Pathology

The term tethered cord syndrome, as used in this chapter, signifies a pathologic fixation of the spinal cord in an abnormally low position so that the spinal cord, with activities and growth, undergoes mechanical stretching, distortion, and ischemia.¹ Many conditions can cause tethering of the spinal cord, including tight filum terminale, split cord malformations, lipoma, dermal sinus, and meningomyelocele. The remainder of this section addresses the care and treatment of these conditions, excluding meningomyelocele.

Presentation

The patient with a tethered cord may be either symptomatic or asymptomatic. Both groups often, but not invariably, have a midline cutaneous dorsal abnormality such as a dimple, a hairy patch or faun’s patch, a hemangioma, a lipoma, or a skin tag (Figs. 116-1 and 116-2). If there is no external manifestation, the problem usually goes unrecognized until symptoms begin to develop.

FIGURE 116-1 Skin tag and angiomatous changes in a child with a tethered cord and spinal lipoma.

FIGURE 116-2 5-year-old girl with faun’s patch.

Symptoms, when present, can be grouped into three general areas: sensorimotor, sphincteric, and orthopaedic. Sensorimotor symptoms can include pain, delayed walking, sensory loss (usually in the dermatomes of the lumbosacral roots), and motor weakness of the distal leg or foot (the most common symptom, presenting in 76% of cases).² Sphincteric symptoms usually are insidious, with frequent urinary tract infections secondary to incomplete emptying, hydronephrosis with renal involvement secondary to reflux, and fecal incontinence. In addition, these patients may develop urinary incontinence or become impotent. The orthopaedic problems are related to gait disturbance and abnormalities of the foot or scoliosis. Adult patients with tethered cords also may present with back pain that may radiate to the legs, urinary difficulties, and lower extremity weakness.³ Patients may present either as asymptomatic in childhood and symptomatic in adulthood or as having a progression of symptoms once they reach adulthood, perhaps due to repeated microtrauma to the cord.⁴ In adults, the disease may have an insidious onset or may have a predisposing factor such as exercise, lifting heavy loads, or even birth trauma.^3,5

Diagnostic Aids

Any infant or child with a midline cutaneous lesion such as a dimple, hair patch, or hemangioma or symptoms mentioned earlier may be suspected of having a tethered cord.⁶ The intention of the workup is to determine the anatomy of the anomaly.

A careful neurologic examination with attention paid to evaluation of motor and sensory function as well as sphincter tone is imperative. If the history indicates possible bladder involvement, a urologic evaluation is indicated. Plain radiographs, which may be obtained as a screening procedure, may show a widened interpedicular distance and defects at one or more levels. The procedure of choice is MRI (Fig. 116-3). Often, this is the only necessary imaging study, and treatment can be planned on the basis of MRI alone. It is prudent to image the entire spinal cord at least once to rule out other associated anomalies, such as a syrinx or a type I Chiari malformation. If there are any remaining concerns or issues, a myelogram with subsequent CT may be helpful.

FIGURE 116-3 T1-weighted MRI demonstrating a tethered spinal cord.

Treatment

Once the problem is identified, the treatment of the tethered cord is surgical. Although controversy existed in the past (over the concept of prophylactic surgery of asymptomatic patients), most surgeons now believe that the risk of waiting for deterioration to begin is not justified, because the deficit often is not reversible. Therefore, surgery is recommended, even in the asymptomatic patient,⁷ although a recent study⁸ suggests that careful follow-up and monitoring for upper motor neuron signs using urodynamic assessments, in order to time surgical intervention to coincide with the appearance of upper motor neuron signs, may be possible. In adults, patients with back pain and lower extremity pain seem to benefit more than those with sphincter problems.³

The goal of surgery is to untether the spinal cord and to avoid incurring further neurologic deficit. It is crucial to expose areas of normal anatomy and then proceed to the abnormal area. The optimal surgical approach should be as low as possible in the lumbar spine so as to decrease risk of injury to the conus. Spinal deformity or instability after multilevel lumbar or thoracolumbar total laminectomy is not uncommon in children and adolescents. Limiting laminae removal and facet destruction may decrease this incidence. Fusion may be required to correct postlaminectomy deformity and to stabilize the spine. An operative approach at the L3-4 junction may be preferable to the L5-S1 level because the lumbosacral junction may present increased risk of postoperative instability.

Surgical treatment may include a single-level laminectomy with partial superior and/or inferior laminotomies. One alternative surgical approach is bilateral laminotomies at one or two lumbar levels with the use of a high-speed drill. The inferior interspinous ligament is then resected. Elevation of the lamina is done in a lobster-tail–like fashion to expose the tethered cord elements. Following surgical resection, the lamina may be replaced and sutured in place to restore the dorsal spinal column and tension band.

Care should be exercised intraoperatively because the periosteum under the lamina may appear as a separate layer and be mistaken for the dura. Once exposed, the filum often is identified in the midline and may appear fatty, fibrous, or thickened. Care should again be taken to inspect the filum to rule out the presence of attached nerve roots. Vascular supply should be identified on the ventral surface and cauterized before resection, because subarachnoid blood may potentiate nerve root adhesions. After release has been completed, and following watertight dural closure, the surgeon should assess for CSF leak by having the patient perform a Valsalva maneuver. Closure may be reinforced with fibrin glue or dural glue to further reduce the probability of CSF leak.

Electrophysiologic monitoring, especially the use of intraoperative electromyography (EMG), may be helpful. This is performed by inserting needle electrodes into the appropriate muscles and the anal sphincters under anesthesia. During surgery, the level of each root is accurately established by intradural nerve root stimulation, and before dividing any structure, it is stimulated to confirm that no neural structures are involved. The same probes that are used for a dorsal rhizotomy may be employed, although any nerve stimulator may be used. The advantage of this technique is that the response to EMG is very rapid and provides a substantial safety margin during the surgery for the tethered cord syndrome.

Complications

Several complications may occur during surgery. As with any clean neurosurgical procedure, however, the incidence of infection should be low.

CSF leak is a very uncommon complication. After careful dural closure, fibrin glue is used, and a Valsalva procedure usually is performed to check for leakage. If postoperative cutaneous leakage occurs, reoperation is mandatory. The patient should be kept flat for 3 days after surgery, with sedation if necessary.

Another potential complication is urinary retention. A Foley catheter always is inserted after induction of anesthesia and removed on the third to fifth day. If the child is unable to initiate micturition after catheter removal, intermittent catheterization is used until normal voiding is reestablished. If the child does void, a postvoid residual urine volume should be checked to verify adequate emptying. No more than 10 to 30 mL of urine should be left after the child has voided.

Outcome

The shorter the duration of symptoms, the better the prognosis.⁵ A study by Archibeck et al.⁹ demonstrated a 50% revision rate by 5 years after initial revision and a 57% revision rate by 2 years after the second release. In addition, 50% of patients required at least one orthopaedic procedure after tethered cord release.⁹ In a study by Cornette et al.⁸ of 12 patients operated on for tethered cord, none required a second operation. However, the series is small, although the follow-up period was reasonable (58 months). In terms of urologic outcome, improvement in symptoms as well as urologic dynamic parameters is expected in most patients, although few if any will return to normal.¹⁰ Improvements may be noted in detrusor function, EMG recordings, and pressures.¹¹

Embryology and Pathology

The term split cord malformation (SCM) was introduced by Pang et al.¹² in 1992 to describe diastematomyelia based on the dural tube and the nature of the septum. Two types of SCM exist: type I, diastematomyelia with septum, and type II, diastematomyelia without septum. Type II is more common.

By the end of the second week of gestation, the human embryo normally consists of a bilaminar structure: (1) an epiblast, or layer of cells next to the amnion, and (2) a hypoblast, or layer of cells next to the yolk sac. From there, the cells divide to form the primitive streak. During gastrulation, the embryo becomes trilaminar as adjacent epiblastic cells migrate medially toward the primitive streak to become mesoderm. The primitive streak begins to regress by day 16,¹² and the notochordal process begins. As the notochord elongates, it canalizes, initially forming a connection through the embryo to join the amnion and yolk sac. This connection is then lost as the open notochord separates from the endoderm and again forms a blind tube.¹²

In the split cord malformations, an adhesion forms between the ectoderm and endoderm, leading to the formation of an “accessory neurenteric canal around which condenses an endomesenchymal tract that bisects the developing notochord and causes formation of two hemineural plates.”¹² Whether a type I or type II SCM is formed depends on what happens to the endomesenchymal tract. If it develops toward bone and cartilage, the result will be two dural sacs and a type I SCM. If the tract regresses or leaves a fibrous septum, a type II SCM will develop.¹³

The spinal cord above and below the split is normal. The two hemicords themselves usually are the same size, but in 10% of patients, they are grossly asymmetrical. When this occurs, the spinal cord itself, above and below the bifurcation, is asymmetrical, being smaller on the side of the smaller hemicord.

The anterior spinal artery and the central canal bifurcate to accompany each hemicord,¹⁴ so that each has its own blood supply. The two hemicords give rise to the spinal nerve roots on their respective sides. Although splitting of the spinal cord at more than one site and cases of incomplete splitting of the spinal cord with a resultant partial cleft cord have been reported, most cases involve a single, complete cleft through the spinal cord and meninges. In cases in which there are two hemicords without an intervening septum, a single dural sac surrounds both. In such cases, symptoms may result from tethering of the cord by fibrous bands or a thickened filum terminale.

In cases in which the meninges themselves also are bifurcated, there almost always is an intervening septum. Its position is at the caudal end of the split; therefore, ascent of the neural elements is prohibited. The septum, or spur, usually is attached to both the dorsal elements and the dorsal aspect of the vertebral body. Because of the incidence of spina bifida, the spur may continue dorsally between unfused laminae. These spurs may present anywhere along the spine, but in 70% of cases they are between L1 and L5. They are less likely to occur in the thoracic spine and have only a 1% incidence in the cervical spine.^15,16 The spur initially is cartilaginous and may mature to calcified bone with time.

Other associated anomalies, such as vertebral body abnormalities, may occur. Hemivertebrae, butterfly vertebrae, blocked vertebrae, and spina bifida may contribute to a kyphoscoliosis. The scoliotic defect and segmental vertebral anomalies commonly are located near the level of the split cord malformation. In addition, many children with these anomalies have hypertrichosis over the level of the spur, clubfoot, or pes cavus. Twenty percent of cases are associated with other abnormalities of the spine, including hydromyelia, lipoma, dermal sinus, and neurenteric, epidermoid, and arachnoid cysts. Unless a preexisting myelomeningocele exists, Chiari malformations usually are not associated with split cord malformations.

Pathophysiology

The clinical symptoms most likely evolve from traction of the spinal cord against the restricting septum or bony spur.^1,17 As with other forms of tethered spinal cord, ascent of the cord within the dural sac and spinal canal is prohibited. The average age of presentation is 6½ years, with neurologic symptoms first becoming evident with the onset of walking.¹⁶ With the onset of walking, however, increased traction of the distal spinal cord against the restricting septum results in new symptoms. To support this finding, Yamada et al.¹⁸ have studied the oxidative metabolism of the distal spinal cord and have found a decrease when the cord is under axial tension.

Presentation

Boxes 116-1 and 116-2 include some of the presenting symptoms and physical signs of patients with split cord malformations.¹⁹ In general, signs and symptoms fall into three categories: (1) cutaneous abnormalities, (2) pain, and (3) neurologic deficits (from spinal cord traction).

In newborns and infants in whom neurologic deficits may not yet have developed, cutaneous lesions bring the child to the attention of the neurosurgeon. Most commonly, a patch of hair or hypertrichosis is noted in the thoracic or lumbosacral midline dorsally. This hair, usually coarse and long, is sometimes referred to as faun’s tail. The surrounding skin is associated with an intradermal angiomatous malformation, giving the skin a pinkish blue color. In addition, a dermal sinus, lipoma, abnormally protuberant spinous process, or meningocele may be associated with the spur.

As the child develops, begins to walk, and acquires bowel and bladder control, the neurologic sequelae of split cord malformations usually appear. Hypoplastic lower extremity and foot deformities sometimes are present at birth. Progressive kyphoscoliosis also may become noticeable. With the onset of walking, a limp, an ulcer secondary to areas of anesthesia, and the new onset of bowel or bladder incontinence after a period of normally developed continence all indicate tethering. Although spasticity and other long-tract signs are not common, hyporeflexia and loss of sensation in the sacral dermatomes are common.

If the patient with split cord malformation has successfully progressed through development with few or none of the aforementioned symptoms, the most common complaint, particularly in older children and adults, is back or leg pain.²⁰ This pain may be due to subtle concomitant scoliosis or to the spinal bony deformity itself. The presence of unilateral symptoms is a key difference in the presentation of split cord malformation versus tethered cord syndrome.

Diagnostic Aids

Aids that confirm the diagnosis of split cord malformation usually are radiologic. Although plain radiographs or unenhanced CT scans of the spine may reveal the bony spur, a widened interpedicular distance, spina bifida occulta, or other segmental vertebral anomalies, MRI in all three axes can be more revealing and is the preferred procedure (Figs. 116-4 to 116-7). Associated lipomas, hydromyelia, and other intraspinal and intradural defects also may be observed incidentally, allowing for a more focused treatment approach. If there are any questions or further clarification is required, myelography and postmyelographic CT best delineate the hemicords, the dural sac, and the presence and extent of the intervening bony septum (Fig. 116-8).²¹ Plain and CT myelography may reveal aberrant nerve roots, intradural bands, a thickened filum terminale, or a concomitant intradural lipoma. In addition, a recent study²² reported an incidence of abnormal urologic dynamic studies as high as 75% in patients with SCM, despite a lack of symptoms. Therefore, obtaining preoperative and postoperative urologic dynamic studies may be of some benefit. Again, as in the tethered cord syndrome, the entire spinal cord should be imaged.

FIGURE 116-4 T1-weighted MRI demonstrating a tethered spinal cord with associated lipoma.

FIGURE 116-5 Plain axial CT scan revealing the bony septum (arrowhead) of split cord malformation.

FIGURE 116-6 Sagittal T2-weighted MRI demonstrating split cord malformation, with the septum spanning from the ventral to the dorsal elements (arrowhead).

FIGURE 116-7 Axial T1-weighted MRI revealing the septum (arrowhead) and hemicords of split cord malformation.

FIGURE 116-8 Axial postmyelogram CT scan demonstrating split cord malformation resulting from a nonossified fibrous septum (arrowhead). Note the delineation of the hemicords.

Treatment

With the exception of incidental findings of split cord malformation in newborns, any patient with signs or symptoms referable to split cord malformation should be promptly untethered to relieve symptoms, preserve function, and possibly reverse neurologic deficits. In the otherwise normal newborn, surgery should be delayed for about 3 months, because the older child will be larger, will tolerate anesthesia better, and will have developed more resilient meninges and soft tissues, allowing for more secure surgical closure.

The goal of surgery is to untether the hemicords by removing the cartilaginous, fibrous, or bony septum, as well as the dural tunnel about the septum, which itself tethers the cord. In addition, any surrounding dural adhesions restricting the motion of the spinal cord should be lysed.

Under general anesthesia, the patient should be positioned as for a laminectomy, in the prone position, either on chest rolls or on a flexed frame with adequate room for abdominal wall motion. Unless an associated tethered filum terminale that requires incision is expected, there is no need for preoperative placement of sphincter and lower extremity EMG electrodes.

Before making a standard midline incision to the lumbosacral fascia, the spine should be palpated. Occasionally, a protruding spinous process or bony spur may be felt, allowing for a more localized incision. In addition, a localizing plain radiograph with a skin marker is used and correlated with the preoperative MRI. If a cutaneous lesion, such as a patch of hair, is present, it may be beneficial to create an elliptical incision circumferentially around the defect. Because the underlying bony and soft tissue defect may not be clear, it is helpful to incise the fascia and perform a subperiosteal reflection of the paraspinous musculature at the levels above and below the level of the lesion, understanding that midline fusion defects may also exist here. The monopolar electrocautery should be used cautiously in retracting the muscles, because areas of expected protective bone may be missing. After the laminae above and below the lesion are exposed, their spinous processes are removed using a rongeur. A partial laminectomy is then performed at the caudal aspect of the lamina above the septum and the rostral aspect of the lamina below the septum. After careful curettage of the underside of both these laminae, the ligamentum flavum, if still intact, is elevated laterally with Penfield forceps and incised longitudinally through its outer layer. A blunt instrument is then gently inserted through the remaining ligament, and a small cottonoid patty is placed between the dura mater and the ligamentum flavum for protection. A small, angled Kerrison punch is then used to remove the ligamentum flavum until the dura mater is completely exposed laterally. With a no. 4 Penfield dissector, the septum is then felt over the dura from above and from below. A small-mouthed rongeur or angled Kerrison punch or high-speed drill is used to remove the lamina and overhanging bone of the involved level until only the spur is left. Because of the substantial epidural venous plexus associated in and around the bony spur and deep to the two hemicords, control of bleeding and cauterization of these vessels should be performed prior to and during the removal of the spur (Fig. 116-9). After decompression, with movement of the hemicords, it may be very difficult to maintain hemostasis. Any extruding segment of spur is removed using a rongeur, and a small dissector is used to probe and dissect the dural sheath away from the bony spicule down to the level of the dorsal vertebral body. A high-speed diamond-bit drill is then used to carefully thin down the spicule as far as possible.

FIGURE 116-9 Intraoperative photograph showing the two hemicords (arrows) in a patient with split cord malformation.

At this point, the dura mater is opened along the midline above and below the spur and elliptically around the spur remnant. After the dural edges are tacked up, the two hemicords become evident. The ventral dural sac is then incised along the midline above the spicule and circumferentially around it. The dural “chimney” is removed, and the resultant dural edges are teased laterally so that the remainder of the spur may be completely drilled off, with the spinal cord being protected at all times. With the removal of the spur, the entire cord may migrate rostrally. Microinstruments should be used to break or cut any additional adhesions that may be tethering the spinal cord to the dura mater. A watertight closure of the dorsal dural opening is then performed, with interrupted sutures used over the elliptical incision to avoid the unraveling of a running stitch over this area of tension; again, fibrin glue is used. The ventral dura mater does not require closure. A small drain is inserted, and the soft tissues are closed in anatomic layers. The patient should then be kept flat for 3 days postoperatively to avoid CSF leak.

Complications

If there are persistent CSF leaks from the suture line, the suture line should be oversewn or patched until the leak stops. It is not uncommon to have a small amount of CSF leakage from the suture holes of the dural closure. If it is minimal, it should not pose a problem. Although infrequent, a persistent CSF leak must be addressed to avoid incision breakdown, infection, and, possibly, meningitis. If a CSF leak from this source should occur, placing a lumbar subarachnoid drain above the incision and having the patient lie flat in bed for 3 days should allow for closure. If the leak is from the dorsal dural suture line, a drain may again avoid reoperation, but failure to stop the leak will necessitate reexploration and primary dural closure. If the meningeal tissues are found to be weak and nonresilient, a pericardium, fascia lata, or other dural graft may be required. Fibrin glue or a dura substitute (DuraGen [Integra Life Sciences, Plainsboro, NJ]) may also be of some value.

As always, postoperative epidural hemorrhage is possible. Because of the apparent increased venous drainage in the defect, hemostasis may be difficult. A delayed postoperative neurologic deficit should be worked up immediately, with either MRI or myelography. Both epidural hematomas and subdural extension secondary to ventral durotomy should be evacuated immediately. Normal coagulation studies and adequate hematologic status should be verified, and corrected if necessary. As mentioned earlier, the placement of a drain in the epidural space for 24 hours may be considered at the time of initial repair, although this should not replace meticulous surgical technique. This may prevent the accumulation of a mass lesion, and rarely does it induce leakage of CSF from the underlying suture line.

Maintaining a sterile field, administration of perioperative antibiotics, and gentle handling of tissues will aid in preventing meningitis and wound infections. If these should occur, appropriate intravenous antibiotics should be administered. The goal is to prevent a subsequent epidural or subdural empyema.

Iatrogenic traction, contusion, or direct injury with an instrument or a drill may inadvertently occur intraoperatively. Care should be exercised continuously to prevent unnecessary manipulation and traction on the spinal cord.

Outcome

In symptomatic patients, bowel and bladder dysfunction may improve up to 40% of the time; stabilization of progressive urologic symptoms also may be noted.²³ Neurologic sensorimotor deficits return to normal only 5% to 10% of the time. Patients whose main complaint is pain in general improve. Also included in improvement of pain is the dysesthetic component. A higher surgical morbidity has been reported in cases in which the bony septum is present, perhaps due to removal of the bony septum.²³ It is important to remember that preserving neurologic function is as important as improving it. Recent studies^23,24 also report that untethering the cord may have no effect on the neuro-orthopaedic syndrome (e.g., lower limb asymmetry, foot deformities), perhaps due to irreversible changes in the ligaments.

Pathology and Embryology

Dorsal congenital dermal sinus, a subtype of spinal dysraphism not associated with spina bifida or bony abnormalities, is defined as an epithelium-lined tract from the skin of the back, usually the lumbosacral midline (although it also may occur in the thoracic and cervical spine),¹³ that passes through the soft tissues toward the spine, the thecal sac, and even into the neural elements. These tracts, which usually are very thin, are thought to develop because of adhesion and failure of separation between the superficial cutaneous ectoderm and the neural tube.²⁵ This failure of separation usually occurs at the fourth week of fetal development, after neurulation of the tail bud. At that time, the attachment between skin and spinal cord is lengthened and thinned with the ascent of the cord and fixation of the skin. Because of its small size, the surrounding bone-forming mesoderm may produce little or no spina bifida. Also, the remainder of the vertebral body at the affected levels usually is normal. Although not usually a cause of tethering of the spinal cord, the cutaneous origin of the sinus tends to be two to three vertebral levels caudal to its adhesion to the neural elements. Congenital dermal sinus is present in 1.2% of the neonatal population.

Because of the epithelial lining and potential communication with the skin, the dermal sinus may result in an expanding dermoid or epidermoid tumor in the subdural or epidural space, in the same manner that such tumors arise from iatrogenic implantation of such elements with spinal needles.²⁶ These tumors may present as mass lesions or as a simple dermal sinus associated with a possible communication between the skin and subarachnoid space. Therefore, they are a nidus for infection, meningitis, and possibly abscess formation. Microscopically, they consist of dermal elements, such as sweat and apocrine glands and hair follicles.

Most dorsal dermal sinuses are lumbosacral; occipital sinuses are less common. Although about 60% of dermal sinuses end in dermoid or epidermoid tumors, only 30% of these tumors are associated with dermal sinuses.^27,28 In addition, the dermal sinus may be involved in tethering of the spinal cord, not by itself but either by a thick band of tissue attached to the spinal cord or conus medullaris or as a result of inflammatory scarring.

Pathophysiology and Presentation

Because of the low incidence of tethering with dermal sinuses, symptoms result from infectious etiologies. Bacterial causes may include Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, and even Proteus species.²⁹ The tract from the bacteria-laden epidermis to the intraspinous space and even the subarachnoid space provides opportunities for intermittent, chronic, and acute infections.^30,31 Although the incidence of meningitis is higher with the presence of a concomitant dermoid or epidermoid tumor, simple tracts carry this risk as well.³² In fact, dermal sinuses may be the cause of infection in up to one quarter of cases of intramedullary spinal abscesses, with approximately 70% of patients having neurologic deficits.³³ Because of the dimple formed in the skin over the tract, infections are noticed and addressed, often before clinical symptoms have surfaced. The dimple, or the external ostium of the sinus, usually is in the midline and may be associated with a hemangioma, a nevus, or short tufts of hair protruding from the sinus. Parents, caretakers, or physicians may notice caseous discharge from this area or perhaps some erythema or inflammation. Because of the usual lumbosacral location, fecal and skin organisms may be the cause of meningitis. Frank neurologic symptoms or neurogenic pain rarely is present unless there is compression from an associated tumor or tethering of the spinal cord or conus, as mentioned previously.

On examination, neurologic function is usually found to be preserved. The dimple should not be probed, in order to avoid lodging infection-forming organisms into the deeper sinus. Manipulation and pulling the skin around the ostium in different directions often reveal further umbilication of the tract, thus confirming that the margins of the tract are fixed to the deeper tissues. Palpation of the spine itself may reveal prominent or absent spinous processes, indicating a dorsal element malformation. It is important, however, to distinguish a low-lying dermal sinus pore from a simple pilonidal dimple, which usually is more caudal, is near the tip of the coccyx, and usually requires no further workup. However, if the dimple is over the sacrum or higher, further evaluation is indicated.

Diagnostic Aids

Although the index of suspicion for congenital dermal sinus is high solely on the basis of the examination, neuroimaging is helpful to discern its extent and the presence of associated tethering and tumors. Injection of contrast material or probing the tract is not recommended because of the risk of inducing meningitis and infection.

Ultrasound may be used to assess for the presence of spina bifida and also to evaluate for the presence of possible cyst. However, it is useful only for infants 4 to 5 months of age. The limited ability of ultrasound to detect underlying abnormalities has kept it from being used as an initial diagnostic tool.

MRI is the initial imaging modality of choice (Fig. 116-10). It is useful to follow the denser, low-signal sinus through the high-signal subcutaneous fat toward the dura mater. The intraspinal course of the tract, however, may not be well displayed by MRI in all cases. Dermoid and epidermoid tumors have variable signal intensity on MRI. Dermoids, with increased T1 and T2 signals compared with water (because of the cholesterol, fat, and protein content), image well. However, if chemical meningitis has been caused by leakage of the tumor, detection may be more difficult. Although epidermoid tumors contain only epithelial elements, they are equally well defined by MRI.

FIGURE 116-10 Sagittal T1-weighted MRI revealing a tethered spinal cord that appears to be contiguous with a dermal sinus tract (arrowhead). Note the overdistended, previously undiagnosed, neurogenic bladder.

Axial CT imaging reveals the dense sinus tract through its course from the skin into the dura mater. Occasionally, the lumen of the tract may be visualized as a more hypodense line within the tract. CT scanning with intra-arachnoid myelographic contrast medium is the most useful method for imaging the course of the sinus in the subdural space as it ascends toward the conus medullaris.³⁴ Water-soluble contrast or air should be used to avoid leaving oily droplets that could act as potential foreign bodies. A lucent mass along the tubular filling defect may represent dermoid or epidermoid tumors. The use of contrast material has been reduced dramatically since the advent of MRI, however. Even in the presence of a normal MRI, if a cutaneous lesion seems to truly represent a dermal sinus in a suspicious location, surgical exploration may be warranted.

Treatment

The treatment of a true dermal sinus is surgical. Even if the MRI is normal, the sinus tract should be excised. Fewer than 25% of sacral sinuses at birth will regress to a simple dimple if left untreated. Technically, surgical excision of the dermal sinus may range from a very simple procedure (when it ends in the soft tissue) to a very difficult and complex operation (if it is adherent to the conus medullaris or is associated with one or more dermoid or epidermoid tumors). When a patient presents with soft tissue infection or meningitis, the infection first should be treated adequately with intravenous antibiotics. The surgery should not be performed until the CSF is sterile. In any case, antibiotics should be used both perioperatively and postoperatively.

Loupe or microscope magnification should be used after the dura mater is exposed. The patient is placed on chest rolls or on a frame after general anesthesia, as for a laminectomy. The skin and ostium of the sinus should be sterilized, and access should be available from the spinous process of T11 to the coccyx. A midline incision is then performed from at least two spinous processes rostral to the dimple to one spinous process below it. An elliptical incision is made around the ostium itself, including any associated nevus or other cutaneous abnormality. The skin edges around the sinus may then be secured and manipulated with a heavy silk suture or a clamp with teeth.

Use of traction on the elliptically incised skin and ostium allows for easier dissection along the tract. The tract is followed with Metzenbaum-type scissors as it dives into the soft tissues, until the lumbosacral fascia is reached. A blunter instrument, such as a small curved hemostat, is then used to follow the tract through the fascia, caudally to rostrally, while holding traction on the tract in the rostral direction. Care should be taken not to incise or avulse the tract, to prevent the remaining stump from retracting under the fascia, which makes it very difficult to locate. Care also should be exercised to avoid undue traction on the sinus tract in case it is contiguous with the neural elements. When the direction and location of the subfascial tract have been determined, the fascia above and below may be incised on either side of the spinous processes above and below the tract. Again, using subperiosteal dissection, either with a gauze and periosteum elevators or with traction and a monopolar electrocautery, the laminae are exposed. This leaves the line of spinous processes, usually with the dermal sinus tract disappearing into the interspinous ligament of the involved level. In the case of dorsal element abnormalities and spina bifida, care should be taken, particularly with the electrocautery, to protect the underlying dura mater and its contents. A no. 4 Penfield dissector is then used to probe the tract as it enters the spine, to determine the tract’s position and direction. The flavum ligamentum, under upward traction with forceps, can be carefully incised. Often, the sinus tract may end in the interspinous ligament, and amputation here can be followed by thorough washing of the incision and closure. If the tract does indeed pierce the ligamentum flavum, a laminectomy above this should be performed, with rongeurs and an angled Kerrison punch. As with a standard laminectomy, if there is no access to the epidural space after bony removal, an incision through the ligamentum flavum, under traction with subsequent protection of the dura mater with a cottonoid, is warranted. The ligamentum flavum is then removed completely laterally and caudally to the tract. A small cottonoid patty should first be advanced under the ligamentum flavum and above the dura mater to avoid incision of the tract as it is neared or to avoid performing an incidental durotomy. The same is accomplished at the lamina below the tract, although only the rostral half of the spinous process, lamina, and ligamentum flavum need to be removed, because intradurally the sinus tract rarely travels caudally.

If, at this time, the tract appears to extend rostrally beyond the level of the laminectomy, the next lamina should be removed. Further exposure may be necessary until the tract is not visualized or until its termination at the conus medullaris can be visualized. The dura mater is then incised along the midline at the rostral end of the exposure, well away from the site of entry of the tract, and carefully extended in the direction of the tract. To approach the entry site, an elliptical incision is made circumferentially around the tract, and the midline incision finally is continued to the caudal end of the exposed dura mater. The dural ellipse that is incised around the tract should be kept as small as possible to preclude the need for use of graft material when closing the dura mater. Care should be taken to keep the force of suction minimal to avoid inadvertent aspiration of free-floating nerve roots. If the arachnoid has been preserved after the durotomy, it may be opened in the midline with a small hook or knife. The dural leaves may then be tacked to the musculature with #4-0 silk or woven nylon sutures.

With gentle traction on the tract, microinstruments are used to dissect away any adhesions between the tract, the dura mater, and the nerve roots. Some adhesions, particularly postmeningitic scars, may require incision. In this way the tract is followed to its attachment, usually dorsally above the tip of the conus. The sides of the tract are completely identified by using a small blunt hook, and microscissors are used to detach it. If the stump bleeds, it is lightly coagulated with bipolar forceps. After verifying the absence of other mass lesions or areas of tethering, the subarachnoid space is copiously irrigated with warm saline, and the dura mater is closed in a watertight fashion with #4-0 silk or woven nylon suture, using a graft if necessary. Fibrin glue is then applied. A drain may be placed in the epidural space and brought through the skin for 24 hours. The soft tissues are then closed in anatomic layers. Some undermining of the subcutaneous tissue occasionally is necessary to bring together the skin edges in the area of the elliptical excision. The patient should remain flat in bed for 3 days, and antibiotics should be administered postoperatively for 3 days, or longer if a previous infection was present.

If a dermoid or epidermoid tumor is encountered preoperatively or during the course of the procedure, it should be completely extirpated, with great care taken to avoid a rupture of its contents. If the tumor itself is at the end of the tract and situated within the substance of the spinal cord, it cannot be completely removed; it should be amputated with an adequate stump to avoid injuring the spinal cord.

Complications

As with other intradural procedures, CSF leakage may occur postoperatively. A Valsalva maneuver (safe to 40 mm Hg) after closing the dura mater is helpful for revealing any obvious areas of leakage. To avoid the problem of wound breakdown and postoperative meningitis, particularly with a dermal sinus and exposure to its intraluminal debris, a watertight closure (again using fibrin glue, saline irrigation before dural closure, and antibiotic irrigation after dural closure) is helpful. If meningitis should occur, the appropriate antibiotics should be administered. In the case of a superficial wound infection, the area should be opened, drained, and packed, and the patient should be treated with antibiotics. If, however, the fascia has been violated by the infection, reoperation is necessary to open the fascia, to verify a clean epidural and, if necessary, subdural space, and to close the fascia primarily. All efforts should be made to avoid leaving the dura mater exposed to the environment.

Iatrogenic injury to the conus medullaris, spinal cord, and nerve roots is minimized by gentle handling of the tissues, avoidance of traction, and protection of the neural elements. If it is thought preoperatively that the sinus may end in the conus medullaris or in a tumor adherent to the conus, somatosensory evoked potentials may be monitored during the procedure. Division of a nerve root, particularly in the absence of EMG monitoring and uncertainty as to its function, should be primarily repaired with #8-0 or #9-0 absorbable monofilament sutures. Because the incidence of postoperative neurologic deficits from dermal sinus surgery is small, no evidence supports the use of perioperative or postoperative corticosteroids for neural protection.

Emphasis should be placed on complete excision of the sinus tract. Because of its epithelial lining, the potential for dermoid or epidermoid formation remains if part of the tract remains. If a lumen is apparent in the stump of the tract after it is incised near the spinal cord or conus medullaris, it should be trimmed until no lumen is apparent, and then well cauterized.

Outcome

Most patients with simple tracts do well, and the incidence of neurologic injury is low. Even in the case of spinal cord infection, a positive outcome may be obtained. In a report by Morandi et al.,³⁵ 10 of 16 patients (62%) with spinal cord abscess had complete recovery.

Pathology and Embryology

Although associated with other forms of occult spinal dysraphism, spinal lipomas are connective tissue and fat collections that are distinct, partially or completely encapsulated, and definitely attached to the spinal cord.³⁶ It is thought that during the process of primary neurulation, improper disjunction of surface ectoderm and neuroepithelium may lead to inclusion of fat.³⁶ Distinct from lipomyelomeningoceles, isolated lipomas technically are fibrolipomas of the filum terminale or dural fibrolipomas, as defined by Emery and London.³⁷ In simple lipomas, the neural elements remain within the spinal canal, whereas lipomyelomeningoceles are marked by herniation of the neural elements out of the canal into the subcutaneous portion of the lipoma.³⁸ Strictly intradural lipomas associated with an intact dura mater are lesions of subpial fat found in the cervical and thoracic spinal cord.³⁹ In a large series reported by McLone and Naidich,⁴⁰ 4% of the lipomas treated surgically were intradural lipomas. More common, however, are lipomas that involve the dura mater and extend from the spinal cord to the subcutaneous tissue.^39,41,42

Lipomas are associated with more severe bony changes than is the previously described dermal sinus, including scalloping of the dorsal vertebral body, widening of the interpedicular space, hemivertebrae, or even hypoplasia of the iliac wing.³⁸ These sequelae of the mass effect associated with lipomas suggest that resultant neurologic deficits occur not only by spinal cord tethering (see Pathophysiology and Presentation) but also by direct neural compression.

Pathophysiology and Presentation

Spinal lipomas, accounting for up to 35% of skin-covered lumbosacral masses, may extend to the superficial subcutaneous tissues and present in the infant as a visible and palpable mass.⁴³ As with other forms of the tethered cord syndrome, children with spinal lipomas may present with several complaints. At this age, before the onset of walking and the development of bladder control, a concomitant hairy patch may accompany an otherwise unnoticed lipoma. If the condition is unnoticed or disregarded, the infant without neurologic abnormalities may, with age, develop sphincter disturbance, postural and lower extremity deformities and weakness, or even verbalized discomfort.⁴⁴

As mentioned, lipomas may cause neurologic symptoms through a combination of neural compression and spinal cord tethering. Lipomas, therefore, may present in much the same manner as do split cord malformations, a thickened filum terminale, and a congenital dermal sinus. As always, failure to attain developmental landmarks, as well as progressive loss of neurologic function, particularly lumbosacral function, should alert the health care provider to investigate the spine.

Classification

Lipomas of the conus medullaris may be classified into three categories: dorsal, caudal, and transitional. First described by Chapman, alone⁴⁵ and with Davis,⁴⁶ two distinct forms as well as a transitional form were categorized.

The dorsal variant is a lipoma that arises through a fascial defect and attaches directly to the dorsal aspect of the caudally descended conus medullaris. All nerve roots emerge from the ventral or lateral surface of the neural tissue and lie in the subarachnoid space. The lateral nerve roots are sensory, while motor roots are found more medial.

The caudal variant or terminal lipoma exits the area of the terminal filum so that the cord becomes progressively larger caudally. In this form the nerve roots may transgress the lipoma. Many of these nerve roots are thought to be nonfunctional and may be sacrificed after stimulation and monitoring. Although this caudal variant is difficult to re-form into a tubular structure, the cut end of the lipoma may retract sufficiently cephalad to reduce the probability that retethering will occur postoperatively.

The final form of lipoma described is the transitional form, which includes elements of both the dorsal and caudal variants described earlier in this chapter. Viable nerve roots pass through significant amounts of the lipoma prior to exiting. These typically are asymmetrical and are associated with a rotational component of the spinal cord. The process of distinguishing among these three types of lipoma usually is straightforward with adequate imaging, including MRI.

Diagnostic Aids

Lipomas constitute only 1% of primary intraspinal tumors and almost always are associated with dysraphic spines.⁴⁷ Varying from intramedullary to extradural, their histologic nature and relative position to the spinal canal make them definable by both CT and MRI.⁴⁸

Although spina bifida occulta, widening of the interpedicular distance, hemivertebrae, and vertebral body scalloping may all be observed on plain radiographic studies in the patient with a lipoma, these radiographs may be difficult to interpret. As with other spinal anomalies, lipomas usually are best worked up with MRI as the initial study (Fig. 116-11). CT usually is reserved for better definition of bony anatomy, if needed. Although not as detailed, myelograms may help define the extent of the mass (Figs. 116-12 and 116-13). Intradural lipomas are low density or even radiolucent on CT and have a high signal on T1-weighted MRI.⁴⁹ Both techniques are useful, however, in axial section. Extradural lipomas usually are more diffuse and contiguous with the nearby epidural fat. Even when epidural lipomas are not clearly visualized on CT scan and MRI, their presence should be considered.

FIGURE 116-11 Sagittal (A) and axial (B) MR images of a tethered spinal cord with an intradural spinal lipoma (arrowheads).

FIGURE 116-12 Anteroposterior lumbar myelogram revealing the filling defect caused by an intradural lipoma, causing a tether of the cord in the lumbosacral region.

FIGURE 116-13 Sagittal T1-weighted image demonstrating large intradural spinal lipoma.

McLone and Naidich⁵⁰ also support the use of ultrasonography in managing these lesions. The lack of calcium in immature bones allows for penetration and evaluation of structural detail, often well enough to verify the lesion, determine the extent of tethering, and proceed straight to surgery without the need for additional radiographic studies.

Treatment

Whether by tethering of the filum terminale and spinal cord, direct neural compression, or both, surgery for spinal lipomas is warranted in the child with neurologic deficits. Operating on the asymptomatic patient has been controversial. However, most neurosurgeons now believe that if possible, lipomas and lipomeningoceles should be operated on before neurologic sequelae occur.^44,51,52 Unlike split cord malformations, in which involvement of the neural elements in the substance of the pathology is rare, such involvement is common in spinal lipomas. Therefore, even with detailed and defining preoperative studies, intraoperative electrophysiologic studies may be something that should be considered during resection of spinal lipomas. Not all surgeons consider this necessary. We, however, find it quite useful and do use intraoperative EMG monitoring, in which we insert needle electrodes in the muscles of the lower extremities as well as in the sphincter. Stimulation is performed with the same probes that are used for dorsal root rhizotomy. With this type of monitoring, one can ascertain whether there is undue traction on the conus medullaris and if it is safe to incise tissues that are near nerves or the spinal cord.

After sterilization of the skin, a midline incision should be performed over the palpable or visible subcutaneous portion of the lipoma. If there is no evidence of such a superficial lesion, needle localization with anteroposterior and lateral radiographs is useful. After incision of the fascia and lateral exposure of the laminae, microscopic enhancement, either with loupe magnification or the operating microscope, should be used. As with all occult dysraphic spines, care should be taken with both the scalpel and the electrocautery, because the unformed or bifid laminae may offer no protection for the thecal sac. Before the bony structures are reached, the extradural portions of the lipoma may require resection. If so, circumferential dissection of the lipoma is important so as to allow complete resection. The actual resection of the extradural portion of the lipoma usually is fairly straightforward. However, the location of the nerve roots relative to the lipoma-cord junction vary with the type of attachment that the lipoma has with the conus medullaris.⁵³ If the lipoma attaches to the dorsal surface of the conus, the nerve roots are ventral to the lipoma-conus interface. If, however, the lipoma is a caudal extension of the conus medullaris, the course of the nerve roots through the lipoma can be variable, and great care has to be exerted to prevent damage to neural structures (Fig. 116-14).

FIGURE 116-14 Intraoperative photograph showing the continuity of this large subcutaneous lipoma with the spinal cord (arrowhead). Note the caudal attachment, making dissection of the nerve roots very difficult.

At the time of the durotomy, all anesthetic muscle relaxants should be avoided. Extradural lipomas that traverse the thecal sac into the intradural compartment may require resection of some dura mater. At this point, although some surgeons advocate the use of the carbon dioxide laser to vaporize the fatty lesion, we also suggest the use of the ultrasonic aspirator. Although relief of neural compression is one principle of surgery, particularly for lipomas on the dorsal surface of the spinal cord, the primary goal is to untether the spinal cord. Therefore, the surgery should be directed at accomplishing this goal. In such cases, sectioning of the filum terminale caudal to the sacral nerve roots may be required. Electrophysiologic monitoring is particularly useful at this point, not only to determine where the filum terminale may be incised and to ensure that no neural structures are coursing within the filum, but also to take care to avoid excessive manipulation of the functional nerve roots and the spinal cord. Often it is not possible or even necessary to resect the lipoma completely (Fig. 116-15). Prudence should be the rule, and one should not take any chance of injuring the conus medullaris or the nerve roots. It should be remembered that the primary goal of surgery is to untether the spinal cord and at the same time cause no deficits.

FIGURE 116-15 Intraoperative photograph demonstrating residual lipoma at the lipoma-conus interface, stimulation of which verified the presence of functional neural elements (arrowhead). Note the slack nerve roots, verifying successful untethering of the cord from the dura.

After removal of the lipoma, untethering of the surrounding neural structures, and possible sectioning of the filum terminale, dural closure must be watertight. Although reapproximation of two of the three meningeal layers may decrease the change of retethering, the pia and arachnoid often are incompetent after removal of the lipoma. Therefore, careful closure of the dura mater, which may include a graft, is important. As with other intrathecal operations, leaving the child flat in bed for 3 days allows for tissue healing and helps avoid collection of CSF, particularly if a large “dead space” has resulted from excision of an extradural component of the lipoma. Special techniques of closure to prevent retethering have been reported, but the patients have not been followed long enough to determine whether these techniques are better than conventional methods of closure.⁵⁴

Complications

Infection, CSF leaks, and iatrogenic neurologic deficits are all complications that should be avoidable with meticulous surgery and disciplined technique. Preoperative administration of antibiotics is paramount, although postoperative doses depend on the surgeon’s preference. Although the literature does not firmly support the use of antibiotics after elective, clean surgery, we prefer to give three postoperative doses to cover common skin flora. If postoperative infection should occur, appropriate drainage and debridement are necessary. Prolonged antibiotic treatment is indicated, and concerns about subsequent meningitis must be addressed with lumbar puncture. In the absence of obvious superficial infection, prolonged fever, or progressive worsening of neurologic function, MRI is necessary to rule out epidural abscess. In such cases, well-intentioned lumbar punctures may result in unintended meningitis.

Avoidance of CSF leaks, as previously mentioned, depends on watertight dural closure, anatomic approximation of all tissue planes, and placement of the child in the supine position postoperatively for recovery. A Valsalva maneuver at the end of the initial surgery may help expose any occult areas of leakage. Continued leaking requires decompression of the intradural pressure with a lumbar subarachnoid drain. Persistence despite these measures requires prompt reexploration, repair of obvious areas of CSF escape, and the appropriate use of dural substitutes and cryoprecipitate-based fibrin glue, with the possible addition of a lumbar subarachnoid drain. In extreme cases of dural incompetence, particularly in the face of a soft tissue defect and a potential space for CSF collection, a rotated or free-pedicle tissue flap may be required.

Immediate postoperative neurologic deficits often resolve. Manipulation of the distal spinal cord and nerve roots may result in traction injury and edema, resolution of which should imply return of neurologic function. This is not always the case, and prudence should be used while handling neural tissue during surgery. Deficits that result from definitive sectioning of the filum terminale or nerve roots are not reversible. With time, however, motor and sensory function may improve with reorganization of cortical neurons.

Outcome

Successful resection and untethering of these lipomas is now associated with little or no morbidity and mortality.^40,53,55 The best outcomes may be achieved with respect to pain, with most of the pain decreasing or disappearing within 3 months. Bladder dysfunction also may respond to resection of the lipoma in 20% to 30% of patients; of these patients, those with a spastic bladder respond best.⁵⁶ In a study by La Marca et al.,⁵¹ 213 patients were operated on over a 20-year period, from 1975 to 1995. In patients with filum lipomas, 28 were asymptomatic and 27 were symptomatic. None of the asymptomatic patients worsened after surgery (mean follow-up, 3.4 years), and of the symptomatic group, there were no further deteriorations noted (follow-up, 6 months to 9 years). Of the group with conus lipomas, 9 of the 71 children (12.7%) operated on prophylactically later deteriorated (mean follow-up, 6.2 years) and required a second untethering operation. Symptoms of deterioration included urinary retention, pain, gait difficulty, urinary incontinence, and spasticity. In the symptomatic group (87 patients), 36 patients (41%) further deteriorated and required further surgery. At the final follow-up, however (mean, 6.6 years), 51% remained at clinical baseline and 26% improved. In a study by Xenos et al.,⁵⁷ the reoperation rate was 12% for signs of recurrent spinal cord tethering.

Minimally Invasive Surgery

Minimally invasive or endoscopic surgical techniques have been used to treat a number of spinal pathologies in recent years. With spinal dysraphism and the tethered spinal cord presenting clinically and often repaired at a young age, minimally invasive operative techniques may be of great benefit to the patient. Recently, it has been determined that tethered spinal cords may be safely and effectively untethered using minimally invasive surgery.⁵⁸ This technique provides the advantage of reduced soft tissue injury, reduced postoperative pain, reduced postoperative scarring, and lower postoperative risk of retethering.

Minimally invasive untethering incorporates an approximately 3-cm incision with a muscle-splitting (tissue-sparing) approach, one vertebral body level below the conus. Benefits of this technique include sparing of midline structures, reduced postoperative complications, and full visualization of the filum terminale. Up to 11% of tethered cords may be found off the midline, and this approach provides visualization of the bilateral extent of the canal and full neural elements.

Carbon Dioxide Laser Detethering

Detethering of the spinal cord currently employs a number of instruments for sharp dissection during the surgical release of the tethered cord. Safely dissecting the nerve roots from the surrounding tissues requires precise microsurgical technique, although various laser technologies have been used in the past, and first were described as early as 1966.

Carbon dioxide (CO₂) lasers contain many features that make them favorable to neurosurgical procedures. CO₂ laser dissection is beneficial, in part, because CO₂ is almost completely absorbed by water and therefore does not penetrate tissue, reducing potential collateral tissue injury. Recent advancements have allowed the CO₂ laser to be passed through optical fiber; this, along with improved ergonomics, has allowed further implementation of this technology into operative practice. The unique physical properties, minimal depth of penetration, and minimal collateral nontargeted tissue damage of CO₂ laser energy make it a particularly useful tool in the release of tethered cord during the delicate operative course.

Cavitron Ultrasonic Aspiration

Cavitron ultrasonic aspiration (CUSA; Valleylab., Inc., Boulder, CO) is one alternative to the CO₂ laser. The original ultrasonic aspirator was developed in 1947 for the removal of dental plaque and was first applied to the field of eye surgery in 1967. Ultrasonic techniques became widely used in the medical and biochemical industries for sterilization, homogenization of solutions, and welding of materials. The use of the ultrasonic aspirator in neurosurgery was first reported in 1978, for the removal of intra-axial and extra-axial tumors such as meningiomas, schwannomas, and gliomas. The ultrasonic aspirator has since become a valuable tool in the neurosurgical armamentarium for the excision of intracranial and intraspinal tumors.⁵⁹ As with the CO₂ laser, the CUSA system provides efficient resection of target tissue with minimal injury to surrounding tissue or structures.

The ultrasonic aspirator has two tissue-disruptive effects at the tissue interface.⁶⁰ The first is caused by a suction effect that couples tissue to the tip and forces impacted tissue to vibrate, accelerate, and decelerate with the tip, eventually fragmenting away from unaffected tissues. The second important effect is cavitation. In cavitation, the rapidly oscillating tip produces localized pressure waves, which cause vapor pockets around cells in tissues with high water content; the collapse of these pockets then causes the tissue cells to rupture. Tissues with weak intracellular bonds, such as tumors and lipomas, are easy to fragment, whereas tissues with strong intracellular bonds, such as nerves and vessel walls, are difficult to fragment. The speed of fragmentation depends on the amplitude setting of the system.