Patient Safety: Electrodiagnostic Findings After Chemodenervation

From the Quality and Patient Safety Committee

A 43 year old presents for evaluation of cervical radiculopathy. Symptoms include neck pain and radicular right arm pain. Medical history includes refractory chronic migraines that are managed by serial Onabotulinum toxin A injections using standard protocol. Electrodiagnostic testing is requested by the spine surgery team to evaluate for the presence of a cervical radiculopathy.

Question: Which of the following statements best describes the expected EDX findings related to treatment?

A: EDX findings are supportive of a widespread neuromuscular disease
B: Decremental responses with repetitive nerve stimulation responses at low frequencies
C: Reinnervating motor unit action potentials in the right deltoid
D: Fibrillation potentials in the right upper trapezius muscles

Explanation:
The correct answer is D. Onabotulinum toxin is an effective treatment for chronic migraine, and its use has increased since FDA approval for this indication. Providers performing electrodiagnostic testing should be vigilant of the location and timing of therapeutic chemodenervation when designing and interpreting data of electrodiagnostic tests.

Onabotulinum toxin binds to SNARE proteins on the surface of synaptic vesicles and the presynaptic membrane, which in turn prevents the release of acetylcholine into the synaptic cleft. Persons with chronic migraine will experience effects in the motor units injected even though this is not the final target for the treatment. Since the motor unit is affected, electrodiagnostic (EDX) studies of the injected muscles will demonstrate findings consistent with a pre-synaptic neuromuscular junction lesion, similar to systemic botulism. EDX findings that have been reported include lack of decrement with 3 Hz repetitive nerve stimulation and facilitation with 20 Hz repetitive nerve stimulation; progressive increasing of jitter; early appearance of fibrillations; small and short motor unit action potential in the first 3 weeks, followed by increasing of MUAP amplitude and duration, with polyphasic morphology. Case reports by neuromuscular experts have also identified similar findings in distal muscles, suggesting that some patients experience distant spread.

These EDX findings can be seen in both patients with and without known neuromuscular disease, and have been reported to peak at 15 to 30 days post-treatment. As such, caution should be practiced when offering chemodenervation as a management option to patients with neuromuscular conditions, particularly if disease state is severe or poorly controlled.

Author: Sarah M. Jones, MD

Sources:

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2. Lispi L, Leonardi L, Petrucci A. Longitudinal neurophysiological assessment of intramuscular type-A botulin toxin in healthy humans. Neurol Sci. 2018;39(2):329-332. doi:10.1007/s10072-017-3191-3
3. Sanders DB, Massey EW, Buckley EG. Botulinum toxin for blepharospasm: single-fiber EMG studies. Neurology. 1986;36(4):545-547. doi:10.1212/wnl.36.4.545 
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