Intracortical motor conduction is associated with hand dexterity in progressive multiple sclerosis

Background: Hand dexterity dysfunction is a key feature of disability in people with progressive multiple sclerosis (PMS). It underlies corticospinal tract (CST) and cerebellar integrity, as well as disruption of cortical networks, which are hardly assessed by standard techniques. Transcranial magnetic stimulation is a promising tool for evaluating the integrity of intracortical motor pathways. Objective: To investigate neurophysiological correlates of motor hand impairment in PMS. Methods: Antero-posterior (AP) stimulation of the primary motor cortex activates the CST indirectly through polysynaptic pathways, while a direct CST activation occurs with latero-medial (LM) directed current. Thirty PMS and 15 healthy controls underwent dominant hand motor evoked potentials (MEP) using AP and LM-directed stimulation, and a clinical assessment of dexterity (nine-hole peg test) and strength (MRC scale, grip and pinch). Results: PMS with AP-LM latency difference 2.5 standard deviation above the mean of controls (33%) showed worse dexterity but no difference in upper limb strength. Accordingly, AP-LM latency shortening predicted dexterity (R2 = 0.538, p < 0.001), but not strength impairment. On the contrary, absolute MEP latencies only correlated with strength (grip: R2 = 0.381, p = 0.014; MRC: R2 = 0.184, p = 0.041). Conclusion: AP-LM latency shortening may be used to assess the integrity polysynaptic intracortical networks implicated in dexterity impairment.