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When to Order an EMG/NCS and other FAQs

When to order an EMG/NCS?
Electromyogram (EMG) and Nerve Conduction Studies (NCS) provide information about the function of the peripheral nervous system above and beyond what can be gleaned from a neurological exam. Specifically, it can determine:

  • a particular disease localization
    • myopathy

    • neuromuscular junction
    • generalized neuropathy
    • focal neuropathy
    • plexopathy
    • radiculopathy
    • anterior horn cell disease
  • chronicity
    • severity
      • pathophysiology (e.g. axonal vs. demyelinating neuropathy)
        • particular nerve fibers involved, even if subclinical (sensory/motor/autonomic)
          • occasionally a very specific diagnosis can be rendered (e.g. distal myotonic discharges
            and myopathic changes would be pathognomonic of myotonic dystrophy)

          How specific of a diagnosis can be rendered with an EMG/NCS?
          For the most part, EMG/NCS identifies the localization of the pathology (e.g. polyneuropathy vs. myopathy) rather than the etiology (diabetic neuropathy vs. uremic neuropathy, etc.). However, occasional pathognomonic findings are present (see Question 1 above).

          Should I order an EMG, an NCS, or both?
          For nearly all indications, both EMG and NCS are required to provide the most accurate assessment of the situation. For example, many physicians think it is sufficient to simply order just NCS in the evaluation of carpal tunnel syndrome (CTS). Although slowing of median conduction across the wrist is the most specific electrodiagnostic finding in CTS, EMG is helpful to provide additional information such as the degree of axonal involvement, the presence of active vs. chronic denervation, and distinguishing CTS from other disorders that may clinically look the similar (cervical radiculopathy, proximal median neuropathy, thoracic outlet syndrome, etc).

          What is the difference between NCS and NCV?
          NCS (Nerve Conduction Study) and NCV (Nerve Conduction Velocity) both refer to the same procedure. “Nerve Conduction Velocity” is an older term, used originally when slowed conduction was the only parameter being evaluated. The more modern term “Nerve Conduction Study” reflects the notion that there are many other parameters besides just conduction velocity that are being assessed.

          What is the sensitivity of the test?
          The sensitivity of detecting any given condition with these tests varies depending on several factors, such as severity, timing, presence of co-existent neuromuscular abnormalities, and the particular studies chosen by the electromyographer. For example, the sensitivity for detecting carpal tunnel syndrome is excellent (above 95%), but only if the person performing the analysis takes the time to do additional procedures (e.g. median-vs.-ulnar comparison studies) in addition to the routine tests. For radiculopathies, the EMG/NCS can be entirely normal if only the sensory root is involved.

          In the case of radiculopathies, does an EMG/NCS need to be ordered if an MRI has already been done?
          Imaging studies, like MRIs, show the anatomy of the relevant structures whereas EMG and NCS are functional tests. As such, they provide complementary types information, and often both imaging and EMG/NCS may be required to arrive at the most accurate assessment of a particular situation.

          When is the best time to order an EMG/NCS after an abrupt onset of symptoms?
          Following an acute axonal injury, a variety of different abnormalities typically appear in a relatively characteristic sequence. Some abnormalities can appear immediately (e.g. abnormal F-waves) whereas other changes can take longer (e.g. fibrillations after several weeks, neurogenic motor units after several months). In general, waiting at least 3 weeks following an acute injury is recommended in order to allow time for the appearance of findings that are most helpful in localizing the lesion.

          What tests can be ordered in the evaluation of neuromuscular junction disorders
          (e.g. myasthenia gravis)?

          Repetitive nerve stimulation (RNS) takes a few minutes to perform but only has a sensitivity of approximately 70% in myasthenia gravis, depending on the severity of the weakness; RNS serves as a useful quick screen for these conditions. “Single fiber EMG” (SF-EMG) is generally considered to be the most sensitive test for myasthenia gravis, but requires a needle to be in the tested muscle for approximately 2 hours. The anti-acetylcholine receptor antibody assay is more specific than SF-EMG and only requires venipuncture. In general, SF-EMG is most useful if myasthenia gravis is clinically suspected but both antibodies and RNS are negative. The Tensilon test can also provide a diagnosis of myasthenia gravis, but only if the patient has a reliable, objective deficit, such as ptosis or ophthalmoparesis. Tensilon tests are performed with continuous EKG monitoring at the Cedars-Sinai Neurophysiology Laboratory (310-423-6841).

          Are these tests painful?
          Routine nerve conduction studies require brief electrical stimulations on the skin surface. Mostpatients tend to find them a little startling at first, and describe the sensation as mild discomfort. The EMG requires and thin sterile needle (usually 28 gauge) to be placed for around 30 seconds in different muscles; it is also described by most as mild discomfort. However, every patient tolerates these procedures differently. People always have the option of immediately stopping the test, and there should be no lasting discomfort. Most patients do not require pretreatment with medication, but some do benefit from their referring physician prescribing them a mild analgesic or anxiolytic to be taken an hour before the test (assuming they have a ride to and from the office).

          Can an EMG be done if the patient is anticoagulated?

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