Erfan Bashar
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ALS — Clinical Phenotypes

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Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.

ALS looks different depending on which motor neurons fail first. Six phenotypes describe that spectrum. Phenotype predicts pace, so recognising the pattern guides prognosis and planning.

The six phenotypes

Figures below are cohort approximations. Individual courses vary widely.

PhenotypeKey featureTypical prognosis
Classical ALSCombined UMN and LMN signs across multiple segmentsAround 3–4 years from first symptoms; about 1 in 10 live 10 years or more
Progressive bulbar palsySpeech and swallowing fail first, with dysarthria usually preceding dysphagiaVariable; often spreads to generalised ALS with shorter survival
LMN-predominant disease, including progressive muscular atrophyLMN signs only at onsetSlower on average than classical ALS
Primary lateral sclerosisUMN signs only: spasticity and hyperreflexiaMarkedly slow; often a decade or more, with near-normal lifespan in many series
Flail-arm syndromeLMN weakness largely confined to the armsMedian around 5–6 years, roughly double classical ALS
Flail-leg syndromeLMN weakness largely confined to the legsMedian around 5–6 years, with a majority alive at 5 years

Primary lateral sclerosis is rare, confidently demarcated in under 3% of motor neuron disease. Flail phenotypes progress roughly twice as slowly as classical ALS but do not carry the near-normal course sometimes claimed for them.

How the disease starts

First symptoms fall roughly into thirds. Some patients notice foot drop or gait disturbance. Others lose hand dexterity. A third group starts with dysarthria or dysphagia.

Respiratory onset and isolated neck flexor weakness are uncommon. Bulbar onset is relatively more common in older women.

One bedside heuristic compares bulbar ALS with myasthenia gravis. In bulbar ALS, dysarthria usually precedes dysphagia. In myasthenia, dysphagia often comes first, reflecting fatigable transmission rather than neuron loss. This contrast is a heuristic, not a rule.

Staging the course

Two systems stage progression. King’s staging counts anatomical spread: one region, then two, then three. Stage 4 marks nutritional (4A) or respiratory (4B) failure. Stage 5 is death.

MITOS tracks loss of independence across four functions: movement, swallowing, communication, and breathing. It draws on ALS Functional Rating Scale scores.

What modifies prognosis

Non-invasive ventilation and timely nutritional support extend survival. Executive dysfunction predicts shorter survival, plausibly because it complicates decisions about gastrostomy and ventilation.

Evidence anchors

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