Excitability of somatosensory cortex is increased in ALS: A SEP recovery function study

Neuronal loss in the somatosensory, as well as the motor cortex in amyotrophic lateral sclerosis (ALS), indicative of a structural abnormality has been reported. Previously we have shown that afferent inhibition was impaired in ALS, suggestive of sensory involvement. In this study, we aimed to evaluate excitability changes in the somatosensory cortex of ALS patients.

ALS patients underwent a paired pulse somatosensory evoked potential (SEP) paradigm at various interstimulus intervals (ISI). The amplitude ratio obtained by dividing the amplitude of paired pulse SEP stimulation S2 (paired pulse stimulation) to S1 (the single pulse stimulation) was considered the somatosensory cortex excitability parameter. Findings were compared to the results obtained from healthy controls. Resting motor threshold (RMT) was also assessed in the ALS group.

An increased S2/S1 ratio was found in the ALS group in every ISI examined. Additionally, the reduced inhibition correlated negatively with forced vital capacity, Medical Research Council sum score, median nerve compound muscle action potential amplitude, while there was a positive association with Penn upper motor neuron score and sural nerve conduction velocity. No correlation existed with RMT.

Our findings demonstrated increased somatosensory cortical excitability in ALS, which was associated with clinical parameters such as reduced pulmonary function and motor strength.

Somatosensory cortical excitability is impaired in ALS. Whether this is associated with increased motor cortical excitability requires further studies.