"Breathing arm" - respiratory synkinesis

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Aberrant reinnervation occurs after nerve injury when axons that originally innervated one muscle mistakenly regrow into the neural tubes leading to a different muscle.(1) In synkinesis, aberrant regeneration results in co-contraction of muscles that do not normally activate together.

The most familiar form of aberrant reinnervation is facial synkinesis following Bell’s palsy.(2) Axons destined for one muscle regrow to innervate another, creating abnormal twitching of the face outside the area of intended movement. On blinking or winking, the corner of the mouth may twitch. On smiling, the eye may close.

In aberrant reinnervation of the third cranial nerve, conditions that mechanically disrupt the nerve may result in regenerating sprouts growing into the wrong tubes and eventually innervating some structure other than the one originally intended.(3) For instance, fibers that originally innervated the medial rectus may reinnervate the levator palpebrae, causing the lid to retract on adduction because of synkinesis between the medial rectus and the levator.

Respiratory synkinesis sometimes occurs after brachial plexopathy when proximal involvement results in misdirected phrenic nerve axons ultimately reinnervating the biceps or other brachial plexus components. In the "breathing arm," an EMG needle electrode records motor units in the biceps firing synchronously with diaphragm contraction.(4) In the video, the needle electrode in the biceps records a burst of motor unit action potentials with each inspiration. (Video)

Video courtesy of Dr. William S. David.


1. Campbell WW. Essentials of electrodiagnostic medicine. 2nd ed. New York: Demos Medical, 2014.

2. Kanaya K, Ushio M, Kongo K, Hagisawa M, et al. Recovery of facial movement and facial synkinesis in Bell's palsy patients. Otol Neurotol. 2009;30:640-4.

3. Gold DR, Shin RK, Bhatt NP, Eggenberger ER. Aberrant regeneration of the third nerve (oculomotor synkinesis). Pract Neurol. 2012;12:390-1.

4. Swift TR. The breathing arm. Muscle Nerve. 1994;17:125-9.