Case 25

Published on 03/03/2015 by admin

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Case 25

HISTORY AND PHYSICAL EXAMINATION

A 70-year-old woman had a slowly progressive motor weakness, which started 20 years ago. At 50 years of age, she noted weakness of the right hand, which did not respond to surgical carpal tunnel release. Right hand weakness progressed to the point that she could not flex her thumb and index finger. She was relatively stable until age 65, when she began to trip and realized that her left foot was weak. Neurologic examination revealed weakness of left foot eversion and dorsiflexion (Medical Research Council [MRC] 4-/5), significant weakness of the right hand long finger flexors, particularly those to the thumb and index finger. There was 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 that was slightly more rapid than the earlier course. At 68 years of age, the patient noted weakness of the left hand and right shoulder. She has had increasing difficulty abducting her right arm, using her left hand, and controlling her left foot. There have been no bulbar or sphincteric symptoms.

Neurologic examination at 70 years of age revealed normal cranial nerves and sensation. There was atrophy of the right thenar eminence. No fasciculations were observed. Tone was normal. Muscle strength was as follows (modified MRC scale):

	Right	Left
Shoulder abduction	2/5	5/5
Elbow flexion	3/5	5/5
Elbow extension	4−/5	5/5
Pronation	0/5	3/5
Fingers flexion	0/5	3/5
Wrist flexion	1/5	1/5
Wrist extension	2/5	5/5
Finger extension	3/5	4−/5
Finger abduction	4−/5	3/5

	Right	Left
Hip flexion	5/5	5/5
Hip extension	5/5	5/5
Knee extension	5/5	5/5
Knee flexion	5/5	5/5
Foot dorsiflexion	5/5	1/5
Toe dorsiflexion	5/5	0/5
Plantar flexion	5/5	5/5
Ankle inversion	5/5	5/5
Ankle eversion	5/5	1/5

Deep tendon reflexes revealed absent right brachioradialis and biceps reflexes, as well as both ankle jerks. All other reflexes were normal. Sensation was normal. Gait was impaired by left footdrop. Romberg test was negative. An electrodiagnostic (EDX) study was performed.

Please now review the Nerve Conduction Studies and Needle EMG tables.

QUESTIONS

1. Based on clinical grounds, the differential diagnosis should include all of the following except:

A. Motor neuron disease.

B. Chronic inflammatory demyelinating polyneuropathy.

C. Neuropathy associated with anti-myelin-associated glycoprotein (anti-MAG) antibodies.

D. Neuropathy associated with anti-HU antibodies.

E. Neuropathy associated with immunoglobulin M (IgM) gammopathy.

2. Laboratory abnormalities that are potentially associated with this disorder include all of the following except:

A. IgM monoclonal gammopathy.

B. Anti-ganglioside M1 (anti-GM1) antibody.

C. Anti-YO antibody.

D. Anti-MAG antibody.

Case 25: Nerve Conduction Studies

Case 25: Needle EMG

3. The EDX findings observed in this patient are seen least commonly in:

A. Classic amyotrophic lateral sclerosis.

B. Chronic inflammatory demyelinating polyneuropathy.

C. Acute inflammatory demyelinating polyneuropathy.

D. Multifocal motor neuropathy.

EDX FINDINGS AND INTERPRETATION OF DATA

Relevant EDX findings in this case include:

1. Normal sensory nerve action potentials (SNAPs) amplitudes, distal latencies, and conduction velocities throughout.

2. Multifocal conduction blocks, some partial and others near complete. The motor nerves with conduction blocks are the following:

• Both median nerves in the forearms.

• Right radial nerve between the elbow and upper arm (Figure C25-1).

• Left ulnar nerve between the axilla and Erb point (Figure C25-2).

• Right musculocutaneous nerve between the axilla and Erb point (Figure C25-3).

• Left peroneal nerve between the knee and fibular head.

3. Severe impairment of recruitment with scattered fibrillation potentials and increased motor unit action potential (MUAP) duration and polyphasia in muscles that follow multiple peripheral nerve distribution; this correlates anatomically with the sites of conduction block.

Figure C25-1 Right radial nerve motor conduction studies, recording the extensor digitorum communis. Note the severe conduction block (>50% CMAP amplitude reduction and >50% CMAP area reduction) between the elbow (waveform 1) and spiral groove (waveform 2) stimulations. Sensitivity = 2 mV/division.

Figure C25-2 Left ulnar nerve motor conduction studies, recording the abductor digiti minimi, shown superimposed. Waveform 1 = wrist; waveform 2 = elbow; waveform 3 = axilla; waveform 4 = Erb point. Note the conduction block (>50% CMAP amplitude reduction and >50% CMAP area reduction) between the axilla and Erb point stimulation. Sensitivity = 2 mV/division.

Figure C25-3 Right musculocutaneous motor conduction studies, recording the biceps. Note the severe conduction block (>50% CMAP amplitude reduction and >50% CMAP area reduction) between the axilla (waveform 1) and Erb point (waveform 2) stimulations. Sensitivity = 2 mV/division.

In summary, this patient has evidence of multifocal motor neuropathy, with multiple definite conduction blocks. Conduction blocks are common in the chronic acquired demyelinating neuropathies, such as chronic inflammatory demyelinating polyneuropathy (CIDP). However, the preservation of sensory nerve conductions, particularly through nerve segments with motor conduction blocks (such as of the median nerves in the forearms), is a unique feature which is diagnostic of multifocal motor neuropathy (MMN). This motor disorder is not consistent with anti-HU or anti-YO antibody-associated paraneoplastic syndromes associated usually with a sensory neuronopathy (ganglionopathy) or subacute cerebellar degeneration, respectively.

DISCUSSION

Definition and Pathogenesis

Multifocal motor neuropathy (MMN), described in the mid-1980s, is a rare disorder with a prevalence of 1 to 2 individuals per 100 000. It is characterized by specific EDX finding, i.e., motor conduction blocks, which is the gold standard for diagnosis. The disorder is important to recognize since it is treatable and responsive to immunomodulating therapies, and may mimic amyotrophic lateral sclerosis (ALS) which has a poor prognosis for survival.

Many patients with MMN have circulating IgM antibodies to ganglioside M1 (GM1), a glycosphingolipid-incorporating sialic acid residue that is present in both the axolemma and the myelin sheath. Anti-GM1 antibodies frequently recognize the terminal disaccharide moiety of GM1, Gal(1-3)GalNAc, which possesses sialic acid. Although anti-GM1 antibodies bind to motor neurons and the spinal cord, there is ample evidence that the node of Ranvier may be the major site of the effects of anti-GM1 antibodies on peripheral nerves. These antibodies may interfere with sodium channel function localized at the node of Ranvier, as evidenced by the diffuse impairment of nodal resting Na⁺ conductance.

Multifocal motor neuropathy is an immune-mediated neuropathy based on the frequent association with anti-GM1 antibodies and the improvement observed in most patients after immune therapies, particularly intravenous immunoglobulin (IVIG). Also, human sera from patients with MMN produce conduction block when injected in vivo into the peripheral nerves of animals. Pathologic findings at the site of the conduction block include evidence of endoneurial edema, a variable degree of lymphocytic infiltration, demyelination, and onion bulb formation.

CLINICAL FEATURES

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