What is the role of EMG in the diagnosis of focal muscular atrophy (FMA)?

Updated: Feb 20, 2018
  • Author: Sridharan Ramaratnam, MD, MBBS; Chief Editor: Helmi L Lutsep, MD  more...
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Answer

EMG is useful in differentiating a myopathic from a neurogenic disorder.

It can detect anterior horn cell involvement. Findings in a patient with FMA due to atypical anterior horn cell disease can be seen in the images below.

EMG at rest from the right quadriceps muscle of a EMG at rest from the right quadriceps muscle of a patient with atypical anterior horn cell disease and isolated atrophy of the right quadriceps; EMG shows spontaneous activity.
EMG on voluntary effort from the right quadriceps EMG on voluntary effort from the right quadriceps muscle of a patient with atypical anterior horn disease and isolated atrophy of the right quadriceps; EMG shows motor unit potentials that exhibit prolonged duration and polyphasia.
EMG on maximal effort from the right quadriceps mu EMG on maximal effort from the right quadriceps muscle of a patient with atypical anterior horn disease and isolated atrophy of the right quadriceps; EMG shows an impaired interference pattern.

Paraspinal EMG may be valuable in spinal root lesions.

Spontaneous activity (eg, fibrillations, fasciculations) may be seen in ALS and to a lesser degree in SMA and PPMA.

Kennedy disease may be characterized by the presence of grouped repetitive motor unit discharges on needle EMG examination of the facial muscles, such as the mentalis muscle, which are present at rest but become prominent with mild activation of the facial muscles, such as with pursing the lips or whistling. Because these discharges occur with voluntary contraction rather than spontaneously, they are distinguished from myokymic or neuromyotonic discharges. [43]

Long-duration, high-amplitude motor unit potentials (which indicate a chronic denervation with reinnervation) are seen in PPMA and, to a lesser extent, in ALS and other anterior horn cell diseases such as SMA and monomelic amyotrophy.

Myopathic pattern with fibrillations suggests an inflammatory myopathy.


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