Heteronymous H reflex in temporal muscle as sign of hyperexcitability in ALS patients

The stimulation of the masseteric nerve elicits a homonymous and a heteronymous H reflex in the masseter muscle and in the temporalis one. The presence of the H reflex may be considered a sign of upper motor neuron (UMN) involvement in amyotrophic lateral sclerosis (ALS) patients. The aim of this study was to evaluate the presence of the heteronymous H reflex in patients with ALS and compare it with normal subjects.

We enrolled 36 ALS patients and 52 healthy subjects. We stimulated the masseteric nerve in the infratemporal fossa and recorded the muscle responses ipsilaterally to the stimulation.

The heteronymous temporalis H reflex was elicitable in 88.9% of ALS patients and in none of the controls.

The heteronymous H reflex does not disappear when the stimulation intensity is increased. It can be used as sign of UMN involvement and may prove useful in patients with suspected MND/ALS with purely lower motor neurons (LMN) signs and no signs of UMN involvement.

The heteronymous H reflex is present far more often in ALS patients than in healthy people. It is a simple test that may be used to detect UMN involvement in patients in whom the only evident signs are LMN impairment, improving diagnosis of ALS.