16.2 Motor Neuron Disease and Multifocal Motor Neuropathy

Why motor-neuron and MMN patterns are an NCS safety issue

Outline VI.B.1 groups motor neuron disease — amyotrophic lateral sclerosis (ALS), other anterior-horn processes such as spinal muscular atrophy (SMA) and residual poliomyelitis — with multifocal motor neuropathy (MMN) with conduction block. Independent OpenExamPrep teaching puts them in one section because the worksheet can look similar if you only glance at a small CMAP: weak wasted hands, relatively spared sensation. They are not the same localization. True motor conduction block is the electrical fork in the road. This resource follows published outline topics. It is not an official AAET manual and does not publish unpublished exam cut scores.

Needle EMG (fasciculation potentials, recruitment, mixed cervical-thoracic-lumbosacral and bulbar myotomes, upper-motor-neuron interpretation) is physician work. R.NCS.T. candidates still must know what nerve conduction studies (NCS) should show so they do not skip sensory studies, under-stimulate proximal sites, or fail to look for block in non-entrapment segments when the referral is “rule out motor neuron disease.”

ALS: motor neurons, not demyelinating block

ALS is a motor neuron disease affecting anterior-horn cells (lower motor neuron) and corticospinal pathways (upper motor neuron). Sensory neurons in the DRG are not the target. Typical NCS teaching:

• SNAPs typically normal. Abnormal SNAPs should trigger a search for coincident polyneuropathy, neuronopathy, plexopathy, or a different diagnosis — not a shrug that “ALS can do anything to SNAPs.”
• CMAPs may be low in wasted, denervated muscles once enough motor axons are gone. Distal latencies and velocities of remaining fibers are not in a CIDP/CMT demyelinating range. A 1 mV APB CMAP with a normal median SNAP and a normal velocity is an axonal motor-loss picture, not conduction block.
• No demyelinating conduction block. Proximal and distal CMAP amplitudes may both be small. The ratio across a segment does not show a preserved distal generator with a collapsed proximal volley. If you think you see block in ALS, first exclude submaximal proximal stimulation, anomalous innervation, co-stimulation, and dispersion. Do not relabel ALS as MMN without a real block pattern.
• F waves may be sparse or unstable when motor-neuron pools are depleted; that is not the same as acquired demyelinating F-wave prolongation with preserved distal CMAPs.
• Sensation on the worksheet matches the bedside: patients are not primarily numb. If they are, record the SNAPs and believe them.

ALS can begin in one limb and mimic a radiculopathy or a mononeuropathy. The NCS job in that phase is still to show spared SNAPs, no demyelinating block, and motor amplitudes that follow wasting, not a single entrapment site. Widespread physician EMG later expands the map; the technologist’s early SNAP panel is what keeps a treatable sensory or demyelinating localization from being missed.

MMN with conduction block: the treatable look-alike

Multifocal motor neuropathy with conduction block is an immune motor neuropathy, not a motor-neuron degeneration. Typical electrodiagnostic teaching:

• Motor conduction block in non-entrapment segments (forearm, upper arm, rarely nerves that are not the everyday cubital tunnel or fibular head). Distal CMAP relatively preserved; proximal CMAP amplitude and/or negative-peak area drops. Compact proximal waveforms with collapsed area support block more than a hugely dispersed peak (Chapter 7).
• SNAPs spared in the same nerve that shows motor block. That is the contrast with Lewis–Sumner / MADSAM (multifocal acquired demyelinating sensory and motor neuropathy), which is a sensorimotor multifocal demyelinating cousin, and with ordinary CIDP, which is usually more symmetric and sensory as well as motor.
• Weakness often upper-limb, named-nerve or patchy (finger extensors, wrist drop mimicking radial neuropathy, asymmetric hand wasting). Legs can be involved, but the classroom case is an arm.
• Anti-GM1 IgM antibodies are sometimes present. They support the clinical picture; they are not a substitute for demonstrating block, and a negative antibody does not erase a clear motor-block map.

Why finding true CB matters: MMN is treatable (typically intravenous immunoglobulin in clinical practice). ALS is not an immunoglobulin-responsive demyelinating neuropathy. Calling MMN “motor neuron disease” because the patient is weak and the SNAP is normal withholds a localization that changes care. Calling every technical proximal amplitude drop “MMN” overcalls a treatable disease. The technologist’s contribution is a supramaximal, temperature-adequate, anomaly-aware motor study at proximal sites, plus the sensory studies that stay normal.

Typical demyelinating-neuropathy literature discusses amplitude drops greater than about 20% as possible partial block and stricter drops (often discussed near 50% in some motor-nerve conversations) as more definite, with duration limits so temporal dispersion is not miscounted. Those figures are classroom ranges from published EDX teaching, not unpublished official AAET cut scores. State the idea: abnormal drop across a non-entrapment segment, distal CMAP preserved, SNAPs spared, mimics excluded.

MMN versus entrapment versus ALS — a worked stimulation set

Median motor to APB:

• Wrist CMAP 8 mV, compact
• Elbow CMAP 2 mV, still relatively compact, area falls with amplitude
• Median SNAP to digit 2 normal
• Ulnar motor across the elbow without focal slowing at the groove, but a forearm ulnar block appears in flexor carpi ulnaris territory on a careful map

Independent OpenExamPrep reading: this is not ALS (ALS should not produce a preserved 8 mV distal CMAP that collapses at the elbow in a demyelinating-block pattern). It is not carpal tunnel (that would prolong distal latency, not drop the elbow response relative to a large wrist response). It is the MMN-style motor block pattern until technical mimics are excluded. A Martin–Gruber anastomosis can imitate ulnar “block” in the forearm; know that anomaly (Chapter 17) before you celebrate a block.

If instead wrist and elbow CMAPs were both 1 mV, SNAP normal, no segmental drop: that is axonal motor loss (ALS, SMA, axonal radiculopathy, or motor neuropathy without block) — not conduction block, because there is no preserved distal generator.

SMA and poliomyelitis: chronic motor axon loss

Spinal muscular atrophy and residual poliomyelitis (including the weak, wasted limb of remote polio and some post-polio referrals) are anterior-horn localization without ALS’s progressive mixed upper- and lower-motor-neuron exam in new regions.

NCS teaching:

• SNAPs spared. Polio and SMA are motor-neuron / motor-axon disorders, not DRG diseases.
• CMAPs low in chronically denervated, reinnervated, or wasted muscles. Distal latencies and velocities of surviving axons are not uniformly demyelinating.
• No acquired multifocal demyelinating CB as the disease mechanism. Old polio can have very small CMAPs; do not manufacture block by under-stimulating a deeply wasted limb.
• Side-to-side comparisons matter in asymmetric old polio. A “normal” opposite limb is the control.

West Nile, other viral anterior-horn attacks, and some paraneoplastic motor neuropathies can mimic this motor-axonal, SNAP-sparing picture. The outline’s named cluster is MMN with CB, ALS, and other polio/SMA. Put viral and toxic motor-neuron-like illnesses in the same electrical bin: motor amplitudes down, SNAPs up, no demyelinating block.

Cervical radiculopathy is the third actor

A wasted hand is not automatically ALS or MMN. Cervical radiculopathy (section 16.1) can drop CMAPs in a myotome, spare SNAPs, and produce pain and reflex changes. Distinguishing points on NCS:

• Radiculopathy is root-territory, often painful, often one myotomal cluster (for example C8–T1). F waves may be unimpressive. No multifocal motor block in unrelated non-entrapment segments of other nerves.
• ALS is more widespread than one root over time, typically painless, with upper-motor-neuron signs the physician examines, still without demyelinating CB and with normal SNAPs.
• MMN is painless or crampy motor weakness in named nerves, with motor CB, without upper-motor-neuron signs, with spared SNAPs.

If you only stimulate the wrist and the ankle, you cannot find MMN block and you cannot help the ALS-versus-radiculopathy SNAP story. Proximal stimulation sites exist for a reason.

ALS versus MMN versus cervical radiculopathy

Typical electrodiagnostic teaching. Not unpublished official AAET cut scores.

Technical traps that fake ALS or fake MMN

1. Skipping SNAPs in a “pure motor” referral. You cannot say SNAPs are normal if you did not record them.
2. Submaximal Erb’s-point or axilla stimulation. A small proximal CMAP is not block until intensity has plateaued without co-stimulating neighbors.
3. Anomalous innervation (Martin–Gruber) mimicking ulnar forearm block.
4. Recording a wasted muscle off the motor point, producing a tiny CMAP that you then compare with a better distal site.
5. Cold limb changing latency and amplitude (Chapter 17), not a new motor-neuron disease.
6. Assuming one low APB CMAP equals C8 radiculopathy without the ulnar/MABC SNAP panel and without looking for block in other nerves.

Technologist expectations versus physician EMG

The interpreting physician may needle thoracic paraspinals, tongue, and multiple limbs to support ALS. None of that replaces a technically complete NCS. Independent OpenExamPrep study of VI.B.1 is a short checklist:

1. Record SNAPs in clinically involved limbs.
2. Measure distal and proximal CMAPs on nerves that could hide non-entrapment block.
3. Describe axonal motor loss without block (ALS/SMA/polio/radiculopathy bin) versus motor CB with spared SNAPs (MMN bin).
4. Do not assign the diagnosis from a single millivolt, and do not invent an official numeric cutoff the handbook does not publish.

If you remember one sentence: normal SNAPs plus low CMAPs is motor-neuron or motor-root territory until you prove motor conduction block; motor conduction block with spared SNAPs is MMN territory until you prove a technical mimic. That sentence is why this outline item exists beside ALS rather than inside a generic “weakness” chapter.