1. (B): The sensory responses are very small, and technical factors and electrical noise may distort the waveforms. For sensory conduction studies, the gain is usually set at 10 to 20 μV. The gain is set at microvolts; millivolts are used to measure motor conduction results. (Preston and Shapiro 1998, p. 27)

2. (C): The sensory nerves have a low threshold for stimulation compared to motor fibers and require a lower current (5 to 30 mA) to achieve supramaximal stimulation. The recording electrodes are placed in line over the nerve with recommended interelectrode distance of 3 to 4 cm. The active electrode is placed closest to the stimulator. Unlike motor studies, in sensory studies a conduction velocity can be calculated using one stimulation alone as the recording electrode is measuring a nerve action potential. (Preston and Shapiro 1998, p. 29)

3. (A): The antidromic technique results in stimulation of the entire mixed nerve including the motor fibers resulting in CMAP along with SNAP. This may become a problem when SNAP imbeds in CMAP. At times if SNAP is absent, one can misread CMAP as SNAP. The amplitude with antidromic technique is higher because the recording electrodes are closer to the nerve and is less subjected to noise or artifact. (Preston and Shapiro 1998, p. 32)

4. (B): The myelinated fibers conduct at a velocity of approximately 65 m/s, while the unmyelinated fibers conduct at a velocity of 1 to 2 m/s. Most fibers lie between these two extremes. (Preston and Shapiro 1998, p. 37)

5. (D): The drop in amplitude in proximal stimulation (conduction block) compared to the distal stimulation is seen with focal demyelinating neuropathies. Marked slowing of conduction velocity (slower than 75% of the lower limit of normal) is seen. Marked prolongation of distal latency (longer than 130% of upper limit of normal) is also a feature of demyelinating conditions. The CMAP amplitude can be normal or reduced in demyelinating neuropathies depending on the degree of distal demyelination or superimposed conduction block. (Preston and Shapiro 1998, p. 39)

6. (B): In patients with typical carpal tunnel syndrome (CTS), the median distal motor and sensory latencies, and minimum F-wave latencies are moderately to markedly prolonged. However, there is a group of patients with clinical symptoms and signs of carpal tunnel syndrome in whom these routine studies are normal (10% to 25%). In such patients, the electrodiagnosis of CTS may be missed. (Preston and Shapiro 1998, p. 238)

7. (D): A CTS study should show the median nerve orthodromic sensory latency delayed by >0.2 ms compared to the ulnar nerve. The median CMAP amplitude is usually much higher than ulnar. The picture depicts an orthodromic sensory response. The median nerve orthodromic sensory absolute latency >3 ms is usually abnormal, however, the absolute latency can be delayed because of multiple factors and therefore, absolute latency delay is usually not taken as a marker of CTS. (Preston and Shapiro 1998, p. 238)

8. (D): In comparison studies, identical distances are used between the stimulator and recording electrodes for median and ulnar nerves. These techniques create an ideal internal control in which several variables that are known to affect conduction time are held constant, for example, distance, temperature, age, and nerve size. If the distal motor and sensory latencies are normal then comparison studies often are very helpful because they rely on patient’s own nerve rather than on population normal values. (Preston and Shapiro 1998, p. 238)

9. (D): In comparison studies, very small difference (0.4 ms) between median and ulnar nerve is considered abnormal. Attention must be paid to all technical factors to avoid false-positive results. Technical factors such as distance measurement, stimulus artifact, supramaximal stimulation, and electrode placement can all affect the results. It is essential to avoid over stimulation because it can cause unintentional stimulus spread to an adjacent nerve. (Preston and Shapiro 1998, p. 238)

10. (A): The absence of a sural sensory response is not seen in tarsal tunnel syndrome. Tarsal tunnel syndrome is the compression of distal tibial nerve under the flexor retinaculum on the medial side of ankle. This results in a delayed or absent response in the medial and lateral plantar nerve responses. The absence of sural sensory response indicates a lesion above the tarsal tunnel syndrome and can be affected by a sciatic nerve lesion, lumbosacral plexopathy, and peripheral neuropathy. (Preston and Shapiro 1998, p. 331)

11. (A): Evaluation of tarsal tunnel syndrome is greatly simplified if one side is abnormal as the other side acts as a control (Fig. 13.3). Surface plantar sensory studies are difficult to perform in healthy subjects. Orthodromic stimulation of the plantar sensory nerve produces very small amplitude responses, making it necessary to average many potentials. Medial and lateral plantar sensory potentials are unobtainable even in healthy subjects and similarly bilateral absent plantar sensory responses in middle-aged or older individuals have no significance. (Preston and Shapiro 1998, p. 331)

12. (C): Only a few peripheral neuropathies preferentially affect small fibers. Diabetes, Fabry disease, and Tangier disease all cause small fiber peripheral neuropathy while diphtheria does not typically affect the small fibers. Small fiber neuropathy manifests as an autonomic dysfunction and a distal peripheral sensory deficit for pinprick and cold, often associated with painful paresthesia. Routine nerve conduction studies assess only large myelinated fibers and can be completely normal in small fiber neuropathies. (Preston and Shapiro 1998, p. 356)