserting at caudal end of spinal cord and connecting with subcutaneous fat.

At this point, the lipoma usually is found to correspond to one of the types described by Chapman.⁸¹ Lipomas that insert into the conus dorsally can be removed from the dorsal aspect of the cord in a plane superficial to the lateral lines of fusion of lipoma, conus, and meninges, with the nerve roots emerging anteriorly. Simultaneously, the lines of fusion are divided laterally, first on one side and then on the other, with bipolar cautery and microscissors or a knife blade over a dissector (Fig. 31–17). The CO₂ laser has been helpful in shaving down the mass of the lipoma. The filum is identified and divided.

FIGURE 31–17 Lateral attachment between lipoma and dura is sharply divided, while dissector protects underlying nerve roots.

Lipomas that insert caudally into the conus must be sectioned distal to any functional roots, which may be identified using electrical stimulation and EMG. It is unnecessary to remove all the gross lipoma; to attempt this risks damage to the conus and nerve roots. Simple division of the lipoma releases the point of tethering and fulfills the goal of the operation. Pang and colleagues¹⁰⁰ reported a decreased rate of retethering, however, after a very aggressive sharp excision of the lipoma.

After the lipoma has been largely removed from the spinal cord, it is sometimes possible to reapproximate the pial edges of the cord to reconstitute the normal tubular configuration (Fig. 31–18); this does not improve neurologic function but may serve to deter retethering. If the thecal sac is capacious, a fine suture may be used to anchor the conus to the ventral dura to discourage dorsal tethering. If a terminal syrinx is seen on preoperative MRI, a midline myelotomy may be performed to provide communication with the subarachnoid space.

FIGURE 31–18 Lipoma has been largely removed from conus, allowing pial stitches to be placed to reconfigure tubular structure of cord. This may aid in preventing retethering.

(From Sutton LN: Lipomyelomeningocele. Neurosurg Clin N Am 6:325-338, 1995.)

When the cord is free of adhesions, the dura is closed. In some cases, the dura may be approximated and closed with a running locked suture of 4-0 nylon. In many situations, this closure results in stricture of the canal, however, with the likelihood of scar formation and retethering. If there is any doubt, an elliptical graft is sutured to the dural edges. This may be technically very difficult if the dural edge is anteriorly located in the spinal canal. Acellular human dermis (Alloderm) is a convenient graft material. Alternatively, artificial substances such as Gore-Tex (Gore, Newark, DE)¹⁰¹ may be used to deter scar formation, but it is difficult to obtain a watertight closure with these materials, and tethering often occurs to the pseudodura that forms beneath the graft. Often the superficial subcutaneous tissue layers in the closure are inadequate for a truly watertight closure, and the dural repair becomes of primary importance to avoid a CSF fistula. The patient is placed in steep reverse Trendelenburg position, and a Valsalva maneuver is performed to ensure the closure is adequate. It may be useful to seal the suture line with tissue glue.

The fascia is closed if possible. If a large fascial defect prevents approximation, lateral fascial flaps are elevated and reflected medially, as in the fascial closure in the standard myelomeningocele repair. In closing the skin, it may be advisable to tack the subcutaneous layer to the underlying fascia to obliterate any dead space. The skin itself is closed with absorbable suture and tissue glue.

Patients are kept recumbent for 48 hours. Patients who experience difficulty with voiding in the immediate postoperative period are treated with judicious urinary catheterization to prevent unnecessary straining that might threaten the dural closure. Infants are diapered below the dressing to avoid fecal soiling of the incision.

Patients, particularly those in whom a significant mass has been left, are cautioned to avoid excessive weight gain. These patients often require urologic or orthopaedic referral and close neurosurgical follow-up.

Surgery is relatively safe. The major postoperative complications are CSF leaks and wound breakdown because it is difficult sometimes to obtain good tissue for surgical repair. A persistent subcutaneous collection or frank leak requires re-exploration of the operative site and meticulous closure. An external spinal drainage catheter carefully inserted above the lamina defect through a Tuohy needle for 1 week is useful in protecting the closure after re-exploration. Persistent fever with sterile CSF cultures and a cellular reaction postoperatively may reflect a mild allergic reaction to the graft. This allergic reaction may persist for several months if untreated but usually resolves with a short course of corticosteroids. The incidence of a new permanent neurologic deficit after initial surgery for lipomyelomeningocele is 3% to 6% in experienced hands and is usually confined to a lower root.⁸⁰

Re-exploration of previously operated lipomyelomeningoceles is tedious. Tissue planes are obliterated, and nerve roots often pass within the substance of the lipoma and scar tissue. For this reason, definitive surgery is required at the primary procedure.

Surgical treatment for lipomyelomeningocele is basically prophylactic and is aimed at preventing progression of fixed deficits or development of new deficits in infants and asymptomatic children or adults. Data documenting the progressive nature of untreated lipomyelomeningocele are compelling. Hoffman and colleagues¹⁰² reported 12 patients who initially were neurologically and functionally intact and were untreated or inappropriately treated. Only one of these patients remained intact, whereas the others experienced deterioration over time. After appropriate repair, there was a halt in further deterioration, but few patients improved in function. Patients who are discovered at a younger age (because of cutaneous abnormality) are more likely to remain symptom-free if they undergo appropriate surgery than patients in whom surgery is deferred.⁸⁰ Despite these data, other investigators have pointed out that retethering results in late deterioration in 46% of patients with conus lipomas who underwent prophylactic surgery, which is no different from the natural history of the disease.⁹⁵

Lipomas that asymmetrically interface with the cord present a particular challenge because of their lack of “safe space” at the placode-dural interface. Cochrane and colleagues^103,104 reported that an asymmetrically located lipoma inserting laterally or ventrally into dura is more likely associated with earlier neurologic deterioration after surgery compared with symmetrically located placodes.

In symptomatic patients, 20% of patients who undergo an appropriate operation note improvement in stance and gait¹⁰⁵; in about 39%, there may be improved bowel or bladder function.⁹⁸ Deformities of the feet do not improve and may continue to worsen even after successful surgery because abnormal muscular innervation and gait lead to progressive deformity. Continued orthopaedic surveillance and physical therapy are indicated.

The neurologic and urologic status of a patient with a lipomyelomeningocele should remain stable after successful surgery, although orthopaedic deformity may progress because of persistent muscular weakness and imbalance. Any significant deterioration in a patient’s status (e.g., new-onset incontinence, pain, or progressive weakness) should provoke a search for a cause. Possible causes include hydromyelia; retethering; infection; and a separate process not addressed by the previous surgery, such as an unrecognized diastematomyelia superior to the operated lipomyelomeningocele.

Hydromyelia is a frequent accompaniment of lipomas and was seen in 6 of 43 patients evaluated at the authors’ institution by MRI and in 19% of patients reported by Iskandar and colleagues.¹⁰⁶ If noted on the preoperative scan, hydromyelia may be addressed at the same time as the lipomyelomeningocele procedure by terminal ventriculostomy, which connects the hydromyelia with the subarachnoid space. If a symptomatic hydromyelia recurs, treatment is best accomplished by a syringopleural or syringosubarachnoid shunt.¹⁰⁷

Assuming that no other pathologic process is found, one must be concerned about the possibility of spinal cord retethering. The incidence of this phenomenon is unknown. Various authors have reported symptomatic retethering to occur in 20% of cases,¹⁰⁸ but the incidence may be much higher. It is likely that virtually all patients experience anatomic retethering within a few weeks of surgery for lipomyelomeningocele but that the dorsal reattachment is at a higher spinal level, and symptoms are relieved because the spinal cord is no longer stretched. Symptomatic retethering can occur at virtually any time after initial surgery, from weeks to more than 10 years.

The onset of new pain or neurologic or urologic signs or symptoms in a previously operated lipomyelomeningocele patient should prompt repeat MRI of the lumbosacral area, despite the fact that postoperative studies are often unrevealing and difficult to interpret. Immediate postoperative MRI in successfully untethered patients would continue to show a low-lying conus and a dorsally displaced spinal cord, and these findings neither confirm nor rule out retethering in a postoperative patient. Because MRI of the spine must be performed with the patient in the supine position to prevent movement artifact from respiration, the spinal cord tends to migrate to the dorsal position, even if no tethering is present. It has been suggested that ultrasound evaluation through the laminectomy defect is helpful in this situation. The finding of prominent pulsation of the spinal cord is reassuring that retethering has not occurred, and its absence is more worrisome, but the value of this remains to be proved.

Developments in MRI technology have allowed assessment of spinal cord pulsations using phase-motion imaging techniques. McCullough and colleagues¹⁰⁹ reported in a small group of patients that cervical spinal cord pulsation peak velocity is decreased in patients with tethering and that it improves after successful surgery. Such techniques would be valuable in diagnosing retethering in patients in whom static images are nondiagnostic. Experience with this technique at the authors’ institution has been disappointing, in that the range of cervical and lumbar cord pulsation velocities in normal individuals is so broad that there is considerable overlap with patients with obviously tethered cords. Another technique that is helpful in some instances is prone-supine metrizamide CT-myelography. In contrast to MRI, it is possible to perform an adequate image with the patient in the prone position using this technique, and ventral migration of the conus with contrast material seen dorsal to the cord rules out retethering.

Until more definitive radiographic techniques become available, retethering remains a clinical diagnosis. Most authorities re-explore any patient in whom significant radicular pain develops or in whom unequivocal deterioration occurs, regardless of radiologic studies. Strategies to minimize the likelihood of retethering are important, particularly at reoperation. These include (1) removal of as much of the lipoma as is safe, to reduce the bulk of the conus; (2) creation of a capacious thecal sac, using a graft; (3) closure of the pia in dorsally inserting lipomas; (4) the use of a dural graft “sandwich” of Silastic or Gore-Tex sheeting for the inner layer and a biologic dural substitute for a watertight outer dural closure; and (5) the use of stay sutures to center the conus in the thecal sac. Retethering is usually found to be due to dense scar, usually dorsal and lateral to the conus.

Despite the technical challenges inherent in the procedure, reoperation for recurrent tethering is usually rewarding, at least in the short-term. Successful repeat untethering can be performed in approximately 90% of patients.⁹⁴ Pain is relieved in most patients, and gait and motor dysfunction improve in about half of patients. The prognosis for improvement in genitourinary dysfunction and scoliosis is less optimistic.

Sacral Agenesis and Caudal Regression Malformations

Numerous anomalies involving the caudal spine have been described, ranging in severity from simple agenesis of the coccyx to complex malformations of the thoracic and sacral spine, lower extremities, spinal cord, genitourinary system, and gastrointestinal tract. The most common of these include the VACTERL syndrome¹¹⁰ (vertebral anomalies, anal atresia, cardiac anomalies, tracheoesophageal fistula, esophageal atresia, renal and limb anomalies), the OEIS complex (omphalocele, cloacal exstrophy, imperforate anus, and spinal deformities), and sacral agenesis. The most severe form includes fusion of the legs, a condition that has been termed sirenomelia, or the mermaid syndrome.¹¹¹ A subset of patients with these anomalies are at risk for late deterioration, and detailed imaging should be followed by prophylactic surgery in some cases.¹¹²

Because imperforate anus and the genitourinary tract are involved in these malformations, embryogenesis must account for these systems and the spine. Primordia of these three organ systems in the embryo originate in a common structure, the caudal eminence or tailbud. This pluripotential structure arises as a continuation of the primitive streak and ultimately gives rise to all of the structures of the caudal embryo, including the caudal spinal cord. This forms as a solid mass, initially separate from the neural tube, which undergoes programmed necrosis or retrogressive differentiation to form the filum terminale, cauda equina, and conus medullaris. During this process, a localized dilation of the central canal, the ventriculus terminalis, arises in the conus. Non-neural structures including the hindgut (giving rise to the distal colon, rectum, and anus) and urogenital sinus (bladder, urethra, and genitalia) also arise from the caudal eminence. As part of this process, the cloaca is divided longitudinally by the urorectal membrane and the cloacal membrane, which fuse to partition the cloaca into the urogenital sinus and anorectal canal. The cloacal membrane eventually ruptures to become a perforate anus. Cloacal folds ultimately differentiate into male or female genitalia.

Severe multiorgan cases of caudal agenesis presumably arise from an embryonic insult to the caudal eminence. Failure of somite differentiation causes failure of corresponding spinal cord and notochord segments. Failure of retrogressive differentiation gives rise to lipomas, myelocystoceles, and related conditions. Related hindgut and urogenital anomalies arise from disturbances of the cloaca. Early dehiscence of the cloacal membrane can result in cloacal exstrophy, in which the posterior wall of the cloaca herniates through a pelvic defect, everting the central bowel field between two hemibladder fields. Because the caudal eminence induces some mesodermal structures, including the metanephros, renal anomalies may follow as well.^112,113

Evidence suggests extrinsic and genetic factors in the causation of caudal regression syndromes. A history of preexisting or gestational maternal diabetes mellitus is obtained in 16% to 49% of cases,^112,114 although caudal spine anomalies occur in only 1% of infants born to diabetic mothers.¹¹⁵ Most cases are sporadic, although rare familial forms have been reported, and an association with chromosome 7q has been described.¹¹⁶ Drugs have been implicated in some cases.¹¹⁷

The severity of the defect determines the presentation. Newborns may have simple imperforate anus or severe anomalies, including omphalocele, cloacal exstrophy, ambiguous genitalia, and tracheoesophageal fistula. The likelihood of a tethered cord is related to the number of anomalies that are present. Overall, tethering is present in 24% of children with anorectal malformations and in 43% of children with complex malformations.¹¹⁸ Sacral agenesis is suspected by flattening of the buttocks, shortening of the intergluteal cleft, and prominence of the iliac crests. A soft, skin-covered mass overlying the lumbosacral area suggests a myelocystocele or lipomyelomeningocele with tethered spinal cord. The lower extremities may exhibit abnormalities consistent with the involved vertebral segments. Hammer toes, calcaneovalgus or equinovarus ankle deformities, and contractures of involved joints are similar to findings in myelomeningocele or other forms of tethered cord. Paralysis, atrophy, and incompetent sphincter muscles reflect the myotomes of involvement. Sensation is relatively spared, and the sensory level may be several spinal segments below the motor level.¹¹² Anomalies higher in the nervous system generally are absent, but a case of Chiari I malformation has been reported.¹¹⁹

MRI of the lower spine is the most useful study and should be obtained in suspected cases. Fetal diagnosis of myelocystocele has been described, and it is vital to distinguish this lesion from a myelomeningocele with an intact sac (Fig. 31–19).¹²⁰ From the neurosurgical standpoint, the goal is to determine if spinal cord tethering is present and, if so, to delineate the mechanism. Pang¹¹² classified the appearance of lumbosacral agenesis into five types, based primarily on the appearance of the sacrum. From a practical standpoint, one can divide these anomalies into anomalies with a high conus and anomalies with a low conus. High symmetrical sacral agenesis is correlated with a truncated, club-shaped conus ending around T11 or T12. Tethering is not present, although dural canal stenosis has been reported as the cause of delayed deterioration in a few patients. Paradoxically, the lower, asymmetrical forms of sacral dysgenesis are more likely to have a low-lying tethered cord. Mechanisms of tethering include myelocystocele, typical hypertrophied filum terminale, and lipomyelomeningocele. Myelocystocele is particularly associated with these anomalies and is characterized by a trumpetlike cystic dilation of the caudal central canal, which is extensively tethered to the caudal spinal canal (Fig. 31–20).

FIGURE 31–19 Fetal MRI shows lumbosacral mass with thick skin (large arrow). No Chiari malformation is present (small arrow), and α-fetoprotein was normal in amniotic fluid. This was confirmed to be a myelocystocele at birth.

FIGURE 31–20 Sagittal MRI of myelocystocele. S, myelocystocele sac, which is a continuation of central canal, or hydromyelia (H); O, opening through which cerebrospinal fluid communicates to fill sac.

Initial management of infants with complex, multisystem anomalies usually does not involve neurosurgery and instead consists of closure of an omphalocele, bowel and possibly urinary diversion, and reconstructive surgery for tracheoesophageal fistula. MRI of the spine is obtained electively, when the infant is stable. Patients with the conus at a high level do not usually require neurosurgical intervention. Patients with low-lying, tethered cords undergo untethering procedures electively, at about 3 months of age, or when their general condition permits. Even infants with high motor levels should undergo prophylactic untethering. Often the motor level descends over several months, presumably as the infant recovers from spinal cord shock associated with delivery. Untethering is also indicated to preserve sensation. Surgery is similar to surgery for tethered spinal cord without sacral anomalies. Surgery for a myelocystocele consists of defining normal spinal cord anatomy above the lesion, identifying the terminal cyst, resecting the sac and any lipoma, untethering the cord, and reconstructing the dural sac.

Patients who do not have tethered cords and patients who have undergone successful untethering procedures should remain with stable deficits. Patients who experience new neurologic problems later in life should undergo repeat MRI looking for signs of retethering, hydromyelia, or dural stenosis. Patients with complex anomalies require long-term surveillance by a team consisting of a pediatric orthopaedist, urologist, and general surgeon.

Split Cord Anomalies

Split cord malformations are anomalies in which the spinal cord is divided along a portion of its length to form a double tube. Classically, two distinct forms, diastematomyelia and diplomyelia, have been described. The term diastematomyelia derives from the Greek words diastema (“cleft”) and myelos (“medulla”) and refers to the split in the spinal cord and not, as frequently misapplied, to the intervening bony or cartilaginous spur that often separates the two hemicords. It has been distinguished from diplomyelia (from the Greek diplos, meaning “double”), which, strictly speaking, refers to a condition in which the spinal cord is completely duplicated (i.e., two complete sets of anterior and posterior horns, with anterior and posterior roots emerging from medial and lateral surfaces of both hemicords). Investigators working more recently maintain that diplomyelia and diastematomyelia are different manifestations of the same pathologic entity and prefer the unifying term split cord malformation.^67,121,122 The two hemicords in split cord malformation may exist within a single dural envelope (type II) or, more commonly, are enclosed within separate dural tubes (type I).^67,123 The defect may be at any spinal level, but is most common in the lower thoracic or upper lumbar areas.^124–126

The embryogenesis of split cord malformation is not fully understood, but the most logical explanation is that suggested by Bremer¹²⁷ and amplified by others.^121,128 Any viable theory must explain the frequent association of diastematomyelia with vertebral anomalies, spina bifida, and gut malformations and the equal involvement of the ventral and the dorsal aspects of the neural tube.

In early embryonic development, the neurenteric canal connects the yolk sac (primitive intestinal cavity) with the amniotic cavity through the proliferating primitive knot, which is destined to become the brain and spinal cord. If this canal or an accessory canal persists, the entodermal lining of the yolk sac can herniate through the fistula and split the notochord, neural tube, and migrating mesenchymal elements that are to form the vertebrae. A temporary persistence of this communication results in hemivertebrae: If the herniated entoderm continues to differentiate, it results in a neurenteric cyst; if the tract attaches to the gut, intestinal malrotation or duplications may result. The cuff of pluripotential mesenchymal cells that condense around the entodermal sinus determines the nature of the split cord. The mesenchyme may differentiate into bone, cartilage, or fibrous tissue to form the median septum, which may or may not induce its own investing meninges. A thin membranous septum may be found within the cleft, which may not be seen on imaging studies, suggesting a diagnosis of true diplomyelia.

Whether or not the hemicords possess medial rootlets depends on the location of the neural crest cells within the spinal cord. If they are far lateral and unaffected by the split, only one set of dorsal roots develops on the lateral aspect of the hemicord. If the cell mass is bisected by the split, each hemicord has its own set of paramedian dorsal roots. If the neurenteric tract persists further dorsally to involve the cutaneous ectoderm, a true myelomeningocele occurs. This is also the explanation for the cutaneous signature that often accompanies the malformation. Because normal ascent of the spinal cord continues through this sequence of events, the septum usually lies cephalad to the vertebral defect, and the septum is found at the caudal end of the cleft at surgery.

Clinically, split cord malformation occurs predominantly in girls.^123–125 The largest series reported a female-to-male ratio of 1.5:1.¹²⁹ The condition may be asymptomatic, or patients may present with symptoms of spinal cord dysfunction or scoliosis. The clinical findings are no different from findings in other cases of spina bifida occulta. Most patients have a midline thoracic or lumbar cutaneous abnormality. The incidence is 50% to 80%.^{63,124,125,130} The level of the skin lesion does not correspond to the level of the split cord malformation. The most common finding is a hairy patch (40%), but other abnormalities include dimples, hemangiomas, lipomas, sinus tracts, or myelomeningocele or meningocele.¹²⁵

Historically, virtually all patients were reported to have significant congenital spinal deformity^124,126 averaging 60 degrees during the growth years. Some modern series report a lower incidence, perhaps because of more widespread imaging.^129,131 The incidence increases with age throughout childhood and is more likely to occur with higher lesions.^123,126 Scoliosis is widely attributed to associated vertebral anomalies such as hemivertebrae, spina bifida, or spinal canal widening rather than to spinal cord dysfunction. In instances of tethering, neural traction may also influence curve development, however.¹³² The incidence of split cord malformation among patients with congenital scoliosis is estimated to be 4.9%.¹²⁴ The type of scoliosis is nonspecific, apart from the equal incidence of right-sided or left-sided curves, and some authors have recommended obtaining imaging studies of the spinal cord before undertaking surgical treatment of patients with congenital scoliosis.^124,125

Neurologic symptoms occur as a result of spinal cord tethering and may not arise until adulthood. Symptoms are typical of the tethered cord syndrome and include back pain, gait disturbance, muscular atrophy or deformity of the calf or foot, and spasticity or sensory abnormalities below the level of the lesion with trophic skin changes. Urologic abnormalities occur in 30% to 50% of cases.^129,133 These abnormalities are nonspecific, and other disease entities, such as spinal cord tumor, Friedreich ataxia, and syringomyelia, must be considered. The traction force exerted on the spinal cord may result in brainstem or spinal cord symptoms as the result of an acquired Chiari malformation.¹³⁴ Similarly, the spinal traction associated with corrective casts for scoliosis or operative intervention may lead to precipitous paraparesis.¹³⁵ Late deterioration in adults has been reported after a blow to the back,¹³⁶ after saddle block anesthesia,¹³³ or simply with associated spondylosis.¹³³ Adult patients may also show progressive myelopathy without a precipitating event, which may recover after surgery.⁷⁰ Pain is present in some children and in virtually all adults. It is usually sharp and localizes to the level of the affected spinal segment.

Split cord malformations are now diagnosed in the fetal period using ultrasonography and fetal MRI.¹³⁷ Associated visceral anomalies occur and include horseshoe or ectopic kidney, utero-ovarian malformation, and anorectal malformations. Associated spine anomalies include myelomeningocele and Klippel-Feil syndrome.¹³⁸ The classic radiographic appearance of split cord malformation is a fusiform interpedicular widening of the spinal canal on the anteroposterior view with a midline oval bony mass projecting posteriorly from the vertebral body. The spike may be invisible on lateral views. Particularly in early infancy, the spur may be invisible and may not be in continuity with the vertebral body or laminae, supporting the view that the septum is calcified from a separate ossification center.¹²⁶ Even in childhood and adult cases, the septum may be cartilaginous and invisible on plain radiographs. In such cases, widening of the neural canal should alert the clinician to the possibility of the diagnosis, and generally the adjacent pedicles should not be widened, as would be seen with an expanding intraspinal tumor.¹²⁵ In addition to the widened canal, spina bifida is frequently associated, as are other vertebral anomalies. Scoliosis and kyphosis are common, and their incidence increases with age.¹²⁵ This observation reinforces the importance of early detection because the chance of scoliosis developing increases the longer that patients are left without treatment.

MRI is the current procedure of choice for evaluation of split cord malformation (Fig. 31–21). A coronal study shows the split nicely, but sagittal views may not show each hemicord separately, and the study may mistakenly be considered normal. Severe scoliosis may make MRI difficult to interpret because the twisted cord weaves in and out of the imaging plane. In these cases, myelography with axial CT slices after the injection of contrast material is the procedure of choice. The entire spine should be evaluated for the possibility of secondary lesions, such as an associated tight filum terminale, a lipoma, a dermoid cyst or teratoma,¹³⁹ or a second spur.

FIGURE 31–21 T2-weighted MRI in sagittal plane shows bony spike penetrating through spinal cord (type I split cord malformation).

Indications for surgery include progressive neurologic deficit and prophylactically before treatment of scoliosis. Application of a cast or halo pelvic traction for severe kyphoscoliosis may result in paraplegia if the point of tethering is not relieved.¹³⁵ Similarly, it is advisable to remove the bony spike as a separate procedure before operative correction of scoliosis. Although it is technically feasible to perform both procedures at a single sitting,¹⁴⁰ removal of the septum may result in early loss of evoked potentials owing to spinal cord manipulation, which reduces the safety of the corrective procedure. In addition, instrumentation of the spine is best avoided with intradural procedures when there is a possibility of a CSF leak.

Management of an asymptomatic patient with split cord malformation is controversial. Most investigators working more recently have urged that asymptomatic children and children who are neurologically stable should undergo prophylactic surgery within the first 2 years of life and cited tethering as the cause of progressive symptoms.^67,122 The older literature states that surgery carries potential risk of neurologic worsening and noted that many patients with split cord malformation remain asymptomatic (or with fixed deficit) throughout growth, favoring a conservative approach.^130,141 A compromise would be to consider the relationship of the septum to the intermedullary cleft in deciding the likelihood of progression owing to tethering. An asymptomatic patient may be followed nonoperatively if the septum is separated from the inferior portion of the cleft, but if the septum is at the caudal end of the cleft or higher, prophylactic surgery is advisable. To stratify risk of neurologic deterioration after surgery, Mahapatra and Gupta¹²⁹ proposed a classification scheme based on the amount of bony septum that straddles the intramedullary cleft. Logically, the highest risk cases are cases in which the bony septum maximally occupies the region of cord split.

The role of surgery in a patient with a cleft but no apparent septum is also unclear. In such patients, there seems to be no reason for tethering, but Pang^67,122 found that at operation these patients essentially always have an unrecognized midline fibrous point of tethering and recommended that these lesions be surgically explored. Finally, the indications for surgery in a patient with myelomeningocele and associated split cord malformation are unclear, although it seems logical to explore such a patient and untether the cord in the face of progressive symptoms referable to the lesion.

The surgical technique described by Matson and colleagues¹⁴² and Meacham¹⁴³ is employed. Intraoperative evoked potential recording and rectal manometry have been described,¹⁴⁴ but their value is unproved. In the thoracic spine, the level of the lesion is marked radiographically before the procedure. The patient is positioned as for a standard laminectomy. The paraspinal muscles on each side of the midline are freed and retracted laterally as in any standard laminectomy, but vigorous blunt dissection with a periosteal elevator is avoided because a spina bifida may coexist with the bony septum. The laminectomy is initiated at least one segment above and below the septum and carried out around the bony spike itself, exposing the dural cleft. The cleft usually extends cephalad to the spur but hugs it tightly caudad. A septal elevator is used to free the septum from the surrounding dura, and the superficial portion of the septum is removed by a rongeur or high-speed drill with a diamond bur within the investing dural sheath, which serves to protect the spinal cord. The inferior portion of the spur may extend anterior to the spinal cord at the inferior portion of the cleft, and in this case final removal is deferred until the dural compartment has been opened.

The bony spike frequently has large blood vessels within it, and bleeding is managed with bone wax and cautery. The bleeding associated with the epidural venous plexus surrounding the bony spur and deep to the two hemicords may be substantial and should be controlled with bipolar cautery as the spur is removed. When the cleft is decompressed and the cord moves cephalad, bleeding may be difficult to control.

The dura is opened circumferentially around the cleft, and all intradural adhesions at the cleft side are divided (Fig. 31–22). The island of dura and the remainder of the spike are removed to the level of the anterior spinal canal (Fig. 31–23). A small midline myelotomy at the caudal portion of the cleft may be required to accomplish this. Type II lesions, in which the two hemicords reside within a common dural sleeve, are managed initially by laminectomy over the region of the abnormality. The dura is opened in a paramedian manner because the hemicords are often tethered dorsally at the midline. Fibrous bands arise from the caudal end of the cleft and are displaced inferiorly as they tether to the dorsal dura. These are sharply divided. Median rootlets are often present and are nonfunctional. They may be sectioned if necessary to release the hemicords.¹²² It is neither necessary nor desirable to close the anterior dura. The posterior dura is closed with a graft if necessary. If there is an associated tethering lesion apart from the split cord malformation, this should be addressed at the same time. A separate laminectomy or laminotomy may be required to section a filum terminale.

FIGURE 31–22 Operative exposure of split cord malformation (type I). Dura has been opened around bone spur, which has been partially removed.

FIGURE 31–23 A and B, CT scans before (A) and after (B) removal of bone spur. The entire spur and its investing dura must be removed to untether cord and prevent regrowth.

The procedure should be considered largely prophylactic, although there may be improvement in half of symptomatic patients.⁹⁹ Long-standing deficits and scoliosis are unlikely to improve, but pain is almost always alleviated. Bladder and autonomic symptoms are least likely to improve.¹²⁹ Complications include CSF leakage and worsening of neurologic status. Cases of type II malformation in which there is an oblique septum and a small, delicate hemicord seem to have a higher risk of cord injury.⁶⁷ Failure to improve or late deterioration after operation may be due to failure to remove the tethered spur entirely, failure to address a second lesion such as a hypertrophied filum, or (rarely) regrowth of the septum.¹⁴⁵

Anterior Sacral Meningocele and Currarino Triad

Anterior meningocele is a rare condition in which there is a herniation of the dural sac through a defect in the anterior surface of the spine, usually in the sacrum. The sac is composed of an outer dural membrane and an inner arachnoid membrane. It contains CSF and occasionally neural elements. If the sac is large, it may manifest as a pelvic mass.

Since its original description in 1837 by “a distinguished surgeon” who preferred to remain anonymous,¹⁴⁶ several hundred cases of anterior sacral meningocele have been reported. In 1981, Currarino and colleagues¹⁴⁷ described the association of anorectal anomalies, sacral bony anomalies, and a presacral mass, which has been termed the Currarino triad. The presacral mass in these cases is an anterior sacral meningocele, a teratoma, or both. The triad is a form of split notochord malformation and is inherited as an autosomal dominant trait in 50% of patients.¹⁴⁸ The varied presentations may bring patients to the attention of a wide range of specialists, including urologists, colorectal surgeons, gynecologists, pediatric surgeons, orthopaedic surgeons, and neurosurgeons. When the condition is properly diagnosed and treated, the cure rate is high; if it is improperly managed, meningitis may result in death.

Most anterior sacral meningoceles are congenital, as evidenced by their frequent appearance in infants and young children, the associated anomalies of pelvic organs, and familial incidence.¹⁴⁹ In contrast to the typical posterior myelomeningocele, there is no accompanying hydrocephalus or association with Chiari malformations. In the Currarino triad, there is an abnormal adhesion between endoderm and ectoderm, or there are abnormalities of the notochord that cause a failure of the mesoderm to fuse in the midline. The notochord and somites fuse to form vertebral bodies, separating the endoderm and ectoderm. If the notochord is split, anterior fusion of the vertebral body fails, and a fistula between the gut and the spinal canal is formed. Partial resorption of this connection forms an anterior sacral meningocele or a retrorectal enteric cyst. The combination of mesodermal tissue with these enteric and neuroectodermal elements leads to formation of a presacral teratoma.¹⁵⁰ Genes in the terminal region of chromosome 7q have been linked with sacral dysgenesis^116,151 and may play a role in these malformations.

The lesion is detected far more commonly in women than in men, but in children the sex incidence is approximately equal. Most likely, there is really no sex predilection, and the lesion simply remains undetected in many men.¹⁵² Most clinical manifestations are caused by pressure of the sac on adjacent pelvic structures such as the rectum, bladder, uterus, and sacral nerve roots. Constipation is the most common symptom, particularly in infants and children. Many patients report use of laxatives and enemas. Urinary difficulties may be due to direct pressure of the mass or to pressure on sacral nerve roots. Midline lumbosacral back pain occurs in some patients and may radiate to the perineal area or inner thighs. Some patients report relief after a bowel movement. Headache may be due to either high or low intracranial pressure. Classically, the headache begins in childhood with sudden onset with Valsalva maneuver, as during straining or defecation. In later life, it may accompany coitus.¹⁵³ The headache is reproduced when the pelvic mass is compressed on digital examination. When the patient stands, the meningocele fills with fluid, which may cause a secondary low-pressure headache.

Meningitis may be either septic or aseptic and may be recurrent.¹⁵⁴ Bacterial meningitis may be spontaneous owing to microperforations of the rectum allowing passage of bacteria into the sac, or may result from traumatic rupture of the sac during childbirth. Meningitis may also result from attempted needle aspiration of the sac. Aseptic meningitis has been described, presumably from irritation secondary to an associated dermoid.¹⁵⁵ In women of childbearing age, anterior meningocele may result in obstructed labor or rupture of the sac during delivery.¹⁵³

The cardinal sign is a smooth, cystic mass palpable on rectal or pelvic examination. It is usually adherent to the sacrum. The mass is detectable on routine abdominal examination only when it is enormous. The differential diagnosis of masses includes dermoids, chordomas, and osseous or metastatic tumors.¹⁵⁶

Radiologic evaluation of the sacrum usually shows some abnormality, classically the “scimitar” sacrum. The anteroposterior radiograph shows a well-circumscribed lucent defect on one side of the sacrum corresponding to the sac, and there is no surrounding bony destruction.¹⁵³ Urologic contrast studies often show compression of the bladder and uterus and duplication of the ureters or renal pelvis. CT-myelography of the pelvis may be indicated in some cases and is performed with water-soluble contrast medium to show a small communication between the subarachnoid space and the meningocele sac. MRI is often sufficient to show the communication (Fig. 31–24). Heterogeneous signal intensity on T1-weighted and T2-weighted sequences may suggest a teratoma.¹⁵⁷

FIGURE 31–24 Mid-sagittal MRI of anterior sacral meningocele in a child. The communication was oversewn by posterior approach.

Surgical treatment of symptomatic lesions is generally advisable because there is no possibility of spontaneous regression, and in untreated patients there is a 30% mortality rate owing to pelvic obstruction at the time of labor or to erosion into the rectum followed by meningitis.^152,153 An asymptomatic lesion may be followed without the need for surgery if there is no possibility of pregnancy and if the lesion does not enlarge on repeated rectal examination.

Aspiration through the rectum or vagina or percutaneously should not be performed. If a cyst is discovered at laparotomy for other reasons and CSF is obtained on aspiration, the operation should be terminated, and radiologic studies should be performed to define the extent of the lesion and the nature of its communication with the subarachnoid space.

Surgical treatment via laparotomy has been described historically and has been advocated more recently.^152,158 Most modern authors favor sacral laminectomy as the initial approach, however.¹⁵⁹ This approach allows visualization of the intraspinal contents and resection of adhesions or division of an abnormal filum terminale. Nerve roots can be protected, and there is usually good visualization of the opening into the meningocele to allow suturing. The goal of surgery is to untether the spinal cord, decompress the sac, and obliterate the CSF fistula. It is unnecessary to remove the lining of the pelvic cyst, which may be densely adherent to the rectum. If the dura is fragile or cannot be mobilized sufficiently to allow suture even with a fascial graft, a posterior pelvic operation or laparotomy should be performed with the assistance of an experienced abdominal surgeon.¹⁶⁰

The technique of sacral laminectomy has been reviewed.¹⁶¹ Antibiotic coverage and bowel preparation are begun 48 hours before surgery in case the rectum is inadvertently entered during the procedure. Under general anesthesia, the patient is placed in the prone position with the chest and abdomen supported, and the anus is draped out of the field with a plastic sheet. A lumbosacral laminectomy is performed from L5 to S4, and the posterior dura is opened longitudinally.

Nerve roots within the dural canal are carefully retracted laterally, and the filum terminale is divided, exposing the anterior defect in the dura leading to the sac. If no roots enter the sac and the neck is narrow, CSF may be suctioned from the sac, and the anterior dura may be oversewn with 4-0 nonabsorbable suture. If the sac inserts caudad and is merely a terminal extension of the dural canal without nerve roots within it, the terminal sac may be amputated and the caudal sac reconstructed. If the anterior defect is wide and cannot be mobilized into the field sufficiently for primary closure, digital collapse of the sac through the rectum as the sac is ligated may be helpful, or a fascial graft may be sewn to the edges of the defect. If roots exit through the ostium, the dura has to be plicated around them to permit their exit. If it proves impossible to close a widened stalk, a second procedure with an abdominal approach is necessary. The posterior dura is closed in a watertight fashion. Postoperatively, special care must be taken to protect the wound from fecal contamination, and stool softeners may be given to prevent straining.

The lithotomy position is used in case an abdominal procedure is required.¹⁵² After a urethral catheter is placed, a midline incision is extended from the symphysis pubis to above the umbilicus. The procedure is carried out similar to an abdominoperineal resection of the rectum. The wall of the meningocele is exposed by opening the peritoneum on the right side of the rectum and dissected between the wall of the cyst and the fibrous capsule, keeping the sac intact. When the base has been identified, the sac is opened, and nerve roots are identified. The meningocele is resected at its base, leaving enough membrane to effect a watertight closure. The incision is closed without drains.

Surgical results reported in the recent literature have been generally good.^158,162 Complications include meningitis or neurologic deterioration in cases in which nerve roots have been found within the sac.

Congenital Dermal Sinus and Dermoid Cysts

The term dermal sinus describes a group of congenital malformations in which a tubular tract lined with squamous epithelium extends from the skin overlying the spine inward to varying depths. Termination of the sinus may be in the subcutaneous tissue, bone, dura, subarachnoid space, or filum terminale; within an intradural dermoid expansion; or within a neuroglial mass inside the spinal cord. These cysts may occur in the occipital region of the head or at any level of the spine above the coccygeal area but are most commonly in the lumbosacral area, where they may be confused with pilonidal sinuses or coccygeal dimples that have no connection with the spinal canal.

In early development, the terminal portion of the neural tube and the coccygeal vertebrae are intimately related and perhaps fused to the overlying epithelium. As the spine begins to elongate, the sacrococcygeal ligament applies traction on the overlying skin, producing a dimple in the skin overlying the sacrococcygeal region. There is no connection with the spinal canal, but MRI may show the fibrous connection to the coccyx. These coccygeal pits are reported to occur in about 4% of children.¹⁶³ Some authors have suggested that a “traction test” to determine the rostral or caudal direction of the tract may be useful,¹⁶⁴ but a more useful test is simply to feel the tip of the coccyx directly beneath the dimple. No treatment is necessary except local cleanliness.

The embryonic defect in true congenital dermal sinus is believed to occur early in fetal life at the time the ectoblast differentiates into cutaneous and neural ectoderm. The cleavage between the two layers of ectoderm is incomplete at this point (incomplete dysjunction), giving rise to the sinus, and as the neural groove closes to create the neural tube, cutaneous ectodermal elements are invaginated within the neural tube, which may subsequently develop into an intraspinal dermoid growth. As the conus ascends, the sinus tract elongates. When there is an associated defect of mesodermal organization around the tract, spina bifida results.

More superficial sinus tracts that do not penetrate to the spinal canal are explained as failure of fusion of the cutaneous ectoderm after it has separated from the neuroectoderm. Focal enlargement of the tract may occur at any point to form dermoid or epidermal inclusion cysts. The dermal elements are typically intramedullary in the thoracic region and adherent to multiple nerve roots of the cauda equina in the lumbosacral region. Similar dermal inclusions occur in myelomeningoceles¹⁶⁵ and after lumbar puncture. Pilonidal sinuses are acquired lesions in adults and are believed to result from repeated trauma or an inflammatory reaction secondary to penetration of broken strands of hair. It is possible that some pilonidal sinuses in adults represent chronic infection within a superficial congenital dermal sinus, and the distinction between these entities may be blurred. In practice, the term pilonidal sinus should be reserved for acquired lesions in adults.

Congenital dermal sinuses may be detected during routine examination of an infant or child or deliberately sought after recurrent bouts of meningitis or local infection at the level of the skin.¹⁶⁶ The sinus occurs in the skin as a deep depression, usually in the lumbar region, which may be accompanied by nevi or hypertrichosis. Sinus tracts may occur in the cervical or thoracic regions.¹⁶⁷ Occasionally, multiple sinuses are encountered. There may be a history of an accompanying purulent or clear discharge. Local infection of the sinus tract with staphylococci or Escherichia coli is common. The opening of the sinus tract is invariably in the midline and may be extremely small. Such a sinus opening occurring above the sacrococcygeal region should be explored and excised. Probing and injection of dyes are inadvisable.

Patients may also present with signs of spinal cord or cauda equina compression, from either an expanding dermoid within the spinal canal or an intraspinal abscess. Whether the condition is of gradual or sudden onset, the patient may present with lower extremity weakness, sphincter incontinence, or root irritation and meningismus. Pain in the back and legs is usual with abscess formation or meningitis owing to rupture of the irritating contents of the dermoid cyst. In chronic cases, motor weakness, foot deformity, and reflex changes occur. A young child may refuse to walk.

Dimples below the top of the intergluteal crease require no investigation beyond physical examination. Ultrasound may be a useful screening test,¹⁶⁸ but MRI is indicated in higher lesions to follow the depth of the tract, to determine the level of the conus, and to evaluate the spinal canal for inclusion cysts or abscesses (Fig. 31–25).¹⁶⁹ The tract may be difficult to visualize, and surgical exploration of pores above the sacral region may be indicated even if radiologic studies do not clearly show a communication with the thecal sac. Sinus tracts in the cervical region should be evaluated with cranial MRI because the dermoid tumor and tract may extend into the posterior fossa.

FIGURE 31–25 MRI of spinal congenital dermal sinus tract and lumbar dermoid inclusion cyst. C, conus; E, epidermoid.

Prophylactic surgery is performed as early as possible to excise the entire tract.^166,167 In asymptomatic patients, the lesions may be explored and the tract followed to its termination. A surgeon undertaking such an operation must be prepared to perform an extensive intradural dissection because the tract may extend for a considerable distance. The operation is begun with an elliptical skin incision surrounding the sinus opening and any abnormal skin surrounding it. The tract is sharply dissected and followed through the fascia. A laminectomy is performed if the tract appears to continue to the dura. If the tract attaches to the dura, the dura is opened lateral to the entrance point, and the intradural contents are inspected. Any intradural tract must be followed to its termination even if this involves an extensive laminectomy to the conus because remaining tissue has the capacity to grow into a large dermoid inclusion. Occasionally, it may be unclear if an intradural tract is a hypertrophied filum or a true sinus tract, and frozen section may be useful.

Intradural cysts are completely removed without violating the capsule if possible. If the cyst has ruptured or is infected, a dense arachnoiditis with scarred nerve roots prevents complete excision.¹⁷⁰ In this event, judicious intracapsular removal of purulent material and dermoid elements is performed, but no attempt is made to remove the scarred capsule wall from the nerve roots. If a cervical or thoracic tract terminates in a neuroglial mass, the tract is amputated at the mass, and no attempt is made to remove it. A watertight dural closure is made except in patients in whom closure would compress residual infected dermoid cyst material, in which case the dura may be left open, and the muscle and fascia are closed.

The outcome is usually favorable when all elements of the tract and associated dermal cysts are excised. Subtotally resected dermoids with root attachment warrant follow-up.

Filum Terminale Syndrome

The hypertrophied filum terminale constitutes a relatively uncommon cause of tethered cord syndrome. In its pure form, the conus is bound into a low position by a thickened filum terminale, but more complex variations include filum lipomas, traction bands, and meningocele manqué, a term used to describe aborted or atrophic meningocele sacs that tether the conus.

Patients present with a typical tethered cord syndrome. Cutaneous abnormalities are noted in approximately half of the patients¹⁷¹ and may consist of a sinus tract, a hairy nevus, a hemangioma, an accessory appendage (tail), or an area of atretic skin. Other symptoms include orthopaedic deformity (scoliosis, leg or foot atrophy), progressive weakness, and urologic dysfunction (urinary incontinence, bladder distention, loss of perineal sensation). Back pain is often prominent in teenage and adult patients. The condition is rarely familial.^171,172 This condition is often suspected in association with imperforate anus, and an incidence of 35% has been reported.⁷³ The condition is also suspected in children with voiding dysfunction, particularly if there is associated impaired bladder sensation or poor bladder emptying. The value of screening MRI is controversial in these patients.^173,174

Radiologic investigation usually shows a simple posterior lumbosacral spina bifida but may reveal other vertebral anomalies. Further workup consists of studies to evaluate the level of the conus and may include MRI or supine myelography with CT. Barson¹⁷⁵ measured the level of conus termination in normal infants and children and reported that on average the adult level (L1-2) is reached by 2 months of age. A conus tip below the L2-3 interspace in a child older than 5 years is definitely abnormal. A filum thicker than 2 mm at myelography is also abnormal. Although MRI may suggest the diagnosis with the finding of a low conus on sagittal and axial views, the filum itself may be poorly seen. If the clinical history is suggestive, myelography in the supine position with a follow-up metrizamide CT scan may be worthwhile even in the face of a normal MRI study. A few patients have been reported who have the typical skin and neurologic manifestations of tethered cord syndrome yet have the conus at the normal spinal level on imaging studies. At operation, these patients had thickened or fatty fila and evidence of tethering.^79,176

The term minimal tethered cord syndrome is sometimes applied to such a scenario,¹⁷⁷ in which the patient exhibits urologic dysfunction but lacks the MRI evidence of tethering. In these cases, division of the filum to relieve urinary symptoms must be guided by strong clinical suspicion. In adults, fat in the filum within 13 mm of the conus has been correlated with neurologic dysfunction, independent of conus position.⁷⁴ Conversely, the presence of an asymptomatic fatty filum terminale on MRI is a normal variant, occurring in 0.24% of patients. If the conus is at a normal level, no further workup or treatment is required.¹⁷⁸

If there is evidence of spinal cord tethering (a low conus or clinical symptoms), prophylactic surgery is recommended at the time the entity is discovered to prevent progression of symptoms and to relieve chronic back pain. A few children with spinal cord tethering have been reported to show no deterioration over a period of a few years,¹⁷⁹ but most investigators believe that most patients eventually show progression of symptoms and that prophylactic surgery is indicated. A laminectomy is performed above or below the bifid segment, and the dura is opened. The filum is coagulated and sectioned as low as possible (Fig. 31–26). In some reported cases, clips placed at the cut ends of the filum have separated by 2.5 cm on postoperative radiographs.¹⁷¹ If it proves difficult to identify the filum clearly, it can be electrically stimulated or followed to its termination by performing an extensive sacral laminectomy. Often, small but functional rootlets are intimately attached to the filum and must be identified and dissected away before sectioning.

FIGURE 31–26 Intraoperative photograph of hypertrophied filum terminale. After stimulating fatty filum and eliciting no response on electromyography, filum was sectioned. D, dural sac; F, thickened filum terminale.

Terminal syringohydromyelia is reported with meningocele manqué and with tight filum anomalies and may be symptomatic.^180,181 This condition is usually treated by untethering the cord and repeating imaging a few months later. If the terminal syrinx persists and is believed to be the cause of symptoms, shunting the cyst to the subarachnoid space, pleura, or peritoneum may be required.

The results of surgery are favorable in relieving back pain and preventing the progression of existing deficits, and often some long-standing neurologic deficits may improve. In the series by Anderson,¹⁰⁵ urinary incontinence improved in 14 of 33 patients, and weakness and numbness of the legs lessened postoperatively in one third. Back and leg pains were relieved in virtually all children and adults in several series.^69,105,182 Retethering is rare but is reported.¹⁸³

Neurenteric Cyst

Although neurenteric cysts are quite rare, they are well-described entities consisting of cystic structures within the spinal canal or anterior to the vertebral bodies in the mediastinum, abdomen, or neck. Ventral cysts may have connections via a stalk to the meninges and spinal cord through a tunnel-like defect in the vertebral bodies. The cyst wall resembles foregut tissue histologically, and in the literature these lesions are also referred to as enterogenous cysts.¹⁸⁴ They may occur in isolation or as part of a more complex group of spinal anomalies, including Klippel-Feil syndrome, spinal lipoma, and syringomyelia.^185,186 They may occur at any spinal level, but most are between C3 and T7.¹⁸⁷

Early in the development of the human embryo there is a communication between the yolk sac (destined to become the gut) and the dorsal surface of the embryo called the neurenteric canal. This fistulous tract transiently connects the future enteric cavity with the neural groove in the region of the coccyx. Bremer¹⁸⁸ pointed out that abnormal accessory neurenteric canals may persist cephalad to the coccygeal tip and can give rise to cysts along the persistent tract. The tract also gives rise to associated vertebral anomalies: a widened vertebral body resulting from bony proliferation in an attempt to fill the gap after disappearance of the tract or a circular defect in the vertebral body produced by the persistent tract that forces bone to form around it. Because the accessory neurenteric tract can be elongated and stretched during growth, the canal may become divided into noncommunicating diverticula that lie at some distance from each other (e.g., in the thoracic cavity and spinal canal). The tendency for the cysts to lie to the right of the vertebral column has been ascribed to the developmental process of gastric and gut rotation.¹⁸⁹ Persistence of only a portion of the tract results in a cyst only within the spinal canal.¹⁹⁰ The cysts are usually anteriorly located within the canal (Fig. 31–27), although dorsally located ones are described.¹⁹¹

FIGURE 31–27 MRI of cervical spine shows anteriorly placed neurenteric cyst compressing spinal cord and causing myelopathy.

Neurenteric cysts within the spinal canal or ventral masses are similar pathologically. They may be thick, well-defined capsules or delicate, clear walls that may be confused with spinal arachnoid cysts. They are described as strawberry-colored¹⁹² or whitish¹⁹⁰ and when in the spinal canal may be intradural-extramedullary¹⁹³ or intramedullary.¹⁹⁴ The cyst fluid is usually described as clear or milky and quite gelatinous. There may or may not be a ventral dural defect.

Histologically, the cyst wall typically is formed of endodermal ciliated columnar epithelium resting on a basement membrane and may contain gastric parietal and chief cells or mucin-producing goblet cells (Fig. 31–28).¹⁹³ The epithelial cells are positive for keratin markers. Some investigators have made a distinction between neurenteric cysts, with well-differentiated gastrointestinal epithelium and a clear connection of the spinal cyst and the chest and abdomen, and “teratomatous cysts,” which have less differentiated epithelium and no ventral connection.^195,196

FIGURE 31–28 Light microscopy with hematoxylin and eosin staining of neurenteric cyst. Epithelium resembles gut.

Neurenteric cysts are usually detected in childhood,^{190,193,194,197,198} although adult patients have been reported.^197,199,200 Neurenteric cysts have been detected by prenatal MRI or ultrasound.²⁰¹ The diagnosis is suggested by the combination of a thoracic cystic mass with vertebral anomalies. In early childhood, the lesion may manifest as a chest or abdominal mass with cardiorespiratory compromise^202,203 or as a neck mass with tracheal compression or may be detected because of vertebral anomalies seen on a radiograph. Adults and older children tend to present with signs of spinal cord compression in the cervical or thoracic regions,²⁰⁴ which may be acute or follow trivial trauma.²⁰⁵ In addition to motor weakness, burning dysesthesias may occur when aggravated by coughing or sneezing or recumbency. Rarely, a neurenteric fistula may lead to meningitis with bowel organisms.²⁰⁴ Associated malrotation of the gut²⁰⁶ and diaphragmatic hernia²⁰⁷ may be found.

Plain radiographs may show widening of the neural canal, cleft vertebrae, or fusion of vertebral segments.²⁰⁷ An anterior soft tissue mass may be seen in the chest, neck, or abdomen, and the association of such a mass with vertebral anomalies should suggest the diagnosis. The presence of the classic circular defect through the vertebral body, although diagnostic, is not essential.²⁰⁷ Patients in whom the diagnosis is suspected should undergo MRI²⁰⁸ and possibly myelography with water-soluble contrast medium and postmyelogram CT to define the bony anatomy and establish the presence or absence of a communication between the spinal canal and a ventral mass. MRI shows a cystic mass and its relationship with the spinal cord.¹⁹⁰

The treatment of neurenteric cysts varies with location and presentation. An intraspinal lesion can be corrected first, at a separate procedure, or as part of a combined procedure, such as thoracotomy, to excise the prevertebral lesion. Because the spinal mass is largely cystic, the traditional treatment has been a wide laminectomy with cyst drainage and conservative resection of cyst wall.¹⁹⁷ Some authors have advocated radical resection of the entire cyst wall, however, which may require a lateral approach²⁰⁰ or corpectomy^189,199 to approach lesions located ventral to the spinal cord. A cyst-subarachnoid shunt has been described,²⁰⁶ but the thick nature of the fluid and its possible irritation of the meninges make this unlikely to be useful in most cases. If there is an anterior dural defect, this may be plugged with a muscle or fascial graft from a posterior approach or ligated or sutured if a thoracotomy is performed.

Patients generally recover neurologic function if the deficit is not severe at the time of operation. Good outcomes can still be attained with subtotal resection.²⁰⁹ Recurrence has been noted after incomplete resection, but the reported recurrence rate in subtotally resected cysts ranges from 11% to 75%.^204,209 In children in particular, delayed kyphosis or swan neck deformity may follow extensive laminectomy.