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CASE 40: DISTAL LIMB WEAKNESS AND NUMBNESS
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A 54-year-old man has had 6 years of slowly progressive numbness and weakness of his distal limbs. Symptoms began as a constant painless tingling in his feet; after a few years, similar sensations appeared in his fingers, and the paresthesias in his feet changed to numbness. During the next 4 years, sensory symptoms spread to his knees and wrists, and he had increasing unsteadiness walking and difficulty writing or manipulating objects. He attributes this difficulty to both weakness and incoordination. There are no symptoms referable to the autonomic nervous system.
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On examination, there is atrophy of muscles below his knees and in his hands. Strength of toe and ankle dorsiflexion and intrinsic hand muscles is graded 3/5 bilaterally; a lesser degree of weakness affects plantar flexion and forearm muscles. Vibratory sensation and proprioception are decreased in his toes, ankles, and fingers; pinprick, touch, and temperature sensation are reduced distal to his elbows and knees. Gait is broad based and unsteady, and he can neither walk tandem nor stand with his feet together and eyes closed. Tendon reflexes are absent throughout. Mentation and cranial nerves are normal.
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Motor nerve conduction velocity of the median nerve in the forearm is 25 m/second, with a compound muscle action potential of 3.0 mV (both markedly reduced). Fibular nerve stimulation fails to generate a potential from the toe extensors, and the median nerve sensory action potential is absent at the wrist. Fibrillations and positive waves are present at electromyographic study of the anterior tibial and opponens muscles. Cerebrospinal fluid (CSF) protein is elevated at 95 mg/dL. Serum protein electrophoresis reveals a γ-globulin spike. The serum immunoglobin M (IgM) level is 960 mg/dL (normal range 50–311 mg/dL); immunoglobin G and A levels are normal. Serum immunofixation identifies an IgM κ paraprotein, and immunoblot reveals that it binds to myelin-associated glycoprotein (MAG). A bone marrow biopsy is normal, and further studies do not identify malignant plasma cell dyscrasia or lymphoma. Sural nerve biopsy reveals IgM with κ light chains on myelin sheaths but no amyloid. Teased fiber preparations and electron microscopy reveal segmental demyelination and remyelination with lesser degrees of axonal loss and Wallerian degeneration.
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He receives plasmapheresis every few days, and within a month, his serum IgM levels decline and his strength and coordination improve. Attempts to increase the interval between plasmaphereses to 2 weeks result in a rise in IgM levels and a worsening of symptoms. Receiving small doses of cyclophosphamide, he is able to increase his plasmapheresis intervals to every few weeks.
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Sensorimotor peripheral neuropathy has produced glove-and-stocking sensory loss, greatest for modalities conveyed by large myelinated nerves, plus distal limb weakness in this patient. The unsteady gait and incoordination of his hands are the result of both weakness and impaired proprioception. Electrodiagnostic studies confirm disease of peripheral sensory and motor ...