ALS and Motor Neuron Disease
NEURO · EP 02 · NEUROLOGY
Before You Listen
- Prerequisites: upper vs lower motor neuron sign vocabulary; phrenic nerve C3-C5 anatomy; basic needle EMG terminology (fibrillations, fasciculations, motor unit action potentials, recruitment).
- Runtime: 1 hour 30 minutes 9 seconds.
- Topic in one line: the diagnostic hallmark of combined upper and lower motor neuron signs in the same body region, the El Escorial / Awaji criteria, the EMG signature, riluzole + edaravone + tofersen pharmacology, and the anticipatory equipment cascade that defines PM&R management.
Vignette. A 63-year-old man presents with 8 months of asymmetric right hand weakness and tripping. Exam shows wasted right thenar eminence with preserved hypothenar bulk, fasciculations across the deltoid and tongue, brisk biceps reflex on the wasted right arm, bilateral Babinski signs, and brisk jaw jerk. Sensation, eye movements, and bowel/bladder are normal. EMG shows fibrillations, positive sharp waves, fasciculations, and large polyphasic motor units in cervical, thoracic paraspinal, and lumbar muscles; sensory studies are normal. Forced vital capacity (FVC) is 62% predicted.
What is the diagnosis, the El Escorial category, the named bedside hand sign, and the next two highest-impact interventions?
Section 1 — The Diagnostic Hallmark and Selective Vulnerability
Bottom line: ALS is the simultaneous degeneration of upper and lower motor neurons in the same body region; eye movements, sensation, and sphincter function are spared, and any deficit in those domains forces the diagnosis to be reconsidered.
Amyotrophic lateral sclerosis is a progressive, fatal neurodegeneration of both upper and lower motor neurons. The name encodes the pathology: amyotrophic = muscle wasting from anterior horn cell loss, lateral sclerosis = gliotic hardening of the lateral corticospinal tracts from Betz cell loss. Co-occurrence of UMN and LMN signs within the same body region is the clinical hallmark; a brisk reflex in a wasted, fasciculating limb is nearly pathognomonic.
Axial FLAIR MRI demonstrating bilateral, symmetric T2/FLAIR hyperintensity of the corticospinal tracts at the posterior limb of the internal capsule, the classic ALS signature on cross-sectional imaging that, when traced superiorly through the corona radiata and inferiorly through the cerebral peduncles, produces the “wine-glass sign” on coronal reconstruction.
Source: Frank Gaillard, “ALS cross.jpg”, via Wikimedia Commons, CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:ALS_cross.jpg
Annual incidence is ~2 per 100,000 with a slight male predominance (1.3-1.5 : 1). Mean age of onset is 55-65 years; ~30,000 people in the United States are affected at any moment. Ninety percent of cases are sporadic and ten percent familial, though the genetic contribution is likely underestimated because apparently sporadic cases may harbor pathogenic variants. Risk factors include military service (Gulf War veterans carry approximately twice the expected risk), cigarette smoking, and possibly heavy physical activity, though the exercise association remains controversial.
| Sign category | UMN | LMN |
|---|---|---|
| Tone | Spasticity (velocity-dependent ↑) | Flaccidity |
| Reflexes | Hyperreflexia, clonus | Hyporeflexia |
| Plantar response | Babinski present; Hoffmann present in UE | Absent / withdrawal |
| Bulk | Preserved (or disuse only) | Atrophy, often dramatic |
| Spontaneous activity | None | Fasciculations |
| Pseudobulbar feature | Pseudobulbar affect, brisk jaw jerk, hyperactive gag | Tongue fasciculations + atrophy |
What is spared is as testable as what is involved.
| Spared in ALS | Anatomic basis |
|---|---|
| Eye movements (CN III/IV/VI) | Oculomotor neurons resistant — possibly differences in calcium-buffering proteins and excitatory input |
| Bowel and bladder | Sacral motor neurons in Onuf nucleus are resistant |
| Sensation | Dorsal root ganglia and sensory pathways unaffected — ALS is a pure motor disease |
| Cognition (mostly) | Largely intact in the majority; 15-20% develop concomitant frontotemporal dementia (FTD); an additional 35-50% have executive dysfunction |
If a patient has significant sensory loss, reconsider the diagnosis: Kennedy disease (X-linked spinal-bulbar muscular atrophy) involves sensory neurons and produces abnormal sensory nerve action potentials, gynecomastia, and testicular atrophy; ALS does not. If eye movements are involved, the diagnosis is essentially never ALS. If early bowel/bladder failure is present, look elsewhere.
Onuf nucleus (S2-S4 sacral motor neurons innervating the external urethral and anal sphincters) is selectively spared, as are oculomotor neurons in CN III, IV, and VI, likely from differences in calcium-buffering protein expression and excitatory input profiles. The clinical consequence is durable: even an end-stage ALS patient on a ventilator retains continence and eye movement, the substrate for eye-tracking communication devices.
The hallmark physical examination finding deserves its own line: simultaneous UMN and LMN signs in the same region. The combination of a brisk reflex in a wasted, fasciculating muscle is one of the most specific signs in clinical neurology and should anchor the differential immediately on ALS.
Clinical Pearl — The classic ALS clinical portrait
Sketch the prototypical board vignette: a man in his sixties or beyond who first noticed one wasted hand, a foot that catches on the carpet, or visible muscle twitching, then progressed to slurred speech and trouble swallowing solids, with episodes of involuntary laughing or crying out of proportion to mood. On exam, the wasted limb has brisk reflexes, the unmistakable mixed UMN/LMN signature in the same body region. What is absent seals the diagnosis: continence is preserved, sensation is normal, and eye movements remain full. Any of those three abnormalities should redirect the workup.
Board Trap — “Sensory loss + LMN weakness = ALS”
Wrong: sensory involvement excludes ALS by definition; think Kennedy disease (gynecomastia, abnormal sensory nerve action potentials [SNAPs], X-linked inheritance) or a superimposed peripheral neuropathy. A pure motor syndrome with normal sensory studies is the gateway requirement.
High Yield — Section 1
- Diagnostic hallmark: UMN + LMN signs in the same body region.
- Spared: eye movements, sphincters (Onuf), sensation, mostly cognition.
- Demographics: 55-65 years, slight male predominance, 90% sporadic.
- The brisk reflex in a wasted, fasciculating limb is nearly pathognomonic.
- Sensory loss, ophthalmoplegia, or early sphincter failure → reconsider diagnosis.