Demyelinating Neuropathy

F-wave Temporal dispersion Motor conduction block Prolonged distal latency

F-WAVES

From: M. Al-Lozi F-Waves: Varied & Prolonged Latencies in CIDP

F-waves: General Late motor response: Occurs after CMAP Physiology Mechanism: Pure motor, single axon circuitry 1st phase: Antidromic impulse along motor axon 2nd phase: Depolarization of cell body 3rd phase Orthodromic impulse along motor axon Occurs after refractory period ends No synapses involved Stimulus needed: Maximal Latencies: Distal stimulation Arms: 25 to 32 ms Legs: 45 to 56 ms Varied from stimulus to stimulus Chronodispersion: Arms 4 ms; Legs 6 ms Longer: Increased patient height Pre-term Infants 1 Increasing gestational age: Shorter latency Amplitude: 1% to 5% of CMAP F-wave persistence Definition: # F-waves/# Stimulations Normal: Usual 80% to 100%; Always > 40% Lowest in: Peroneal nerve (40%) Other features May be difficult to elicit in some normals Reduced or absent with Sleep or sedation Reduced CMAP amplitude Increased with Jendrassik maneuver (Reinforcement) Utility Most useful: Nerves to distal extremities Useful for: Conduction in proximal areas of axons F-waves: Disorders Demyelinating neuropathy Number: Reduced Impersistence Latencies: Prolonged & Variable Demyelinating GBS Loss may be earliest, or only, sign of demyelination SBMA: Giant F-waves Also see: H-reflex

TEMPORAL DISPERSION

Temporal dispersion (TD): CMAPs Proximal stimulation (Lower trace) Amplitude: Reduced Phases: Multiple Duration: Prolonged More proximal stimulation: More TD Pure motor TD: Area unchanged or Reduced Reduction may relate to phase cancellation Cause: Varied conduction velocity of motor axons Normal: CMAPs from Proximal & Distal stimulation Similar configuration

---

Return to Neuromuscular Home Page
Return to CIDP

References
1. Childs Nerv Syst 2018 Apr 10

4/20/2018