Fibromyalgia (FM) is a chronic condition characterized primarily by widespread musculoskeletal pain, profound fatigue, sleep disturbances, and cognitive difficulties. These symptoms severely impact daily life. A common question among those affected is whether FM can directly cause a low-grade fever. While the medical community does not generally consider fibromyalgia an inflammatory disease that generates a true fever, the experience of feeling feverish or having temperature instability is widely reported. This article explores the physiological reasons for this sensation and clarifies the important distinction between subjective feelings and a clinically measured fever.
Identifying Core Fibromyalgia Symptoms
The defining characteristic of fibromyalgia is chronic, widespread pain affecting multiple body regions for at least three months. This pain is often described as a constant, dull ache, though patients may experience periods of heightened severity. To aid in diagnosis, clinicians use the Widespread Pain Index (WPI), which scores the number of painful areas out of 19 specified body regions.
Diagnosis also relies on the Symptom Severity Scale (SSS), which assesses the severity of other common symptoms. These include chronic fatigue and unrefreshed sleep, meaning waking up tired even after a full night’s rest. Cognitive issues, often called “fibro fog,” involve problems with memory, concentration, and processing speed. The current diagnostic criteria use a combination of the WPI and SSS scores.
The Link Between Fibromyalgia and Body Temperature
The medical consensus is that fibromyalgia does not directly cause a sustained, clinically measurable fever because it is not an inflammatory disease, unlike conditions such as rheumatoid arthritis. A true fever is defined by an oral temperature of 100.4°F (38°C) or higher, generally indicating the body is fighting an infection. A low-grade fever (LGF) is a persistent elevation between 99.6°F and 100.4°F.
Although a true, sustained LGF is not an established symptom of FM, patients frequently report a subjective feeling of being feverish or experiencing temperature fluctuations. This feeling can include abnormal sweating, sudden chills, or an internal heat that does not register on a thermometer. This temperature instability is more common than a sustained fever and should not be attributed to a fibromyalgia diagnosis.
Explaining Autonomic Nervous System Dysfunction
The subjective temperature instability experienced by people with fibromyalgia is often attributed to dysregulation of the Autonomic Nervous System (ANS). The ANS is the part of the nervous system that controls involuntary bodily functions, including heart rate, blood pressure, digestion, and crucially, thermoregulation. This system operates automatically, managing the body’s internal environment without conscious thought.
The ANS consists of the sympathetic and parasympathetic branches, which ideally maintain a constant internal balance. Dysfunction in this system, particularly a dominance of the sympathetic, or “fight or flight,” branch, can disrupt the body’s ability to maintain a steady core temperature. The area of the brain responsible for regulating temperature, the hypothalamus, may become hypersensitive or dysregulated. This can lead to inappropriate responses, such as the body attempting to cool itself with excessive sweating or causing sudden chills when the external temperature has not significantly changed.
When to Consult a Doctor About a Low Grade Fever
Since fibromyalgia itself is not the cause of a high or sustained fever, any persistent or rising low-grade fever must be investigated by a physician. A fever is a clear physical sign that the body is reacting to an internal process, and its presence requires a differential diagnosis to rule out other medical conditions. Ignoring a measured fever, even a low-grade one, can lead to a delay in diagnosing a treatable underlying problem.
A persistent LGF may be a sign of a chronic infection, such as tuberculosis or endocarditis, or the initial manifestation of an autoimmune disorder. Conditions like systemic lupus erythematosus or rheumatoid arthritis can coexist with fibromyalgia and are known causes of chronic low-grade fever. Immediate medical consultation is required if the fever climbs above 101°F, is accompanied by a new rash, or is unresponsive to over-the-counter fever reducers.