Interside amplitude asymmetries in the median nerve somatosensory evoked potential: normative data

Short latency median nerve somatosensory evoked potentials (SEPs) were recorded from a group of 31 adults in order to establish the boundaries between normal and abnormal interside amplitude asymmetries. The potentials studied were the cervical SEP (N13) and the primary cortical response (N20). In order to more accurately analyse and compare intersubject data, the amplitude of the lesser potential was expressed as a percentage of the greater potential. The limit for normality was defined as 1 SD (of the mean amplitude asymmetry) below the lowest individual value observed among normal subjects. It is concluded that if a cervical potential from one side is no more than 60% of the potential from the other side, then this represents a significantly abnormal amplitude asymmetry. In contrast, a cortical potential must have an amplitude which is less than 36% of the larger potential in order to be considered significantly abnormal.