Evoked Potentials

Visual Evoked Potentials

The speed and amplitude of impulse conduction in the visual pathway are tested by a technique known as pattern reversal or pattern shift visual evoked potential (VEP). With one eye covered at a time, the patient stares at a spot in the centre of a screen illuminated in a black-and-white checkerboard pattern (small optic nerve lesions can be detected by the technique of multifocal VEP where separate responses can be obtained from up to 60 different regions of the central visual field). Once or twice per second, the pattern is reversed (to white and black), for a total of 100 repetitions. Averaging is performed on the first 500 ms of data from a bipolar recording at the occipital and parietal midline EEG sites (OZ and PZ).

The wave peak of interest is called P1 (or P100). In healthy subjects it is a positive deflection 100 ms post stimulus ( Fig. 30.1 ). In the clinical example shown, taken from a patient with a presumptive diagnosis of multiple sclerosis (MS), the normal P1 wave from the right-eye test indicated that both optic tracts and both optic radiations were clear. The P1 wave from the left eye was both delayed and of reduced amplitude, suggesting the presence of a defect in optic nerve conduction, anterior to the optic chiasm. This type of abnormality is encountered in demyelinative optic neuropathies such as one caused by myelin degeneration from multiple sclerosis and in this example, involving the left optic nerve. ( Note: On screen and in printouts, it is now customary for these waveforms to be ‘flipped’, with positive responses registering as upward deflections.) Conduction defects caused by demyelination are more often expressed in the form of latency delays of the kind shown than in the form of amplitude abnormalities.

In the absence of any evidence for MS elsewhere, an abnormal P1 amplitude from one eye may be caused by an ocular disease such as glaucoma or by compression or ischemia of the optic nerve; visual evoked potential abnormalities do not specify aetiology. Bilateral abnormal P1 recordings can indicate pathology in one or both optic radiations.

Brainstem Auditory Evoked Potentials

Remarkably, it is possible to follow the sequence of electrical events in the auditory pathway, step by step, from cochlea to primary auditory cortex. Following placement of temporal scalp recording electrodes, 0.1 ms click sounds are presented at approximately 10 Hz to each ear in turn through conventional audiometric earphones. Click intensity is adjusted to 60 to 65 decibels (60 to 65 dB nHL; decibels relative to the threshold of a control population) above the click hearing threshold for the ear being tested. The contralateral ear is ‘masked’ by white noise. (The number of stimuli necessary to elicit clear waveforms is in the order of several thousand, in part because of their small relative amplitude.)

A sequence of seven averaged-out waves (I to VII) constitutes the brainstem auditory evoked response (BAER). They are accounted for in the caption to Fig. 30.2 .

Pathology anywhere along the auditory pathway results in reduction or abolition of the wave above that level. The technique is a sensitive screening test for acoustic neuroma. A diagnostic feature here is I to III interpeak latency separation. ( Interpeak latency refers to the time interval between the recorded waveforms; s eparation refers to extension of the interval, in this case between waves I and III, which is caused by a conduction delay along the affected cochlear nerve that also causes a characteristically reduced amplitude wave II. While the absolute latency of subsequent waves is delayed, the interpeak latency between wave III and V is normal.)

In about 30% of patients who have MS with no clinical evidence of brainstem lesions, BAER is abnormal. The most frequent abnormalities are reduced amplitude of wave V and overall slowing of conduction indicated by increased interwave intervals.

Another clinical application of the BAER technique is the assessment of cochlear function in infants under suspicion of congenital deafness. Assessment of brainstem auditory evoked potentials is also important in the medicolegal domain, to assess deafness induced by environmental noise in industry.