Single-fiber EMG (SFEMG) and repetitive nerve stimulation (RNS) are specialized techniques for evaluating neuromuscular junction (NMJ) disorders — myasthenia gravis, Lambert-Eaton myasthenic syndrome, congenital myasthenic syndromes, and botulism. They test the integrity of synaptic transmission, complementing standard EMG and NCS which test the muscle and nerve respectively. RNS detects post-synaptic NMJ dysfunction (decrement in MG) or pre-synaptic dysfunction (facilitation in LEMS). SFEMG quantifies jitter — the variability of synaptic delay — and is the most sensitive electrodiagnostic test for NMJ disease. This page covers the techniques, interpretation, and clinical applications.

Neuromuscular Junction Physiology Review

Presynaptic terminal: voltage-gated Ca²⁺ channels (P/Q type) open with action potential → Ca²⁺ enters → vesicle fusion via SNARE proteins → ACh release.
Synaptic cleft: ACh diffuses across.
Postsynaptic membrane: nicotinic acetylcholine receptors (nAChR); ACh binding → Na⁺/K⁺ flux → end-plate potential (EPP).
EPP must reach threshold to trigger muscle action potential.
Safety factor: normally many more ACh molecules than needed for threshold; provides buffer.
Acetylcholinesterase rapidly hydrolyzes ACh.

Repetitive Nerve Stimulation (RNS)

Slow RNS (2–5 Hz) for Postsynaptic NMJ Disease

Stimulate motor nerve at 2–5 Hz for 5–10 stimuli.
Record CMAP amplitude with each stimulus.
Compare 4th–5th stimulus to first.
Normal: amplitudes stable (variation <10%).
Decrement: drop in amplitude by >10% (some labs >15% or >20%) — supports postsynaptic NMJ disease (MG).
Mechanism: with repetitive stimulation, ACh stores in presynaptic terminal partially deplete; with reduced AChRs (MG), some end-plates can’t reach threshold → drop in number of muscle fibers activated → reduced CMAP.
Best in clinically affected proximal muscles: deltoid, trapezius, biceps.

Fast RNS (20–50 Hz) for Presynaptic NMJ Disease

Stimulate at 20–50 Hz for short trains.
Record CMAP amplitude.
Normal: minor variation.
Facilitation: increase in amplitude >100% (some labs >60%) — supports presynaptic NMJ disease (LEMS).
Mechanism: in LEMS, presynaptic Ca²⁺ channels are reduced; baseline ACh release is poor → small CMAP. With high-frequency stimulation, Ca²⁺ accumulates faster than it clears → more vesicle release → larger CMAP.
Also seen after brief voluntary contraction (post-exercise facilitation).

Post-Exercise Facilitation (Alternative for LEMS)

Baseline CMAP recorded.
Patient performs 10-second maximal voluntary contraction.
CMAP recorded immediately after.
>100% increase supports LEMS.
Easier than fast RNS in awake patient.

RNS Pitfalls

Temperature: cool muscle can produce false-positive decrement (always warm to 32°C).
Submaximal stimulation: not all axons activated; misleading.
Movement during stimulation: artifact mimicking decrement.
Severe weakness: low baseline CMAP makes decrement hard to quantify.
Anticholinergic effect: pyridostigmine masks decrement; consider holding before testing.
Cold limbs: produce false decrement.

Single-Fiber EMG (SFEMG)

Concept

Specialized needle with small recording surface (25 μm vs 0.5 mm standard).
Records action potentials from individual muscle fibers.
Two adjacent fibers from the same motor unit are recorded simultaneously.
Jitter: variability in time between the two fiber action potentials.

Jitter Physiology

Both fibers are innervated by the same motor neuron.
Time between firing of the two fibers depends on conduction time differences (small) and synaptic transmission time variability (the main source).
Normal NMJ: synaptic transmission is rapid and uniform; small jitter.
Abnormal NMJ: synaptic transmission delayed or variable; large jitter.
Severely abnormal NMJ: blocking (one fiber fails to fire some trials).

Measurements

Mean consecutive difference (MCD): average difference between consecutive intervals.
Normal MCD: typically <30–50 μs (muscle-specific).
Abnormal MCD: >50 μs.
Blocking: any blocking is abnormal; supports NMJ disease.
20 fiber pairs analyzed per muscle; 10% (or 2 pairs) abnormal is positive.

Technique

Patient gently contracts muscle.
Needle positioned to record from individual fibers.
Stimulation SFEMG: alternative technique using electrical stimulation; doesn’t require patient cooperation; useful in children or uncooperative patients.
Time-consuming; expertise required.

Sensitivity

MG: SFEMG sensitivity ~95% (more sensitive than RNS or antibodies in mild/ocular disease).
LEMS: SFEMG sensitive but RNS findings often more clinically useful.
Botulism: SFEMG can detect early NMJ dysfunction.
Specificity: lower; abnormalities can occur in other disorders (chronic neurogenic disease with reinnervation).

Myasthenia Gravis (MG)

Pathophysiology

Autoantibodies against AChR (~85% of generalized MG) or MuSK (~10%).
Reduced functional AChRs → reduced safety factor.
Fatigable weakness pattern.
Ocular onset common; can stay ocular or generalize.

Electrodiagnostic Findings

RNS: decrement >10% in affected muscle.
Postexercise facilitation: NOT typical of MG (it’s seen in LEMS).
SFEMG: increased jitter, blocking; most sensitive test.
Routine NCS: normal except for low baseline CMAP if severe disease.
EMG: normal at rest; sometimes brief activation produces myopathic-appearing changes.

Clinical Approach

Anti-AChR antibodies: first test; positive in ~85% of generalized.
RNS: sensitive in generalized MG (~50–70%).
SFEMG: most sensitive (~95%); useful in ocular-only or seronegative cases.
Treat with pyridostigmine + immunotherapy (steroids, azathioprine, mycophenolate, rituximab, complement inhibitors, FcRn inhibitors).

Lambert-Eaton Myasthenic Syndrome (LEMS)

Pathophysiology

Autoantibodies against presynaptic P/Q-type Ca²⁺ channels.
Reduced Ca²⁺ entry during depolarization.
Reduced ACh release per action potential.
Often paraneoplastic (small cell lung cancer ~50%); idiopathic autoimmune in others.

Clinical

Proximal weakness, dry mouth, hyporeflexia.
Strength improves transiently with brief sustained effort (“warm-up phenomenon”).
Ocular involvement less prominent than MG.

Electrodiagnostic Findings

Routine motor NCS: SMALL baseline CMAP amplitudes (in clinically weak muscles).
Post-exercise facilitation: >100% increase after brief contraction.
Fast RNS (20–50 Hz): facilitation pattern.
Slow RNS: also shows decrement (mimics MG).
SFEMG: increased jitter, blocking.
The combination of small baseline CMAP + post-exercise facilitation is highly suggestive of LEMS.

Management

Search for malignancy (small cell lung cancer).
3,4-Diaminopyridine (amifampridine): symptom relief.
Immunotherapy.
Cancer treatment if paraneoplastic.

Congenital Myasthenic Syndromes (CMS)

Genetic NMJ disorders.
Variable phenotypes (postsynaptic AChR defects, presynaptic, synaptic).
Electrodiagnostic features similar to acquired MG.
Treatment varies by subtype:
- Slow-channel syndromes: quinidine or fluoxetine.
- Fast-channel: pyridostigmine.
- DOK7 mutations: β-agonists (albuterol, ephedrine).
Genetic confirmation required.

Botulism

Botulinum toxin cleaves SNARE proteins.
Prevents ACh release from presynaptic terminal.
Clinical: descending paralysis, autonomic dysfunction.

Electrodiagnostic findings:

Small CMAP amplitudes.
Post-exercise facilitation (similar to LEMS but more profound).
Fast RNS: facilitation.
SFEMG: increased jitter.
Diagnostic distinguishing from LEMS by clinical context (food exposure, wound, infant).

Tick Paralysis

Tick toxin causes NMJ blockade.
Electrodiagnostic findings similar to botulism.
Removal of tick → rapid recovery.

Distinguishing NMJ Disorders

Feature	MG	LEMS	Botulism
Baseline CMAP	Normal (or low if severe)	Small	Small
Slow RNS (2–5 Hz)	Decrement	Decrement (similar to MG)	Decrement
Fast RNS (20–50 Hz)	No facilitation	Facilitation >100%	Facilitation
Post-exercise facilitation	Absent	Present (>100%)	Present
SFEMG	Increased jitter, blocking	Increased jitter, blocking	Increased jitter
Clinical pattern	Fatigable weakness, ocular onset, ptosis	Proximal weakness, dry mouth, transient strength improvement with exercise	Descending paralysis, autonomic
Cause	Anti-AChR autoantibodies	Anti-P/Q VGCC; paraneoplastic SCLC common	C. botulinum toxin

Drugs Affecting NMJ Testing

Pyridostigmine: masks decrement; hold for 12+ hours before testing if possible.
Aminoglycosides: can cause NMJ blockade; document and consider effect on results.
Magnesium: NMJ blockade; affects results.
D-penicillamine: can induce MG.
Botulinum toxin (therapeutic): can affect distant muscles via diffusion.

🔍 Did You Know?

Single-fiber EMG is the most sensitive electrodiagnostic test for myasthenia gravis, with sensitivity of approximately 95% for both generalized and ocular MG — substantially higher than acetylcholine receptor antibody testing (~85% in generalized, ~50% in ocular) or repetitive nerve stimulation (~50–70%). This makes SFEMG particularly valuable in patients with classic MG presentation but negative AChR antibodies (so-called “seronegative MG”) and in ocular-only MG. The test is technically demanding — requiring specialized needles, expertise to interpret jitter, and patient cooperation — and time-consuming (often 1–2 hours per study). But for the patient with fatigable ptosis, diplopia, or proximal weakness whose initial workup is negative, SFEMG can establish the diagnosis when other testing has failed. The clinical implications are profound: a patient with classic MG symptoms and negative AChR antibodies should have SFEMG considered before MG is ruled out. Conversely, in established generalized MG with positive AChR antibodies, SFEMG is often not needed for diagnosis but can quantify NMJ dysfunction for monitoring. For practicing neurologists, the take-home is that “negative MG workup” should not end the evaluation when clinical suspicion remains high — referral to a specialized electrodiagnostic laboratory for SFEMG can change the diagnosis. The same principle applies to MuSK-positive MG (RNS often normal due to different pathophysiology) where SFEMG complements antibody testing.

Pitfalls and Pearls

RNS slow (2–5 Hz): decrement >10% = postsynaptic NMJ disease (MG).
RNS fast (20–50 Hz) or post-exercise: facilitation >100% = presynaptic NMJ disease (LEMS).
MG: decrement on slow RNS; SFEMG most sensitive (~95%).
LEMS: small baseline CMAP + facilitation; search for SCLC.
Botulism: similar to LEMS but with descending paralysis; SFEMG also abnormal.
SFEMG: measures jitter (MCD) and blocking; specialized needle; expertise required.
Normal MCD: typically <30–50 μs depending on muscle.
Stimulation SFEMG: alternative for children/uncooperative patients.
RNS technical: warm to 32°C; supramaximal stim; patient relaxed.
Pyridostigmine masks decrement: hold 12+ hours before RNS if possible.
AChR antibodies: positive in ~85% generalized, ~50% ocular MG.
MuSK antibodies: ~10% of generalized; particularly bulbar; RNS often normal.
P/Q VGCC antibodies: LEMS; check for SCLC.
Anti-cholinesterase test (Tensilon): less commonly used now.
Ice pack test: ptosis improves with cold; bedside MG test.
Specificity of SFEMG: lower; abnormalities in chronic neurogenic disease too.
NMJ testing always includes RNS + EMG + SFEMG when clinically warranted.

References

Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
Stalberg E, Trontelj JV. Single Fiber Electromyography: Studies in Healthy and Diseased Muscle. 2nd ed. Raven Press; 1994.
Sanders DB, Stalberg EV. AAEM minimonograph #25: single-fiber electromyography. Muscle Nerve. 1996;19(9):1069-1083.
Howard JF Jr. Electrodiagnosis of disorders of neuromuscular transmission. Phys Med Rehabil Clin N Am. 2013;24(1):169-192.
Titulaer MJ, Lang B, Verschuuren JJ. Lambert-Eaton myasthenic syndrome: from clinical characteristics to therapeutic strategies. Lancet Neurol. 2011;10(12):1098-1107.
Sanders DB, Wolfe GI, Benatar M, et al. International consensus guidance for management of myasthenia gravis. Neurology. 2016;87(4):419-425.