Startle Reflex in Primary Lateral Sclerosis (PLS): A Comparison With Amyotrophic Lateral Sclerosis (ALS)

There is a lack of information about startle reflex (SR) in primary lateral sclerosis (PLS). This study examined the presence and prevalence of SR in PLS and compared findings with amyotrophic lateral sclerosis (ALS).

46 PLS and 54 ALS participants were assessed through structured interviews in this cross-sectional study. Fisher's exact test was used to compare reported SR prevalence. Multivariable linear regression was utilized to study associations between disease group and SR frequency in response to sudden stimuli.

SR differed markedly between the two groups, with a higher prevalence in PLS (93.5%) than ALS (20.4%; p < 0.001). Among ALS patients, SR was present in all upper motor neuron (UMN)-predominant cases, which accounted for 54.5% of the SR-positive ALS group, but only 10.4% of probable/definite ALS cases. In SR-positive patients, response frequency to sudden stimuli exceeded 60% in both ALS and PLS, most often triggered by auditory stimuli. Younger age, shorter disease duration, and PLS diagnosis were associated with more frequent SR.

SR is significantly more common in PLS than in ALS. Notably, UMN-predominant ALS, although limited in number, showed a higher prevalence of SR (6 out of 6, 100%), indicating that predominant UMN involvement may be a key determinant of SR across both conditions. These hypothesis-generating findings suggest that SR may serve as a novel clinical marker in PLS and UMN-predominant ALS, warranting further validation through prospective studies.