To study event-related potentials and particularly the P300 wave in a group of patients with amyotrophic lateral sclerosis (ALS), as changes in the P300 wave have been observed in many cases of brain damage—inducing cognitive impairment.
Twenty patients with ALS were compared with 20 healthy control subjects, with no significant age range and no significant difference in their education level.
Main Outcome Measures:
Amplitudes and latencies of long-latency auditory evoked potentials for the N100, P200, N200, and P300 waves were measured in the "oddball paradigm." Mental status was assessed by the Similarities subtest of the Wechsler Adult Intelligence Scale and by the Rapid Evaluation of Cognitive Function that allows for a mini-neuropsychological testing. The degree of disability was scored on the ALS Severity Scale, which rates bulbar and spinal functions.
The latencies of the N100 and P200 waves did not appear to differ between normal and affected subjects; in contrast, the N200 and P300 latencies were significantly longer in the group with ALS. Sixty percent of the patients had a P300 latency more than 2 SDs above the theoretical norm that was established by the linear curve for control subjects. The N100, P200, N200, and P300 amplitudes showed no significant difference between the control subjects and the group of patients with ALS. There was no significant correlation between the N200 and P300 latencies and the disease duration or between the N200 and P300 latencies and the degree of motor impairment. The scores from the Rapid Evaluation of Cognitive Function were significantly lower in the group of patients with ALS than in the group of control subjects.
A subtle cortical cognitive dysfunction is frequently observed in patients with ALS. These findings point also in the same direction as did previous neuropsychological, histological, and positron emission tomographic studies of ALS. Limb motor deficits and speech difficulties make it difficult to study the time course of changes in intellectual function in patients with ALS by using psychometric methods; thus, in the future, cognitive evoked potentials should constitute a fruitful method of testing cognitive function in patients with ALS to follow up their development over time.