Segmental motor neuron dysfunction in amyotrophic lateral sclerosis: Insights from H reflex paradigms

In amyotrophic lateral sclerosis (ALS), the role of spinal interneurons in ALS is underrecognized. We aimed to investigate pre- and post-synaptic modulation of spinal motor neuron excitability by studying the H reflex, to understand spinal interneuron function in ALS.

We evaluated the soleus H reflex, and three different modulation paradigms, to study segmental spinal inhibitory mechanisms. Homonymous recurrent inhibition (H'_RI ) was assessed using the paired H reflex technique. Presynaptic inhibition of Ia afferents (H'_Pre ) was evaluated using D1 inhibition after stimulation of the common peroneal nerve. We also studied inhibition of the H reflex after cutaneous stimulation of the sural nerve (H'_Pos ).

Fifteen ALS patients (median age 57.0 years), with minimal signs of lower motor neuron involvement and good functional status, and a control group of 10 healthy people (median age 57.0 years) were studied. ALS patients showed reduced inhibition, compared to controls, in all paradigms (H'_RI 0.35 vs. 0.11, p = .036; H'_Pre 1.0 vs. 5.0, p = .001; H'_Pos 0.0 vs. 2.5, p = .031). The clinical UMN score was a significant predictor of the amount of recurrent and presynaptic inhibition.

Spinal inhibitory mechanisms are impaired in ALS. We argue that hyperreflexia could be associated with dysfunction of spinal inhibitory interneurons. In this case, an interneuronopathy could be deemed a major feature of ALS.