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Electromyography (EMG) forms one of the cornerstones of the assessment of patients with neuromuscular disease. Needle EMG gives insight into the firing pattern and anatomical arrangement of individual muscle fibres and entire motor units. Characteristic changes in these variables occur in a range of neuromuscular disorders. The resulting changes in the EMG signal allow the underlying pathophysiology to be determined.
In the previous review article, I discussed the fundamentals of nerve conduction studies, and here I hope to give an insight into the technique of electromyography (EMG). In reality, treating these as two separate topics is somewhat artificial; they give complementary information, and it would be very rare to perform EMG without first performing nerve conduction studies. Similarly, I not infrequently go back to check a couple of further nerves on the basis of what I find on EMG. Thus, the examination becomes a fluid (some might say balletic) process involving both techniques, as hypotheses as to the site or nature of a lesion are formed and refined.
EMG records the electrical activity arising within muscles. You can record this activity with surface electrodes, known as surface EMG. This samples a large region of the muscle, and while it gives a good indication of which muscle groups are active in for example tremor or dystonia, it gives little information as to the fine structure of those muscles. For this, you need needle EMG, which allows recording from within the muscle (figure 1). The rest of this review relates solely to needle EMG.
The most obvious risk is in performing EMG in patients with abnormal clotting. This is by no means an absolute contra-indication, but for some deep muscles the consequences of haematoma formation could be catastrophic. The …
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