2016 Volume 65 Issue 6 Pages 690-694
Jerk-locked back averaging (JLA), a back averaging technique for EEGs that are time locked to the onset of jerks on the EMG, is used to detect the cortical spikes that precede jerks of cortical origin. JLA is mainly used to diagnose cortical myoclonus. In this report, we discuss a case in which we used JLA to detect cortical spikes preceding myoclonic jerks. We report the case of a 14-year-old boy who was diagnosed with cortical myoclonus by JLA. For 9 years and 8 months, whole body tonic seizures emerged after head injury, but CT and MRI revealed no abnormality. The patient was admitted to our hospital because of myoclonus and tremors in the hand from the age of 11 years. A trigger was evoked from the right abductor digiti minimi muscle of the hand. Concurrently, EEGs of the cerebrum area of the right hand were obtained from 140 ms before to 60 ms after the trigger. The JLA results revealed a positive–negative biphasic sharp wave, starting about 15.8 ms before the trigger, with the maximum amplitude observed in the left parietal region and the vicinity of the longitudinal cerebral fissure in the parietal region. Median nerve stimulation of somatosensory evoked potential (SEP) results revealed giant SEP and extended central conduction time (CCT). In this case report, we describe a positive–negative biphasic sharp wave that preceded a myoclonus, giant SEP and extended CCT. These observations suggested that the origin of the myoclonus is subcortical dysfunction.
