How do you recognize ALS on a vignette?

ALS is suggested by mixed upper and lower motor neuron findings with preserved sensation. Classic clues are progressive weakness plus fasciculations, muscle atrophy, hyperreflexia, and spasticity in the same patient. Bulbar symptoms such as dysarthria and dysphagia also fit, and extraocular movements are usually spared until very late. The combination of UMN signs, LMN signs, and no sensory loss is the key pattern.

Why does ALS cause both UMN and LMN signs without sensory loss?

ALS causes both UMN and LMN signs because it degenerates the lateral corticospinal tracts and the anterior horn cells. Lateral corticospinal tract damage produces upper motor neuron findings like hyperreflexia and spasticity, while anterior horn cell loss causes lower motor neuron findings such as atrophy and fasciculations. Sensation is typically spared because the dorsal columns and spinothalamic pathways are usually intact.

How do you tell ALS from cervical cord compression?

ALS is more likely when there are mixed UMN and LMN signs without sensory deficits, whereas cervical cord compression often has sensory and structural clues. Neck pain, radicular symptoms, a sensory level, and bowel or bladder issues point toward cervical myelopathy rather than ALS. MRI of the cervical spine is the classic study to evaluate cord compression, while ALS workup often includes EMG and MRI to exclude structural lesions.

What tests support the diagnosis of ALS?

The main supportive test is EMG with nerve conduction studies, used after clinical suspicion based on mixed UMN and LMN findings. EMG can show active denervation with fibrillations, positive sharp waves, and common fasciculation potentials, helping demonstrate widespread lower motor neuron involvement across regions. MRI of the brain and spine is often obtained to rule out structural mimics such as cord compression, tumor, or syrinx.

What is the treatment for ALS and what usually causes death?

ALS treatment includes disease-modifying therapy and supportive care, but respiratory failure is a common cause of death. Riluzole can modestly prolong survival by decreasing glutamatergic transmission, and edaravone may slow functional decline in some patients. Noninvasive ventilation with BiPAP improves survival and quality of life, and PEG feeding is used for dysphagia or weight loss caused by bulbar involvement.

Comparison table: ALS

ALS is a classic “boards love it” neurodegenerative diagnosis because it’s pure motor, hits both UMN and LMN, and spares sensation—a combo that instantly narrows your differential. This post is built as quick-hit, shareable tables + a mnemonic you can picture in your head.

Quick check · free practice questionStep 2 CK

A 49-year-old man comes to the clinic with an 8-month history of progressive voice changes and difficulty swallowing. His dysphagia initially occurred with dry foods such as crackers but has progressed to include liquids such as smoothies and ice cream. He works as a newspaper editor and reports increasing difficulty writing with his dominant hand. His wife says that he snores and drools in his sleep. Medical history is notable for hypertension and a bicuspid aortic valve, for which he takes hydrochlorothiazide. On examination, there is atrophy of the right hand. His speech is slow. Cardiac auscultation reveals a systolic murmur at the right upper sternal border. Neurologic examination shows a brisk left patellar reflex, and stroking the lateral plantar surface of the left foot elicits extension of the toes. Which of the following therapies is most likely to slow progression of this patient’s symptoms?

ALS in one line (the Step-ready definition)

Amyotrophic lateral sclerosis (ALS) = progressive degeneration of upper and lower motor neurons → mixed UMN + LMN signs with preserved sensation and cognition (usually), often fatal from respiratory failure.

The visual/mnemonic device: “ALS = Anterior horn + Lateral corticospinal”

Picture the spinal cord cross-section:

A = Anterior horn cell loss → LMN signs
L = Lateral corticospinal tract degeneration → UMN signs
S = Sensation spared (dorsal columns/spinothalamic usually intact)

One-liner: ALS damages motor neuron cell bodies (anterior horn) and descending UMN tracts (lateral columns), so you get both UMN + LMN findings without sensory loss.

Quick-hit comparison table: ALS vs look-alikes (high-yield)

Disorder	UMN signs?	LMN signs?	Sensory loss?	Key distinguishing clues	Classic test/association
ALS	✅	✅	❌ (typically)	Progressive weakness, fasciculations, atrophy, hyperreflexia, spasticity; bulbar symptoms (dysarthria/dysphagia)	EMG: denervation (fibrillations), fasciculations; can be familial (SOD1, C9orf72)
Cervical myelopathy / cord compression	✅ (below lesion)	✅ (at lesion level)	✅ often	Neck pain, radicular symptoms, sensory level, bowel/bladder issues possible	MRI cervical spine
Multiple sclerosis	✅	❌ (usually)	✅ often	CNS demyelination; optic neuritis, INO, relapsing-remitting	MRI plaques; CSF oligoclonal bands
Guillain-Barré (AIDP)	❌	✅	± (often paresthesias)	Ascending weakness, areflexia, autonomic instability; often post-infectious	CSF: albuminocytologic dissociation
Myasthenia gravis	❌	❌ (no true LMN)	❌	Fluctuating weakness, worse with use, ptosis/diplopia; normal reflexes	AChR/MuSK Ab; improves with rest/edrophonium historically
Parkinson disease	❌	❌	❌	Bradykinesia, resting tremor, rigidity; not true weakness early	Lewy bodies (α-synuclein)
Primary lateral sclerosis (PLS)	✅	❌	❌	Pure UMN syndrome (spasticity, hyperreflexia)	Diagnosis of exclusion; slower course
Spinal muscular atrophy	❌	✅	❌	Childhood onset; tongue fasciculations; genetic	SMN1 mutation

USMLE move: If you see UMN + LMN + no sensory deficits, ALS rockets to the top—but always consider cord compression and rule it out (especially if pain, sensory level, or sphincter issues).

Core clinical features (what shows up in vignettes)

UMN signs (corticospinal tract)

Hyperreflexia
Spasticity
Clonus
Babinski sign

LMN signs (anterior horn cells)

Muscle atrophy
Fasciculations
Weakness
Hyporeflexia (can be regionally present even when other areas are hyperreflexic)

Bulbar involvement (high-yield)

Dysarthria, dysphagia
Emotional lability (pseudobulbar affect can occur)
Tongue fasciculations/atrophy may appear

Sensory and other systems

Sensation is typically preserved
Extraocular movements usually spared until very late
Bowel/bladder usually spared early (if prominent, think spinal cord pathology)

ALS vs MS in one sentence (common confusion)

ALS = motor neuron degeneration (UMN + LMN), no sensory loss.
MS = CNS demyelination (UMN + sensory/visual), lesions separated in time/space.

Diagnosis: what to know for USMLE

Best next step logic

Clinical suspicion (mixed UMN/LMN signs) → EMG/NCS to support LMN involvement and exclude mimics
MRI brain/spine is often used to rule out structural lesions (cord compression, tumor, syrinx)

EMG pearls

Shows active denervation: fibrillations, positive sharp waves
Fasciculation potentials are common
Helps show widespread LMN involvement across regions

Pathology & genetics (high-yield snippets)

Degeneration of motor neurons in:
- Primary motor cortex
- Brainstem motor nuclei
- Anterior horn of spinal cord
Some familial cases:
- SOD1 mutation (free radical handling)
- C9orf72 repeat expansion (also linked with frontotemporal dementia)

Treatment & prognosis (Step 2–leaning but testable)

Disease-modifying (modest benefit)

Riluzole (decreases glutamatergic transmission; can modestly prolong survival)
Edaravone (free radical scavenger; may slow functional decline in some patients)

Supportive care that matters

Noninvasive ventilation (BiPAP) improves survival and quality of life
PEG feeding for dysphagia/weight loss
Multidisciplinary ALS clinic care is associated with better outcomes

Cause of death (very commonly tested)

Respiratory failure from diaphragmatic/respiratory muscle weakness

Rapid-fire “If you see this → think ALS” checklist

Progressive weakness + fasciculations
Hyperreflexia + muscle atrophy in the same patient
No sensory loss
Bulbar symptoms (dysarthria/dysphagia)
Normal eye movements early

Mini-mnemonic recap (shareable)

“AL(S)” = Anterior horn (LMN) + Lateral corticospinal (UMN) + Sensation spared.