Use primary lateral sclerosis (PLS), progressive muscular atrophy (PMA), and flail arm syndrome to frame the working diagnosis and next step; use it to sharpen the differential and avoid a false label. Make progressive muscular atrophy (PMA) the checkpoint that determines whether you escalate testing, narrow the differential, or change treatment.

PLS is a pure upper motor neuron syndrome; PMA is the lower motor neuron counterpart and can eventually threaten respiration; Flail arm and flail leg syndromes are lower motor neuron-predominant ALS variants; Progressive asymmetric weakness without sensory loss is a major ALS warning sign; Sensory loss, prolonged plateaus, or bowel and bladder symptoms should prompt reconsideration of ALS