Fibromyalgia is a chronic condition in which the nervous system amplifies pain signals, causing widespread body pain, deep fatigue, and cognitive difficulties. It affects about 4 million adults in the United States, roughly 2% of the adult population, and women are twice as likely to develop it as men. Despite decades of research, no single cause has been identified. Instead, fibromyalgia appears to result from a combination of nervous system changes, genetic vulnerability, stress responses, and environmental triggers that together shift the body into a state of heightened pain sensitivity.
How Fibromyalgia Is Diagnosed
There is no blood test or imaging scan for fibromyalgia. Diagnosis relies on a structured assessment of where you hurt and how severely other symptoms affect your daily life. Doctors used to check 18 specific “tender points” on the body, but that approach was replaced in 2010 with a questionnaire-based system that better captures the full picture of the condition.
The current criteria, revised in 2016 by the American College of Rheumatology, use two scores. The first is a widespread pain index that asks whether you’ve had pain in 19 different body areas over the past week, including both shoulders, both hips, both upper and lower arms, both upper and lower legs, the neck, the upper and lower back, the chest, the abdomen, and both sides of the jaw. The second is a symptom severity scale that rates the impact of fatigue, unrefreshing sleep, and cognitive problems on a 0 to 3 scale, then adds points for headaches, abdominal pain or cramping, and depression.
To meet the diagnostic threshold, your pain must be present in at least four of five body regions, and your combined scores must be high enough to reflect a significant burden. Symptoms also need to have persisted for at least three months. Importantly, a fibromyalgia diagnosis doesn’t rule out other conditions. You can have fibromyalgia alongside arthritis, lupus, or other pain-causing illnesses.
Central Sensitization: The Core Problem
The leading explanation for fibromyalgia centers on a process called central sensitization, in which the brain and spinal cord become overly responsive to pain signals. In people with fibromyalgia, the nervous system essentially turns up the volume on incoming signals, so stimuli that wouldn’t normally register as painful (like light pressure on a muscle) start to hurt. This produces two hallmark features: hyperalgesia, where mildly painful things feel much worse than they should, and allodynia, where things that aren’t painful at all, like the pressure of clothing or a gentle touch, cause genuine pain.
Several measurable changes in brain chemistry accompany this process. Substance P, a chemical messenger involved in transmitting pain signals in the spinal cord, is found at levels up to three times higher than normal in the spinal fluid of people with fibromyalgia. Glutamate, another excitatory brain chemical, is also elevated. Together, these substances lower the threshold at which spinal cord neurons fire pain signals to the brain, making the entire system more reactive. At the same time, serotonin and norepinephrine, two chemicals the brain uses to dampen and regulate pain signals coming up from the body, are lower than normal. The result is a nervous system that amplifies pain more aggressively while simultaneously losing its ability to turn pain down.
This isn’t just a chemical imbalance. Immune cells in the brain called microglia also play a role. When activated, these cells release inflammatory molecules that keep neurons in a hyperactive state, reinforcing the cycle of sensitization. This neuroinflammatory process helps explain why fibromyalgia pain feels so persistent and widespread rather than localized to one injury site.
Nerve Damage in the Skin
For years, fibromyalgia was considered a purely central nervous system problem, with no detectable abnormality in the body’s peripheral nerves. That picture has shifted. Skin biopsy studies now show that 45% to 50% of fibromyalgia patients have small fiber neuropathy, a condition where the tiny nerve endings in the skin are damaged or reduced in number. These are the nerve fibers responsible for sensing temperature, touch, and pain.
This finding is significant because it suggests that for a substantial portion of people with fibromyalgia, the pain isn’t only a problem of the brain misinterpreting signals. There may also be abnormal signaling from the nerves themselves. Small fiber neuropathy can contribute to widespread pain, fatigue, cognitive dysfunction, and problems with automatic body functions like heart rate and digestion, all of which overlap heavily with fibromyalgia symptoms.
Genetic Vulnerability
Fibromyalgia runs in families. Researchers have observed consistent familial clustering, meaning that if a close relative has the condition, your own risk increases. This pattern points to a genetic component, though no single gene causes fibromyalgia on its own.
One of the most studied genetic links involves a gene called COMT, which produces an enzyme responsible for breaking down stress hormones like adrenaline and noradrenaline. A common variation in this gene, where the amino acid valine is swapped for methionine at a specific position, reduces the enzyme’s activity by about fourfold. People who carry two copies of this slower variant produce an enzyme that is much less efficient at clearing stress chemicals from the brain. In one study, this genotype was found exclusively in fibromyalgia patients and not in healthy controls. The buildup of stress hormones that results from sluggish COMT activity may prime the nervous system for the kind of heightened sensitivity that defines fibromyalgia.
A Disrupted Stress Response
Your body’s main stress management system involves a communication loop between the brain and the adrenal glands, which produce cortisol, the hormone that helps regulate inflammation, energy, and the body’s response to threats. In fibromyalgia, this system appears to be underactive. Multiple studies have found that people with the condition have lower baseline cortisol levels and reduced 24-hour cortisol output compared to healthy individuals.
Low cortisol is a plausible contributor to fibromyalgia symptoms because other low-cortisol conditions share many of the same features: persistent fatigue, excessive sleepiness, and muscle and joint pain. At the same time, the picture is complicated. Some fibromyalgia patients show a loss of the normal daily cortisol rhythm, where levels should peak in the morning and taper off at night, and about 35% fail a standard cortisol suppression test. This suggests the stress response isn’t simply too low; it’s dysregulated, sometimes behaving as if cortisol is too high while other markers point to deficiency.
The practical effect is a body that responds poorly to physical and emotional stress. Rather than mounting a proportional response and then returning to baseline, the system stays off-balance, which may feed into the fatigue, sleep disruption, and pain amplification that characterize the condition.
Triggers That Set It Off
Many people with fibromyalgia can trace the onset of their symptoms to a specific event. Common triggers include physical trauma such as car accidents or surgeries, severe emotional stress, repetitive injuries from sports or physically demanding jobs, and viral infections. Not everyone has an identifiable trigger, but when one exists, it often involves a significant demand on the body’s stress and immune systems.
These events don’t cause fibromyalgia in the way a virus causes the flu. Instead, they appear to act as a tipping point in someone whose nervous system is already genetically or biologically primed for sensitization. The trigger overwhelms the body’s ability to recalibrate, and the heightened pain state that would normally resolve after healing instead becomes self-sustaining.
What Fibromyalgia Feels Like Day to Day
The diagnostic criteria hint at the symptom profile, but they don’t fully convey the daily reality. The pain is typically widespread and persistent, described by many people as a deep ache or burning sensation that shifts location but never fully disappears. It affects both sides of the body and spans from the neck and shoulders down through the back, hips, and legs.
Fatigue in fibromyalgia is not ordinary tiredness. Many people wake up feeling as though they never slept, regardless of how many hours they spent in bed. This unrefreshing sleep is one of the most disabling aspects of the condition and is scored separately in the diagnostic assessment for good reason. Cognitive symptoms, often called “fibro fog,” involve difficulty concentrating, trouble finding words, and short-term memory lapses that can interfere with work and daily responsibilities. Headaches, abdominal cramping, and depression frequently accompany the core symptoms.
Because fibromyalgia involves the central nervous system rather than visible tissue damage, standard lab work and imaging come back normal. This has historically led to dismissal or misdiagnosis, though growing recognition of the neurological basis of the condition has improved diagnostic accuracy. The current questionnaire-based approach allows patients to report the severity of their own symptoms directly, which reduces the risk of underdiagnosis that occurred when assessment depended entirely on a physician’s interpretation.