Single Fiber EMG

The single-fibre electrode and the recording principle

The classical single-fibre electrode is a modified concentric needle carrying a 25-µm recording surface mounted on the side of the cannula, some distance back from the bevelled tip. The combination of the small leading-off surface and an aggressive high-pass filter (typically 500 Hz, considerably higher than the routine concentric setting) restricts the recording radius to roughly 300 µm, so that the electrode resolves the action potentials of only one or two muscle fibres belonging to the same motor unit. A fibre is accepted for analysis when its potential is biphasic, exceeds a defined amplitude (conventionally >200 µV) and has a rise time short enough (<300 µs) to confirm proximity to the recording surface. Because reusable single-fibre electrodes raise reprocessing and infection-control concerns, much contemporary practice substitutes a disposable concentric needle with a low-frequency filter raised to ~1–2 kHz; this records a slightly larger territory and therefore demands its own, narrower reference values.

Two parameters are extracted. Jitter quantifies the stability of transmission; fibre density quantifies the spatial clustering of fibres within the motor unit and thus indexes reinnervation. The two are conceptually orthogonal, and a complete study reports both.

Jitter: the variability of neuromuscular transmission

When a single axon discharges, it activates each of its terminal branches, and every endplate must depolarise its muscle fibre to threshold. The interval between the firing of two fibres of the same motor unit — the inter-potential interval — is not fixed. It fluctuates from discharge to discharge because the endplate potential rises to threshold at a slightly variable rate, governed by the stochastic release of acetylcholine quanta and the instantaneous height of the safety factor. The standard deviation of the latency to threshold, propagated across two endplates, manifests as jitter.

Jitter is quantified not as a simple standard deviation — which would be corrupted by slow trends in firing rate — but as the mean consecutive difference (MCD): the mean of the absolute differences between successive inter-potential intervals. The MCD is robust to gradual drift in discharge rate, capturing only the beat-to-beat instability that reflects junctional physiology. Normal values are muscle-specific and age-dependent, but for many limb muscles the upper limit of normal mean jitter sits near ~35–55 µs; the orbicularis oculi and frontalis, frequently studied in suspected ocular myasthenia, carry their own tabulated limits. Expressing a result demands a reference table for the exact muscle and electrode used — there is no universal threshold.

Fibre density and the reinnervation signal

Fibre density is the mean number of single-fibre potentials, time-locked to the same trigger, recorded at 20 systematic electrode positions within a muscle. In normal muscle the figure is low (typically 1.3–1.8, muscle-dependent) because fibres of one motor unit are sparsely scattered and only one usually falls within the recording radius. After denervation and collateral reinnervation, surviving axons adopt orphaned fibres, packing more fibres of a single unit into a small territory; fibre density rises. Elevated fibre density is therefore a sensitive marker of a chronic neurogenic or reorganising process and must be interpreted alongside jitter — newly formed, immature endplates on reinnervated fibres themselves transmit unstably, so reinnervation elevates both parameters.

Voluntary versus stimulated SFEMG

In voluntary SFEMG the patient maintains a minimal, steady contraction and the electrode is positioned to capture two fibres of one unit firing semi-rhythmically; the inter-potential interval is measured over 50–100 discharges. This is the reference standard but requires cooperation, sustained activation, and a steady firing rate — impossible in young children, encephalopathic patients, or tremor.

Stimulated SFEMGsubstitutes intramuscular microstimulation of the nerve twig for voluntary effort. Here jitter is measured between the stimulus and a single fibre's response (involving one endplate rather than two), so the reference values are systematically lower — roughly the voluntary value divided by √2. Stimulated technique extends SFEMG to the uncooperative patient and fixes the discharge rate precisely, but it introduces artefacts of its own: subthreshold or fluctuating stimulus intensity produces spurious jitter and apparent blocking that mimic disease.