ALS motor phenotypes: a revised 'OPM' classification

Defining motor phenotypes in amyotrophic lateral sclerosis (ALS) is important for individualized care and optimal therapeutic trial design. The "ALS-OPM" classification is based on the onset region (O), the propagation of motor symptoms (P), and the degree of clinical upper (UMN) and/or lower (LMN) motor neuron dysfunction (M).

An international ALS expert focus group was held in September 2025, followed by a consensus process through which revisions of the OPM classification were finalized.

Onset (O1-4) identifies first motor symptoms as relating to the head (O1), distal/proximal arm (O2d/p), respiratory/axial trunk (O3r/a), or distal/proximal leg (O4d/p). Onset symptoms are defined by weakness or slowed, poorly coordinated voluntary movements in the muscles of the head, arm, trunk, or leg, including dysarthria, dysphagia, dysphonia, dyspnea, and axial instability. Propagation (P1(n)) or absence of propagation (P0(n)) of motor symptoms from the onset region to another body region are designated, where n denotes the number of months from onset to propagation or assessment. The degree of UMN dysfunction (slowed, poorly coordinated voluntary movements, hyperreflexia and/or spastic muscle tone, emotional lability) and/or LMN dysfunction (weakness with associated muscle atrophy) is classified as follows: balanced UMN and LMN dysfunction (M0); dominant (M1d) or pure UMN dysfunction (M1p); dominant (M2d) or pure LMN dysfunction (M2p); and dissociated UMN/LMN dysfunction (M3), in which the arms and legs predominantly show LMN and UMN involvement, respectively.

The revised ALS-OPM classification aims to make it routine, practical and feasible to capture phenotype in clinical practice and therapeutic trials.