Uhthoff Phenomenon – Mechanism — SCE Neurology MCQ

Correct answer: C — Increased body temperature slows or blocks conduction in demyelinated axons — even a 0.5°C rise can impair conduction because the safety factor for impulse propagation is already reduced in demyelinated fibres; cooling restores conduction

Uhthoff phenomenon occurs because demyelinated axons have a reduced safety factor for action potential propagation. Temperature increases (even 0.5°C) further slow or block conduction through the demyelinated segment by affecting sodium channel kinetics (faster inactivation at higher temperatures). This causes transient worsening of pre-existing neurological deficits. It is NOT a new attack — no new inflammation or demyelination occurs. Symptoms resolve completely with cooling. It was originally described with exercise but also occurs with hot baths, fever, hot weather, and hot meals. A: No new lesion. C: Not simple muscle fatigue. D: Not dehydration. E: Has a clear physiological basis.

Reference: NICE NG100 MS; Clinical Neurophysiology Principles