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Arm-diaphragm synkinesis

Electrodiagnostic studies of aberrant regeneration of phrenic motor neurons

Medical College of Georgia, Augusta, GA.

Ann-diaphragm synkinesis may occur after injury to the proximal portion of the brachial plexus or cervical nerve roots. Regenerating axons of phrenic motor neurons are misdirected to supply limb muscles. Electrodiagnostic investigations of three patients with symptoms referable to upper brachial plexus or cervical roots revealed motor units in either biceps or triceps muscles that discharged during inspiration. These units did not fire during forced or passive expiration or Valsalva maneuver. Activation of these units could not be achieved by volitional contraction of the appropriate arm muscles. The synkinetic motor units were of increased amplitude and duration. Some units contained late components that demonstrated jitter and blocking, as is characteristic of reinnervated motor units.

The unwary electromyographer may misinterpret these synkinetic units as incomplete muscle relaxation or some form of abnormal repetitive discharge. Correct identification of these units provides objective evidence of antecedent nerve root or brachial plexus injury.

Address correspondence and reprint requests to Dr. Swift, Electromyography Laboratory, Department of Neurology, Medical College of Georgia, Augusta, GA 30912.

Accepted for publication August 13, 1979.

This study was supported in part by a Local Chapter Grant of the Muscular Dystrophy Association.

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