Types of Occult Spinal Dysraphism

Lipomyelomeningocele and Spinal Cord Lipoma

Congenital abnormalities of the spinal cord containing fat are of three different types: lipomyelomeningocele, spinal cord lipoma and fatty filum terminale. Each can cause progressive neurologic dysfunction through the mechanism of spinal cord tethering. The lipomyelomeningocele is defined as a subcutaneous lipoma that passes through a defect in the lumbosacral fascia, vertebral lamina, and dura to merge with a low-lying spinal cord. The spinal cord is usually partially open (similar to a true myelomeningocele) and the lipoma-cord boundary is outside the spinal canal. The intradural fatty masses where the spinal cord remains within the spinal canal are defined as spinal cord lipomas (Fig. 5). These are distinct entities but some lesions clearly straddle the boundary between lipomyelomeningoceles and spinal cord lipomas. This lesion is not associated with other abnormalities of the bone or skin. The fatty filum, or fibrolipoma of the filum terminale, refers to a fatty infiltration of part or all of the filum terminale. It can be associated with a short, thick filum and occurs as an incidental finding in approximately 1.5% to 5% of the normal adult population.

Figure 5. A) A T1-weighted sagittal MR image showing a prominent fibro-fatty filum terminale extending from the conus of the spinal cord to the distal end of the thecal sac. B) A T1-weighted sagittal MR image showing a terminal spinal cord lipoma with direct extension of the lipoma into the subcutaneous tissues. The spinal cord is low and within the spinal canal. The presence of a deficiency in the posterior elements suggests that this lesion falls between a true spinal cord lipoma and a lipomyelomeningocele.

Figure 5. A) A T1-weighted sagittal MR image showing a prominent fibro-fatty filum terminale extending from the conus of the spinal cord to the distal end of the thecal sac. B) A T1-weighted sagittal MR image showing a terminal spinal cord lipoma with direct extension of the lipoma into the subcutaneous tissues. The spinal cord is low and within the spinal canal. The presence of a deficiency in the posterior elements suggests that this lesion falls between a true spinal cord lipoma and a lipomyelomeningocele.

Clinical Presentation

For patients with a lipomyelomeningocele, 50% to 90% present with a back mass, 30% to 60% have bladder dysfunction, and 10% to 60% present with foot deformities, neurological symptoms, or other orthopedic abnormalities. All lipomyeloschisis lesions may cause progressive neurological and orthopedic dysfunction by spinal cord tethering. Patients with spinal cord lipoma often present with signs and symptoms resulting from spinal cord compression.

Radiological Evaluation

MRI and computed tomography (CT) are useful in demonstrating lipomatous abnormalities of the spinal cord. MRI and CT myelography can also demonstrate an abnormally low conus medullaris.

Treatment

The aim of surgical treament is to improve symptoms caused by spinal cord tethering or by cord compression from progressive fat deposition. Early surgical intervention is important to prevent further decline in neurological function. Some orthopedic abnormalities, such as foot deformities, progress regardless of surgery. Surgical correction of the lipomyelomeningocele includes resection or debulking of the fatty tumor, untethering of the spinal cord and closure of the dura. Overall, 19% of patients will experience an improvement in symptoms, 6% will worsen, and 75% will be unchanged.

Fatty Filum Terminale Features

A fatty filum terminale, or fibro-fatty filum, refers to a condition in which the spinal cord is tethered by the presence of an abnormal filum. The normal filum is usually not visible on routine MRI, although some fat is present in approximately 5% of normal individuals. The tethered spinal cord syndrome originally referred to symptoms of cord tethering (Table 1), a low-lying conus medullaris, and a terminal filum greater than 2 mm in diameter without evidence of other tethering conditions. However, it is most useful to view the fatty filum terminale as one cause of the tethered cord syndrome.

Treatment

The indications for surgical treatment of a fatty filum terminale are controversial. In the presence of clearly progressive symptoms, most surgeons would proceed with surgical division of a fatty filum terminale. For some surgeons, the coexistance of a low conus medullaris and a fatty filum in an asymptomatic patient is sufficient reason to proceed with a prophylactic procedure to prevent the possibility of future neurological deterioration. For asymptomatic patients with an isolated fatty filum and a normally positioned conus, the recommendation is less clear, since the natural history of this condition is unclear. Some surgeons feel that this anomaly will become symptomatic in a sufficient number of patients and that prophylactic surgery is indicated.

A baseline preoperative cystometrogram is recommended, as postoperative changes in bladder function can occur. Since the terminal filum acts as the tethering agent in this condition, a limited lumbosacral laminectomy with sectioning of the filum terminale is the procedure of choice. The filum terminale is identified by the presence of tortuous vessels on its surface, as well as a whiter appearance than surrounding nerve roots. Intraoperative neurological monitoring and anal sphincter electromyogram (EMG) recording are used to definitely differentiate functional nerve fibers from the filum.

Surgical sectioning of the fatty filum has been advocated for both symptomatic and asymptomatic patients. In the symptomatic group, 43% will have improvement or resolution of neurological or orthopedic abnormalities, while 58% will have a stabilization of their symptoms. The overwhelming majority of asymptomatic patients remain asymptomatic after surgery.

Split Cord Malformation

Features

Split cord malformations (SCM), also refered to as either diastematomyelia or diplomyelia, are believed to occur due to a failure in gastrulation preceding neural tube closure. Mesodermal remnants persist dorsal to the notocord leading to a split spinal cord. This condition accounts for up to 25% of cases of occult spinal dysraphism. There are two types of SCM, each occurring with the same incidence. In each type of SCM, cord tethering occurs as a result of either a thickened filum terminale, or from fibrous bands that act to tether the hemicords and dura.

Type I SCM is a split cord in which each hemicord lies within a separate dural tube, with a bony septum in between (Fig. 6). Other spinal abnormalities, such as absence of an intervertebral disc, or hypertrophic bone, often exist at the site of the SCM. An overlying area of hypertrichosis can also be seen, as can orthopedic foot deformities, and scoliosis in 50% of cases. Type II SCM refers to a split cord in which the two hemicords are contained within a single dural tube, separated by a fibrous median septum. Each hemicord has nerve roots exiting from it, and there is usually no spinal abnormality at the level of the split. Patients present with symptoms from cord tethering. The hemicords and septum are best demonstrated by CT myelography or MRI.

Treatment

Surgical intervention for Type I SCM involves untethering of the spinal cord at the level of the defect and at the filum, removal of the bony septum and reconstruction of the dura into a single tube. It is important to resect the median septum prior to sectioning of the filum so as to prevent the cord from retracting against the bony septum after the filum is cut. The treatment for Type II SCM involves untethering the cord at the level of the split, as well as dividing the thickened filum.

Dermal Sinus Tract

Features

A dermal sinus tract is an epithelial-lined structure beginning at the surface of the skin, most often of the lumbosacral region, and extending to the dura, subarachnoid space, or spinal cord. This cutaneous portion of the tract is visible as a midline dimple with either normal or pigmented skin surrounding the dimple. The tract always courses cephalad as it travels inward. Moreover, this tract can widen at any point to form an epidermoid cyst if lined with stratified squamous epithelium, or a dermoid cyst if lined with dermis. Bladder dysfunction resulting from cord tethering is usually the first presentation of a dermal sinus tract. In addition, the sinus tract serves as a potential pathway for intradural infection, and can result in meningitis and/or an intrathecal abscess.

When a dermal sinus is noticed at birth, ultrasound is the modality of choice to evaluate for occult spinal dysraphism. MRI is essential to demonstrate both the extraspinal and intraspinal course of the sinus tract (Fig. 7).

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Figure 6. Several CT scan images (bone windows) showing an asymmetric midline bony spur dividing the spinal cord in two with separate dural sacs. The vertebral body is also abnormal. This is an example of a Type I SCM.

Treatment

Sinus tracts should never be probed or injected, as this may cause an infection or chemical meningitis. All sinuses should be surgically explored and excised prior to the onset of neurological dysfunction or infection. The deep attachment of the tract to the spinal cord or filum must be released.

Terminal Myelocystocele

Features

The terminal myelocystocele is a rare form of occult spinal dysraphism and refers to the presence of a CSF-containing terminal cyst of the lower spinal cord (Fig. 8). This cyst communicates with the central canal of the cord and is surrounded by an outer dural sac. Tethering results from the attachment of the terminal myelocys-tocele to the spinal cord.

Figure 7. A T1-weighted sagittal MR image demonstrating a dermal sinus tract extending from the skin through the spinous process and into the spinal canal. If there is a patent lumen within the tract, CSF can leak through the tract.

Figure 8. A T1-weighted sagittal MR image demonstrating a terminal myelocystocele. The end of the spinal cord flares into a dysplatic, cystic placode that terminates in the fat-containing subcutaneous tissues.

Signs and symptoms are those of the tethered cord. Patients with this lesion typically have very poor bowel and bladder function and marked lower extremity weakness. These lesions are also associated with various congenital defects, including the imperforate anus, cloacal extrophy, omphalocele, unilateral renal agenesis and ambiguous genitalia. MRI or ultrasonography is useful in demonstrating the terminal myelocystocele.

Treatment

Surgical intervention requires separating the spinal cord from the terminal my-elocystocele, recreating the neural tube, and reconstructing the subarachnoid space by incising and reapproximating the dura.

Neurenteric Cyst Features

Neurenteric cysts are spinal cysts lined with alimentary tract mucosa. They may be extradural and extend into the abdomen or mediastinum, or may be intramedul-lary. These cysts are formed by entrapment of endoderm between a split notochord. Neurenteric cysts may be associated with other dysraphic elements, or may exist as a solitary lesion. Patients with these lesions can present with symptoms of acute spinal-cord compression.

Treatment

Gross total resection of the cyst is the treatment of choice. Intraoperative ultrasound is useful in localizing the entirety of the lesion. Patients who undergo a subtotal cyst excision have been found to have an 11% cyst recurrence rate.

Meningocele Manque

This condition refers to the presence of fibrous bands of neural tissue tethering the spinal cord to the dura or surrounding structures. Patients present with signs and symptoms of cord tethering. MRI and/or CT myelography are used to visualize the fibrous tethering bands. Surgical treatment involves sectioning of the fibrous tethering bands.

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