Neurological Assessment of Tethered Spinal Cord

4 Neurological Assessment of Tethered Spinal Cord

Sanford Schneider

A nonambulatory patient in his thirties had been a long-distance runner in college. Over a period of a dozen years he had progressively lost motor strength in his lower extremities until he became wheelchair bound. Multiple prior studies had been nondiagnostic, but on careful neurological examination, motor and sensory losses implicated the spinal cord and, subsequently, a myelogram revealed findings compatible with tethered cord syndrome (TCS). Unfortunately, surgical intervention yielded no significant improvement. TCS diagnosis had never been considered by the numerous physicians who had examined him over those many years. This scenario will, it is hoped, not repeat itself given that neurologists, internists, and urologists are now well sensitized to the existence of this disorder. Clinical awareness of TCS and its multiple clinical presentations now leads to neurological referral to “rule-out TCS.” Increasingly, patients with TCS will be diagnosed prior to the onset of irreversible cord damage. However, it is important for the examining physician to be aware of the subtle historical and neurological features that warrant further diagnostic studies to establish this diagnosis.

Although widespread awareness of TCS has been a recent phenomenon, the syndrome was recognized long ago. Chute, in 1921, reported the association of urinary retention and traction on the conus medullaris.1 In 1940, Lichtenstein utilized the all-encompassing term spinal dysraphism for several syndromes associating the failure of the posterior neuropore to close, resulting in neural dysplasia both with and without mesenchymal or dermatological manifestations.²

Bassett in 1950 reported the association of lipomas and conus traction. He noted improvement in seven of nine patients following surgical resection of a lipomatous sac from the conus.³ Garceau reported three teenagers with progressive paraparesis, two with scoliosis, due to a restrictive filum terminale (“cordtraction syndrome”) without associated vertebral malformation.⁴ Hoffman et al, several years later, described a “tethered” spinal cord after meningomyelocele repair, with significant improvement in symptoms following secondary surgical intervention.⁵ Of 442 patients with spina bifida seen over a decade, six patients were diagnosed with a tight filum terminale; neurological dysfunction improved after section of the filum.⁶ Anderson, in 1968, described the clinical presentation of progressive weakness and deformity of lower extremities associated with loss of bowel and bladder control in children with a tight filum terminale, either as an isolated anomaly or associated with spinal dysraphism.⁷ By the 1970s, Hoffman and his associates had operated on 31 children with tethered spinal cords in a 5-year span, generally with gratifying improvement.⁸ Refer to the definition of tethered spinal cord and tethered cord syndrome described in Chapter 3, Pathophysiology of Tethered Cord Syndrome.

Over the last 2 or 3 decades clinical recognition of TCS as a distinct syndrome has evolved. This diagnosis was supported by the development of imaging techniques since the mid-1970s, first initiated by computed tomography (CT), then CT myelography, to the present-day utilization of magnetic resonance imaging (MRI), which readily allows noninvasive diagnosis.9 However, selection of patients for MRI examination must be based on careful historical and neurological features. In one series of 23 symptomatic children with refractory voiding dysfunction, 16 children (70%) had spina bifida occulta but only two children (9%) had an abnormal MRI.¹⁰ Similar to the old axiom that an operating surgeon should occasionally remove a normal appendix when the diagnosis is suggestive, so should a certain number of spinal cord MRIs prove to be normal. However, given the expense of this technology, such imaging should be limited to those patients in whom thoughtful screening, careful history taking, and meticulous neurological examinations all suggest the possibility of TCS.

Patient Evaluation

History

Clinical assessment is initiated through history. The age of the patient will determine whether the pertinent history is obtained from the patient or the caregiver. A newborn presents without significant history but with clinical features: a sacral midline hair tuft (“faun’s tail”), a sacral lipomatous mass, or possibly orthopedic (musculoskeletal) deformities such as shortening or atrophy of a limb or a club foot. The historical features are relatively limited and may include a poor urinary stream or urinary continuous dribbling. In the neonate the decision for further investigation is based on the clinical features, whereas historical features seldom significantly contribute to the decisionmaking process. The ambulatory child, however, develops a symptom complex due to the combination of upper and lower motor neuron deficits. Careful questioning will reveal whether the child has had an alteration of gait that was previously normal, impairment or awkwardness while running, and the tendency to wear out the tip or sides of generally one shoe, due to peroneus or anterior tibialis weakness. An astute parent may report that one leg seems thinner than the other (stork leg). Uncommonly, a parent may report that a child refuses to bear weight due to pain or that the child refuses to flex or rotate his or her back because of pain. Sensory loss is generally minimal in a child, so that trophic scarring is seldom noted. Rapid development of scoliosis may be the chief complaint of the adolescent child undergoing a growth spurt. Although, congenital club foot may be noted in the newborn, later development of a unilateral pes cavus or talipes may be due to TCS. The combination of scoliosis, exaggerated lumbosacral lordosis (in older children and adults), and lower extremity deformity should raise the possibility of TCS.¹¹ In the preteen years, the major symptoms include lower extremity weakness, gait disturbances, incontinence or reemergence of eneuresis, and development of mild foot deformities. The teenager may develop a rather rapid and progressive picture of scoliosis and urinary incontinence frequently associated with gelastic leakage (often misinterpreted as due to a urinary tract infection that defies treatment with antibiotics). The urinary symptoms are frequently investigated with multiple urological studies, and even surgery to eliminate an obstruction, before a spina bifida occulta is noted on the abdominal x-ray.

Adult patients can usually be divided into two presenting groups: those with previously diagnosed and repaired spinal dysraphism and an occult group with slowly progressive and insidious symptomatology or a catastrophic acute precipitating event. This acute event may have been precipitated by the sudden stretching of the conus or lumbosacral cord (during childbirth, intercourse, vigorous sporting events, or extensive exercise) or direct trauma. Pain is the most common presenting symptom in adults, usually in the perineal-saddle region.11,12 Lower extremity sensory loss, leg weakness, and bladder disturbances often accompany the pain.¹³ Many of the patients with TCS will have a spina bifida occulta in the lumbosacral region noted on spine x-ray; however, the vast majority of patients with spina bifida occulta do not have TCS. Most patients with urinary dysfunction will have a hypotonic bladder secondary to lower motor neuron dysfunction. Although a small spastic bladder can result from cord dysfunction above the conus medullaris, spasms are often caused by bladder wall irritation in the presence of the lesion in the conus, which still preserves an adequate number of functional neurons for spinal reflex. Despite urinary dysfunction being common in this population, urinary tract infection is relatively rare in the adult but common in children.¹⁴

Patients with a prior history of repaired spinal dysraphism have at least a 15% chance of a TCS developing later in life.¹⁴ Patients with prior meningomyelocele repairs should be frequently monitored for development of TCS by inquiries as to whether changes in bowel or bladder function have occurred, walking is more difficult, or new motor or sensory disturbances have developed.

Physical Examination

Careful visual examination of the lumbosacral region may reveal a hair tuft, a dermal sinus, a lipomatous mass, a midline nevus, or hypertrichosis, which may the only suggestion of embryonic failure of differentiation between the midline ectoderm and mesoderm occurring in the first trimester of pregnancy. Cutaneous manifestations occur in ∼40% of patients with TCS. Orthopedic deformities, particularly if they are asymmetrical, are also common. Scoliosis, a high-riding asymmetrical hip while standing, pes cavus, and equinovarus skeletal abnormalities may be the presenting feature of TCS. A unilateral stork leg is very uncommon, but subtle asymmetries of the lower extremities are quite common. Careful measurements of the midthigh and midcalf circumferences should be made by marking the midpatella bilaterally, then measuring similar distances above and below the mark with the patient preferably standing; a 10% or 3 cm difference (be aware that the dominant limb is usually slightly greater in bulk) is suggestive of lower extremity atrophy. Differences greater than 3 cm should be considered abnormal, suggesting TCS. In the nonstanding patient, the recumbent position is satisfactory if both legs are flat and equally relaxed. If the measurements are abnormal, it is important to similarly measure the arms because a minimal hemiatrophy of cerebral origin will have asymmetrical measurements. Such subtle circumferential differences as 3 cm are virtually nondetectable by visual inspection. The technique should be routinely performed as part of the assessment of TCS, and the measurement differences should be reproducible on several repeat measurements. This simple test has proven, on many patients, to be a sensitive indicator of TCS.

With a patient of any age, the neurological examination has to be meticulously performed when there is a possibility of TCS, with modification of the exam for the infant or very young child who is nonambulatory. Observation of gait is mandatory and can be assessed watching the child at play while estimating spasticity or weakness. Generally, it is possible to cajole children as young as 3 to heel and toe walk by mimicking the examiner. Emphasis needs to be placed on the motor and sensory examination of the lower extremities. Presentations of TCS are notorious for asymmetrical motor and sensory examinations with “skip areas” of normalcy. The gastrocnemius, anterior tibialis, peroneus, quadriceps, hamstring, and hip rotators and adductor muscle groups need to be individually assessed. Sensation measured by pin and light touch, as well as posterior column function, vibration, and joint position sense are readily performed in patients above the age of 4. As noted earlier, sensation, particularly pinprick, may involve noncontiguous dermatomes, so that each dermatome needs to be examined. The anal wink must be assessed for presence and symmetry. TCS generally produces lower or upper motor neuron signs or a mixture of these signs. Accordingly, deep tendon reflexes are usually reduced or absent, particularly at the ankles, or brisk and asymmetrical. A Babinski sign may be present in one or both extremities but can be normal or silent. In patients with upper motor neuron signs, the concomitant syrinx or diastematomyelia in a higher-lumbar or thoracic level must be considered. Thus clinical suspicion depends on a combination of orthopedic deformity, sacral cutaneous manifestations, asymmetrical lower extremity atrophy, hypotonia or hypertonia, and deep tendon reflex asymmetries, all combined with a history of gait disturbance, bladder dysfunction, and a frequent description of pain in the older child or adult.

Laboratory Studies

The neurodiagnostic capability of MRI noninvasive imaging does not relieve the clinician of thoroughly evaluating the patient by history and careful neurological examination. In one study of 23 children with chronic urological dysfunction, 16 had spina bifida occulta, but MRI spine studies in all 23 patients yielded only one patient with symptomatic TCS and one patient with a syrinx.10 Indiscriminate MRI studies as a screening tool would result in an unacceptably low yield for a relatively expensive study. Many patients with TCS will have radiological demonstration of lumbar, lumbosacral, or sacral incomplete neural arch deformities, generally at L4, L5, or S1. The older literature relates that nearly all pediatric patients with TCS will have a neural arch deformity.^15,16 Newer studies report that the syndrome can often occur in the absence of bony defects,^4,17 particularly in adults and older children.¹¹ The presence of an occult spina bifida is now only suggestive of TCS because 22% of the total population have an occult spina bifida and only an extremely small fraction of that population will have TCS.^15,18 As CT metrizamide myelography replaced myelography as the diagnostic tool of choice; MRI has largely supplanted CT myelography. The normal range of the filum thickness has been established during posterior rhizotomy procedures for control of cerebral palsy-related spasms in children who showed no signs of TCS.¹⁹ Multiple studies have confirmed the diagnostic sensitivity of MRI in children.²⁰ Rarely, high-resolution MRI studies may be necessary to confirm the diagnosis. In the newborn with lipoma, meningomyelocele, and spina bifida, ultrasonography by knowledgeable radiologists is a useful screening tool in determining the extent of the malformation.²¹ The majority of newborns with a low-lying conus can be diagnosed by ultrasound.²²

In the author’s opinion, electromyography and nerve conduction studies are seldom of much practical clinical value in patient assessment of TCS. Often, electromyographic studies are normal, which may mislead the clinician into believing the patient is also normal. Somatosensory evoked studies of the posterior tibial nerve have frequently been useful as a screening procedure and may be a useful tool for following postoperative long-term outcome.²³

Occasionally, urological symptoms, including incontinence, impotence, urgency, and rarely infection may be the heralding symptoms of TCS. Awareness of the typical urodynamic features of typically hypotonic and the less common hypertonic bladder due to TCS should be understood by the urological clinician.

Differential Diagnosis

Although, the classic presentation, as described earlier, may occur, it is the rare patient who presents with a complete constellation of historical and neurological dysfunction. Prior to MRI, the diagnosing physician had to have an extremely high probability of TCS before submitting the patient to an invasive myelogram. However, even with the ease of obtaining a spine MRI, the neurologist should be reasonably certain that TCS is a possibility.

In the infant or younger child TCS can be confused with a subtle hemiparesis due to cerebral palsy. Brain damage in the superior distribution of the anterior cerebral artery may result in weakness and spasticity primarily of the lower extremity. Eighty to 90% of children with cerebral palsy have no history of birthing difficulty, so often a difficult delivery history may not be present. However, a history of prematurity should alert the clinician to the possibility of cerebral palsy. Careful history taking may reveal that the child has had a marked hand preference early in infancy and that motor and language milestones are delayed. Examination of the child with hemiparetic cerebral palsy generally demonstrates unilateral spasticity and hyperreflexia of both the upper and the lower extremities and unilateral long tract signs such as an extensor plantar response. Spastic diplegic patients will have corticospinal tract spasticity limited to the lower extremities and Babinski sign, generally bilaterally symmetrical. In contrast, patients with TCS usually present with lower motor rather than upper motor neuron signs, and sometimes with their combination, manifested by asymmetrical dysfunction in the lower extremities.

Friedreich ataxia, particularly in the young child, can be difficult to diagnose because of the child’s inability to describe a loss of posterior column function. Deep tendon reflexes can be brisk with bilateral extensor plantar responses. A positive family history of progressive weakness, ending by being wheelchair bound in middle age, aids in distinguishing between the two disorders. A DNA study for the expanded nucleotide is diagnostic. The many other forms of spinocerebellar degenerative disorders have a very slow progression, a cerebellar and peripheral neuropathy component, and a positive family history. Chronic polyneuropathy syndromes should be readily distinguished by distal weakness and absent deep tendon reflexes. The static spinomuscular atrophies, such as spinal musculature atrophy III disease, generally have marked lower extremity atrophy, fasciculations, absent deep tendon reflexes, intact bladder and sensory function, and a history of marked delay in walking. Although a family history is usually lacking due to its autosomal recessive inheritance pattern, a DNA study can confirm the diagnosis.

Intrinsic spinal cord pathology can mimic the clinical presentation of TCS. A cord arteriovenous malformation, syrinx cavity, partial cord transection (Devic demyelination), and a cord tumor, such as an ependymoma or glioma, can present with both upper and lower motor neuron dysfunction and sensory disturbance. Tumors tend to be localized in the central part of the cord with sparing of the sacral sensation, whereas TCS tends to commonly involve the bladder function and anal sensation.

Poliomyelitis, now nearly eradicated worldwide, can mimic some features of TCS. An older patient who survived childhood polio is still rarely encountered. The virus attacks the anterior horn cells, which results in asymmetrical weakness and atrophy. A well-known syndrome of exacerbation of weakness may occur decades after the original insult. Careful assessment of the asymmetrical weakness, lack of sensory involvement, and absence of long tract signs should allow ready diagnosis. Rarely, other enteric viruses, particularly Coxsackie, may have an acute, poliolike presentation. A slowly progressive spinal bone infection such as tuberculosis (Pott disease) can present as a slowly evolving spinal cord epidural mass, which clinically would have some features that resemble TCS.

Congenital disorders, including diastematomyelia, can mimic TCS, particularly as functional tethering may occur. A fibroadipose filum terminale or lipomas extending from the caudal conus throughout the sacral canal are part of the symptom complex of TCS.24 Sacral dysgenesis, occasionally found in children of diabetic mothers, may present with a symptom complex similar to TCS. A low-lying, imperforate anus is frequently associated with TCS.²⁵ Type 1 neurofibromatosis, due to an aberrant gene in chromosome 17, may develop a dumbbell schwannoma through a vertebral foramen, which can result in both upper and lower motor, as well as sensory, dysfunction. Extremely rare intraspinal anomalies may have similar clinical presentations.^17,26

The adult patient, generally starting in late middle age and on, has a risk of progressive spinal stenosis. The symptoms and findings are similar to patients with TCS. Although back pain is a symptom common to both disorders,11,12,27 spinal stenosis can be clinically distinguished from TCS by the later age of onset, the frequency of back pain as the presenting symptoms, the relatively rapid development of symptoms, the frequent episodes of precipitating trauma, and often a normal neurological examination.

Treatment

Following diagnosis with the aid of MRI, surgical intervention is necessary in the symptomatic patient. Complex studies, undertaken over many years by Yamada,²⁸ proved that the progressive motor and sensory dysfunction caused by stretching of the cord is not secondary to the histological changes but is due to the deterioration of mitochondrial midcord oxidation that occurs when the function of the lumbosacral spinal cord is compromised. The linear adolescent growth spurt would seem to be the time of greatest vulnerability. The tendency has been to divide patients in symptomatic and nonsymptomatic categories. The nonsymptomatic patients have little or no complaints and show no apparent neurological signs, and suspicion for potential TCS is based on evaluation of a lumbar hair tuft, a slightly asymmetrical gait, or a lipomatous mass. Previously in these stable children the tendency was to wait until the child was older, generally age 5 to 7, before surgical intervention. This was based on anesthetic risk, blood volume percentage loss, small operative field, and a feeling of increased vulnerability to surgical intervention in the infant and young child. Presently, with operating theaters designed for children, and improved monitoring and increasingly successful microscopic surgery, surgery is performed soon after diagnosis at any age.^29,30 However, significantly symptomatic patients, including newborns with spinal dysraphism or adults with acute onset need urgent untethering and should be treated as emergencies.

As the surgical risk lessens and surgical technique and knowledge expand there is little need for hesitation following diagnosis. Notwithstanding reports of series to the contrary,³¹ this author seldom sees postsurgical improvement in patients with significant signs and symptoms. A lost neurological function tends to lack noticeable recovery; thus the earlier the surgical intervention, which almost always prevents further deterioration, the better the patient’s prognosis.^14,32 Particularly in the adult population, or the rapidly growing teenager with accelerating progressive scoliosis, neurological loss can be noted in weeks and months, not years.

After untethering surgery, the patient needs to be periodically but indefinitely followed. The possibility of retethering or symptomatic retractive scarring is significant, particularly if the patient has undergone multiple surgeries at the same site.¹⁴ Careful neurological examination, inquiry regarding bowel and bladder function, and periodic midthigh and calf measurements are necessary. Occasionally, if the patient becomes symptomatic, repeat MRI studies are indicated. Serial somatosensory studies of the posterior tibial nerve may be useful in early prediction of recurrence prior to the occurrence of clinical symptoms.²³

Families with a child with spinal dysraphism will need specific genetic counseling because they have an increased risk of a second child with spina bifida. Additionally, the mother needs to be counseled in regard to potentially teratogenic drugs and the need for folic acid supplementation prior to another pregnancy. If the mother does become pregnant, she needs to be monitored in a high-risk clinic with periodic ultrasounds and a-fetoprotein monitoring.