1. (A): Only approximately 15% of patients have the restricted ocular form of the disease. If a patient’s symptoms remain restricted to the ocular muscles for 2 years, then there is a high probability of it being restricted as ocular myasthenia. Neonatal myasthenia develops when maternal autoantibodies cross the placenta and result in clinical syndrome in a newborn infant. Neonatal myasthenia is mild, self-limited, and disappears after the first few months of life. Penicillamine can cause myasthenia, including the presence of acetylcholine receptor antibodies. Myasthenia gravis may also be seen in patients treated with penicillamine. The clinical syndrome is similar to myasthenia gravis, including the presence of acetylcholine receptor antibodies except thatmost patients slowly improve with penicillamine discontinuation. (Preston and Shapiro 1998, p. 506)

2. (C): In any patient suspected of myasthenia gravis, routine motor and sensory nerve conductions must be done. Particular attention must be paid to normal CMAP amplitude in myasthenia in comparison to LES, where baseline CMAP is usually diffusely low. A decrement on repetitive nerve stimulation can also be seen in various conditions, for example, neuropathies, motor neuron disease, inflammatory myopathies, and myotonic disorders. A decrement on repetitive nerve stimulation of the ulnar nerve may be seen in a severe ulnar neuropathy with denervation; such a finding in the context does not imply a primary neuromuscular junction disorder. (Preston and Shapiro 1998, p. 506)

3. (B): RNS is typically abnormal in 50% to 70% of patients with generalized myasthenia gravis. In most patients with ocular myasthenia RNS is normal. In normal subjects, slow RNS (3 Hz) results in little or no decrement of CMAP, whereas in myasthenia gravis a CMAP decrement of 10% or more is characteristically seen. The diagnostic yield increases with proximal nerves. If baseline RNS in myasthenia gravis is inconclusive, a postexercise after 1 minute shows significant decrement in the CMAP. (Preston and Shapiro 1998, p. 506)

4. (C): LEMS is quite rare. It affects males more than females usually older than 40 years. It presents with generalized weakness more in the proximal muscles. The deep tendon reflexes are characteristically reduced or absent, which is unusual in myasthenia gravis.

Single stimuli produce a reduced release of acetylcholine and at rest many of the end-plate potentials do not reach threshold, resulting in small-amplitude CMAPs on routine motor nerve conduction studies. (Preston and Shapiro 1998, p. 510)

5. (D): The electrophysiology of LEMS is diagnostic. Rapid RNS (30- to 50-Hz) or brief intense exercise produces a marked increase in the CMAP amplitude due to calcium accumulation in the presynaptic nerve terminal with subsequent enhancement of release of acetylcholine. The CMAP commonly increases in amplitude by >200%. Brief intense exercise is preferred over rapid RNS because rapid RNS can be painful and is not easily tolerated by the patients. This marked postexercise facilitation of CMAP is the electrical correlate of the clinical facilitation of muscle strength and reflexes seen after brief exercise. (Preston and Shapiro 1998, p. 510)

6. (D): The pathophysiology of botulism is presynaptic blocking of acetylcholine.

Like Lambert-Eaton myasthenic syndrome, CMAP amplitudes are decreased with normal latencies and conduction velocities. A decremental response is seen with slow RNS (3 Hz). An incremental response occurs after brief exercise and fast RNS in early or mild cases. However in severe botulism, if amount of acetylcholine release has dropped severely below threshold, even facilitation with rapid RNS or brief exercise may not result in a threshold response and therefore no increment occurs in the CMAP amplitude. Lack of incrementing response to rapid RNS or brief exercise cannot rule out the diagnosis of botulism. (Preston and Shapiro 1998, p. 512)

7. (B): Routine nerve conduction studies should always be done in patients with suspected myopathy. Sensory conduction studies are always normal unless there is a coexistent neuropathy. Motor conduction studies are usually normal as myopathies usually involve proximal muscles that are not tested in routine motor nerve conduction studies. However, if the myopathy is severe enough to affect distal and proximal muscles or one of the rare myopathies that preferentially affects distal muscles, motor nerve studies may be abnormal and show decreased CMAP with normal latencies and conduction velocities. (Preston and Shapiro 1998, p. 527)

8. (C): The most commonly encountered anomaly in the upper extremity is a crossover of median-to-ulnar fibers, the Martin-Gruber anastomosis (MGA) seen in approximately 15% to 30% of patients. There are three types of MGA anomalies seen: (a) to innervate hypothenar muscles (shows decrease ulnar CMAP amplitude [recording at abductor digiti minimi] with below-elbow stimulation compared to wrist stimulation), (b) to innervate the first dorsal interosseous muscle (this is the most common MGA anomaly and shows decrease in ulnar CMAP amplitude [recording at first dorsal interosseous] with below-elbow stimulation compared to wrist stimulation, and (c) to innervate thenar muscles (shows decrease in increased median CMAP amplitude [recording at abductor policis brevis] with below-elbow stimulation compared to wrist stimulation). (Preston and Shapiro 1998, p. 78)

9. (D): The most common anomalous innervation in the leg is an accessory peroneal nerve in the lateral calf. The extensor digitorum brevis (EDB) muscle gets extra innervation by an anomalous motor branch originating from the superficial peroneal nerve. During the routine nerve conduction studies, this anomaly is recognized during peroneal motor studies. The accessory peroneal nerve travels down the lateral calf, posterior to the lateral malleolus. If anomaly is present, the peroneal CMAP amplitude recording the EDB is higher stimulating below the fibular head and lateral malleolus than at the ankle. (Preston and Shapiro 1998, p. 81)

10. (C): The blink reflex study measures the entire reflex arc between the trigeminal and facial nerves, including proximal segment of facial nerve. To get the response, both orbicularis oculi muscles are recorded simultaneously. On each side the ipsilateral supraorbital nerve is stimulated over the medial eyebrow. It is crucial to record 4 to 6 supramaximal stimulation trials. The ground electrode is placed on the chin. It is extremely important that the patient be in a relaxed state to eliminate any signal noise.

The R1 latency represents conduction time along the fastest fibers of the afferent pathway of ipsilateral trigeminal nerve and R2 latency represents conduction time from ipsilateral trigeminal nucleus to both ipsilateral and contralateral facial nerve nuclei and in turn to facial nerves. Normal R1 and R2 latencies are 10 to 12 ms and 30 to 40 ms, respectively. (Preston and Shapiro 1998, p. 58)

11. (D): For each blink response, the absolute R1 and R2 latencies are compared with normal controls as well as with the contralateral side. Normal R1 and R2 latencies are 10 to 12 ms and 30 to 40 ms, respectively. Stimulating the affected side results in absence of ipsilateral R1 and R2 potentials but a normal contralateral R2 potential, whereas stimulating the unaffected side results in a normal ipsilateral R1 and R2 but absent contralateral R2, suggests a unilateral facial nerve lesion. In this pattern all potentials on the affected side are abnormal, regardless of which side is stimulated. In demyelinating peripheral polyneuropathy, all potentials of the blink response may be markedly delayed or absent, reflecting slowing of either or both motor and sensory pathways. (Preston and Shapiro 1998, p. 60)

12. (A): The tracings in Figure 15.1A showed a delayed median antidromic sensory latency compared to radial sensory latency. Figure 15.1B showed a prolonged median distal motor latency and absence of conduction block. These findings are consistent with a right carpal tunnel syndrome. MGA is the most commonly encountered anomaly in the upper extremity with a crossover of median-to-ulnar fibers. The MGA is seen in approximately 15% to 30% of patients. The most common MGA anomaly shows decrease in ulnar CMAP amplitude (recording at first dorsal interosseous) with below-elbow stimulation compared to wrist stimulation. We do not see this in the figure hence it is not a correct option. (Preston and Shapiro 1998, p. 78)

13. (B): This above tracing in Figure 15.2 shows ulnar motor response at wrist and at elbow. The ulnar distal motor latency is severely delayed with decreased amplitude. This is consistent with ulnar demyelinating ulnar motor neuropathy with secondary axonal involvement (indicated by low amplitude). There is no conduction block seen across the elbow. This effectively rules out cubital tunnel syndrome. The best option would be an ulnar demyelinating mononeuropathy that is not localizable with the above tracing. The left ulnar forearm is less likely and left brachial plexopathy is less likely because most brachial plexopathies are axonal in nature. (Preston and Shapiro 1998, p. 273)