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Electrophysiology

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Abstract

Electrophysiological studies record the electrical activity of muscles or neural structures as a function of time. This activity can be spontaneous as in electroencephalography and electromyography, or it may be the consequence of stimulation as in nerve conduction velocity measurements and evoked potential studies. Electrophysiological studies evaluate neural tissue, the neuromuscular junction and muscle function. They are minimally invasive but require sedation and frequently anaesthesia. The cost of the equipment and the experience needed for conducting these studies limit their use to academic and referral clinics. this chapter looks at types of recorded potential, evaluation of the peripheral motor system, evaluation of the peripheral sensory system, evaluation of central afferent pathways, special senses, electroencephalography.

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Figures

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4.3 Relative sizes of the concentric needle electrode and the muscle fibre. The amplitude of the electrode potential decreases exponentially with distance; the electrode is not influenced by muscle fibres more than 1000 µm from its tip. The larger the recording electrode surface, the smaller the potential decay as a function of the distance – non-insulated needles or surface electrodes (alligator clips) give a more comprehensive picture of the underlying muscle activity. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.4 Spontaneous muscle fibre activity. An intact relaxed normal muscle is electrically silent, except in the endplate region where endplate noise and spikes are found (top tracing). In a denervated muscle, spontaneous electrical activity, such as fibrillation potentials (F) and positive sharp waves (P), may be recorded at various locations (bottom tracing). Vertical = 100 µV/div; horizontal = 10 ms/div; positivity downwards. (Courtesy of N Olby)
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4.5 Spontaneous activities detected in a dog suffering from muscle disease associated with hyperadrenocorticism. Complex repetitive discharges. Slowly waxing activity sometimes called ‘pseudomyotonia’ (positivity downwards). (Reproduced from with permission from the )
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4.6 Two sites of placement for the stimulating electrodes for maximum motor NCV measurement at proximal and distal locations along the ulnar nerve and the sciatic–tibial nerve. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.7 Maximum motor NCV. The sciatic–tibial nerve is stimulated at a proximal and a distal location and the resulting CMAPs from the plantar interosseous muscles are recorded (left). The onset latency (1) of the evoked muscle potential measured with the distal stimulation (lower tracing) is subtracted from the onset latency (1) measured with the proximal stimulation (upper tracing). The distance (mm) between the two stimulus locations (yellow pathway on photograph) is divided by the latency difference (ms) to give the maximum motor NCV (m/s) between the two stimulation points. The tracings on the right show a normal muscle response to distal stimulation but a low amplitude, polyphasic potential with proximal stimulation, suggesting conduction changes in the proximal part of the nerve. Vertical = 2 mV/div; horizontal = 2 ms/div; positivity downwards.
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4.9 F wave study. When a motor nerve is stimulated, action potentials travel orthodromically (distal conduction time; green pathway on photograph) and trigger the direct muscle response. The action potentials also travel antidromically toward the spinal cord and may induce a backfiring in some motor neurons from the ventral horn cell. A successful backfiring takes 1 ms. These secondary action potentials travel to the target muscle and evoke F waves. The F wave latency represents the conduction time along the yellow pathway on the photograph and includes the 1 ms backfiring delay. In the tracings on the right, an F wave is not observed on each stimulation and those that are present are dispersed, indicating conduction blocks and conduction slowing at the level of the motor root. Vertical = 0.2 mV/div; horizontal = 5 ms/div; positivity downwards.
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4.10 Tracing from a Polish Shepherd dog with myasthenia gravis using repetitive stimulation of the distal tibial nerve at 3 Hz. The amplitude of the responses diminishes rapidly as the train of stimuli proceeds. The amplitude of the tenth response is 24% less than that of the first response.
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4.11 Sensory NCV. The skin on the dorsal aspect of the paw is stimulated (cathode proximal) and the action potential recorded in the radial nerve at the level of the humeral epicondylar crest. The latency is measured at the tip of the first peak and is divided by the distance between the stimulating cathode and the recording electrode. Vertical = 20 µV/div; horizontal = 1 ms/div; positivity downwards; average of 256 responses; two recordings superimposed.
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4.12 At low stimulus intensity, IA fibres, which are the largest afferents coming from the muscle spindles, may be stimulated in isolation in some dogs to give a pure H-reflex. At higher stimulus intensity, antidromically conducted action potentials are elicited in the alpha-motor neurons and cancel the reflexively triggered potentials (tibial nerve, plantar interosseous; positivity downwards). Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.13 Stimulation of the infraorbital nerve (stimulation 2) may trigger reflex responses in the orbicularis oculi muscle (CN V–VII reflex) innervated by the facial nerve. The efferent branch of the reflex should first be assessed by stimulating the auriculopalpebral branch of the facial nerve (stimulation 1). (Modified from ). Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.14 SSEPs recorded along the lumbar and thoracic spine in response to tibial nerve stimulation (two repetitions are superimposed). Caudocranially, the root component (L7–S1 and L6–L7), the large interneuronal component (from L5–L6 to L3–L4) and the ascending evoked potential (from L2–L3) can be recognized. Note that positivity is up in these tracings. (See text for details about waveforms.) (Reproduced from with permission from the )
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4.15 EIPs in the vicinity of a severe disc herniation. Caudocranial recordings (from bottom to top) over a distance of 2 cm, demonstrating the EIP waveform change from a biphasic to a monophasic character that takes place at the conduction block location. Note that positivity is up in these tracings. (Reproduced from with permission from the )
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4.16 Set up for recording BAEPs. The recording electrodes are seated at the vertex, the reference electrode in the mastoid area and the ground electrode in the neck area. The transducers are connected to silicone tubing ending with a polyurethane foam cylinder fitted in the external auditory meatus. A 0.1 ms rectangular electrical pulse generates a complex pressure wave that may begin with a pressure rise (condensation click, C) or a pressure drop (rarefaction click, R) according to its polarity. The upper tracing is the electronic sum of the two pressure tracings (a zero line), proving that the transducer faithfully reverses the signal.
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4.17 Auditory nuclei and pathways in the brainstem. All deflection peaks (I–V) of the short latency auditory evoked potentials are generated by structures distal to the caudal colliculi ( ). Except for peak I, which mostly originates from the auditory nerve, no simple relationship exists between a given peak and a structure. All information eventually crosses the midline, although it can do so at different levels. AN = auditory nerve; BCC = brachium of the caudal colliculus; CC = caudal colliculus; DCN = dorsal cochlear nucleus; MGN = medial geniculate nucleus; NLL = nuclei of the lateral lemniscus; NT = nucleus of the trapezoid body; OC = olivary complex; VCN = ventral cochlear nucleus. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.18 Short latency auditory evoked potentials in response to click stimuli of decreasing intensity from 90 to –6 dB normal hearing level (left panel). The right panel records the latency of wave V as a function of the stimulus intensity. The slope of the regression line fitting the points in the lower intensity range may have diagnostic value in detecting partial deafness. (Reproduced from with permission from the
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4.19 Generation of extracellular voltage fields from excitatory and inhibitory synaptic activity. The relationship between surface polarity and the site of dendritic postsynaptic potentials is also shown. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.20 Position of EEG scalp electrodes in relation to the cortical pyramidal neurons. Note that the EEG signal is modified by the electrical conductive properties of the tissues between the electrical source and the recording electrode, the orientation of the cortical generator to the recording electrode, and the conductive properties of the recording electrode. Only radially oriented neurons closest to the skull significantly influence the recording electrodes. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.21 Position of the EEG electrodes on the scalp. Reference and ground electrodes are located in neutral positions. Illustration created by Allison L. Wright, MS, CMI, Athens, Georgia, USA.
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4.22 Diffuse K complexes (arrowed) observed in a dog during sleep. Note the initial negative peak followed by a slow positive component. Referential montage; timescale = 50 mm/s; time line interval = 1.0 s.
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4.23 Physiological artefacts recorded in a dog: eye blink (red box), muscle activity (black box) and heartbeat (green box). Referential montage; timescale = 10 s/page; time line interval = 1.0 s.
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4.24 Tracing from an 8-month-old Chihuahua presented with transient episodes of abnormal demeanour associated with myoclonic twitching of the head and nose. Note the 4 Hz spike and generalized wave discharges (black box), suggestive of absence seizures. Bipolar montage; timescale = 30 mm/s; time line interval = 1.0 s.
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4.25 Tracings from a 3-year-old mixed-breed dog presented with focal seizures secondary to an intracranial meningioma located in the left cerebral hemisphere. Interictal focal epileptiform activity (spikes) on the left cerebral hemisphere characterized by true phase reversal. Bipolar montage; timescale = 10 s/page; time line interval = 1.0 s. The site of maximum negativity is circled. Referential montage; timescale = 10 s/page; time line interval = 1.0 s.
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4.26 Tracing from a 3-year-old Boston Terrier presented in status epilepticus. The recording was obtained during treatment with a constant rate infusion of propofol, which was used in an attempt to suppress pathological cortical electrical activity. The tracing shows epileptiform activity, characterized by a burst of polyphasic waveforms (purple oval), spikes (black oval), sharp waves (red oval) and spike and waves (green oval). Bipolar montage; timescale = 5 s/page; time line interval = 1.0 s.
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4.27 Tracing from an 8-month-old Dachshund diagnosed with hepatic encephalopathy secondary to a portosystemic shunt. The patient was awake. Note the diffuse, bilaterally symmetrical slow (delta) waves (black box). The EEG waveforms returned to normal frequency and morphology after successful correction of the portosystemic shunt. Bipolar montage; timescale = 20 mm/s; time line interval = 1.0 s.

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