58 4 Ancillary Tests in Neurology

								the pyramidal pathway to the muscles. Surface elec-
								trodes placed on an arm or leg muscle are used to record
Stimulation								the summed motor potentials. These potentials are
right								larger and easier to record when the subject lightly con-
								tracts the corresponding muscle beforehand. An abnor-
								mality of the MEP implies a lesion in the peripheral or
			P100: 147ms					central portion of the motor pathway (see Fig. 4.21).
								Epilepsy, cardiac pacemakers, and ferromagnetic in-
								tracranial implants are contraindications for trans-
Stimulation								cranial magnetic stimulation for any purpose, including
								MEP.
left
						2µV		Electromyography
				50ms
		P100: 100ms						Principle. Electrical activity is recorded from a muscle
Fig. 4.19 Visual evoked potentials (VEP). A 38-year-old woman								through bipolar needle electrodes, first at rest, and then
with multiple sclerosis and right optic neuritis. The cortical re-								with light and maximal voluntary muscle contraction.
sponse on the right side is significantly delayed compared to the								The recorded potentials are displayed visually on an
normal left side.								oscilloscope and also converted into audible signals
								from a loudspeaker. When the muscle is lightly con-
Somatosensory evoked potentials (SSEP). When a re-								tracted, the potentials arising from individual motor
petitive electrical stimulus is applied to the skin, im-								units can be observed. (A motor unit is the set of muscle
pulses are generated at the terminal sensory branch of a								fibers innervated by a single motor anterior horn cell by
peripheral nerve and conducted centrally via the pe-								way of its multiple axon collaterals.) When the muscle is
ripheral nerve, nerve root, posterior columns/								strongly or maximally contracted, a large number of
spinothalamic tract, medial lemniscus, and thalamocor-								motor unit potentials come together to form an interfer-
tical connections. A lesion at any point along this path-								ence pattern.
way can alter the evoked potentials, which are recorded								Insertional activity and spontaneous activity. The re-
first over Erb point (for the median n.) or the lumbar
spine (for the tibial n.), and then through a scalp elec-								sting muscle is normally electrically silent; when the
trode in the parietal region on the side opposite the								needle is inserted, there are normally only a few positive
stimulation. An example of delayed conduction in the								sharp waves or fibrillations. Pathological spontaneous
central somatosensory pathway is shown in Fig. 4.20.								activity of a muscle is manifested as prolonged in-
								sertional activity as well as pathological fibrillation
Motor evoked potentials (MEP). In this technique, a								potentials and positive sharp waves (Fig. 4.22). This spon-
rapidly alternating magnetic field produced by a ring-								taneous activity reflects denervation of the muscle.
shaped magnetic impulse generator induces a stimulat-								Fasciculations and complex repetitive discharges are
ing electrical current in the motor cortex. Action poten-								further forms of pathological spontaneous activity, as
tials are then generated in the cortex and travel down								are myotonic repetitive discharges.

Stimulation: right tibial n.

P40: 39.2ms

Cortical recording

1µV

Lumbar recording

2µV

Stimulation: left tibial n.

N22: 21.6ms

P40: 58.4ms

Cortical recording

1µV

Lumbar recording

2µV

N22: 21.2ms

10ms

Fig. 4.20 Somatosensory evoked potentials of the tibial n. A 44-year- old woman with multiple sclerosis. Normal lumbar N22 potential on both sides. The cortical P40 potential appears at a normal latency of 29.2 ms on the right, but is significantly delayed on the left, with a latency of 58.4 ms, and also abnormally small. These findings indicate impaired conduction in the spinothalamic pathways.

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Electrophysiological Studies	59

Fig. 4.21 Motor evoked potentials in a 61-year-old man with cervical syringomyelia. Recording of motor potentials from the abductor digiti minimi m. after electrical stimulation of the ulnar n. at the wrist, the forearm, and the C8 root (tracings a−c). After cortical stimulation (d), the recorded motor evoked potential is reduced in amplitude and somewhat delayed. The calculated central motor conduction time (CMCT) of 9.2 ms is prolonged in comparison to the normal value of 8.7 ms. These findings suggest impaired conduction in the pyramidal tract in the cervical spinal cord.

Stimulus:

aWrist

bArm

c	Root		5 mV

16.1 ms

d Cortex

1.5 mV

	25.3 ms
CMCT =				10 ms
9.2 ms

4

Ancillary Tests

300μV	5000μV

a	b	c	d	e
	20ms			20ms

Fig. 4.22 Different types of potentials in an electromyogram. a Normal motor unit potential. b Fibrillation potential in denervation. c Positive sharp waves in denervation. d Fragmented poly-

Electrical activity with voluntary contraction. Muscle action potentials are observed when the muscle is voluntarily contracted. The amplitude and duration of individual motor unit potentials are proportional to the size of the motor unit, i. e., the number of muscle fibers it contains. The more strongly a muscle is contracted, the more motor units will be recruited. When a large number of motor units are active, their potentials can no longer be seen individually. Instead, they summate to form a (complete) interference pattern (Fig. 4.23a).

The size and shape of electromyographic potentials are altered by many different types of neuromuscular disease. Myopathy is characterized by a diffuse loss of individual muscle fibers throughout the affected

phasic low-amplitude potential, as seen in reinnervation. e Abnormally prolonged and high-amplitude motor unit potential (“giant potential”) in chronic anterior horn cell disease.

muscle(s). Each motor unit potential is therefore of lower amplitude and shorter duration (Fig. 4.23d). In principle, all of the motor units are still present, but they contain fewer muscle fibers than before; thus, on maximal voluntary contraction of the muscle, the interference pattern is full, but of lower than normal amplitude. In contrast, in a neuropathic process (chronic denervation of a muscle), the motor units are larger than normal because of repeated denervation and reinnervation. When the nerve fiber to a particular motor unit degenerates, axon collaterals sprouting from the nerves of adjacent motor units take over the muscle fibers of the denervated unit, so that the surviving motor units actually contain more muscle fibers than before. Their

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60 4 Ancillary Tests in Neurology

20ms

a

2mV	20ms

b

1s	2mV	10ms

c

0.2mV	20ms

d

2mV

Tabelle 4.6 Indications for EMG and ENG

Condition/suspected pathology

Suspected anterior horn cell disease

Suspected nerve root lesion

Suspected plexus lesion (differentiation from peripheral nerve lesion)

Focal peripheral nerve lesion

Polyneuropathy

Myopathy

Ischemic muscle damage

Myasthenia gravis

++	=	indicated test
+	=	may be additionally useful

Fig. 4.23 Various EMG findings. a Normal electromyogram with full interference pattern. b Individual oscillations in the reinnervation stage after a peripheral nerve injury. c Total denervation. Fibrillation potentials and positive sharp waves are seen. d Myopathy. Despite muscle weakness, there is a complete interference pattern. The individual potentials making up the interference pattern are of low amplitude; some of them are polyphasic and fragmented.

motor unit potentials are usually polyphasic and of increased amplitude and duration (Fig. 4.23b). Because of the reduced number of motor units, maximal voluntary contraction of a denervated muscle yields a markedly attenuated interference pattern, in which the individual action potentials of the remaining motor units appear as large oscillations.

		Electrical activity at the motor end plate. Abnormali-
		ties of the motor end plate affecting neuromuscular
		transmission are also revealed by EMG. On repetitive
	0.4mV	electrical stimulation of a peripheral motor nerve, the
		recorded muscle action potential becomes smaller with
		each stimulus (decrement phenomenon, Fig. 4.12, p. 53).
		Indications. In disorders affecting muscle, EMG can be
		used to determine whether the underlying pathological
		process is located in the muscle itself (myopathic
		process), in the nerve innervating it (neuropathic
		process), or at the neuromuscular junction. It can also be
		used to grade the severity of muscle denervation and
		the extent of reinnervation. In combination with elec-
		troneurography (see below), EMG is a very important
		type of ancillary study for the diagnosis of neuromuscu-
		lar diseases. The indications for these two methods are
		listed side by side in Table 4.6.

Electroneurography

Principle. Electroneurography is a method of measuring the motor and sensory conduction velocities of peripheral nerves. The result of measurement is always the

EMG (needle	ENG	Remarks
myography)
++	Negative
+	++ (F wave)	Imaging studies may be
		more important
+	++ (F wave)
++	++	Severity of injury, signs
		of regeneration, localiza-
		tion of injury
+	++
++	Normal
++
++		Repetitive stimulation,
		jitter phenomenon

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