Beyond Jitter: Spike Count Analysis for Differentiating Botulinum Toxin and Myasthenic Effects on Neuromuscular Function.

PURPOSE Single-fiber electromyography is the most sensitive tool for diagnosing neuromuscular diseases but is limited in differentiating between presynaptic and postsynaptic neuromuscular junction involvement with increased jitter. With rising botulinum toxin (BoNT) use for therapeutic and cosmetic applications, referrals for electromyography because of myasthenia-like symptoms have increased, complicating differential diagnosis. This study examines whether spike count measurements from single-use concentric needle electrodes can distinguish BoNT effects from neuromuscular junction diseases such as myasthenia gravis. METHODS We analyzed 49 patients and 26 controls, assessing jitter and spike count with concentric needle electrodes in the frontalis muscle. Groups included those exposed to BoNT (>1 month and <1 month prior) and patients with myasthenia gravis, with normal jitter controls for comparison. Data were analyzed for jitter, spike count, and additional electrophysiologic parameters using standard statistical tests (P < 0.05). RESULTS Results showed that spike counts were significantly different across the groups. Higher spike counts with lower single spike frequency were found in patients with BoNT exposure >1 month, suggesting a differentiation point from primary neuromuscular involvement. In contrast, patients with myasthenia gravis exhibited increased jitter without increased spike counts. CONCLUSIONS These findings indicate that spike count analysis with concentric needle electrodes may aid in differentiating iatrogenic BoNT effects from primary neuromuscular junction disorders, such as myasthenia gravis. However, further studies with larger sample sizes are necessary to validate these results.