Through our years of practice, it has become evident that much confusion exists pertaining to electrodiagnostic testing (EMGs). We have been frequently questioned as to indications for testing as well as what is involved in the procedure itself.

The goals of this article are to:

• Improve one’s understanding of when electrodiagnostic testing can be most helpful.
• Describe limitations of electrodiagnostic testing.
• Familiarize individuals with the testing procedure itself.
• Understand how to best order an EMG.

When EMG is most helpful

Compression neuropathies. EMG is quite sensitive and is the only diagnostic test for determining whether and to what extent a compression neuropathy is present. Results are quantitative: distal latencies, nerve conduction velocities, and/or amplitudes of evoked potentials. The test can often be utilized to prognosticate, or determine the need for surgical intervention, or follow progression. Examples: carpal tunnel syndrome, facial neuropathies, peroneal palsy.

Localization of a lesion. EMG can often determine the site of an injury or disease process when such information is not clinically apparent. Examples: In a patient with footdrop, testing can distinguish between involvement of the common peroneal nerve, lumbosacral plexus, lumbar radiculopathy, or systemic process such as motor neuron disease.

Differentiation between a neuropathic lesion and soft tissue injury. EMG can determine whether weakness and atrophy is due to lower motor neuron involvement or tearing of muscles/tendons. Examples: A patient with weakness and wasting of rotator cuff musculature could have a C5,6 radiculopathy, upper trunk plexopathy, suprascapular nerve palsy, or rotator cuff tear.

Neuromuscular junction disorders. EMG utilizing repetitive stimulation has approximately a 90 percent sensitivity in ruling out disorders of the neuromuscular junction, the most common of which is myasthenia gravis.

Guidance after traumatic nerve injuries. In the acute setting (example: post ORIF of a fracture), one can determine whether there is neural continuity when the surgeon is considering exploration and possible decompression. Six months to two years after a traumatic nerve injury, EMG can help determine the need for a tendon transfer or nerve graft by assessing renervation.

Other uses for EMG

Radiculopathy. EMG can be helpful in localizing the level of a radiculopathy, ruling out more distal lesions, and distinguishing between polyradiculopathy rather than monoradiculopathy. The test is less helpful in prognosticating or determining who will require surgery, but adds clinical perspective to a CT or MRI, which are purely anatomical.

Peripheral neuropathies. EMG can detect the presence of a polyneuropathy and place it into one of several broad categories. This can guide the clinician in his work-up of a polyneuropathy. EMG can also be useful in prognosticating in patients with Guillian-Barre syndrome.

Motor neuron disease. EMG can add information that assists in the diagnosis of motor neuron disease, but the test is not absolutely diagnostic.

Myopathies. EMG can document the presence of myopathy, although the test is often negative early in the course of most myopathies. EMG will occasionally be diagnostic of a specific myopathy, ex., myotonic dystrophy. More often, the clinical picture determines the type of myopathy present.

Documentation. For legal purposes, patients with personal injuries often require documentation of the presence or absence of a lesion. Other situations require documentation as well.