The advantages and limits of electromyography
Steven A. Day, M.D.
General neurologist, Providence Peripheral Nerve Services
The electrodiagnostic examination – also known as electromyography, or EMG – is an essential tool in the diagnostic evaluation of peripheral neurological disorders. Unfortunately the nature of the test, its utility and its limitations are not always clear to many physicians.
Electromyography is composed of two subtests: the nerve conduction study and the needle electrode examination. Each provides different information about the peripheral nervous system, which when analyzed together allows for a diagnostic conclusion.
The nerve conduction study involves electrically stimulating the nerve and recording its response or the muscle it innervates.
The needle electrode examination involves placing a small needle directly into muscles and recording the electrical activity generated by the muscle at rest and with voluntary activation.
An EMG is a helpful diagnostic adjunct when there is concern about mononeuropathy (carpal tunnel syndrome, wrist drop, foot drop); polyneuropathies; radiculopathies (when MRI does not identify clear nerve root compression); disorders of the neuromuscular junction (myasthenia gravis); and myopathies or muscular dystrophies.
Unfortunately EMG has limitations that can reduce the clinical yield in some cases. Technical limitations arise in the setting of obesity and advanced age, which can limit the diagnostic certainty in some clinical situations. Lack of patient cooperation can also limit diagnostic yield.
When the clinical concern is radiculopathy, a notable limitation is the inability to comment on the presence (or absence) of a sensory radiculopathy. Due to the anatomical relationships of the peripheral nervous system, radiculopathy can be detected only if there is involvement of the motor fibers.
When ordering an EMG, it is important to specify the clinical syndrome of concern and the limb to be studied (e.g. carpal tunnel syndrome, right arm). It is also helpful to briefly review the test with the patient.
Due to the somewhat uncomfortable nature of the study, it is important that patients have a sense of what to expect. If it is not clear that an EMG would help, a neurological consultation can always be requested first.