Background: The demonstration of upper motor neuron (UMN) dysfunction in patients with amyotrophic lateral sclerosis (ALS) with predominantly lower motor neuron (LMN) signs is clinically sometimes difficult.
Methods: We analyzed the possible role of F and H waves in the diagnosis of UMN dysfunction in 36 patients with different clinical forms of ALS and 20 controls. In both lower limbs we evaluated F-wave persistence and the F/M ratio of tibial nerves, Hmax/Mmax ratio, vibratory and recurrent (paired) inhibition of the H-reflex.
Results: The persistence of the F-response was decreased significantly in the LMN group only. The F/M amplitude ratio, vibrated-H/rest-H amplitude ratio and conditioned H/rest H amplitude ratio were significantly increased in patients with predominantly UMN, as well as in those with predominantly LMN signs. H/M amplitude ratios did not differ between the various patient groups.
Conclusion: These findings show that ALS patients, including those without clinical evidence of UMN involvement, have a marked disinhibition of anterior horn motor neurons. The simple tests described could support an UMN abnormality when clinical signs are lacking, and help to establish a diagnosis sooner and more accurately.