Often neurophysiological studies can determine the site of the lesion more precisely than examination alone, but they do not determine the cause. If you can localise the lesion with confidence on clinical examination alone, will you gain additional useful information from requesting NCS/EMG? If you cannot frame your question in anatomical terms, which is how the neurophysiologist will try to answer it, then wonder why you are ordering the test at all.
NCS/EMG is particularly helpful in localising a peripheral nervous system deficit found on clinical examination. NCS/EMG and clinical examination are both operator dependant. When examining a patient you are often able to discount certain clinical findings that relate to known prior pathology-for example, a mild foot drop from a previous L5 radiculopathy the neurophysiologist has to do the same so please tell them about it. If it is difficult to obtain cooperation in the clinical examination it is likely to be more so when doing neurophysiology-especially EMG which requires a high level of patient cooperation. These same problems arise neurophysiologically-a more significant lesion is easier to localise as the neurophysiological abnormalities are more clear cut. Similarly “soft” sensory signs tend to be more difficult to localise. It is usually more straightforward to localise a significant weakness clinically than a milder degree of weakness where you tend to be less certain in distinguishing mild weakness from normal strength. There are a few obvious parallels with clinical examination: Neurophysiology can be thought of as the clinical examination with the ability to “probe” nerves and muscles in a different manner. Neurophysiologically you not only examine the distribution but also the type of abnormalities detected in the nerve conduction studies and EMG. In clinical examination you determine the site of the lesion by assessing the distribution of weakness, reflex changes, and sensory loss. However, neurophysiology is very definitely used in the same way as clinical examination to solve clinical problems, and complements the clinical evaluation rather than replacing it.
These observations may seem implausible to those who do not perform these tests and view electrodiagnostic tests as a “black box” from which answers magically appear. NCS/EMG are an extension of the clinical assessment. The tests do not replace a careful history and examination of the patient Very early on in just about every textbook or article on NCS/EMG the following two observations are made:
ELECTRODIAGNOSTIC STUDIES ARE AN EXTENSION OF CLINICAL EXAMINATION
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