The Rolandic mu rhythm: a clinical study of the atypical group

We studied 241 patients whose electroencephalograms (EEGs) showed 7-13 Hz arch shaped wave patterns (mu rhythm) that are known to appear in the Rolandic area. The patients were then classified into two groups depending on the conditions of appearance of the mu rhythm. Group I (typical group) consisted of 171 cases. In this group, the presence of the wave patterns was not affected by the opening of the eyes, and it was blocked by spontaneous movements, or when sensorimotor stimulation was applied. The characteristic symptoms for this group were observed in patients diagnosed as having well-controlled epilepsy, psychiatric disorders, collagen diseases, etc. In Group I, the peak lay between the ages 6 through 15. Group II (atypical group) consisted of 70 cases. In contrast to Group I, the presence of the wave patterns in this group was reinforced by drowsiness, photic stimulation and hyperventilation. The characteristic symptoms for this group were observed in patients diagnosed as having intractable epilepsy or organic brain disorders. In Group II the peak lay between ages 11 through 15. However, in Group II the cases were almost equally distributed among the various age groups. Paroxysmal abnormal EEG patterns were found to be jointly present with more frequency in the Group II sample of epileptic patients than the Group I sample. Therefore, when the mu rhythm associated with conditions indicating Group II type patients is observed, care must be exercised in the observation of further progression of the illness, and in searching for the possibility of organic brain disorders.