Visual evoked potentials

The speed and amplitude of impulse conduction in the visual pathway are tested by the technique known as pattern reversal or pattern shift. With one eye covered at a time, the patient stares at a spot in the center of a screen illuminated in a black-and-white checkerboard pattern. Once or twice per second the pattern is reversed (to white and black), over a period 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 P100. In healthy subjects it is a positive deflection 100 ms poststimulus (Figure 31.1). In the clinical example shown, taken from a patient with a presumptive diagnosis of multiple sclerosis, the normal P100 wave from the right-eye test indicated that both optic tracts and both optic radiations were clear. The P100 wave from the left eye was both delayed and of reduced amplitude, suggesting presence of one or more plaques of myelin degeneration in the left optic nerve. (Note: On screen and in printouts, it is now customary for the waveforms to be ‘flipped’, with positive responses registering as upward deflections.)

Figure 31.1 Visual evoked potentials. The patient’s right eye has been tested and is now shielded. The left eye is fixated on the spot in the center of the checkerboard during pattern reversal episodes. The pattern from the right eye is normal, showing a positive deflection at 100 ms poststimulus. In the recording from the left eye the P100 is both delayed and reduced in amplitude. The combined results indicate presence of a lesion in the left eye or left optic nerve.

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 P100 from one eye may be caused by an ocular disease such as glaucoma or by compression or ischemia of the optic nerve.

Bilateral abnormal P100 recordings can indicate pathology in one or both optic radiations. In such a situation, it is usual to take recordings from electrode pairs placed a few centimeters to one side of the midline and then a few centimeters to the other side. Should the (say) right optic radiation be at fault, any P100 abnormality is likely to be more pronounced in recordings from that side of the midline.

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 65–70 decibels above click hearing threshold for the ear being tested. The contralateral ear is ‘masked’ by white noise.

A sequence of seven averaged-out waves (I–VII) constitutes the BAER (brainstem auditory evoked response). They are accounted for in the caption to Figure 31.2.

Figure 31.2 Brainstem auditory evoked potentials.

Sources of evoked potentials: 1 distal cochlear nerve (cochlear hair cells); 2 cochlear nerve (proximal); 3 from cochlear nucleus; 4 from lateral lemniscus; 5 from inferior colliculus (inferior brachium); 6 from medial geniculate body (auditory radiation); 7 primary auditory cortex.

Pathology anywhere along the auditory pathway results in reduction or abolition of the wave above that level. The technique is the most sensitive screening test available for acoustic neuroma. A diagnostic feature here is I–III latency separation. (Latency refers to the time interval between stimulus and response; separation refers to extension of the interval between waves I and III, caused by delay during passage along the affected cochlear nerve during a characteristically reduced amplitude wave II.)

In about 30% of patients who have multiple sclerosis (MS) with no clinical evidence of brainstem lesions, the BAER is abnormal. 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 veracity of claims of deafness induced by environmental noise in industry.

Somatosensory evoked potentials

Somatosensory evoked potentials are the waveforms recorded at surface landmarks en route from the point of stimulation of a peripheral nerve to the contralateral somatic sensory cortex. The rate and amplitude of impulse conduction provide valuable information about the status of myelinated nerve fibers in both peripheral nerves and central pathways.