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Uhthoff as Positive Sign – Axonal Preservation — SCE Neurology MCQ

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HardMultiple Sclerosis & CNS InflammationUhthoff as Positive Sign – Axonal PreservationSCE Neurology

A 40-year-old man with known MS and a history of optic neuritis notices that when he exercises vigorously, his vision in the previously affected eye temporarily worsens. This resolves within 30 minutes of resting and cooling. His neurologist explains that this is a positive prognostic sign. Why?

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Reveal the answer and explanation

Correct answer: B — Uhthoff phenomenon in the optic nerve indicates that the demyelinated fibres are still intact and capable of conducting — the temporary conduction block from heat shows that axons are present but vulnerable to temperature; if the axons had been completely destroyed, there would be no worsening with heat (nothing left to fail)

This is a subtle but clinically important point: Uhthoff phenomenon paradoxically indicates axonal preservation. If axons had been completely destroyed (Wallerian degeneration), heating would not produce temporary worsening — there would be a fixed deficit regardless of temperature. The fact that heat causes transient worsening proves that demyelinated but intact axons are still conducting under normal conditions and failing only when heated. A: No new damage occurs. C: Retina is not involved. D: No new inflammation — this is a physiological phenomenon. E: The deficit is in the optic nerve, not visual cortex.

Reference: Clinical Neurophysiology; Uhthoff Phenomenon Physiology