54 4 Ancillary Tests in Neurology

Electrophysiological Studies

Fundamentals

Electrophysiological processes are an intrinsic part of all cellular activity (p. 4). Differences in electrical potential and changes in these differences over time can be amplified, displayed on an oscilloscope, and recorded on paper or in digitized form. Electroencephalography records the activity of cortical neurons and neuronal populations and electromyography that of muscle cells. The conduction of spontaneous or induced impulses in peripheral nerves is assessed by electroneurography. Repeated stimulation of the receptors of a particular sensory system (e. g., the retina, by visual stimuli) and simultaneous measurement of the resulting cortical activity enables determination of the conduction velocity within the sensory system in question (evoked potential studies). The complex electrophysiological phenomena that occur during sleep are registered by somnography (sleep studies). These electrophysiological diagnostic

techniques offer a practically riskfree means of assessing the functional state of the nervous system, though some of them are rather unpleasant for the patient. Despite the absence of risk, they should only be performed for strict indications, in accordance with the general principles outlined above on p. 45.

The techniques discussed in this chapter are in widespread use and belong to the diagnostic armamentarium of any clinical neurophysiologist.

Electroencephalography (EEG)

Principle. The surface EEG registers fluctuations in electrical potential that are generated by the cerebral cortex. These represent the sum of the excitatory and inhibitory synaptic potentials.

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		b
a

			50μV						50μV
	1sec						1sec

Fig. 4.15 Normal EEG. a Monopolar recording, b bipolar recording.

Electrophysiological Studies

55

4

Ancillary Tests

Technique. Electrodes are placed on the scalp according to the internationally standardized 10−20 system (Fig. 4.14). The potential fluctuations at each electrode are recorded, either in bipolar mode (i. e., differences in potential between adjacent electrodes) or in unipolar mode (i. e., differences in potential between each electrode and a reference electrode). Their magnitude at the scalp is 10−100 μV. They are amplified and recorded on paper in 12 parallel channels. Fluctuations in electrical potential are classified by frequency. Certain maneuvers, e. g., opening and closing the eyes, hyperventilation, and rhythmic photic stimulation, affect the EEG tracing in characteristic ways and may induce pathological waves in patients with epilepsy.

Evaluation. A mainly occipital alpha rhythm is the major component of the EEG tracing in a normal, awake individual. There is a progressive slowing of frequencies during sleep, depending on the sleep stage (depth of sleep). The following EEG changes indicate a pathological process in the brain:

Fig. 4.14 Placement of EEG electrodes according to the 10−20 system (a−c from Masuhr K.F., Neumann M.: Neurologie, Hippokrates, Stuttgart 1992; d from Künkel H.: Das EEG in der neurologischen Diagnostik, in Schliack H., Hopf H.C.: Diagnostik in der Neurologie, Thieme, Stuttgart 1988). a Lateral view. The electrodes are placed at fixed percentage intervals between the nasion and the inion. b Frontal view. The preauricular points serve as reference points for the placement of the central transverse row of electrodes. C2 is the intersection of the central transverse and longitudinal rows. c Superior view. d Names of the electrodes in the 10− 20 system.

General changes. Slowing of the background rhythm in the awake patient is abnormal, as is acceleration of background activity (e. g., in the form of a beta rhythm). The latter is often due to medication use.

Focal findings. Slowing of background activity (e. g., in the form of theta or delta waves) limited to a circumscribed area of the brain reflects focal cortical disfunction. Findings of this type are often due to structural lesions of the brain (e. g., tumors).

Sharp waves and spikes. These characteristically shaped abnormal potentials are seen in persons with epilepsy. During a seizure, characteristic seizure-related potentials appear (spikes with a prolonged following wave— the “spike and wave” pattern). Pathological EEG changes are not necessarily demonstrable between seizures; thus, a normal interictal EEG does not rule out epilepsy.

An example of a normal EEG is shown in Fig. 4.15 and the most important graphoelements of the EEG are shown schematically in Fig. 4.16.

Indications. The main indications for EEG are summarized in Table 4.5. EEG changes are also seen in many other processes affecting the brain. The most important pathological EEG rhythms are shown in Fig. 4.16.

Polysomnography

Technique. Polysomnography is a special application of EEG in which the EEG is recorded simultaneously with a number of other electrophysiological parameters. It is used to assess sleep and sleep disturbances. The EEG

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56	4 Ancillary Tests in Neurology
		Table 4.5 The main indications for electroencephalography	changes that normally occur during sleep are related to
			the progression of the individual through various sleep
		Confirmation of the diagnosis of epilepsy
			stages, including deep or REM sleep (REM = “rapid eye
		Determination of the type of epilepsy that is present
			movement”). The recorded parameters include eye
		Brief, episodic impairment of consciousness of unknown etiology
			movements (by electro-oculography), respiratory ex-
		Longer-lasting disturbances of consciousness, delirium
			cursion, airflow in the nostrils, muscle activity (by sur-
		Metabolic disturbances	face EMG), cardiac activity (by ECG), and the partial
		Creutzfeldt−Jakob disease	pressure of oxygen (by transcutaneous pulse oximetry)
		Sleep studies (e. g., in suspected narcolepsy)	(Fig. 4.17). These are displayed together with the EEG in
			a polygraph recording (polysomnogram).

	Designation	Morphology	Definition
1 β rhythm			Regular sequence of
			waves at 14–30 Hz
2 Spindles			Regularly waxing and
			waning waves
			at 14–30 Hz
3	α rhythm		Regular sequence of
			waves at 8–13.3 Hz
4	ϑ rhythm		Regular sequence of
			waves at 4–7 Hz
5	δ rhythm		Regular sequence of
			waves at 1–3.5 Hz
6	δ activity		Irregular sequence of
			polymorphic waves
			at 1–3.5 Hz
7	Subdelta		Wave with duration
	wave		> 1 s
8	Steep waves		Conspicuous, blunt,
	(steep
			steep individual waves
	potential)
9	Sharp waves		Sharp and steep waves
			of 80–250 ms duration,
	(sharp		ascending phase
	potential)		usually steeper than
			descending phase
10	Spike		Sharp and steep wave
			of duration < 80 ms
11	Polyspikes		Compact series
			of spikes
12	Spike–wave		Complex consisting
			of a spike and slow
	complex
			wave
13	Rhythmic		Sequence of regular
	spikes and		spike–wave complexes
	waves		at about 3 Hz
			Sequence of comp-
14	Sharp and		lexes of sharp waves
			and slow waves of
	slow waves
			500–1000 ms dura-
		2s
			tion, often rhythmic

Fig. 4.16 The most important graphoelements in EEG: designations, morphology, and definitions (from Schliack H., Hopf H.C.: Diagnostik in der Neurologie, Thieme, Stuttgart 1988).

Indications. The most important indication for a sleep study is a clinical suspicion of sleep apnea syndrome (p. 171) on the basis of a characteristic history obtained from the patient or bed partner, together with related physical findings and a low partial pressure of oxygen measured during sleep by pulse oximetry. The typical polysomnographic finding in such patients is shown in Fig. 4.18. Polysomnography is also indicated for the diagnosis of narcolepsy, as well as for the assessment of excessive fatigue and daytime somnolence.

Evoked potentials

General principles. Evoked potentials are used to assess the integrity of individual functional systems (visual, auditory, somatosensory, or motor). The system under study is activated with a repeatedly delivered stimulus. The resulting fluctuations of electrical potential in the brain can be detected by summation of the potentials that are recorded when the excitatory stimulus has been delivered a large number of times. Evoked potentials provide evidence of whether impulse conduction in the system in question is intact from the site of stimulation all the way to the cerebral cortex. Sometimes a partial or total conduction block can be localized precisely between two relay stations for neural transmission within a particular system. In addition, evoked potentials may reveal subclinical lesions. The most important types of evoked potential for clinical practice are outlined in the following paragraphs.

Visual evoked potentials (VEP). The patient fixates on a video screen displaying a checkerboard pattern in which the white and black fields are regularly and periodically inverted, while electrical potentials are recorded through a needle electrode in the scalp at the occiput. Evoked potentials are obtained by summation; the largest fluctuation is a positive wave that appears 100 milliseconds after the stimulus. Delay of this wave is found early in the course of optic neuritis and persists thereafter (Fig. 4.19).

Auditory evoked potentials (AEP). A click stimulus delivered periodically to one ear induces the generation of neural impulses that travel along the auditory nerve to the brainstem, the thalamus, and finally the cerebral cortex. The electrophysiological response is measured from the vertex of the head in relation to a reference electrode on the earlobe. The normal AEP contains five different waves, each of which is generated by a different structure along the chain of impulse transmission.

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				Electrophysiological Studies		57
Fig. 4.17 Recording scheme for				C3-A1
polysomnography.		Video recording			EEG
				01-A1
	EEG	C4-A2		ROC	EOG
		2-A2		LOC
	Respiratory		Nasal	Masseter	EMG
	volume		Air flow	“Submentalis”
			Oral
				Body position
			Thoracic		ECG
	Respiratory		breathing	Left
	effort		Abdominal		EMG
				Biceps brachii
			breathing
				Right
						4
	Oxygen		Oximetry	Left		Tests
	saturation			Tibialis anterior	EMG	Ancillary
				Right

Fig. 4.18 Hypnogram. Polysomnography in a patient with REM-sleep-associated obstructive sleep apnea syndrome. 1 EEG frequency analysis. 2 Rapid eye movement (REM) sleep. 3 Submental muscle activity measured through a surface electrode. 4 Sleep stages. AWK = awake, REM = REM sleep, 1−4 = sleep stages 1−4. 5 Time axis. 6 Nasal/oral air flow and count (cnt) of apneic and hypopneic episodes per minute. 7 Transcutaneously measured oxygen saturation (upper curve) and frequency of desaturations by 4 % or more (lower curve). 8 ECG (bpm = beats per minute) and number of tachycardias, bradycardias, or extrasystoles. 9 Surface EMG from the masseter m. 10 Surface EMG from the right tibialis m. 11 Surface EMG from the left tibialis m. 12 Body position.

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