Multifocal Motor Neuropathy with Conduction Block

Diagnosis

Clinical criteria

1. Slow or stepwise progressive limb weakness

2. Asymmetric limb weakness

3. Fewer than seven affected limb regions (upper arms, lower arms, upper legs, or lower legs)

4. Decreased or absent tendon reflexes in affected limbs

5. Signs and symptoms more pronounced in arms than in legs

6. Age 20–65 years at disease onset

7. No objective sensory abnormalities except for vibration sense

8. No bulbar signs or symptoms

9. No upper motor neuron features

10. No other neuropathies

11. No myopathy

Laboratory criteria

1. Cerebrospinal fluid protein <1 g/L

2. High GM1-specific IgM antibody titer

3. High signal intensity on T2-weighted MRI of the brachial plexus

Electrodiagnostic criteria

1. Definite motor conduction block:

a. CMAP area reduction on proximal versus distal stimulation of at least 50% over a long nerve segment (between Erb’s point and the axilla, upper arm, lower arm or lower leg), or

b. CMAP amplitude reduction on proximal versus distal stimulation of at least 30% over a short distance (2.5 cm), detected by inching. CMAP amplitude of at least 1 mV on stimulation of the distal part of the segment with motor conduction block.

2. Probable motor conduction block:

a. CMAP amplitude reduction on proximal versus distal stimulation of at least 30% over a long segment of an arm nerve. CMAP amplitude of at least 1 mV on stimulation of the distal part of the segment with motor conduction block.

3. Slowing of conduction compatible with demyelination:

a. Motor conduction velocity 75% of LLN; distal motor latency or shortest F wave latency 130% of ULN, or absence of F waves (all after 16–20 stimuli). CMAP amplitude on distal stimulation of at least 0.5 mV.

4. Normal sensory nerve conduction in arm segments with motor conduction block; normal sensory neuron action potential amplitudes on distal stimulation.

Criteria:

• Definite MMN

o 1–11 on clinical criteria, 1 on laboratory criteria, and 1 and 4 on electrodiagnostic criteria

• Probable MMN

o 1–3 and 6–11 on clinical criteria, 1 on laboratory criteria, and 2 and 4 on electrodiagnostic criteria

• Possible MMN

o 1 and 7–11 on clinical criteria, 2 or 3 on laboratory criteria, and 3 and 4 on electrodiagnostic criteria

DDX:

	MMN	ALS	Lower motor syndrome	CIDP	Lewis-Sumner (MADSAM)
Distribution of weakness	Asymmetric	Asymmetric	Asymmetric	Symmetric	Asymmetric
Prominent sensory symptoms	No	No	No	Yes	Yes
Tendon reflexes	Normal or decreased in weakened muscles	Increased in weakened muscles	Decreased in weakened muscles	General hyporeflexia or areflexia	Decreased in weakened muscles
Disease Course	Slowly progressive	Rapidly progressive	Slowly or rapidly progressive	Progressive or relapsing	Progressive or relapsing
CSF protein >1g/L	No	No	No	Yes	Rare
GM1-IgM ab	Common	Rare	Rare	Rare	Rare
Abnormal MRI of brachial plexus	Asymmetric	No	No	Symmetric	Asymmetric
Response to IVIG	Yes	No	No	Yes	Yes
Response to steroids	No (may worsen	No	No	Yes	Yes