Interpretation
| Pitfall | Explanation |
|---|---|
| CN, clinical neurophysiologist; LEMS, Lambert Eaton myasthenic syndrome; MG, myasthenia gravis; PNE, peripheral neurophysiological examination; SNAPS, sensory nerve action potentials. | |
| Overinterpretation of results | • A limited numerical abnormality must be reported but placed in the context of the clinical problem—for example, the finding of a subclinical carpal tunnel abnormality on NCS does not make the diagnosis of carpal tunnel syndrome • An NCS abnormality may relate to a previous and currently irrelevant injury or operation |
| Over simplification of results | • All studies should be interpreted together in case multiple diagnoses are present. Occum’s razor may be too sharp. For example, in a patient with obvious polymyositis, NCS should ideally be performed to delineate any additional coexisting neuropathy |
| Misinterpretation of NCS results | • If the FMNCV is normal, the patient may still have slowing of conduction of other nerve fibres related to a neuropathy • If the SNAPS are normal in all respects the patient may still have a small fibre sensory neuropathy producing symptoms and signs • If a nerve is reported unstimulatable, think of anomalous innervation or anatomical variation of nerve position before concluding pathology |
| Misinterpretation of RNS results | • If a decrement continues throughout the RNS train, this is probably artefact • If a decrement is found in RNS, it is not pathognomic of myasthenia gravis. It has been found to be abnormal in a number of conditions with abnormal nerve terminal function (motor neurone disease, active axonal neuropathy) or muscle membrane instability (polymyositis) • While MG and LEMS are the most common NMT disorders, the unwary will miss correctly diagnosing congenital myasthenic syndromes, botulism, drug and toxins (for example, organophosphates), all of which have characteristic PNE patterns when all tests are interpreted together with the clinical picture |
| For the referrer | • If the referring doctor has not asked a specific question of the CN investigator, they should not be surprised if occasionally their request for a PNE is misunderstood • A PNE request should never be couched in “please exclude” terms. While probabilities may be discussed, total exclusion of a diagnosis depends on non-biological errors such as sampling • In the UK, CN are a scarce resource, often struggling with large waiting lists. Thus all referrals should be considered in terms of: (a) what knowledge will I gain about my patient from this investigation? (b) Will the result change my management? (c) How urgent should the request be? |