Occult Spinal Dysraphism and the Tethered Spinal Cord

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Chapter 116 Occult Spinal Dysraphism and the Tethered Spinal Cord

In this chapter the term occult spinal dysraphism describes a group of primary embryonic myelodysplastic syndromes that present with or without cutaneous manifestations and often, but not invariably, with tethering of the spinal cord. Spinal dysraphism is more common in females than in males by a ratio of at least 2:1. Although there are subtypes of occult spinal dysraphism (e.g., spina bifida associated with congenital lesions of the caudal spinal cord) that do not present with tethering of the distal cord, many do. The symptoms of spinal dysraphism with or without the presence of distal cord tethering are quite similar. Therefore, this chapter includes a discussion of the tethered cord syndrome. In addition to the tethered cord from a thickened filum terminale, the clinical entities covered in this chapter include the structural defects found in split cord syndrome, congenital dermal sinuses, and spinal lipomas.

This overlapping of structural pathology and clinical findings may present with few or minimal neurologic deficits, particularly if tension on the ascending spinal cord is not present. Symptoms such as isolated back or lower extremity pain in the young adult may be the first and only symptom referable to such a congenital lesion. Regardless, the proper surgical treatment of these anomalies is paramount to preserve and improve neurologic function, prevent potential infection, and relieve pain.

Because of the nature of these dysraphic lesions (i.e., breeching of the natural protective barriers of the distal neuraxis, such as incomplete or absent closure of the dorsal elements, enteric or subcutaneous communications with the subarachnoid space, and compromised soft tissues over the spine), repair and treatment sometimes are complicated by infection, cerebrospinal fluid (CSF) leak, and breakdown of compromised tissues. Timing of surgery, meticulous surgical technique, and close postoperative examinations, as always, may aid in avoiding potentially devastating results.

Each entity is discussed according to the following format: (1) a brief discussion of the basic embryology and pathology, (2) the clinical presentation and appropriate workup, (3) the indications for surgery, (4) a description of the technical aspects of the operative treatment (with emphasis on complication avoidance), (5) the recognition and treatment of complications, and (6) outcome.

Tethered Cord Syndrome

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.

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

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.

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

Split Cord Malformations

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.