Average evoked potentials--clinical applications of short latency responses

Many clinical neurophysiology laboratories have added average evoked potential studies to their routine procedures as evoked potential recording methods are non invasive, highly objective and informative. Indeed, short latency brainstem auditory evoked potentials as well as short and intermediate latency cortical evoked potentials, lately have proved to be valuable clinical tools for objectively testing afferent functions in patients with neurological and sensory disorders. The averaged evoked potential responses (EPR) have been widely used in clinical practice to record the changes in the electrical potentials that occur within the central nervous system (CNS) of the patient in response to an external stimulus. Two types of evoked potentials are usually recorded. 1-Stimulus related, short latency evoked potentials, which represent an obligate neuronal response to a given stimulus and both the amplitudes and latencies of these depend on the physical characteristics of the eliciting stimulus. In this category brainstem auditory evoked potentials (BAEPs), visual evoked potentials (VEPs) and somatosensory evoked potentials (SEPs), have normal values for latencies, amplitudes of waves and characteristic wave form. Any abnormality of these reflects excitation, conduction block in the specific pathways in the CNS. Certain abnormalities in EPR reflect subclinical involvement of CNS even before the disease clinically manifests. Abnormality in BAEPs can in addition, depict the exact site of lesion in the brainstem auditory pathways. Same is true for SEPs where abnormalities in far-field or near-field components, reflect lesions at the plexus, spinal cord, brainstem or thalamo-cortical regions respectively. 2- The event related potentials (ERPs) can be recorded in response to an external stimulus to which person is attentive or an event requiring cognition, discrimination, or reaction to the target stimulus. P300 is one such ERP, helpful in distinguishing between disorders such as dementia and depression. This first review gives a bird's eyeview of the essentials, methods, interpretation and clinical applications of stimulus evoked short latency (brainstem auditory, visual and somatosensory) responses in human beings.