Respiratory synkinesis, also known as “the breathing arm” or arm-diaphragm synkinesis, is a relatively uncommon phenomenon seen in electromyography (EMG), felt to likely be due to an aberrant rewiring process after a neurologic insult to the proximal portion of a peripheral nerve in the cervical spine. It can also occur in the setting of a pre-existing common root mass-firing effect, in which muscles with innervation from the same nerve root will co-activate because of loss of normal central control.
This case involves a needle EMG examination of a 70-yr-old female patient. She was referred for an EMG at the request of her shoulder surgeon to help differentiate the cause of chief complaint of lateral right upper arm pain radiating down to the hand in the setting of severe shoulder osteoarthritis. Approximately 2 decades before presentation, she contracted bacterial meningitis, complicated by osteomyelitis of the cervical vertebrae, which required an unknown surgery to the neck. Her previous medical records were unable to be obtained. She did not remember requiring ventilator support or supplemental oxygen during her illness. At the end of that particular hospitalization, she noted that she was unable to move either of her arms in any way. For the following year, she regained approximately 90% of her strength. She had residual permanent hypoesthesia of the right lateral shoulder and bilateral palms of her hands. The EMG finding shown on the video was seen in both the right deltoid and the right biceps brachii, but otherwise, the EMG evaluation showed no additional abnormal acute or chronic findings.
The video can be summarized in three parts. The patient has a needle inserted into her biceps brachii. First, while resting, without activating her biceps, firing of an atypical motor unit can be seen with inspiration. Second, when asked to take deeper breaths, the motor unit increases in frequency with each inspiration. Third, when the breath is held, the unit stops firing.
Respiratory synkinesis, to the authors' knowledge, has not been reported after bacterial meningitis but has been reported in cases after cervical nerve root injury due to brachial plexus injuries at birth,1,2 trauma,2 syringomyelia/syringobulbia,3 and idiopathic causes.2 The patient had cervical spine surgery to address her osteomyelitis with permanent residual hypoesthesia at the right shoulder and both hands, which may have been the sequela of an associated cervical root or plexus injury.
This phenomenon is postulated to be most commonly caused by aberrant regrowth of diaphragmatic neurons to other C5 innervated muscles, such as the deltoid or biceps brachii muscles. The motor neuron fibers to the diaphragm typically travel with the phrenic nerve, with primary innervation from C2-C6. An accessory phrenic nerve also contributes to the innervation of the diaphragm in approximately 60% of the population,4 generally originating from the C5 level. The synkinetic pattern seen in our patient could therefore also be the result of a pre-existing common C5 root mass firing effect with aberrant central control of normal innervation after meningitis, rather than aberrant re-innervation itself. Given that the patient had cervical osteomyelitis with unknown surgery, both injury of the cervical nerves resulting in aberrant nerve regrowth and change in central control due to meningitis are possibilities for development of her synkinesis. Synkinetic motor units are reported to always be abnormal in morphology,2 with increased duration and/or amplitude, which was seen in this case and supports aberrant regrowth of the neurons as the likely cause of synkinesis in our case.
In the case of synkinesis, a deep inspiration causes motor units to fire in erroneously wired muscles. Although these findings may often be of limited clinical importance (most patients, including the one presented, are not aware they exist3), they can be mistaken for fasciculations, complex repetitive discharges, or other pathologic findings. The “coming and going” nature of the units, which appear during inspiration and disappear during exhalation in a relaxed patient, are one of the hallmark features of this phenomenon.
1. Friedenberg SM, Hermann RC: The breathing hand: obstetric brachial plexopathy reinnervation from thoracic roots? J Neurol Neurosurg Psychiatry
2. Swift TR, Leshner RT, Gross JA: Arm-diaphragm synkinesis: electrodiagnostic studies of aberrant regeneration of phrenic motor neurons. Neurology
3. Nogués MA, Leiguarda RC, Rivero AD, et al: Involuntary movements and abnormal spontaneous EMG activity in syringomyelia and syringobulbia. Neurology
4. Loukas M, Kinsella CR Jr, Louis RG Jr, et al: Surgical anatomy of the accessory phrenic nerve. Ann Thorac Surg