Spina bifida occulta (Tethered cord):

 
Dr. A. Vincent Thamburaj,  
Neurosurgeon, Apollo Hospitals,  Chennai , India.

Spina bifida is the commonest of neural tube defects. Spina bifida occulta is much more prevalent than open variety. It is a failure of fusion of the vertebral arches, most frequently the fifth lumbar and first sacral, without a protrusion of the cord and meninges. It may involve only one vertebra or may extend over several segments. It is seen as an incidental X-ray finding in 17-30 per cent of the normal population.

 

For the majority it carries no clinical significance but an occasional patient may have an occult spinal dysraphism in the form of a tethered cord, midline spur, lipoma, dermoid or an abnormal dilatation of the sacral end of the dural sac (occult intrasacral meningocoele). The overlying skin may be normal or stigmata of adysraphic state, such as hypertrichosis, haemangioma, skin pigmentation, dimples, sinuses and "pedunculated tail" may be present.

They may be collectively called 'Tethered cord syndrome.

 

Tethered cord syndrome (TCS):

 

Progressive neurological deterioration localized to the lower spinal cord resulting from traction on the conus medullaris has been termed TCS. Tethered cord is associated with low conus below the mid L2 level and a short, thick filum, attached to the dura, or an extradural band may anchor the cord. It is the cord which is tethered, with the nerve roots lying lax and even loosely on either side. The effects of the tethering is maximum close to the site of tethering and when the tethering lesion extends over a distance the maximum effect is on the cord adjacent to the caudal end of the lesion, where the maximum stress lies.

 

Pathology:

Any process that tethers the spinal cord can result in a patient who has tethered cord syn­drome. Children can be born with normal anat­omy and develop a tethered cord (secondary) through an acquired process, such as infection, scarring, or tumor. This section focuses on congenital (primary) causes of tethered cord. Lipomyelomeningocoele and a thick­ened filum terminale accounts for 70 per cent of TCS.

 

Fatty filum is the simplest form of conditions causing TCS. In this condition, the filum terminale, can be thickened, potentially with lipomatous tissue. Hoffman and colleagues have suggested that a diameter of 2 mm or greater should be considered an abnormally thick­ened filum. Impaired canalization of the growing secondary neural tube (the neural cord) with cells capable of growth and differentiation, particularly preadipose tissue, is believed to be the cause of both the thickened and fatty filum terminale. The fatty filum commonly is associated with cases of imperforate anus, suggesting a com­mon timing of pathogenesis during development.

Lipomyelomeningoceles are more extensive lesions and represent a combination of a splayed cord fused with a lipomatous mass, which in turn fuses with the subcutaneous adipose tissue. The neurologic deficits of cord tethering are probably caused by impaired circulation in the stretched cord, as evident by its reduced oxidative metabolism or to abnormal development as the cause of the neurologic deficits as in myelomeningocele.

An intradural lipoma has no anatomic connection with the subcutaneous fat and lies wholly within the dural space in embryologic terms. This is similar to a lipomyelomeningocele except that the neural tube closes after

tethered cord

tethering due to lipoma

the mesenchyme has entered. In some patients, the cord is bound down by lipomatous tumors or fibrous tissue, the sacral roots ascending. Lipomas may attach dorsally to the conus and be sessile or pedunculated, while other lipomas occupy the distal end of the conus, elongating the latter and terminating in a small lipoma with attached nerve roots. The dorsal and caudal lipomas may occur in combination. Filar lipomas occupy an enlarged filum terminale. Lipomas most are commonly subpial, although a small number can be subdural. Subdural lipomas are infrequently associated with tethering and more commonly present like a mass lesion with cord compression. More commonly, however, lipomas of the spinal cord occur in the lumbosacral region and have an associated dural defect.

 

Dorsal dermal sinus is a long, thin, squamous epithelium-lined sinus that extends from the skin surface for a variable distance. Like most dysraphic abnormalities the commonest sits for the defect is the region of the posterior neuropore i.e.the lumbo-sacral region, the next frequent site being the sub-occipital region. It may, however, occur anywhere in the dorsal midline from the nasion to the coccyx. It may rarely be away from the midline. The sinus may terminate extradurally, intradurally or extend into the fourth ven­tricle or the medulla where it may produce an abscess or a dermoid.  The defect arises as a result of failure of separation of the neuroectoderm from the epithelial or surface ectoderm. This separation normally takes place between the 3rd-5th week of intrauterine life. The congenital dermal sinus is mostly superficially located and presents as a dirnple. This sinus may become symptomatic due to an infection if it has a connection within the theca, distorts growth of the neural tissues or compression of neural tissue if associated with a dermoid/epidermoid.

Meningitis is the most common presentation. The cutaneous opening may at times be inconspicuous with surrounding hairy growth, naevus, pigmentation or lipoma, When infected, the opening of the sinus becomes red and inflamed with erythema of the surrounding skin. A clinical picture of spinal arachnoiditis may be seen as a result of recurrent attacks of subclinical infection. Any or all the neurological features observed in spinal dysraphism may be encountered in this condition. Spinal compression, extradural, intradural or intramedullary, may be caused by an associated dermoid or epidermoid cyst or by a secondary localised abscess. The neurologi­cal level of the lesion is usually several segments higher than the external opening, as the dermal sinus runs an oblique course from the skin in a cephalad direction.

 

Split cord malformation (SCM) results from abnormal splitting of the notochord. This new nomenclature was introduced by Pang, et al., in 1992, to elim­inate confusion created by the use of the terms diastema­tomyelia and diplomyelia. Diastematomyelia usually re­fers to a split cord in which the two halves are separated by a bone spicule and contained within separate dural sleeves. In contrast, the term diplomyelia is generally used to describe a condition of two hemicords within one dural sac, often with two complete sets of nerve roots, separat­ed by a fibrous band. Pang, et al., proposed a common origin of both malformations: an adhesion between the ectoderm and endoderm leads to an endomesenchymal tract that bisects the spinal cord. If the tract also contains cells of the precur­sor cells of the meninges (meninx primitiva) the resul­tant malformation would be SCM Type I, or diastemato­myelia. Otherwise, the formation of a separate dural sleeve and bone septum does not occur, and the malforma­tion is a SCM Type II, or diplomyelia.

 

In both SCM I and SCM II, when neural crest cells are also involved in the split, two sets of dorsal roots will develop with each hemicord having paramedian dorsal roots as well. These rootlets may end blindly at the midline septum or go beyond the dura along with a leash of abnormal vessels and fibrous tissue to form the fibroneuro vascular stalk of a myelomeningocoele plaque. The endomesenchymal iract may persist to a variable extent. When it goes upto the cutaneous ectoderm, it may form a fine dermal sinus or a wide myelomeningocoele. It may differentiate to form the various cutaneous stigmata, lipoma, dermoid or epidermoid cysts. In a type ll SCM the fibrous septum may be entirely ventral to the cord, with the dorsal surface of the cord looking normal.

The common site of this defect is the lower thoracic or lumbar region. When the lesion is situated in the lumbar region the cleft often is through an unusually low placed conus.

 

Both types of SCMs represent lesions that tether the spinal cord during growth and movement. The lesion manifests itself commonly in children, with a higher incidence in females. Occasionally cases presenting with symptoms in adult life have been reported. The development of symptoms and signs appears to be related to periods of rapid skeletal growth. Prevention of the normal ascent of the cord by the spur during these periods has been blamed for the development of the neurological deficit. It is the spur and not the divided cord that is the cause of the symptoms.

 

A myelocystocele is a variant of hydromyelic dilatation of the central canal, the cystic cavity being within the cord and the spinal roots originating at the ventral and dorsal outer surface of the cyst wall. A myelocystocele is often associated with defects of the vertebral bodies or intestinal fistula. These types of lesions are often located in the cervical or upper thoracic cord at the level the underlying bony defect. They often are associ­ated with a lipoma (lipomyelocystocele).

 

Neurenteric cysts often are found on the ventral side of the spinal canal and consist of a fluid-filled cyst that may communicate with the gastrointestinal tract through a vertebral defect such as a hemivertebra or butterfly vertebra. The neurenteric cyst itself can cause compression, but its adherent fibrous bands also can result in tethering.They usually are intradural and extramedullary, and their origin is debated, although positive immunoreactivity for carcinoembryonic antigen suggests endodermal origins.

Tethering can occur with meningocele and meningomyelocele, as functional cord attaches itself dorsally ei­ther to dura or to surface ectoderm. An interesting case of meningocele known as the "meningocele manque" (the "missing" meningocele) occurs when a meningocele has formed during embryogenesis but has healed spontaneously or scarred creating a dorsal band. These dorsal bands can extend from intrathecal structure into the dura or outside structures creating a significant tether­ing effect [34]. The dorsal band of meningocele manque may reflect a fibroneurovascular stalk de­rived from the same endomesenchymal tract that is the basis for split cord malformations.

 

Anterior meningocoeles are rare lesions and may occur in the pelvis through a defect in the sacrum, the meninges may protrude into the pelvis causing compression of the pelvic organs or the condition may present as a neurogenic bladder. The sac may harbor benign tumors like a lipoma and the filum terminale attached to the meningocoele may produce a tethered cord syndrome.  Intrathoracic meningocoeles are often associated with neurofibromatosis and are frequently confused with mediastinal tumors. However, meningocoeles cause fewer physical signs than tumors. X-rays of the spine often show erosion of the posterior aspect of the dorsal vertebrae. These meningocoeles are best left alone except when there are compelling reasons forcing surgery.

 

Clinical Features:

The clinical presentations differ widely in children and in adults. Rapid early deterioration within the first weeks after birth characterises those lesions associated with imperforate anus, omphalocoele and exstrophy. The onset of tethered cord syndrome is related to the level of the pathology and the height and age of the patient.

Pain is invariably present in the adult as a diffuse, dysaesthetic "central'' type of pain over the legs and perineum and in some patients may mimick a lumbo-sacral intervertebral disc protrusion. In chidren pain is uncommon feature and is limited to the lumbosacral region.

A slow onset of difficulty in micturition or a limping gait in a child seven or eight years old or more, should arouse the suspicion of a tethered cord.

Cutaneous stigmata of dysraphism are almost always present in a child while only about half the adults have them. They include the midline lumbosacral cutaneous hemangiomas, lumbosacral hypertrichosis, the lumbosacral dermal sinus, the midline lum­bosacral subcutaneous lipoma, and the lumbosacral skin appendage. The cutaneous stigmata are present in approximately 50%70 of patients who present with TCS.

The hallmark of the neurological deficits is their asymmetry. Sensory, motor and bladder involvement are fre­quently seen in children as well as adults. Neurological deterioration with both up­per and lower motor neuron involvement takes months or years and symptoms appear during late childhood. In children motor involvement is in the form of gait distur­bance and in the adult there is actual weakness and wasting.

Bladder symptoms in the form of incontinence or repeated bouts of urinary tract infection may be the only symptom. Orthopedic deformities such as scoliosis or kyphoscoliosis is common. There may also be defor­mities of the feet, e.g., pes cavus, talipes equinovarus or trophic skin lesions. They are in adults.

 

Imaging:

Plain roentgenograms are almost always abnormal. There is usually a spina bifida, a circumscribed median bony opacity, a widening of the interpedicular distance or an associated hemi-or block vertebra.

MRI is well suited to identifying the level of the conus relative to vertebral bodies, the presence of a syrinx, or visualization of other pathologic pro­cesses. MRI also is able to define the anatomy of other causes of tethered cord, such as the anatomy of terminal or multiple lipomas, the pres­ence of congenital lesions (such as dermoids), and the presence of myelomeningocele. Its limitation, however, is the poor quality of images in infants, the difficulty in interpreting the various aspects of a complex malformation, and its inability to define bony ventral abnormalities like the bony septum in a split cord malformation.

 Ultrasonography can have a role as a relatively quick and easy screening tool in young children.

 

Surgical Treatment:

Development or progression of symptoms in patients who have a tethered cord often calls for an untethering operation. Patients who have large spinal lipomas that exhibit mass effect on the spinal cord, and presenting only with pain may be considered for a trial of weight loss before committing to surgical intervention.

 

A group of myelomeningocele patients who have symptoms suggestive of tethered cord syndrome and who also have ventricular shunts. A malfunctioning shunt sometimes can cause signs and symptoms that may mimic a tethered cord, and shunt correction helps.  Orthopedic  deformities may be the presenting symptoms, and untethering should be considered before orthopedic correction. data suggest that the neurosurgeon should recommend unte­thering as treatment of the root cause of scoliosis in selected cases, but that correction of the defor­mity may be limited, and orthopedic involvement may be necessary.

Controversy, however, still surrounds treat­ment options for the asymptomatic patient who has signs of a spinal anomaly, particularly a milder anomaly such as a thickened filum or an asymp­tomatic lipoma.  The risks must be weighed, because lipomas of the filum (or a thick­ened filum) have much better surgical outcomes than more complex ones. Some recommend surgical prophylactic untethering in asymptomatic group, since surgery does not always provide a reversal of dysfunction or abnormality in symptomatic patients.

 

Untethering surgery can arrest the progression of symptoms in the major­ity of patients; a smaller percentage of patients show improvement after untethering. Im­provement is more likely to be seen in patients whose primary symptom is pain, although these patients tend to be an older population including young adults and adults.

 

Finding of a normal bony lamina may allow identification of the dura and subsequent improved understanding of ab­normal tissue planes. This principle holds true intradurally as well, where rostral exposure of normal spinal cord may facilitate safer dissection. Use of laser, if available, helps in complex cases, such as, a lipoma.

Intra-operative neurophysiologic monitoring with com­binations of motor-evoked potentials and sensory-evoked potentials helps in identifying nerve roots.

 

The filum is recogniz­able by its fatty appearance, by its straight midline location, and by its vasculature. It is im­portant to visualize the underside of the filum be­fore sectioning, because nerve roots can travel along with the filum. Once the filum is sectioned, care should be taken that there is no bleeding at the site of sec­tion before the proximal stump is released, be­cause it may retract out of reach.

 

In split cord malformations, once the median spur/septum is removed, the dural sleeves of both hemicords are opened, and the median dura is resected along with the ventral dura with no ventral repair.

 

Dermal sinus tracts  should be explored intradurally and excised in toto. Simple excision of the extradural component of the tract may not alleviate intradural tethering.

 

As much of any intradural lipoma or dermoid is excised as is safely possible. It is not desirable to embark upon a hazardous total excision especially in the caudal and transitional variant of lipomyelorneningocoele. The lipomas dissected all around under the operating microscope, defining its neck in the lumbo-dorsal fascia. The normal dura under the laminectomy is opened and extended inferiorly dividing the constricting fibro vascular band lying around the dura just inferior to the last intact lamina. The band is frequently seen in the dorsal variant and its division leads to an immediate ballooning of the released dura. An associated congenital dermal sinus or split cord malformation is treated. Likewise any intraspinal meningocoele may have to be excised before opening the dura. On opening the dura, extreme care is required to preserve sensory nerve roots that may be adherent to the dura. Careful dissection using magnification and bipolar stimu­lation is required to identify and excise the filum terminale to release the tethered cord. As this is being done one may notice an actual cephalic migration of the cord and a change in the direction of the nerve roots.

Neurenteric cysts often are extremely adherent to the spinal cord and, because of the risk of recurrence, should be resected completely if possi­ble. This may require a ventral approach.

 

A watertight dural closure, with graft if necessary, is mandatory. To avoid retethering, meticulous attention is needed in hemostasis and closure. Dural closure with 4-0 Nurolon is adequate for many straightforward detethering operations. With more complex spinal dysraphisms, a running monofilament suture can be used with good results. In complex lesions, such as extensive lipomas, resection of the maxi­mal amount of pathologic tissue should be per­formed, followed by imbrications of the pial surface to create a smooth surface apposed to the dura.

 

In severe cases of retethering in patients who have myelomeningocele and compromised neural function, transaction of the spinal cord above the neural placode can be performed to prevent the placode from scarring and forming adhesions. Post operatively, patients are nursed prone to minimize adhesions of the cord to the dural suture line. The patients are turned supine and slowly elevated in bed over several days.

 

Retethering is about 20% in complex cases.  Complete hemostasis at surgery, imbrications of the pial surface to create a smooth surface apposed to the dura, and nursing patients in prone position postoperatively help in preventing retethering. Any new or significantly progressive orthopedic, urologic, or neurologic symptom should be evaluated for the possibility of retethering. Although retethering can occur at any time, the risk often decreases once adult stature is reached and growth has stopped. Resurgery may be considered.  

 
 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

 

from Peer Reviewed Resources only

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