Which ALS variant is LMN-only and may eventually develop classic ALS?

• A. Primary lateral sclerosis
• B. Pseudobulbar palsy
• C. Progressive muscular atrophy
• D. Bulbar onset ALS

Answer: (C)

Progressive muscular atrophy is an LMN-dominant motor neuron disease. Clinically, it presents with weakness, muscle wasting, and fasciculations due to lower motor neuron degeneration, while upper motor neuron signs such as spasticity and hyperreflexia are not prominent early on. Because the disease starts with LMN involvement, it can look like another motor neuron disorder at first, but in many people the pathology eventually extends to include upper motor neuron degeneration as well. When that happens, the clinical picture shifts to classic ALS, with both LMN and UMN signs across the body. That potential for evolution from an LMN-only presentation to a combined ALS phenotype is why progressive muscular atrophy is the best answer.

Primary lateral sclerosis, by contrast, involves upper motor neuron degeneration and shows spasticity and hyperreflexia with little or no LMN involvement early. Pseudobulbar palsy describes a syndrome of UMN impairment affecting bulbar functions, not a distinct LMN-only motor disease. Bulbar onset ALS starts with bulbar symptoms but already includes both LMN and UMN signs in the affected regions and typically progresses to involve other areas with mixed signs.