Tests do not replace a careful history and the examination of the patient
NCS/EMG is an extension of clinical assessment
These observations may seem implausible to those who do not take these tests and see electrodiagnostic tests as the “black box” from which answers magically appear. However, neurophysiology is certainly used in the same way as a clinical examination to solve clinical problems, and it complements, rather than replaces, clinical evaluation. On clinical examination, locate the lesion by evaluating the distribution of weakness, reflex changes, and sensory loss. Neurophysiologically, it examines not only the distribution but also the type of abnormalities detected in nerve conduction and EMG studies. Neurophysiology can be thought of as a clinical examination with the ability to “examine” nerves and muscles in a different way.
There are some obvious similarities with clinical examination:
Locating a clinically significant impairment is usually more straightforward than locating a milder degree of impairment as it tends to be less certain in distinguishing between mild weakness and normal strength. Likewise, ‘soft’ sensory signs tend to be more difficult to localize. These problems manifest themselves Neurophysiologically – the most significant lesion is easy to localize as the neurophysiological abnormalities are more pronounced.
If cooperation is difficult to obtain on clinical examination, it is likely even more so when doing neurophysiology – especially electromyography which requires a high level of patient cooperation.
On examining a patient, you are often able to exclude some clinical findings that are related to known prior pathology – eg, mild foot drop from previous L5 radiculopathy; A neurophysiologist should do the same, so please tell them.
Both NCS/EMG and clinical examination is operator dependent.
The NCS/EMG is particularly useful in locating the peripheral nervous system deficit found on clinical examination. So:
If you can’t frame your question in anatomical terms, which is how a neurophysiologist would try to answer it, then wonder why the test is required at all.
If you can confidently localize the lesion on clinical examination alone, will you get additional useful information from the NCS/EMG order?
Oftentimes neurophysiological studies can locate the lesion more accurately than examination alone, but they do not identify the cause. This usually requires further investigations and neurophysiological studies may add nothing to the diagnosis if these other tests are diagnostically positive. For example, neurophysiological confirmation of L5 radiculopathy may contribute little if the magnetic resonance image (MRI) of the lumbosacral spine clearly demonstrates compression of the L5 root. If you are going to have other tests, consider whether the NCS/EMG will add to the diagnosis. In the era of the ‘programmed investigation approach’ remember that ‘mass confinement’ tests and thus failure to value the investigative pathway is a poor and ineffective use of diagnostic resources.
Before looking at the role that NCS/EMG plays in different clinical situations, it is useful to think a little about the tests themselves and make some observations about how we use investigations in general.
Evidence-based medicine suggests that we must understand the special features of diagnostic tests, such as sensitivity (the ability to detect those with disease) and specificity (the ability to detect those who do not). To account for these, we ask that the test be compared to the accepted gold standard.
This leads to the first problem for NCS/EMG. For some conditions – eg, chronic inflammatory demyelinating colonic radiculopathy (CIDP) or multifocal motor neuropathy with conduction block – the NCS/EMG is an essential part of the definition of these conditions making sensitivity and specificity calculations somewhat circular, although Attempts to evaluate different combinations of diagnostic criteria 1 For other conditions, such as carpal tunnel 2 or ulnar neuropathy of the cubital tunnel, 3 While NCS/EMG is not the only way to confirm the diagnosis, it is probably the most robust non-invasive method. In other cases, another method can provide a more accurate diagnosis – eg, cervical radiculopathy where MRI or surgical findings can provide the gold standard.
Paradoxically, this means that the circumstances in which there is the best published evidence of sensitivity and specificity are those in which there is another, more specific method for achieving the diagnosis, this being used as the gold standard against which NCS/EMG can be compared. Cervical and the lumbar radiculopathy are clear examples of this.