Ultrasonography

conduction velocity of the most rapidly conducting fibers in the nerve being studied. The technique involves stimulating and recording electrodes placed at some distance from each other along the course of a peripheral nerve. The measured conduction velocity is then the temporal interval between the delivery of the stimulus and the beginning of the recorded response, divided by the distance between the electrodes. Normal values in the arms are 50−70 m/s, in the legs 40−60 m/s. The amplitude and duration of the recorded response are a function of the number of functioning axons and the degree of dispersion of their conduction velocities. A case illustrating the usefulness of ENG is presented in Fig. 4.24 (localized compression of the common peroneal n. at the head of the fibula).

F wave. When a peripheral motor nerve is stimulated, the resulting impulses travel not only orthodromically (in the normal direction of transmission, i. e., distally, toward the muscle), but also antidromically (toward the spinal cord). The antidromic impulse reaches the ganglion cells of the anterior horn and is then sent back to the periphery in the manner of an echo. This echo is the so-called “F wave.” Thus, two orthodromic impulse waves go down the peripheral nerve, the original wave due to the stimulus and the F wave; compared with the original wave, the F wave is later and smaller in amplitude. Sometimes it is not seen at all. If the F wave is delayed by a longer interval than usual, this may indicate slowed conduction in the plexus or nerve roots.

Other Electrophysiological Studies

Other types of electrophysiological study are used less commonly in neurological diagnosis. We will only briefly mention a few of them here.

Motor neurography

of the common peroneal n.

Ankle

Distal to

the fibular head

2mV

5ms

Popliteal

fossa Stimulus

Fig. 4.24 Electroneurography of the right common peroneal n. in pressure palsy at the fibular head. The farther the stimulating electrode is from the recording electrode (in the peroneal muscles), the longer the latency until the summed muscle potential appears. When the stimulus is delivered in the popliteal fossa, the amplitude of the summed potential collapses. This implies that conduction is blocked in all axons between the popliteal fossa and the stimulation site distal to the fibular head. The finding is typical in pressure palsy.

Oculography is a study of the electrical potentials accompanying eye movements. It can be used for objective documentation of gaze saccades and pathological eye movements. When oculography is used to study vestibular disturbances, it is called electronystagmography. Retinography is mainly used to determine whether the lesion causing a visual disturbance is in the retina or in the optic nerve.

Ultrasonography

There are two main types of ultrasound study: Doppler sonography and duplex sonography.

Principle. The 19th-century Austrian physicist Christian Doppler discovered that the frequency of a wave changes when its source and receiver are in relative motion. Thus, when ultrasound pulses are directed at erythrocytes in flowing blood, the ultrasonic waves reflected back from the erythrocytes are altered in frequency to a degree that depends on the flow velocity. In fact, the Doppler shift is directly proportional to the flow velocity.

Technique. The ultrasound probe contains both a transmitter and a receiver of ultrasonic waves. The angle of insonation should be as steep as possible to minimize angle-dependent variations in the measured values and thus keep the results as consistent as possible from study to study. There are two types of Doppler system: continuous-wave (CW) systems detect all moving wave

reflectors within the cone of insonation, while pulsedwave (PW) systems detect only those at a particular depth, which can be chosen by the examiner. In CW Doppler studies, the signals of different vessels may overlie one another.

The Doppler signal can be represented graphically as a frequency spectrum that changes over time (Fig. 4.25). It can also be transduced into an audible signal. Ultrasound waves are reflected to varying extents by different types of tissue with different acoustic resistance; thus, the profile of reflected echo intensities can be used to construct a two-dimensional sectional image of the insonated tissue. The so-called B image (“brightness mode”) or echotomogram is a gray-scale representation of the tissue (Fig. 4.25a). The combination of Doppler flow measurement with B imaging is called duplex ultrasonography. The velocity of blood flow can be colorcoded and displayed as an overlay on the B image; this is called color duplex ultrasonography (Fig. 4.26).

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4

Ancillary Tests

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62 4 Ancillary Tests in Neurology

ECA

ICA a

b

a

a

Fig. 4.25 Doppler study of a normal carotid bifurcation. a Two-dimensional sectional image (B image) of the carotid bifurcation. b−c Doppler frequency-time spectra in the common carotid (b), internal carotid (c), and external carotid arteries (d).

c

d

b

Fig. 4.26 Color-coded duplex ultrasonography of carotid stenosis. a Duplex ultrasonography of the carotid bifurcation. Rapid flow is coded as bright, slow flow as dark. Flow is abnormally rapid in the internal carotid a. (ICA) because the lumen is narrowed. Atherosclerosis can be seen in the thickened vessel wall (arrow). b Flow spectrum of the internal carotid a. showing elevated maximal systolic and end-diastolic velocities (from the laboratory of the Neurological and Neurosurgical Clinics, University of Berne, Switzerland). ECA = external carotid a., CCA = common carotid a.

b

Fig. 4.27 Color-coded duplex ultrasonography of an occlusion of the left internal carotid a. 3 cm above the carotid bifurcation. a Blood flow can be seen up to the bifurcation. In the internal carotid a. (ICA LT), there is only minimal movement of the blood column. b Doppler ultrasonography reveals no more than a brief forward flow in early systole at greatly reduced maximal speed; backward flow is already seen in early diastole.

Fig. 4.27 c

Mumenthaler / Mattle, Fundamentals of Neurology © 2006 Thieme All rights reserved. Usage subject to terms and conditions of license.