ALS motor phenotypes: a revised 'OPM' classification

Area of Science:

• Neurology
• Clinical Phenotyping
• Motor Neuron Diseases

Background:

• Accurate motor phenotyping in Amyotrophic Lateral Sclerosis (ALS) is crucial for personalized patient care and optimizing clinical trial design.
• The existing ALS-Onset-Propagation-Motor Neuron Dysfunction (ALS-OPM) classification system categorizes ALS based on symptom onset, propagation, and motor neuron involvement.

Purpose of the Study:

• To revise and refine the ALS-OPM classification system.
• To enhance the practicality and feasibility of ALS phenotyping in routine clinical practice and therapeutic trials.

Main Methods:

• An international focus group of ALS experts convened in September 2025.
• A consensus process was employed to finalize revisions to the OPM classification.

Main Results:

• The revised classification details onset regions (O1-4) including head, arm, trunk, or leg.
• Propagation (P0(n) or P1(n)) and the degree of upper motor neuron (UMN) and/or lower motor neuron (LMN) dysfunction (M0-M3) are precisely defined.
• Specific criteria for UMN (e.g., spasticity, hyperreflexia) and LMN (e.g., weakness, atrophy) dysfunction are established.

Conclusions:

• The updated ALS-OPM classification provides a standardized and practical framework for capturing ALS phenotypes.
• This revised system is expected to improve the routine assessment of ALS patients in clinical settings and facilitate more effective therapeutic trials.