Impaired Motor Cortex Plasticity in Patients with ALS

Amyotrophic lateral sclerosis (ALS) is characterized by an extensive loss of neurons in the corticospinal motor system. Neurophysiological studies in ALS patients have demonstrated motor system hyperexcitability with increased corticospinal excitability and reduced intracortical inhibition (ICI). We wanted to examine whether the deficient inhibitory control of intracortical circuits in ALS patients is accompanied by impairment of functional reorganization. The capability of the corticospinal system for externally induced, transient plastic changes was tested using transcranial direct current stimulation (tDCS). Eight patients with ALS and eight age- and sex-matched healthy controls received 7min of anodal or cathodal tDCS in two sessions. Active and resting motor thresholds, corticospinal excitability using single-pulse MEP and ICI and intracortical facilitation (ICF) using paired-pulse stimulation (interstimulus intervals: 2 and 12 ms) were tested with TMS before and twice after tDCS. As reported previously, ICI was reduced in ALS patients compared to controls. In addition, corticospinal excitability in ALS patients remained unchanged after either type of tDCS, while a polarity-specific effect could be observed in controls with anodal tDCS inducing facilitation and cathodal tDCS leading to inhibition. Motor thresholds, ICI and ICF did not change in response to tDCS in both groups. Our data are compatible with the idea that motor-cortical plasticity is impaired in ALS patients.