The pattern of abnormalities of the R1 and R2 responses assist in localizing a lesion to the afferent trigeminal nerve, the efferent facial nerve, or the interneurons in the pons. In Bell palsy there is a delay or absence of R1 and R2 responses only on the affected side. Large acoustic neuromas (vestibular schwannomas) also may interfere with the efferent portion of the response. The test may be helpful in identifying a demyelinating neuropathy when the facial and oropharyngeal muscles are affected. Diseases of the brainstem have yielded inconsistent responses. It is noteworthy that the test is normal in patients with trigeminal neuralgia.
Chap. 46)
This test of the neuromuscular junction is based on Jolly’s observation in 1895 that in myasthenia gravis the strength of muscular contractions progressively declines in response to a train of stimuli. By adjusting the amplitude of a stimulus over a nerve to the supramaximal range, a maximal CMAP may be obtained for each stimulus. With repeated stimuli, each response will have the same waveform and amplitude. In a healthy individual, a muscular response follows each stimulus with rates of stimulation up to 25 per second for periods of 60 s or more before a decrement of the CMAP appears. A decrement of 10 percent or more denotes a failure of a proportion of the neuromuscular junctions.
In certain disorders, notably myasthenia gravis, a train of 4 to 10 stimuli at rates optimally 2 to 3 per second, the amplitude of the motor potentials decreases (Fig. 2-15A). A progressive reduction in amplitude is most likely to be found in proximal muscles, but these are not easily stimulated for which reason the locations most commonly used for clinical testing are the accessory nerve in the posterior triangle of the neck (trapezius), the ulnar nerve (hypothenar muscle), the median nerve at the wrist (thenar muscle), and the facial nerve (orbicularis oculi muscle).
The sensitivity of the procedure is improved by first exercising the tested muscle for 30 to 60 s; a form of post-tetanic potentiation. The full procedure consists of testing the muscle with a train of stimuli before and immediately after exercise (or maximal voluntary contraction) and at 30-s intervals for several minutes. The posttetanic potentiation at first partially compensates for the depletion of ACh during slow rates of stimulation; this is followed by an exaggerated decrease in the transmission through the neuromuscular junction during the approximately 2 to 4 min after exercise. The induced failure of neuromuscular transmission in myasthenia is similar to the one produced by curare and other nondepolarizing neuromuscular blocking agents, and the electrical features of both can be partially corrected with anticholinesterase drugs such as neostigmine and edrophonium. Similar but lesser decremental responses may occur in poliomyelitis, ALS, and certain other diseases of the motor unit or motor nerve, particularly those resulting in the growth of reinnervating nerve twigs.