Pitfalls in electrodiagnosis

This review describes some of the factors that may lead to erroneous interpretations of electromyographic and nerve conduction studies. Such errors may be due either to technical or to biological factors, and it is imperative that the consequent limitations of the methods be considered in a diagnostic setting. Electrodiagnostic findings should always be interpreted in the clinical context, and since they are rarely specific for a particular disorder or pathology, it is necessary to satisfy several criteria to make a specific diagnosis. The aim of electromyographic examination is to ascertain whether weakness is due to a neurogenic lesion or to myopathy. It is, however, not sufficient to show the presence of denervation activity since this may occur in either condition. Therefore the motor unit potentials from both weak and nonaffected muscles should be examined quantitatively. Nerve conduction studies are carried out to ascertain whether motor or sensory myelinated fibers are lost, and whether the primary pathology is due to demyelination or axonal loss or to both. The nerve conduction velocity is of primary importance in this distinction. However, loss of large myelinated fibers leads to slowing of conduction; in some instances the conduction velocity may be normal if only a few large fibers are spared. In addition collateral sprouting in chronic conditions may lead to apparent sparing of motor fibers. Hence an erroneous diagnosis may be made of a sensory neuropathy if additional electromyography or other tests are not carried out. Conduction studies investigate only large myelinated fibers, and therefore in some instances there is discordance between the morphology and physiology. Acquired demyelinating neuropathies are sometimes associated with focal slowing of conduction or with conduction block. The demonstration of conduction block is important, but several requirements must be fulfilled in terms of technique, clinical context, and temporal development in order to avoid errors.