A misdiagnosis of multifocal motor neuropathy

Abstract

A 60-year-old man had progressive asymmetric weakness in his arms and legs for 11 years. The weakness first appeared in the distal muscles and progressed to involve the proximal muscles in the left leg. 5 years into the illness the patient developed distal weakness in his left hand, which spread to involve his left forearm, arm, and shoulder girdle muscles. 8 years after the onset of motor symptoms he developed weakness in his right hand, which gradually affected his right arm, shoulder, and leg muscles over 3 years. He also noticed progressive wasting of muscles, especially in his left arm and both legs. He also complained of frequent fasciculation over all the affected muscles. There were no cranial nerve, sensory, or autonomic symptoms. He was diagnosed with atypical motor-neuron disease in 1993. The diagnosis was confirmed by doctors at another institution in 1994. We first saw the patient in Jan, 2001. His higher mental functions and cranial nerves were intact. We detected hypotonia in the left arm and legs, and winging of the left scapula. Fasciculation was observed in both arms and the left thigh. All deep tendon jerks were hypoactive. There were neither sensory deficits nor incoordination of the limbs. Muscle strength—as assessed by use of the Medical Research Council (MRC) rating scale in ten of the most affected arm and leg muscles (total 20 muscles, maximal score=100)—was 23/100. The baseline modified Rankin disability scale was grade 4. The functional impairment scale for arms and legs was grade 4. The clinical differential diagnosis was progressive spinal muscular atrophy and multifocal motor neuropathy with conduction block. We did nerve conduction studies using a Nicolet Viking IV-D (Madison, WI, USA) and a Cadwell Excel EMG/EP system (Kennewick, WA, USA). The skin temperature was maintained at 34 C. Bilateral median, ulnar, radial, peroneal, and tibial motor nerves were studied. The stimulation sites were at the wrist, elbow, proximal part of the upper arm, and Erb’s point in the upper limb. The peroneal nerve was stimulated at the ankle, and above and below the fibular head in the popliteal fossa. Ankle and popliteal fossa were the stimulation sites for the tibial nerve. The intensity and the duration of the current were raised to the highest levels to ensure supramaximal stimulation. Negative-peak amplitude, area, and duration were measured for the compound muscle action potential obtained from each stimulation site. Distal motor latency, conduction velocity, and F-wave latency were recorded. Antidromic sensory conduction was investigated in the median, ulnar, radial, and sural nerves. Conduction block was detected by use of electrodiagnostic criteria for multifocal motor neuropathy with conduction block. We did the tests with both the electromyography machines, as well as on a healthy volunteer in order to exclude technical artefacts. Conduction block was seen in axilla-to-Erb’s segment in the left median and right radial nerves. In the elbow-to-axilla segment, definite conduction block was observed in the right ulnar and median nerves. In the right median nerve there was definite conduction block in the long segment of wristto-elbow (figure). The F-wave latency was 130% of the normal upper limit in the right tibial nerve. The motor conduction velocity was reduced in the right median nerve. Electromyography showed neurogenic changes in the muscles sampled. CSF examination did not reveal any cells; the protein was 0·6mmol/L (0·15–0·40 mmol/L) with normal sugar. The IgM anti-GM1 ganglioside antibody titre was less than 1 in 600 (Ranbaxy Laboratories, Mumbai, India). MRI of brachial and lumbosacral plexus was normal. Haemogram, blood sugar, renal, and liver function tests were normal. Serum and urine protein electrophoresis were normal. HIV and CSF venereal disease research laboratory serology were non-reactive. A chest x-ray and abdomen ultrasonogram were normal. We started the patient on intravenous immunoglobulins (IVIg) 400 mg/kg/day for 5 days, which were followed by oral cyclophosphamide 1–2 mg/kg. The patient had improved remarkably by the first follow-up examination at 1 month. The total MRC score had increased from 23/100 to 56/100. The modified Rankin disability score increased from grade 4 to grade 2. The functional impairment scale was grade 2. The patient was given IVIg for 2 days (400 mg/kg/day) every 3 months, for 1 year. Recently, IVIg therapy has been spaced to 6 months and his neurological findings are stable without any further worsening of symptoms. We did a repeat electrodiagnostic study after 18 months of IVIg therapy; there was probable conduction block in the right ulnar nerve and both median nerves in the elbow-toaxilla segment. There was pronounced improvement in the distal compound muscle action potential amplitudes compared with the previous study. The patient fulfilled the clinical, laboratory, and electrodiagnostic criteria for definite multifocal motor neuropathy with conduction block. Our patient was initially diagnosed with atypical motor-neuron disease (progressive spinal muscular atrophy) at another hospital.