Functional somatic syndrome is characterized by ambiguous, non-specific symptoms that appear in otherwise-healthy populations. Overlap exists in different syndromes, including gastrointestinal issues, pain, fatigue, cognitive difficulties, and sleep difficulties. Some have proposed to group symptoms into clusters or into one general functional somatic disorder given the finding of correlations between symptoms and underlying etiologies.

Functional somatic syndrome is a term used to refer to physical symptoms that are poorly explained. It encompass disorders such as chronic fatigue syndrome, fibromyalgia, chronic widespread pain, temporomandibular disorder, irritable bowel syndrome, lower back pain, tension headache, atypical face pain, non-cardiac chest pain, insomnia, palpitation, dyspepsia, and dizziness. General overlap exists between this term, somatization, and somatoform.

Irritable Bowel Syndrome (IBS),

Fibromyalgia (FMS),

Chronic Fatigue Syndrome (CFS),

Chronic Pelvic Pain (CPP),

Interstitial Cystitis (IC),

Temporomandibular Joint Pain (TMJ), Functional Neurological Symptom Disorder (FNsD),

Non-Cardiac Chest Pain (NCCP),

Post-Traumatic Stress Disorder (PTSD),

Dysuria (Pain On Urination),

and Multiple Chemical Sensitivity

There are a great number of symptoms experienced by those with a functional neurological disorder. It is important to note that the symptoms experienced by those with an FND are very real , and should not be confused with malingering, factitious disorders, or Munchausen syndrome. At the same time, the origin of symptoms is complex since it can be associated with physical injury, severe psychological trauma (conversion disorder), and idiopathic neurological dysfunction. The core symptoms are those of motor or sensory function or episodes of altered awareness

- Limb weakness or paralysis

- Blackouts (also called dissociative or non-epileptic seizures/attacks) – these may look like epileptic seizures or faints

- Movement disorders including tremors, dystonia (spasms), myoclonus (jerky movements)

- Visual symptoms including loss of vision or double vision

- Speech symptoms including dysphonia (whispering speech), slurred or stuttering speech

- Sensory disturbance including hemisensory syndrome (altered sensation down one side of the body)

In the DSM-5 the disorder has been renamed somatic symptom disorder (SSD), and includes SSD with predominantly somatic complaints (previously referred to as somatization disorder), and SSD with pain features (previously known as pain disorder).

Functional neurological disorders are a common problem, and are the second most common reason for a neurological outpatient visit after headache/migraine. Dissociative (non-epileptic) seizures account for about 1 in 7 referrals to neurologists after an initial seizure, and functional weakness has a similar prevalence to multiple sclerosis.

Whether a given medical condition is termed a "functional disorder" depends in part on the state of knowledge. Some diseases, including epilepsy, schizophrenia, and migraine headaches were once considered functional disorders, but are no longer generally classified that way.

Somatization disorder (also Briquet's syndrome) is a mental disorder characterized by recurring, multiple, and current, clinically significant complaints about somatic symptoms, although it is no longer considered a clinical diagnosis. It was recognized in the DSM-IV-TR classification system, but in the latest version DSM-5, it was combined with undifferentiated somatoform disorder to become "somatic symptom disorder", a diagnosis which no longer requires a specific number of somatic symptoms. ICD-10, the latest version of the International Statistical Classification of Diseases and Related Health Problems, still includes somatization syndrome.

Somatic symptom disorders are a group of disorders, all of which fit the definition of physical symptoms similar to those observed in physical disease or injury for which there is no identifiable physical cause. As such, they are a diagnosis of exclusion. Somatic symptoms may be generalized in four major medical categories: neurological, cardiac, pain, and gastrointestinal somatic symptoms.

BFS is classified in the fourth revision of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV) as a culture-bound syndrome. Individuals with "symptoms" of brain fag must be differentiated from those with the "syndrome" according to the Brain Fag Syndrome Scale (BFSS); Ola "et al" said it would not be "surpris[ing] if BFS was called an equivalent of either depression or anxiety".

Brain fag syndrome (BFS) was described by RH Prince in 1960. It was first discovered in Nigeria, describing high school and university students with symptoms including somatic, sleep-related and cognitive complaints, head and neck pains, difficulty in concentrating and retaining information, and eye pain. It is caused by excessive external pressure to be successful among the young.

The defining symptoms of fibromyalgia are chronic widespread pain, fatigue, sleep disturbance, and heightened pain in response to tactile pressure (allodynia). Other symptoms may include tingling of the skin (paresthesias), prolonged muscle spasms, weakness in the limbs, nerve pain, muscle twitching, palpitations, and functional bowel disturbances.

Many people experience cognitive dysfunction (known as "fibrofog"), which may be characterized by impaired concentration, problems with short and long-term memory, short-term memory consolidation, impaired speed of performance, inability to multi-task, cognitive overload, and diminished attention span. Fibromyalgia is often associated with anxiety and depressive symptoms.

Other symptoms often attributed to fibromyalgia that may be due to a comorbid disorder include myofascial pain syndrome, also referred to as chronic myofascial pain, diffuse non-dermatomal paresthesias, functional bowel disturbances and irritable bowel syndrome, genitourinary symptoms and interstitial cystitis, dermatological disorders, headaches, myoclonic twitches, and symptomatic hypoglycemia. Although fibromyalgia is classified based on the presence of chronic widespread pain, pain may also be localized in areas such as the shoulders, neck, low back, hips, or other areas. Many sufferers also experience varying degrees of myofascial pain and have high rates of comorbid temporomandibular joint dysfunction. 20–30% of people with rheumatoid arthritis and systemic lupus erythematosus may also have fibromyalgia.

Fibromyalgia is classed as a disorder of pain processing due to abnormalities in how pain signals are processed in the central nervous system. The American College of Rheumatology classifies fibromyalgia as being a functional somatic syndrome. The expert committee of the European League Against Rheumatism classifies fibromyalgia as a neurobiological disorder and as a result exclusively give pharmacotherapy their highest level of support. The International Classification of Diseases (ICD-10) lists fibromyalgia as a diagnosable disease under "Diseases of the musculoskeletal system and connective tissue," under the code M79-7, and states that fibromyalgia syndrome should be classified as a functional somatic syndrome rather than a mental disorder. Although mental disorders and some physical disorders commonly are co-morbid with fibromyalgia – especially anxiety, depression, irritable bowel syndrome, and chronic fatigue syndrome – the ICD states that these should be diagnosed separately.

Differences in psychological and autonomic nervous system profiles among affected individuals may indicate the existence of fibromyalgia subtypes. A 2007 review divides individuals with fibromyalgia into four groups as well as "mixed types":

1. "extreme sensitivity to pain but no associated psychiatric conditions" (may respond to medications that block the 5-HT3 receptor)

2. "fibromyalgia and comorbid, pain-related depression" (may respond to antidepressants)

3. "depression with concomitant fibromyalgia syndrome" (may respond to antidepressants)

4. "fibromyalgia due to somatization" (may respond to psychotherapy)

People may vomit or retch six to twelve times an hour, and an episode may last from a few hours to well over three weeks, and in some cases months, with a median episode duration of 41 hours. Acid, bile and (if the vomiting is severe) blood may be vomited. Some sufferers will ingest water to reduce the irritation of bile and acid on the esophagus during emesis. Between episodes the sufferer is usually normal and healthy otherwise but can be in a weak state of fatigue or suffer from muscle pain. In approximately half of cases the attacks, or episodes, occur in a time-related manner. Each attack is stereotypical; that is, in any given individual, the timing, frequency and severity of attacks is similar.

Episodes may happen every few days, every few weeks or every few months. For some there is not a pattern in time that can be recognized. Some sufferers have a warning of an attack. They may experience a prodrome, usually intense nausea and pallor, heightened sensitivity, especially to light, though sensitivity to smell, sound, pressure, and temperature, as well as oncoming muscle pain and fatigue, are also reported by some patients. The majority of sufferers can identify triggers that may precipitate an attack. The most common are various foods, infections (such as colds), menstruation, extreme physical exertion, lack of sleep, and psychological stresses both positive and negative.

A sufferer may also be light-sensitive (photophobic) during an attack, as well as sound-sensitive (phonophobic) and, less frequently, temperature- or pressure-sensitive. Some sufferers also have a strong urge to bathe in warm or cold water. Some sufferers report that they experience a restless sensation or stinging pain along the spine, hands, and feet followed by weakness in both legs. Some of these symptoms may be due to dehydration rather than the underlying cause of CVS.

Masked depression (MD) was a proposed form of atypical depression in which somatic symptoms or behavioural disturbances dominate the clinical picture and disguise the underlying affective disorder. The concept is not currently supported by the mental health profession.

This proposed subtype is characterized by depression occurring as a consequence of psychic, somatic or external stressors. Praag theorizes that if the individual's stress threshold is insufficient or overly sensitive, a prolonged stress response via the HPA axis can trigger anxiety followed by significant lowering of mood, the mechanisms of which act to reenforce each other, keeping the sufferer in a perpetual state of stress arousal, coupled with dysphoria; and that this occurs as a result of the stress response inhibiting the normal expression of neurotransmitters associated with wellbeing and pleasure. He proposes that sufferers may experience panic attacks, depersonalization and other psychic and somatic symptoms common to both anxiety and depression disorders.

According to van Praag, in Anxiety/Aggression-Driven Depression "...dysregulation of anxiety and/or aggression are primordial and mood lowering is a derivative phenomenon."

According to the DSM-IV-TR, which classifies Ganser syndrome as a dissociative disorder, it is "the giving of approximate answers to questions (e.g. '2 plus 2 equals 5' when not associated with dissociative amnesia or dissociative fugue)."

Diagnosing Ganser syndrome is very challenging, because some measure of dishonesty is involved and because it is very rare.

Usually when giving wrong answers, they are only slightly off, showing that the individual understood the question. For instance, when asked how many legs a horse has they might say, "five." Although subjects appear confused in their answers, in other respects they appear to understand their surroundings.

The original description by Sigbert Josef Maria Ganser in 1898 pointed out their hysterical twilight state. They may also describe hallucinations which are usually more than those in schizophrenia. They may also have disorders of sensation similar to those in conversion disorder. They may be inattentive or drowsy.

Some workers believe there is a genuine psychosis underlying this, others believe it is a dissociative disorder, while still others believe it is the result of malingering. Over the years, opinions have seemed to move from the last view more towards the first.

Ganser syndrome is currently classified under dissociative disorders, to which it moved in the DSM IV from the factitious disorders.

Anxiety/aggression-driven depression (also known as 5-HT related depression) is a proposed subtype of Major depressive disorder first proposed by the Dutch psychiatrist Herman M. van Praag in 1996. Van Praag has continued to write on this topic in subsequent academic articles.

Cyclic vomiting syndrome (US English) or cyclical vomiting syndrome (UK English) (CVS) is a chronic functional condition of unknown cause characterised by recurring attacks of intense nausea, vomiting, and sometimes abdominal pain, headaches, or migraines. CVS typically develops during childhood, usually between ages 3 and 7; although it often remits during adolescence, it can persist into adult life.

A somatic symptom disorder, formerly known as a somatoform disorder, is any mental disorder which manifests as physical symptoms that suggest illness or injury, but which cannot be explained fully by a general medical condition or by the direct effect of a substance, and are not attributable to another mental disorder (e.g., panic disorder). Somatic symptom disorders, as a group, are included in a number of diagnostic schemes of mental illness, including the "Diagnostic and Statistical Manual of Mental Disorders". (Before DSM-5 this disorder was split into "somatization disorder" and "undifferentiated somatoform disorder".)

In people who have been diagnosed with a somatic symptom disorder, medical test results are either normal or do not explain the person's symptoms, and history and physical examination do not indicate the presence of a known medical condition that could cause them, though it is important to note that the DSM-5 cautions that this alone is not sufficient for diagnosis. The patient must also be excessively worried about their symptoms, and this worry must be judged to be out of proportion to the severity of the physical complaints themselves. A diagnosis of somatic symptom disorder requires that the subject have recurring somatic complaints for at least six months.

Symptoms are sometimes similar to those of other illnesses and may last for years. Usually, the symptoms begin appearing during adolescence, and patients are diagnosed before the age of 30 years. Symptoms may occur across cultures and gender. Other common symptoms include anxiety and depression. However, since anxiety and depression are also very common in persons with confirmed medical illnesses, it remains possible that such symptoms are a consequence of the physical impairment, rather than a cause. Somatic symptom disorders are not the result of conscious malingering (fabricating or exaggerating symptoms for secondary motives) or factitious disorders (deliberately producing, feigning, or exaggerating symptoms). Somatic symptom disorder is difficult to diagnose and treat. Some advocates of the diagnosis believe this is because proper diagnosis and treatment requires psychiatrists to work with neurologists on patients with this disorder.

Somatic manifestations of MD are distinguished by an extreme diversity and include headaches, back pain, abdominal pain etc. Pathological behaviour masking depression may take the form of compulsive gambling, compulsive work, changes in arousal or orgasmic function, decreased libido or, on the contrary, impulsive sexual behaviour, alcoholism, drug addiction and more.

In medicine and medical anthropology, a culture-bound syndrome, culture-specific syndrome, or folk illness is a combination of psychiatric and somatic symptoms that are considered to be a recognizable disease only within a specific society or culture. There are no objective biochemical or structural alterations of body organs or functions, and the disease is not recognized in other cultures. The term "culture-bound syndrome" was included in the fourth version of the "Diagnostic and Statistical Manual of Mental Disorders" (American Psychiatric Association, 1994) which also includes a list of the most common culture-bound conditions (DSM-IV: Appendix I). Counterpart within the framework of ICD-10 () are the "culture-specific disorders" defined in Annex 2 of the "Diagnostic criteria for research".

More broadly, an epidemic that can be attributed to cultural behavior patterns or suggestion is sometimes referred to as a behavioral epidemic. As in the cases of drug or alcohol abuse or smoking, transmission can be determined by communal reinforcement as well as by person-to-person interactions. On etiological grounds, it can be difficult to distinguish the causal contribution of culture in disease from other environmental factors such as toxicity.

The onset of ORS may be sudden, where it usually follows after a precipitating event, or gradual.

The differential diagnosis for ORS may be complicated as the disorder shares features with other conditions. Consequently, ORS may be misdiagnosed as another medical or psychiatric condition and "vice versa".

The typical history of ORS involves a long delay while the person continues to believe there is a genuine odor. On average, a patient with ORS goes undiagnosed for about 8 years. Repeated consultation with multiple different non-psychiatric medical specialists ("doctor shopping") in an attempt to have their non-existent body odor treated is frequently reported. Individuals with ORS may present to dermatologists, gastroentrologists, otolaryngologists, dentists, proctologists, and gynecologists. Despite the absence of any clinically detectable odor, physicians and surgeons may embark on unnecessary investigations (e.g. gastroscopy), and treatments, including surgery such as thoracic sympathectomy, tonsillectomy, etc. Such treatments generally have no long-term effect on the individual's belief in an odor symptom. If non-psychiatric clinicians refuse to carry out treatment on the basis that there is no real odor and offer to refer them to a psychologist or psychiatrist, persons with ORS typically refuse and instead seek "a better" doctor/dentist.

Conversely, some have suggested that medical conditions which cause genuine odor may sometimes be misdiagnosed as ORS. There are a great many different medical conditions which are reported to potentially cause a genuine odor, and these are usually considered according to the origin of the odor, e.g. halitosis (bad breath), bromhidrosis (body odor), etc. These conditions are excluded before a diagnosis of ORS is made. Although there are a multitude of different publications on topics like halitosis, the symptom is still poorly understood and managed in practice. It is recognized that symptoms such as halitosis can be intermittent, and therefore may not be present at the time of the consultation, leading to misdiagnosis. Individuals with genuine odor symptoms may present with similar mindset and behavior to persons with ORS. For example, one otolaryngologist researcher noted "behavioral problems such as continuous occupation with oral hygiene issues, obsessive use of cosmetic breath freshening products such as mouthwashes, candies, chewing gums, and sprays, avoiding close contact with other people, and turning the head away during conversation" as part of what was termed ""skunk syndrome"" in patients with genuine halitosis secondary to chronic tonsillitis. Another author, writing about halitosis, noted that there are generally 3 types of person that complain of halitosis: those with above average odor, those with average or near-average odor who are oversensitive, and those with below average or no odor who believe they have offensive breath. Therefore, in persons with genuine odor complaints, the distress and concern may typically be out of proportion to the reality of the problem. Genuine halitosis has been described as a social barrier between the individual and friends, relatives, partners and colleagues, and may negatively alter self-esteem and quality of life. Similar psychosocial problems are reported in other conditions which cause genuine odor symptoms. In the literature on halitosis, emphasis is frequently placed on multiple consultations to reduce the risk of misdiagnosis, and also asking the individual to have a reliable confidant accompany them to the consultation who can confirm the reality of the reported symptom. ORS patients are unable to provide such confidants as they have no objective odor.

Various organic diseases may cause parosmias (distortion of the sense of smell). Also, since smell and taste are intimately linked senses, disorders of gustation (e.g. dysgeusia- taste dysfunction) can present as a complaint related to smell, and "vice versa". These conditions, collectively termed chemosensory dysfunctions, are many and varied, and they may trigger a person to complain of an odor than is not present, however the diagnostic criteria for ORS require the exclusion of any such causes. They include pathology of the right hemisphere of the brain, substance abuse, arteriovenous malformations in the brain, and temporal lobe epilepsy.

Social anxiety disorder (SAD) and ORS have some demographic and clinical similarities. Where the social anxiety and avoidance behavior is primarily focussed on concern about body odors, ORS is a more appropriate diagnosis than avoidant personality disorder or SAD. Body dismorphic disorder (BDD) has been described as the closest diagnosis in DSM-IV to ORS as both primarily focus on bodily symptoms. The defining difference between the two is that in BDD the preoccupation is with physical appearance not body odors. Similarly, where obsessive behaviors are directly and consistently related to body odors rather than anything else, ORS is a more appropriate diagnosis than obsessive compulsive disorder, in which obsessions are different and multiple over time.

ORS may be misdiagnosed as schizophrenia. About 13% of schizophrenics have olfactory hallucinations. Generally, schizophrenic hallucinations are perceived as having an imposed, external origin, while in ORS they are recognized as originating from the individual. The suggested diagnostic criteria mean that the possibility of ORS is negated by a diagnosis of schizophrenia in which persistent delusions of an offensive body odor and olfactory hallucinations are contributing features for criterion A. However, some reported ORS cases were presented as co-morbid. Indeed, some have suggested that ORS may in time transform into schizophrenia, but others state there is little evidence for this. Persons with ORS have none of the other criteria to qualify for a diagnosis of schizophrenia.

It has been suggested that various special investigations may be indicated to help rule out some of the above conditions. Depending upon the case, this might include neuroimaging, thyroid and adrenal hormone tests, and analysis of body fluids (e.g. blood) with gas chromatography.