F-Wave-Induced Repetitive Discharges: Expanding the Spectrum of Stimulus-Induced Repetitive Discharges.

INTRODUCTION/AIMS: The clinical significance of stereotyped post-F-wave discharges remains uncertain. We aimed to characterize F-wave-induced repetitive discharges (FIRDs) and their associated clinical and electrodiagnostic findings.

METHODS

We searched the Mayo Clinic electrodiagnostic database (January 1997-December 2025) using "F-wave" and "repetitive discharges" and reviewed retrieved waveforms. FIRDs were defined by repetitive discharges following F-wave activation, reproducible morphology within a train, and regular or semiregular firing.

RESULTS

The study included 20 patients with FIRDs (median age 61 years, range 21-83 years; 10 male). Routine motor conduction attributes were normal in 90% of patients and F-wave latencies were normal in all. FIRDs consisted of brief, reproducible discharges with a regular or semi-regular firing pattern, individual durations of 2-10 ms, firing frequencies of 100-500 Hz, and amplitudes of 120-1000 μV. Neurogenic abnormalities on needle electromyography were seen in the distribution of the FIRD nerve in 65%. FIRDs were transient in a subset of patients who had repeat studies (5/5). Clinical and electrophysiological diagnoses included peripheral neuropathy (30%), radiculopathy (25%), polio (5%), brachial plexopathy (5%), and chronic inflammatory demyelinating polyradiculoneuropathy (5%). Six patients (30%) had no clinical or electrophysiological evidence of a neuromuscular disorder.

DISCUSSION: FIRDs represent an uncommon electrophysiological phenomenon seen in both chronic neurogenic disorders and patients without neurologic disease. Recognition may aid waveform interpretation, but their presence alone does not establish a specific neuromuscular diagnosis.