Evoked potentials

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31 Evoked potentials

Sensory Evoked Potentials

The term sensory evoked potentials is used to define the response of the CNS to specific sensory stimulation. In clinical neurophysiology the specific stimuli relate to vision, hearing, and cutaneous sensations.

A difficulty with these evoked potentials is that their low amplitudes, of 20 µV or even less, render them undetectable in routine EEG recordings because of the background wave pattern. Advantage is taken of the regularity of the response to repeated stimuli of the same type. With repetitive stimulation followed by computer averaging, irregular backround rhythms cancel each other out and the evoked potentials can be clearly seen.

The three basic kinds of sensory evoked potentials are described as visual, auditory and somatosensory.

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.)

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.

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.