A 42-year-old man reports three days of gradual onset, progressively worsening lower extremity weakness. The day before presentation, he lost all strength in his right leg, causing him to fall twice.
He has no back pain or sensory disturbance, denies urine and stool incontinence, and reports no trauma or heavy lifting. He works in a restaurant kitchen, and does not use tobacco, alcohol, or IV drugs. His temperature was as high as 39.7°C, but his vitals were otherwise normal. His cardiac, pulmonary, and peripheral vascular examinations were unremarkable. His alertness, cognitive ability, facial and upper extremity strength, and coordination are normal. His left leg strength is 5/5 at all joints.
RLE. He can abduct and adduct at the hip, but cannot flex against gravity nor can he extend his knee. Dorsal and plantar flexion of the ankle and toes is 4/5. Patellar reflexes are not obtainable. At the ankle 1+. Babinski negative.
I lightly touch his feet and lower legs. Gross sensation is equal. He is able to discern pinprick throughout all dermatomes including the perianal area. Rectal tone is normal, and he has no retention of urine by bladder percussion.
Time constraints and a sensitivity-doctor approach entice us to simplify this case to fever and leg weakness equaling rule-out epidural abscess, and then admit. I believe we can do better.
Instead of “weakness,” we should say, “Dense 2/5 monomelic motor deficits relatively sparing the anterior tibial, extensor hallucis, gastrocnemius, soleus, and flexor hallucis muscle fibers, and unaccompanied by sensory findings or back pain.” This description will steer us to the answer when we walk the neuromuscular circuit from peripheral to central.
Muscle. Myopathies can be structural and affect fibril components or protein interconnections between myocytes or metabolic and involve glucose and lipid metabolism or electrolyte channels. All of these will manifest as weakness without sensory disturbance. But none would be expected to present asymmetrically. The same can be said for noninfectious myositis. Infectious myositis, on the other hand, can be asymmetric, but should have tender muscle groups.
Neuromuscular junction. Motor deficits without sensory findings are the rule here. It can be related to Ach receptor blockade as in myasthenics or impaired presynaptic release as in Lambert-Eaton disease, botulism, or another toxin. Again, monomelia would not be classic.
Peripheral nerve. Neuropathies or neuritis usually produce sensory and motor findings. Occasional cases of demyelination or axonal degeneration of only motor nerves have been described. Proximal predominance of weakness with relative sparing of distal common peroneal and posterior tibial fibers, however, is not characteristic of a neuropathy.
Plexus. The lumbar plexus sits behind the psoas muscle. A network of crisscrossing fibers marks the transition from spinal nerve to peripheral nerve. Any lumbar plexopathy could spare the lower lumbar and sacral origin fibers, allowing for flexion at the ankle and toes. But all would be expected to have sensory deficits because motor fibers run amid pain and proprioceptive axons here.
Nerve root. A multilevel motor radiculopathy involving L2, 3, 4, and sparing L5 and S1 while leaving all dorsal roots and ganglia unaffected? Texts describe avulsion injuries occasionally shearing only the delicate ventral motor roots, leaving the sturdier dorsal roots intact. But our patient had no antecedent trauma or traction injury.
We cross the arachnoid layer and enter the lumbar thecal sac. I am tempted to ascend to L1 and jump into the conus declaring a partial cord syndrome from ischemia, compression, or transverse myelitis. But our patient has LMN flaccid weakness rather than UMN spastic weakness, and lacks the contralateral pinprick deficits or ipsilateral dorsal column dysfunction of a hemicord.
Before ascending along the corticospinal tracts of the brainstem, internal capsule, and corona radiata to the contralateral medial motor homunculus along which lays a map of the proximal leg, I pause at our level of the cord and consider one other component.
Gray matter. The one area within the cord that is still lower motor neuron in origin. The anterior horn of gray matter bulges in the lumbar area and sends axons outward to form the ventral root. Pathology at this particular site can produce LMN monomelic weakness without any sensory findings. It would have to be a targeted molecular or cellular attack, not compression from an abscess or disc contents because such cases would involve the neighboring corticospinal tracts and lateral spinothalamic tracts. Fortunately, the differential at this locale is short. It was polio 100 years ago. We must consider ALS in patients with UMN signs and flaccid weakness not localizable to one CNS site. Given that it is 2014 and the patient has no spasticity, the diagnosis is likely to be another cause of anterior horn failure.
Head CT is negative, and the patient is scheduled for an MRI to exclude infectious or disc-related cord compression and paraspinal plexopathies. These studies are obligatory and will be nondiagnostic. The presence of dense LMN deficits that fade distally within one limb and lack any sensory component say so.
I suppose we should make the diagnosis in the meantime. We perform a lumbar puncture, not for meningitis, encephalitis, or traditional myelitis, but to capture the residual inflammation that has eaten away at anterior horn cells in the right lumbar cord gray matter.
CSF WBC of 213 (68% lymphocytes, 21% monocytes), protein of 89, and normal glucose. Rapid HIV test is negative. CSF IgM for West Nile virus came back at five times the upper limit of normal. Infectious disease confirms the diagnosis.
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