Corticomotoneuronal activity in ALS: changes in the peristimulus time histogram over time

Objective: The primary peak in the peristimulus time histogram (PSTH) reflects the initial rising phase of the excitatory post-synaptic potential (EPSP) evoked at the anterior horn cell. In ALS the primary peak is delayed in onset, increased in duration and desynchronized, abnormalities reflecting dysfunction of the corticomotoneurons. It is not known whether these abnormalities change over time in amyotrophic lateral sclerosis (ALS). Methods: PSTHs were constructed from changes in the firing probability of single, voluntarily activated motor units subjected to subthreshold transcranial magnetic stimuli. We studied 58 motor units in12 patients with ALS on two separate occasions (mean time interval of 10.6±1.6 months). Results were compared with 49 motor units in 11 age matched controls. Results: All the parameters except the amplitude differed significantly between normals and patients. In general the primary peak in ALS was complex, desynchronized and occasionally consisted of a double peak. The abnormalities persisted or were accentuated at the follow up visit. This was reflected by an increase in the number of excess bins, longer duration and latency and decrease of synchrony. Conclusions: Increasing desynchronization of the primary peak over time in ALS reflects dysfunction of the monosynaptic corticomotoneuronal pathway and may also reflect activation of additional slow conducting and/or polysynaptic corticomotoneuronal connections.