Is Fever a Symptom of Multiple Sclerosis, and How Does Fever Affect Relapses vs. Pseudo‑Relapses?

Fever itself is not considered a core symptom caused directly by multiple sclerosis (MS). Instead, fever (and any rise in body temperature) can temporarily worsen existing MS symptoms, a phenomenon often called a “pseudo‑relapse.” ^[1] These temperature‑related symptom flares are typically reversible once the body cools and are not viewed as true disease relapses. ^[1]

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Fever vs. MS Symptoms

• Fever is generally due to another cause, most commonly infections, and not a direct manifestation of MS. ^[2] However, when body temperature rises even slightly MS symptoms can temporarily get worse. ^[1]
• Heat exposure, hot baths, sun, and stress can similarly trigger temporary worsening, as can fever from infections. ^[2] This temporary worsening is distinct from a true relapse because it does not reflect new inflammation in the central nervous system. ^[1]

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What Is a Pseudo‑Relapse?

• A pseudo‑relapse is a short‑term worsening of prior MS symptoms provoked by stressors like fever, heat, or infection. ^[1] These events typically improve after the trigger (e.g., fever) is treated and body temperature normalizes. ^[1]
• In children and adults, heat, fever, infection, fatigue, and stress can increase symptom severity without constituting a relapse. ^[3] Day‑to‑day variability in symptom intensity is common and does not necessarily indicate new disease activity. ^[3]

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True Relapse vs. Pseudo‑Relapse

• True MS relapse (attack) refers to new or clearly worsening neurological symptoms that persist for more than 24 hours and are not explained by fever, infection, or heat. ^[3] In contrast, pseudo‑relapses are transient and linked to external triggers like elevated temperature. ^[1]
• Small increases in body temperature can temporarily worsen MS symptoms, but this is not a true relapse and does not cause permanent damage. ^[1] Heat itself does not cause an MS attack; the worsening resolves when cooling or after the trigger is managed. ^[4]

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Why Does Heat or Fever Worsen Symptoms?

• The classic “Uhthoff’s phenomenon” describes transient worsening of neurological function (often vision) in MS when body temperature rises with exercise, fever, or hot environments. ^[5] This is thought to occur because heat alters conduction in demyelinated nerve fibers (affected by MS), making existing symptoms more noticeable until temperature normalizes. ^[5]
• Historical studies show most people with MS experience increased neurological signs during induced hyperthermia, underscoring how sensitive demyelinated axons are to temperature changes. ^[6] These changes are typically reversible, though responses can vary from person to person. ^[6]

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Practical Guidance During Fever

• Treat the cause of the fever and lower body temperature to help symptoms settle (e.g., appropriate antipyretics if advised and hydration). ^[2] Cooling strategies keeping the environment cool, avoiding hot baths, using cooling devices can help reduce temporary symptom flares. ^[7]
• Because fever often signals infection, clinicians will look for and treat underlying infections, which commonly drive pseudo‑relapses. ^[2] Cooling and managing infections generally restore baseline function without needing relapse‑specific therapy. ^[1]

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When to Seek Medical Care

• If you experience new neurological symptoms or a clear worsening that lasts more than 24 hours without a fever or infection, it may represent a true relapse and warrants medical evaluation. ^[3] If symptoms worsen during a fever or infection, address the trigger first; persistent symptoms after recovery should be reassessed. ^[1]

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Key Takeaways

• Fever is not a primary symptom caused by MS; it usually reflects an infection or other condition. ^[2]
• Even mild temperature increases can temporarily worsen MS symptoms (pseudo‑relapse) and typically improve with cooling and treatment of the fever. ^{[1] [4]}
• True relapses involve new or significantly worsened neurological symptoms lasting more than 24 hours and are not due to fever, infection, or heat. ^[3]

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Quick Comparison Table

Feature	Pseudo‑Relapse	True Relapse
Trigger	Heat, fever, infection, stress	Immune‑mediated CNS inflammation
Symptom pattern	Temporary worsening of existing symptoms	New or markedly worsened symptoms
Duration	Short‑term; improves after cooling or treating trigger	>24 hours, persists without fever/infection
Tissue damage	No evidence of new damage	Potential new inflammatory activity
Management focus	Cool down, treat fever/infection	Medical evaluation; may need relapse therapy

Heat and fever commonly intensify MS symptoms briefly and do not indicate new disease activity, while a true relapse persists beyond 24 hours without temperature‑related triggers. ^{[1] [3] [4] [2]}