Which combination defines Clinically probable ALS?

• A. UMN + LMN signs in 2+ regions AND some UMN signs rostral to LMN signs
• B. UMN + LMN signs in 2+ regions without rostral signs
• C. UMN signs alone in 3 regions
• D. LMN signs only

Answer: (A)

ALS is a disease that affects both upper motor neurons (the pathways that control gross motor tone and reflexes) and lower motor neurons (the motor cells that directly innervate muscles). To call a diagnosis clinically probable, you need evidence of both kinds of signs in multiple body regions, showing that the disease is spreading along the motor pathways. Specifically, there should be UMN and LMN signs in at least two regions, and there should be UMN signs that occur rostral (toward the head) to the LMN signs, indicating progression to higher levels of the neuraxis. This combination reflects the characteristic pattern of contiguous spread that distinguishes ALS from a pure UMN-only process or a pure LMN disease.

Patterns that don’t fit include having UMN signs alone in several regions (no LMN involvement), or LMN signs alone (no UMN involvement), or having both UMN and LMN signs in two regions but without evidence of rostral spread.