Study guidelines
- 1.
Describe the general methodology performed to record sensory evoked potentials, the sensory modalities assessed, and an example of clinical disorders and expected abnormality.
- 2.
Contrast the performance of a motor evoked potential versus a motor nerve conduction study.
- 3.
Provide an example of how motor evoked potentials have been used to detect physiologic changes in the motor cortex.
- 4.
Provide an example of how acupuncture may have its clinical effect.
Sensory evoked potentials
The term s ensory evoked potentials is used to define the response of the central nervous system (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 electroencephalogram (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 background 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 a technique known as pattern reversal or pattern shift. 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. 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 poststimulus ( Figure 31.1 ). 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 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 P1 from one eye may be caused by an ocular disease such as glaucoma or by compression or ischaemia 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 avoked 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 to 70 decibels 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 Figure 31.2 .
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. 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 the veracity of claims of deafness induced by environmental noise in industry.
Evidence for a ‘Where?’ auditory pathway
When recording electrodes are specifically deployed over the temporoparietal region and brief sounds are emitted from loudspeakers placed in the left and right visual fields, a cortical response can be detected over the posterior part of the temporal plane, close to the temporoparietal junction. The right posterior temporal plane gives a stronger response, suggesting a right-sided dominance for auditory and visual space analysis.
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 fibres in both peripheral nerves and central pathways.
The nerve of choice for stimulation in the upper limb is the median at the wrist and in the lower limb, the common peroneal at the knee. Repetitive electrical pulses are delivered to the nerve through a surface or needle electrode. The larger myelinated fibres are stimulated. Computer averaging is required to distinguish the stimulated responses from background noise, notably within the CNS. In the example shown in Figure 31.3 , impulse traffic along the median nerve is detected by a sequence of active electrodes attached to the skin for the purpose of recording speed and amplitude of nerve conduction in sequential segments as follows:
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Over the brachial plexus, to assess the median nerve segment extending from the wrist to the anterior triangle of the neck;
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Over the spine of the C2 vertebra, to record the waveform when it arrives at the dorsal nerve roots and ipsilateral dorsal column (fasciculus cuneatus);
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The ipsilateral scalp over the sensory cortex, to ‘pick up’ stimulus traffic ascending the medial lemniscus;
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Over the contralateral sensory cortex, to detect activity in the thalamocortical projection.
