The motor unit action potential (MUAP) shown here was obtained from a 78-yr-old woman with progressive weakness over an 18-mo period. The weakness started in her proximal lower limbs and then progressed to her upper limbs and subsequently to dyspnea on exertion. She had no sensory or bulbar symptoms. Physical examination revealed absence of muscle stretch reflexes without atrophy or fasciculations. She was referred for electrodiagnostic studies to evaluate for suspected myasthenic crisis after her condition progressed to respiratory failure.
The recording in the figure is remarkable for considerable variability in the time-locked, second potential (Fig. 1). This is analogous to the jitter found in single fiber recordings, which commonly suggests neuromuscular junction (NMJ) dysfunction. However, also note the long duration of the MUAP, more consistent with reinnervation and neuropathic disease.
The remainder of the needle examination was remarkable for evidence of widespread denervation with large amplitude, long duration, polyphasic MUAPs, and very reduced recruitment. Repetitive nerve studies were normal in the ulnar nerve but demonstrated a 13% decrement when studying the spinal accessory nerve recording from trapezius. Motor nerve conduction studies revealed decreased amplitude responses, and F-wave studies showed low levels of persistence. Sensory nerve conduction studies and somatosensory-evoked potentials were normal. She was ultimately given a diagnosis of motor neuron disease.
Although this MUAP recording is suggestive of NMJ impairment, it is important to keep in mind that diseases with rapid denervation and reinnervation can present with unstable MUAPs as a result of new immature NMJs.1,2 Repetitive stimulation can also produce moderate decrements in this situation for the same reason. Correspondingly, acute neurogenic and myopathic diseases can show jitter and blocking on single fiber electromyography. However, a single fiber electromyographic electrode is not required to demonstrate the jitter because it can also be demonstrated with both concentric and monopolar needle electrodes,3,4 as demonstrated here.
In summary, when findings of NMJ impairment such as that shown in the figure are noted on electrodiagnostic studies, one should consider the context of the history, physical examination, and broader electrodiagnostic findings when making a diagnosis. This will help prevent us from labeling patients with an erroneous diagnosis of primary NMJ disease when they actually have a primary neuropathic disorder.
1. Daube JR: Electrodiagnostic studies in amyotrophic lateral sclerosis and other motor neuron disorders. Muscle Nerve
2. Cui LY, Liu MS, Tang XF: Single fiber electromyography in 78 patients with amyotrophic lateral sclerosis. Chin Med J (Engl)
3. Stålberg EV, Sanders DB: Jitter recordings with concentric needle electrodes. Muscle Nerve
4. Buchman AS, Garratt M: Determining neuromuscular jitter using a monopolar electrode. Muscle Nerve