Article Text
Statistics from Altmetric.com
This article covers a variety of techniques that might be thought of as out of the usual run of the mill neurophysiological testing. Investigation of the neuromuscular junction with repetitive nerve stimulation and single fibre electromyography (EMG), a number of quantitative EMG techniques, motor unit number estimation, cervical root stimulation, and some aspects of transcranial magnetic stimulation will be covered.
INVESTIGATION OF THE NEUROMUSCULAR JUNCTION
Neurophysiology offers the most sensitive diagnostic tests for disorders of neuromuscular transmission. The tests are, however, not absolutely specific. Repetitive nerve stimulation shows a decrementing response in myasthenia gravis (MG), the decrement being more pronounced in proximal muscles. Single fibre EMG (SFEMG), however, is much more sensitive; SFEMG of facial muscles detects an abnormality in virtually all cases of MG. In Lambert-Eaton myasthenic syndrome (LEMS), the compound muscle action potential (CMAP) evoked in hand muscles is small and increases dramatically after exercise. Decrement is seen on repetitive nerve stimulation (RNS) and stimulated SFEMG, if required, shows a frequency dependent increase in jitter (table 1).
- In this window
- In a new window
The neuromuscular junction is most simply investigated by recording from a muscle with surface electrodes and repetitively stimulating the peripheral nerve at supramaximal intensity. The size of the evoked CMAP reflects the number of muscle fibres accessible from the nerve and will be reduced by atrophy (from whatever cause), by conduction block in the peripheral nerve or at the neuromuscular junction. RNS stresses the neuromuscular junction. In healthy subjects, the safety factor for transmission is sufficiently high for all impulses to be transmitted across the junction and so the evoked CMAPs are all of identical size. If neuromuscular transmission is compromised, either from a presynaptic abnormality (for example, LEMS) or from a post-synaptic abnormality (for example, MG), then RNS may result in some junctions failing and the …