History and physical examination
A 70-year-old woman has had slowly progressive motor weakness, which started 20 years ago. Initially, at age 50 years, she noted weakness of the right hand, which did not respond to carpal tunnel release. Right hand weakness progressed to the point that she could not flex her thumb and index fingers. She was relatively stable until five years ago when she began to trip, and realized that her left foot was weak. Neurological examination, at age 65 years, revealed weakness of left foot eversion and dorsiflexion (MRC 4-/5), significant weakness of the right hand long finger flexors, particularly to the thumb and index fingers. There was mild atrophy of the right thenar muscles. Deep tendon reflexes and sensory examination were normal. Plantar responses were flexors.

During the next few years she had further worsening, slightly more rapid than the earlier course. At age 68 years, weakness of the left hand and right shoulder started. She had increasing difficulty abducting her right arm, using her left hand, and controlling her left foot. There have been no bulbar symptoms or sphincteric symptoms.

Neurological exam, at age 70 years, revealed normal cranial nerves and sensation. There was mild atrophy of the right hand. No fasciculations were observed. Tone was normal. Muscle strengths were as follows (modified MRC scale):

Deep tendon reflexes revealed absent right brachioradialis, biceps reflexes, and both ankle jerks. All other reflexes were normal. Sensation was normal. Gait was impaired by the left foot drop. Romberg test was negative.

An EMG was done and the relevant findings are:

1. Normal sensory nerve action potentials throughout.
2. Multiple focal conduction blocks, some partial and others complete. The motor nerves with definite conduction block include the following:
   1. Both median nerves in the forearms
   2. Right radial nerve between elbow and upper arm (Figure 1)
   3. Left ulnar nerve between axilla and Erb's point (Figure 2)
   4. Right musculocutaneous nerve between axilla and Erb's point (Figure 3)
   5. Left peroneal between knee and fibular head
3. Severe impairment of recruitment with scattered fibrillation potentials, increase in motor unit potential (MUP) duration, and polyphasia in muscles following multiple peripheral nerve distribution, correlating with the sites of conduction block.

Figure 1 Fig. 1 Right radial nerve motor conduction studies, recording the extensor digitorum communis. Note the severe conduction block between the elbow (waveform 1) and spiral groove (waveform 2) stimulations. Sensitivity = 2 mV/division.

Figure 2 Fig. 2 Left ulnar nerve motor conduction studies, recording the abductor digiti minirni. Note the conduction block and dispersion between the axilla and Erb's point stimulation. 1 = wrist; 2 = elbow; 3 = axilla; 4 = Erb's point. Sensitivity = 2 mV/division.

Figure 3 Fig. 3 Right musculocutaneous motor conduction studies, recording the biceps. Note the severe conduction block between the axilla (1) and Erb's point (2) stimulations. Sensitivity = 2 mV/division.

In summary, this patient has evidence of chronic multifocal motor neuropathy with multiple conduction blocks. Although conduction blocks are common in all chronic acquired demyelinating neuropathies, such as in chronic inflammatory demyelinating polyneuropathy (CIDP), the preservation of sensory nerve conductions, particularly through nerve segments with motor axon blocks, is diagnostic of the recognized disorder, multifocal motor neuropathy with conduction block.

GM1 autoantibodies were moderately elevated in the serum with a titer of 1/1600 (Normal = <1/800). Serum protein electrophoresis and immunofixation were normal. CSF examination revealed normal protein.

She was infused on IVIg (total 2 g/kg) with dramatic improvement of the right shoulder and hand, and left hand. This maximum improvement was in ten days and stabilized for two weeks, but her strength worsened again. She was placed on periodic IVIg every four weeks with good results. Neurological examination six months following the institution of therapy revealed significant improvement of the right upper extremity, particularly of the biceps and the wrist extensors, and the left median innervated muscles. Repeat motor conduction showed significant improvement of conduction blocks (Figure 4A,B). She will continue to receive IVIg, from which she has had a good response, every six-to-eight weeks over the next five years.

Figure 4A

Figure 4B Fig. 4 Right musculocutaneous motor conduction studies, recording the biceps, while stimulating at the axilla (1) and Erb's point (2), before (A) and 6 months after treatment with intravenous immunoglobulin (B). Note that the severe conduction block noted with Erb's point (2) stimulations initially improved significantly, although not completely, after therapy. Sensitivity = 2 mV/division.

This case study provided courtesy of Bashar Katirji, MD of University Hospitals of Cleveland, Cleveland, Ohio