EDX · EP 11 · ELECTRODIAGNOSTICS
Before You Listen
Set the stage
- Prerequisites: comfort with the basic NCS pattern (compound muscle action potential / CMAP and sensory nerve action potential / SNAP); the needle electromyography (EMG) findings of denervation (fibrillation potentials, positive sharp waves, fasciculation potentials) and chronic reinnervation (large-amplitude, long-duration, polyphasic motor unit action potentials with reduced recruitment); recognition that motor neuron disease (MND) selectively destroys motor neurons while sparing sensory neurons; familiarity with the four body regions (bulbar, cervical, thoracic, lumbosacral) used in ALS diagnostic criteria.
- Runtime: 1 hour 6 minutes 45 seconds.
- Topic in one line: the MND spectrum, the evolution of amyotrophic lateral sclerosis (ALS) diagnostic criteria from El Escorial to Awaji-Shima to Gold Coast, the cardinal rule that ALS demands normal SNAPs, the split hand sign, spinal muscular atrophy (SMA) and its three new disease-modifying therapies, Kennedy disease (spinal and bulbar muscular atrophy / SBMA) with its mandatory abnormal SNAPs, primary lateral sclerosis (PLS) vs progressive muscular atrophy (PMA), ALS treatments (riluzole, edaravone, tofersen, Relyvrio withdrawal), respiratory thresholds for noninvasive ventilation (NIV), the ALSFRS-R, and post-polio syndrome with the Halstead-Ross criteria.
Vignette. A 42-year-old man presents with eighteen months of slowly progressive proximal weakness, prominent perioral fasciculations producing a chin-quivering appearance, dysphagia, dysarthria, and prominent muscle cramps. He has gynecomastia and reports decreased libido. Family history reveals a maternal uncle who developed similar symptoms in his fifties and lived another two decades. On exam he has proximal greater than distal weakness, brisk fasciculations in the tongue and limbs, hyporeflexia, and notably preserved muscle bulk in the calves with absent ankle reflexes. Strength has progressed only modestly over the past year. Laboratory studies show creatine kinase (CK) elevated at 1,200 U/L. Standard NCS show reduced CMAP amplitudes in the upper and lower limbs with normal conduction velocities, and reduced SNAP amplitudes diffusely (sural absent, median sensory amplitude 8 microV). Needle EMG shows chronic neurogenic motor unit changes with relatively few fibrillation potentials.
What is the most likely diagnosis, what is the single most important EDX feature that excludes ALS in this case, what is the genetic basis, what physical findings beyond the neuromuscular exam point to the diagnosis, and what is the expected disease course compared with classic ALS?
(Answer at the end of this chapter)
Section 1: The MND Spectrum and the Cardinal Rule (Normal SNAPs in ALS)
Bottom line: motor neuron diseases selectively destroy motor neurons while sparing sensory neurons; the single most consequential EDX rule is that ALS requires NORMAL sensory nerve action potentials, and any abnormal SNAP redirects the differential to Kennedy disease, multifocal motor neuropathy with conduction block (MMN), cervical myelopathy with neuropathy, or polyradiculopathy.
Motor neuron diseases (MNDs) encompass progressive neurodegenerative disorders characterized by selective loss of upper motor neurons (UMNs), lower motor neurons (LMNs), or both. ALS is the prototypical MND, defined by the combination of UMN and LMN degeneration across multiple body regions. The broader spectrum includes primary lateral sclerosis (PLS, pure UMN); progressive muscular atrophy (PMA, pure LMN, likely an ALS variant); spinal muscular atrophy (SMA, hereditary LMN from SMN1 deletion); Kennedy disease (SBMA, X-linked LMN with sensory involvement); and post-polio syndrome.
The electrodiagnostic evaluation confirms widespread LMN involvement, excludes alternative diagnoses, and quantifies extent of disease. One fundamental principle governs the entire evaluation: motor neuron diseases affect motor neurons while sparing sensory neurons. The single most critical EDX finding in ALS, and the finding the board tests most consistently, is NORMAL sensory nerve action potentials (SNAPs). If SNAPs are abnormal in a patient with suspected ALS, the diagnosis must be reconsidered immediately. Specific alternatives include Kennedy disease (sensory neuronopathy component), multifocal motor neuropathy with conduction block (MMN; sensory studies normal but motor conduction block at non-entrapment sites), cervical myelopathy with superimposed polyneuropathy, and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP).
The motor NCS in ALS reflects pure motor neuron loss without primary myelin involvement. CMAP amplitudes are reduced (reflecting LMN loss and atrophy) but vary by stage and region. Conduction velocities are normal or only mildly reduced because surviving axons retain their myelin sheaths; mild slowing can occur from loss of the largest, fastest-conducting motor axons, but velocities never drop into the demyelinating range. Distal motor latencies are normal or mildly prolonged. No conduction block is present (critical for excluding MMN). No temporal dispersion is present. F-wave persistence may be reduced, and repeater F-waves (identical morphology on consecutive stimulations) suggest a depleted motor neuron pool.
The needle EMG is the cornerstone. The hallmark is coexistence of active denervation AND chronic reinnervation in the same muscle. Active denervation is documented by fibrillation potentials, positive sharp waves, and fasciculation potentials (which under Awaji carry equivalent weight when accompanied by chronic neurogenic changes). Chronic reinnervation produces motor unit action potentials (MUAPs) with increased duration (surviving motor neurons sprout to reinnervate denervated fibers), increased amplitude (sometimes giant MUAPs >10-20 mV), increased polyphasia, reduced recruitment, and unstable morphology on successive firings. Both must coexist in the same muscle. Chronic changes alone may represent a remote stable process such as old polio; acute denervation alone may represent a different acute process. The combination across multiple body regions establishes ongoing, progressive motor neuron loss.
The four body regions are bulbar (tongue/genioglossus, masseter, orbicularis oris, mentalis), cervical (deltoid, biceps, triceps, first dorsal interosseous (FDI), abductor pollicis brevis (APB)), thoracic (paraspinal muscles at T6-T10), and lumbosacral (vastus lateralis, tibialis anterior, gastrocnemius, extensor digitorum brevis (EDB)). Thoracic paraspinal EMG is particularly valuable because the thoracic region is rarely affected by other common conditions (radiculopathy, entrapment neuropathy), so denervation there provides an additional independent body region that can upgrade diagnostic certainty. When a patient with suspected ALS has only cervical and lumbosacral abnormalities, thoracic paraspinal examination can supply the third region needed for definite classification.
The AANEM minimum study for suspected MND includes up to 4 motor NCS and 2 sensory NCS plus needle EMG of up to 4 extremities (or 3 limbs plus bulbar muscles).
High Yield — MND spectrum and the cardinal SNAP rule
- MND spectrum: ALS (UMN + LMN), PLS (UMN only ≥4 years), PMA (LMN only), SMA (hereditary), Kennedy disease (X-linked, abnormal SNAPs), post-polio syndrome.
- NORMAL SNAPs are mandatory for ALS. Abnormal SNAPs → reconsider Kennedy, MMN, cervical myelopathy + neuropathy, CIDP, polyradiculopathy.
- Motor NCS: reduced CMAP amplitudes; normal or mildly reduced CV; no conduction block (excludes MMN); no temporal dispersion.
- Needle EMG hallmark: BOTH active denervation (fibs, PSWs, fasciculations) AND chronic reinnervation (large MUPs, reduced recruitment) in the same muscle.
- Four body regions: bulbar, cervical, thoracic, lumbosacral. Thoracic paraspinal EMG is high-yield because the thoracic region is rarely affected by other conditions.
- AANEM minimum: ≤4 motor NCS, ≤2 sensory NCS, EMG of 4 extremities (or 3 limbs + bulbar).
We’re referring to a distinct group of progressive neurodegenerative disorders characterized by the selective, relentless loss of upper motor neurons in the motor cortex, lower motor neurons in the brainstem and spinal cord, or a combination of both. And amyotrophic lateral sclerosis, or ALS, is obviously the prototypical disease on this spectrum.
— EDX-11 podcast, ~02:52