Facial Single-Fiber Electromyography After Prior Botulinum Toxin Exposure: A Retrospective Clinical Cohort

Single-fiber electromyography (SFEMG) is highly sensitive for disorders of neuromuscular transmission but may be confounded by prior botulinum neurotoxin (BoNT) exposure. Although BoNT is known to transiently elevate jitter, the duration of its electrophysiologic effects remains unclear.

Retrospective cohort study of patients 18 years and older who underwent voluntary SFEMG of the frontalis or orbicularis oculi over a 10-year period with a history of local facial BoNT exposure. Patients with coexisting neuromuscular disorders were excluded.

Sixty-one patients were identified, with 51 demonstrating elevated jitter measured as mean consecutive difference (MCD). The median MCD (n = 51) was 72 µs (range: 26-150), with blocking present in 78%. The median time since last BoNT injection was 6 months (range: <3-240); however, 16 patients had elevated MCD despite testing ≥12 months after the last injection. MCD was higher in studies performed <12 months compared with ≥12 months since the last BoNT injection (80 vs. 43 µs; P = 0.007). Common indications for BoNT included cosmetic use (30/51) and migraine (16/51). Six of 51 patients underwent follow-up SFEMG (ranging from 5 to 13 months); five had significant reduction in MCD. Two-Hz repetitive facial nerve stimulation was performed in 44/51; none demonstrated an abnormal decrement. Ten patients had normal MCD despite prior BoNT exposure, with a median interval of 25 months (range: 4-120) since last injection.

Elevated MCD is associated with more recent BoNT exposure but may persist well beyond 12 months in some patients, underscoring the need for cautious interpretation of SFEMG results in patients with prior BoNT exposure.