Diagnosis of amyotrophic lateral sclerosis overturned

Mr. A, age 61, presented with progressive weakness in his lower extremities. Three months earlier, he had undergone bilateral knee replacement for progressive osteoarthritis. One month after post-op rehabilitation, he could walk a mile without support. At that time, Mr. A noticed increasing weakness when he tried to dorsiflex his left foot, which progressed to foot drop over the course of a week. During the next month, he developed weakness in other muscle groups of the lower extremities, left greater than right.

By the time we saw him, Mr. A was able to ambulate only with a walker. He complained of persistent bilateral tingling over the plantar aspects of his toes. He reported no neck or back pain; upper-extremity weakness; any unusual sensation in his body or extremities; changes in vision, speech, balance, or coordination; or change in urinary and/or bowel habits.

Nerve conduction studies and electromyography (EMG) showed extensive denervation with fibrillations and fasciculations. Also noted was motor-unit remodeling involving bilateral lower-extremity as well as upper-lumbar and upper-thoracic paraspinal muscles. At that point, a motor-neuron disease, particularly amyotrophic lateral sclerosis (ALS), was suspected, and Mr. A was admitted for further evaluation.

EXAMINATION FINDINGS

Neurologic exam on Friday showed normal cognition, speech, cranial nerves (including tongue without atrophy and/or fasciculations), and coordination (when corrected for weakness). Motor exam revealed adequate muscle tone with mild atrophy in both distal lower extremities and no fasciculations. In the lower extremities, strength was notably diminished, ranging from 3-5/5 on the right and 1-4/5 on the left, specifically 3/5 in left-hip flexion and adduction, 2/5 in left-knee extension, and 1/5 in left-foot dorsiflexion. Strength was 5/5 in all muscle groups of both upper extremities.

Sensory exam showed decreased vibration and joint-position perception over the toes (right more than left). Deep tendon reflexes (biceps, brachioradialis, triceps, patellar, and Achilles) were 3+ bilaterally. Plantar responses were upgoing bilaterally. Gait was steady with a walker; left foot drop was apparent.

On Monday, the lower-extremity reflex and sensory findings were as noted on initial presentation. Motor examination was remarkable for 4/5 strength in left-hip flexion and adduction and 3/5 strength in left-knee extension and left-ankle dorsiflexion (all improved since the initial exam). An examination on Tuesday was remarkable for 3/5 weakness in left-hip flexion and adduction and 2/5 weakness in left-knee extension and left-foot dorsiflexion, compatible with the original admission evaluation findings.

ANALYSIS

The subacute progression of weakness, presence of both upper and lower motor-neuron signs, and EMG findings supported the diagnosis of ALS. However, lack of upper-extremity weakness and/or atrophy, absence of bulbar symptoms, presence of sensory deficits, and, most importantly, the fluctuating nature of the weakness were not consistent with ALS. We noted that prior to this latest demonstration of marked weakness, a physical therapist had done a thorough motor exam on Mr. A, part of which included walking up and down the hallway.

A pattern to the motor-strength fluctuations began to emerge. Weakness noted on admission improved markedly after two days of bed rest but recurred following exercise with the physical therapist. This pattern was highly inconsistent with a motor-neuron disease. We considered other options, including vascular myelopathy (particularly venous dural malformation) and multilevel cord compression with conus medullaris and/or cauda equina involvement. While both conditions may produce both upper and lower motor-neuron signs and sensory findings, temporary improvement of weakness with rest is almost pathognomonic of spinal dural fistulas. Moreover, while absence of pain (notable in our case) is uncommon in spinal dural malformations, it is extremely rare in spinal stenosis, conus medullaris, and cauda equina syndromes.

IMAGING WORKUP

MRI of the spine showed a normal cervical cord, a dilated thoracic cord with increased signal intensity on the T2-weighted images beginning at T5 and going through the conus, and dilated vascular structures along the dorsal and ventral surfaces of the cord consistent with dilated anterior and posterior spinal veins (Figure 1). On post-contrast imaging, cord enhancement was noted in a heterogeneous patchy pattern from T5 through the conus (Figure 2). Dynamic magnetic resonance angiography revealed dilated veins along the ventral and dorsal surface of the cord.

DISCUSSION

Spinal dural fistulas occur within the dural sleeve of lower thoracic or lumbar nerve roots and represent up to 80% of all spinal arteriovenous malformations (AVMs). AVMs are seen more often in men. Symptoms appear in mid and late adulthood, between 40 and 70 years of age.

The lesions rarely, if ever, result in hemorrhage but almost always cause pain and myelopathy. The pain may be radicular in nature and frequently has a pattern suggestive of neurogenic claudication, in which pain occurs with ambulation and is relieved by rest. The myelopathy results from spinal cord ischemia, as the fistula within the dural sleeve leads to congestion of the venous outflow of the spinal cord, with sec-ondary venous hypertension. This may be followed by a motor-sensory disturbance which can progress steadily or with remissions and exacerbations that may mimic multiple sclerosis. Sphincter disturbance eventually occurs, and untreated patients are usually wheelchair-bound within five years from onset of the leg weakness.

The diagnosis of the spinal dural fistula can be made by either thin-cut CT myelogram after injection of intrathecal contrast material or by spinal MRI with special technique (e.g., standard T1- and T2-weighted sagittal spin-echo images and T2-weighted axial gradient-echo images, as well as post-contrast three-dimensional MRI and T1-weighted sagittal and axial images). If those are unavailable and a spinal AVM is strongly suspected, spinal angiography must be performed.

HOW TO TREAT

Treatment choices include endovascular embolization and surgical ligation of the fistula. Embolization is less invasive but not possible in every patient, as anatomic variants may present risk of spinal-cord ischemia and/or arterial-wall dissection of the feeding vessels during the procedure. Results are excellent and frequently dramatic, with reversal of even very advanced paraparesis, particularly when the deterioration has been relatively acute and the treatment is undertaken early.

Mr. A underwent successful embolization of the fistula, completed a course of acute rehabilitation, and is doing well. He is ambulating independently, with mild residual weakness in extension of his left knee and dorsiflexion of the left foot.

 

Dr. Gorelov is a resident in the Department of Neurology, North Shore University Hospital, Manhasset, N.Y., and Dr. Respicio is a resident in the Department of Medicine, University of Connecticut Health Center, Farmington.

 

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