Chapter 598 Spinal Cord Disorders
598.1 Tethered Cord
Beyond infancy the spinal cord in humans ends in the conus medullaris at about the level of L1. The position of the conus below L2 is consistent with a congenital tethered spinal cord. For normal humans as the spine flexes and extends, the spinal cord is free to move up and down within the spinal canal. If the spinal cord is fixed at any point, this movement is restricted and the spinal cord and associated nerve roots become stretched. This fixing of the spinal cord, regardless of the underlying cause of the fixation, is called a tethered cord. When severe pain or neurologic deterioration occurs in response to the fixation, it is called the tethered cord syndrome.
In its simplest form the tethered cord syndrome results from a thickened filum terminale, which normally extends as a thin, very mobile structure from the tip of the conus to the sacrococcygeal region where it attaches. When this structure is thickened and shortened, the conus is found to end at levels below L2. This stretching between 2 points is likely to cause symptoms later in life. Fatty infiltration is often seen in the thickened filum (Fig. 598-1).
Figure 598-1 Sagittal MRI showing thickening of the filum terminale in a patient with a symptomatic tethered spinal cord.
(Used with permission from Barrow Neurological Institute.)
Any condition that fixes the spinal cord can be the cause of the tethered cord syndrome. Conditions that are well established to cause symptomatic tethering include various forms of occult dysraphism such as lipomyelomeningocele, myelocystocele, and diastematomyelia. These conditions are associated with cutaneous manifestations such as midline lipomas often with asymmetry of the gluteal fold (Fig. 598-2), and hairy patches called hypertrichosis (Fig. 598-3). Probably the most common type of symptomatic tethered cord involves patients who had previously undergone closure of an open myelomeningocele and later become symptomatic with pain or neurologic deterioration. Tethered cord syndrome can also be associated with attachment of the spinal cord in patients who undergo surgical procedures that disrupt the pial surface of the spinal cord.
Figure 598-2 Child with a lipomyelomeningocele demonstrating an extraspinal mass and an asymmetry of the gluteal fold indicative of underlying occult dysraphism.
(Used with permission from Barrow Neurological Institute.)
Figure 598-3 Hairy patch or hypertrichosis usually associated with diastematomyelia.
(Used with permission from Barrow Neurological Institute.)
Clinical Manifestations
Patients at risk for the subsequent development of the tethered cord syndrome can often be identified at birth by the presence of an open myelomeningocele or by cutaneous manifestations of dysraphism. It is important to examine the back of the newborn for cutaneous midline lesions (lipoma, dermal sinus, tail, hair patch, hemangioma, port-wine stain) that may signal an underlying form of occult dysraphism. Dermal sinuses are usually located above the gluteal fold. Cutaneous abnormalities are not found in patients with an isolated thickened filum terminale. Patients who become symptomatic later in life often exhibit an asymmetry of the feet (i.e., one is smaller than the other). The smaller foot will show a high arch and clawing of the toes (Fig. 598-4). Characteristically, there is no ankle jerk on the involved side and the calf is atrophied. This condition is termed the neuro-orthopedic syndrome.
Figure 598-4 Example of the neuro-orthopedic syndrome involving a larger left foot than right foot, a high arch, and absent ankle jerk frequently associated with tethered cord regardless of the etiology.
(Used with permission from Barrow Neurological Institute.)
Three clinical syndromes can occur at the time of deterioration. The most likely clinical presentation is increasing urinary urgency and, finally, incontinence. Deterioration of motor and sensory function in the lower extremities is a compelling reason for intervention. Finally, severe generalized back pain, often radiating into the lower extremities, can occur, particularly in older adolescents and adults.
Diagnostic Evaluation
When patients present with symptoms related to the tethered cord syndrome, a thorough motor and sensory examination of the patient must be documented. Assessment of bladder function with an ultrasound of the bladder and urodynamic studies is useful in analyzing bladder innervation. MRI is the diagnostic study of choice to reflect the anatomy of the tethering lesion and to provide information about the risks of surgical intervention.
Treatment
There are no nonsurgical options for the management of tethered cord syndrome. Because the presence of tethering is most likely to be at least suspected in the newborn, prophylactic surgery to prevent late deterioration has been advocated by some neurosurgeons. This strategy remains controversial and depends to some extent on a careful assessment of the risks compared to the benefits. If surgical intervention is chosen, microsurgical dissection with release of the spinal cord attachment to the overlying dura is the goal of treatment.
Outcome
The outcome of releasing a thickened filum terminale or detethering of patients with diastematomyelia is routinely good, and the chance of recurrent symptoms is very low. Patients with symptomatic tethered cord who undergo repair of a myelomeningocele or a lipomyelomeningocele have a significant possibility of recurrent tethering and recurrent symptoms.
598.2 Diastematomyelia
Diastematomyelia: Split Cord Malformation
Diastematomyelia is a relatively rare form of occult dysraphism in which the spinal cord is divided into 2 halves. In type 1 split cord malformation, there are 2 spinal cords, each in its own dural tube and separated by a spicule of bone and cartilage (Fig. 598-5A). In a type 2 split cord malformation, the 2 spinal cords are enclosed in a single dural sac with a fibrous septum between the 2 spinal segments (Fig. 598-5B). In both cases the anatomy of the outer half of the spinal cord is essentially normal while the medial half is extremely underdeveloped. Undeveloped nerve roots and dentate ligaments terminate medially into the medial dural tube in type 1 cases and terminate in the membranous septum in type 2 cases. Both types have an associated defect in the bony spinal segment. In the case of type 2 lesions, this defect can be quite subtle.
Clinical Manifestations
Patients with both type 1 and type 2 split cord malformations may have subtle signs of neurologic involvement such as unilateral calf atrophy and a high arch to one or both feet early in life, but they are more likely to be neurologically normal. These patients are tethered by the adherence of the spinal cord to the median membrane or dural sac. Later they may develop progressive loss of bowel and bladder function and sensory and motor difficulties in the lower extremities. Back pain is a common symptom in adolescents and adults with split cord malformation but is uncommon in small children.
Cutaneous manifestations of dysraphism are present in 90% of patients with split cord malformations. Large hairy midline patches called hypertrichosis, the most common cutaneous manifestations, are present in about 60% of the cases.
Diagnostic Evaluation
MRI, the study of choice, shows the 2 spinal cords. The frequent association of bony abnormalities in this condition may require further evaluation with radiography or computed tomography.
Treatment
The treatment of split cord malformations is surgical. This abnormality is a form of tethered cord syndrome, and its treatment is to release the spinal cord to move freely with movement of the spine. In type 1 split cord malformations, the 2 half cords are in separate dural sacs with medial attachment to the dura and bony septum. In this case the dura needs to be opened, the bony septum removed, the medial attachments to the dura lysed, and a single dural tube created. For type 2 lesions, the membranous septum should be lysed. An attachment of this membrane to the anterior dura should be explored and lysed as well. Retethering of this type is rare as there is no reason to disrupt the pial layer of the spinal cord.
598.3 Syringomyelia
Syringomyelia is a cystic distension of the spinal cord caused by obstruction of the flow of spinal fluid from within the spinal cord to its point of absorption. There are 3 recognized forms of syringomyelia depending on the underlying cause. Communicating syringomyelia implies that cerebrospinal fluid (CSF) from within the ventricles communicates with the fluid within the spinal cord and is assumed to be the source of the CSF that distends the spinal cord. Noncommunicating syringomyelia implies that ventricular CSF does not communicate with the fluid within the spinal cord. It primarily occurs in the context of intramedullary tumors and obstructive lesions. In the final form of syringomyelia, that is, post-traumatic syringomyelia, spinal cord injury results in damage and subsequent softening of the spinal cord. This softening, combined with the scarring of the surrounding spinal cord tissue, results in progressive distension of the cyst.
Clinical Manifestations
Signs and symptoms of syringomyelia develop insidiously over years or decades. The classic presentation is the central cord syndrome. In this situation the patient develops numbness beginning in the shoulder in a capelike distribution followed by the development of atrophy and weakness in the upper extremities. Trophic ulcers of the hands are characteristic of advanced cases. The central cord syndrome results from damage to the central spinal cord and the orientation of spinal tracts from proximal to distal leading to selective involvement of the upper rather than the lower extremities.
Other forms of presentation include scoliosis that may be rapidly progressive and often can be presumed from the absence of superficial abdominal reflexes. Urgency and bladder dysfunction as well as lower extremity spasticity also may be part of the presentation.
In patients with syringomyelia related to spinal cord injury, the presentation is usually severe pain in the area of the spinal cord distension above the level of the initial injury. There is also an ascending level of motor and sensory dysfunction.
Diagnostic Evaluation
MRI is the radiologic study of choice (Figs. 598-6 and 598-7). The study should include the entire spine and should include gadolinium-enhanced sequences. Specific attention should be paid to the craniovertebral junction due to the frequent association of syringomyelia with Chiari I and II malformations. Obstruction to the flow of CSF from the 4th ventricle can cause syringomyelia; therefore, most patients also should undergo imaging of the brain.
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