A functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.

Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or, occasionally, unimportant for treatment purposes.

Status

Whether a given medical condition is termed a functional disorder depends in part on the state of knowledge. Some diseases, including epilepsy

Epilepsy

Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

, schizophrenia

Schizophrenia

Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

, and migraine

Migraine

Migraine is a chronic neurological disorder characterized by moderate to severe headaches, and nausea...

 headaches were once considered functional disorders, but are no longer generally classified that way.

Current Examples

Tension headache

Tension headache

A tension headache is the most common type of primary headache. The pain can radiate from the neck, back, eyes, or other muscle groups in the body. Tension-type headaches account for nearly 90% of all headaches...

Irritable bowel syndrome

Irritable bowel syndrome

Irritable bowel syndrome is a diagnosis of exclusion. It is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any detectable organic cause. In some cases, the symptoms are relieved by bowel movements...

Chronic fatigue syndrome

Chronic fatigue syndrome

Chronic fatigue syndrome is the most common name used to designate a significantly debilitating medical disorder or group of disorders generally defined by persistent fatigue accompanied by other specific symptoms for a minimum of six months, not due to ongoing exertion, not substantially...