A syndrome is a set of medical signs and symptoms occurring together, constitutes a particular disease or disorder. The word derives from the Greek σύνδρομον, meaning "concurrence". In some instances, a syndrome is so closely linked with a pathogenesis or cause that the words "syndrome", "disease", and "disorder" end up being used interchangeably for them. This is especially true of inherited syndromes. For example, Down syndrome, Wolf–Hirschhorn syndrome, and Andersen syndrome are disorders with known pathogeneses, so each is more than just a set of signs and symptoms, despite the "syndrome" nomenclature. In other instances, a syndrome is not specific to only one disease. For example, toxic shock syndrome can be caused by various toxins; premotor syndrome can be caused by various brain lesions; and premenstrual syndrome is not a disease but simply a set of symptoms.

If an underlying genetic cause is suspected but not known, a condition may be referred to as a genetic association (often just "association" in context). By definition, an association indicates that the collection of signs and symptoms occurs in combination more frequently than would be likely by chance alone.

Syndromes are often named after the physician or group of physicians that discovered them or initially described the full clinical picture. Such eponymous syndrome names are examples of medical eponyms. Recently, there has been a shift towards naming conditions descriptively (by symptoms or underlying cause) rather than eponymously, but the eponymous syndrome names often persist in common usage.

Psychiatric syndromes often called "psychopathological syndromes" (psychopathology is a psychic dysfunction occurring in mental disorder, also it's the study of the origin, diagnosis, development, and treatment of mental disorders).

In Russia those psychopathological syndromes are used in modern clinical practice and described in psychiatric literature in the details: asthenic syndrome, obsessive syndrome, emotional syndromes (for example, manic syndrome, depressive syndrome), Cotard's syndrome, catatonic syndrome, hebephrenic syndrome, delusional and hallucinatory syndromes (for example, paranoid syndrome, paranoid-hallucinatory syndrome, Kandinsky-Clérambault's syndrome also known as syndrome of psychic automatism, hallucinosis), paraphrenic syndrome, psychopathic syndromes (includes all personality disorders), clouding of consciousness syndromes (for example, twilight clouding of consciousness, amential syndrome also known as amentia, delirious syndrome, stunned consciousness syndrome, oneiroid syndrome), hysteric syndrome, neurotic syndrome, Korsakoff's syndrome, hypochondriacal syndrome, paranoiac syndrome, senestopathic syndrome, encephalopathic syndrome.

There are some examples of the psychopathological syndromes used in modern Germany: psychoorganic syndrome, depressive syndrome, paranoid-hallucinatory syndrome, obsessive-compulsive syndrome, autonomic syndrome, hostility syndrome, manic syndrome, apathy syndrome.

Also well known Münchausen syndrom, Ganser syndrome, neuroleptic-induced deficit syndrome, olfactory reference syndrome.

The most common symptoms of Williams syndrome are heart defects and unusual facial features. Other symptoms include failure to gain weight appropriately in infancy (failure to thrive) and low muscle tone. Individuals with Williams syndrome tend to have widely spaced teeth, a long philtrum, and a flattened nasal bridge.

Most individuals with Williams syndrome are highly verbal relative to their IQ, and are overly sociable, having what has been described as a "cocktail party" type personality. Individuals with WS hyperfocus on the eyes of others in social engagements.

Pashayan syndrome also known as Pashayan–Prozansky Syndrome, and blepharo-naso-facial syndrome is a rare syndrome. Facial abnormalities characterise this syndrome as well as malformation of extremities. Specific characteristics would be a bulky, flattened nose, where the face has a mask like appearance and the ears are also malformed.

A subset of Pashayan syndrome has also been described, known as "cerebrofacioarticular syndrome", "Van Maldergem syndrome'" or "Van Maldergem–Wetzburger–Verloes syndrome". Similar symptoms are noted in these cases as in Pashayan syndrome.

The earliest observable symptoms of Williams syndrome include low birth weight, failure to thrive, trouble breastfeeding, nocturnal irritability and gastroesophageal reflux. Facial dysmorphies thought to be characteristic of the syndrome are also present early in development, as is heart murmur. Research on the development of the syndrome suggest that congenital heart disease is typically present at an early age, often at the infant's first pediatric appointment. Heart problems in infancy often lead to the initial diagnosis of Williams syndrome.

Developmental delays are present in most cases of Williams syndrome, and include delay of language abilities and delayed motor skill development. Individuals with Williams syndrome develop language abilities quite late relative to other children, with the child's first word often occurring as late as three years of age. Language abilities are often observed to be deficient until adolescence, in terms of semantics, morphology, and phonology, though not in vocabulary.

Williams syndrome is also marked by a delay in development of motor skills. Infants with Williams develop the ability to lift their heads and sit without support months later than typically developing children. These delays continue into childhood, where patients with Williams syndrome are delayed in learning to walk. In young children, the observed motor delay is around five to six months, though some research suggests that children with Williams syndrome have a delay in development that becomes more extreme with age. Children with motor delays as a result of Williams syndrome are particularly behind in development of coordination, fine motor skills such as writing and drawing, response time, and strength and dexterity of the arms. Impaired motor ability persists (and possibly worsens) as children with Williams syndrome reach adolescence.

Adults and adolescents with Williams syndrome typically achieve a below-average height and weight, compared with non-affected populations. As individuals with Williams syndrome age, they frequently develop joint limitations and hypertonia, or abnormally increased muscle tone. Hypertension, gastrointestinal problems, and genitourinary symptoms often persist into adulthood, as well as cardiovascular problems. Adults with Williams syndrome are typically limited in their ability to live independently or work in competitive employment settings, but this developmental impairment is attributed more to psychological symptoms than physiological problems.

There is no specific treatment or cure for individuals affected with this type of syndrome, though some of the abnormal physical features may be surgically correctable.

There is a range of signs and symptoms including cleft lip or palate, mental retardation and various forms of ectodermal dysplasia. Additional symptoms may include fused eyelids, absent nails, delayed bone growth and dry skin. It is believed that this syndrome follows an autosomal dominant pattern of inheritance with incomplete penetrance, and caused by a mutation affecting the TP63 gene. It has been suggested that this syndrome, AEC syndrome and Rapp–Hodgkin syndrome may be variations of the same disease.

Children are most commonly identified with Aicardi syndrome before the age of five months. A significant number of girls are products of normal births and seem to be developing normally until around the age of three months, when they begin to have infantile spasms. The onset of infantile spasms at this age is due to closure of the final neural synapses in the brain, a stage of normal brain development. A number of tumors have been reported in association with Aicardi syndrome: choroid plexus papilloma (the most common), medulloblastoma, gastric hyperplastic polyps, rectal polyps, soft palate benign teratoma, hepatoblastoma, parapharyngeal embryonal cell cancer, limb angiosarcoma and scalp lipoma.

The Kocher–Debré–Semelaigne syndrome is hypothyroidism in infancy or childhood characterised by lower extremity or generalized muscular hypertrophy, myxoedema, short stature and cretinism. The absence of painful spasms and pseudomyotonia differentiates this syndrome from its adult form, which is Hoffmann syndrome.

The syndrome is named after Emil Theodor Kocher, Robert Debré and Georges Semelaigne.

Also known as Debre–Semelaigne syndrome or cretinism-muscular hypertrophy, hypothyroid myopathy, hypothyroidism-large muscle syndrome, hypothyreotic muscular hypertrophy in children, infantile myxoedema-muscular hypertrophy, myopathy-myxoedema syndrome, myxoedema-muscular hypertrophy syndrome, myxoedema-myotonic dystrophy syndrome.

Kocher-Debre-Semelaigne syndrome gives infant a Hercules appearance.

One of the most prominent and visible symptoms of Nevo Syndrome is the prenatal overgrowth, which continues into the infant and toddler stage. This excessive weight gain can be attributed to the low concentrations of growth hormone and insulin growth factor that are normally present to regulate weight gain. Other common symptoms associated with Nevo Syndrome are the outward wrist-drop, edema in hands and feet, undescended testes, low-set ears, hypotonia, the presence of low muscle tone in children, and long tapered fingers, and a highly arched palate.

West syndrome appears in 1% to 5% of infants with Down syndrome. This form of epilepsy is relatively difficult to treat in children who do not have the chromosomal abnormalities involved in Down syndrome. However, in children with Down syndrome, the syndrome is often far more mild, and the children often react better to medication. The German Down Syndrom InfoCenter noted in 2003 that what was normally a serious epilepsy was in such cases often a relatively benign one.

EEG records for children with Down syndrome are often more symmetrical with fewer unusual findings. Although not all children can become entirely free from attacks with medication, children with Down syndrome are less likely to go on to develop Lennox-Gastaut syndrome or other forms of epilepsy than those without additional hereditary material on the 21st chromosome. The reason why it is easier to treat children with Down syndrome is not known.

If, however, a child with Down syndrome has seizures that are difficult to control, the child should be accessed for autistic spectrum disorder.

The ‘Harlequin Sign’ is unilateral flushing and sweating of the face, neck, and upper chest usually after exposure to heat or strenuous exertion. Horner syndrome, another problem associated with the sympathetic nervous system, is often seen in conjunction with harlequin syndrome.

Since Harlequin syndrome is associated with a dysfunction in the autonomic nervous system, main symptoms of this dysfunction are in the following: Absence of sweat(anhidrosis) and flushing on one side of the face, neck, or upper thoracic area. In addition, other symptoms include cluster headaches, tearing of the eyes, nasal discharge, abnormal contraction of the pupils, weakness in neck muscles, and drooping of on side of the upper eyelid.

Aicardi syndrome is a rare genetic malformation syndrome characterized by the partial or complete absence of a key structure in the brain called the corpus callosum, the presence of retinal abnormalities, and seizures in the form of infantile spasms. Aicardi syndrome is theorized to be caused by a defect on the X chromosome as it has thus far only been observed in girls or in boys with Klinefelter syndrome. Confirmation of this theory awaits the discovery of a causative gene. Symptoms typically appear before a baby reaches about 5 months of age.

Multiple hamartoma syndrome is a syndrome characterized by more than one hamartoma.

It is sometimes equated with Cowden syndrome. However, MeSH also includes Bannayan–Zonana syndrome (that is, Bannayan–Riley–Ruvalcaba syndrome) and Lhermitte–Duclos disease under this description. Some articles include Cowden syndrome, Bannayan–Riley–Ruvalcaba syndrome, and at least some forms of Proteus syndrome and Proteus-like syndrome under the umbrella term PTEN hamartoma tumor syndromes (PHTS).

The most striking sign of Timothy syndrome is the co-occurrence of both syndactyly (~0.03% of births) and long QT syndrome (1% per year) in a single patient. Other common symptoms of Timothy syndrome are cardiac arrhythmia (94%), heart malformations (59%), autism or an autism spectrum disorder (80% who survive long enough for evaluation). Facial dysmorphologies such as flattened noses also occur in approximately half of patients. Children with this disorder have small teeth which, due to poor enamel coating, are prone to dental cavities and often require removal. The average age of death due to complications of these symptoms is 2.5 years.

Atypical Timothy syndrome has largely the same symptoms as the classical form. Differences in the atypical form are the lack of syndactyly, the presence of musculoskeletal problems (particularly hyperflexible joints), and atrial fibrillation. Patients with atypical Timothy syndrome also have more facial deformities, including protruding foreheads and tongues. Finally, one patient with atypical Timothy syndrome had a body development discrepancy wherein her upper body was normally developed (that of a 6-year-old) while her lower half resembled a 2- or 3-year-old.

Children with Timothy syndrome tend to be born via caesarean section due to fetal distress.

Reye syndrome progresses through five stages:

- Stage I

- Rash on palms of hands and feet

- Persistent, heavy vomiting that is not relieved by not eating

- Generalized lethargy

- Confusion

- Nightmares

- No fever usually present

- Headaches

- Stage II

- Stupor

- Hyperventilation

- Fatty liver (found by biopsy)

- Hyperactive reflexes

- Stage III

- Continuation of Stage I and II symptoms

- Possible coma

- Possible cerebral edema

- Rarely, respiratory arrest

- Stage IV

- Deepening coma

- Dilated pupils with minimal response to light

- Minimal but still present liver dysfunction

- Stage V

- Very rapid onset following stage IV

- Deep coma

- Seizures

- Multiple organ failure

- Flaccidity

- Hyperammonemia (above 300 mg/dL of blood)

- Death

The syndrome is a rare clinical disorder.

- Physical

- Overgrowth

- Accelerated skeletal maturation

- Dysmorphic facial features

- Prominent eyes

- Bluish sclerae

- Coarse eyebrows

- Upturned nose

- Radiologic examination

- Accelerated osseous maturation

- Phalangeal abnormalities

- Tubular thinning of the long bones

- Skull abnormalities

- Mental

- Often associated with intellectual disability (of variable degree)

The epileptic seizures which can be observed in infants with West syndrome fall into three categories, collectively known as infantile spasms. Typically, the following triad of attack types appears; while the three types usually appear simultaneously, they also can occur independently of each other:

- "Lightning attacks": Sudden, severe myoclonic convulsions of the entire body or several parts of the body in split seconds, and the legs in particular are bent (flexor muscle convulsions here are generally more severe than extensor ones).

- "Nodding attacks": Convulsions of the throat and neck flexor muscles, during which the chin is fitfully jerked towards the breast or the head is drawn inward.

- "Salaam or jackknife attacks": a flexor spasm with rapid bending of the head and torso forward and simultaneous raising and bending of the arms while partially drawing the hands together in front of the chest and/or flailing. If one imagined this act in slow motion, it would appear similar to the Muslim ceremonial greeting (Salaam), from which this type of attack derives its name.

Harlequin syndrome is a condition characterized by asymmetric sweating and flushing on the upper thoracic region of the chest, neck, and face. Harlequin syndrome is considered an injury to the autonomic nervous system (ANS). The ANS controls some of the body's natural processes such as sweating, skin flushing, and pupil response to stimuli. Such individuals with this syndrome have an absence of sweat skin flushing unilaterally; usually on the one side of the face, arms, and chest. It is an autonomic disorder that may occur at any age. Harlequin syndrome affects fewer than 200,000 people in the United States.

Symptoms associated with Harlequin syndrome are more likely to appear when a person has been in the following conditions: exercising, warm environment, and intense emotional situation. Since one side of the body sweats and flushes appropriately to the condition, the other side of the body will have an absence of such symptoms. This syndrome has also been called the "Harlequin sign," and thought to be one of the spectrum of diseases that may cause Harlequin syndrome.

It can also be the outcome of a one sided endoscopic thoracic sympathectomy (ETS) or endoscopic sympathetic blockade (ESB) surgery.

Harlequin syndrome can also be seen as a complication of VA (veno-arterial) extracorporeal membrane oxygenation (ECMO). This involves differential hypoxemia (low oxygen levels in the blood) of the upper body in comparison to the lower body.

Reye syndrome is a rapidly progressive encephalopathy. Symptoms may include vomiting, personality changes, confusion, seizures, and loss of consciousness. Even though liver toxicity typically occurs, yellowish skin usually does not. Death occurs in 20–40% of those affected and about a third of those who survive are left with a significant degree of brain damage.

The cause of Reye syndrome is unknown. It usually begins shortly after recovery from a viral infection, such as influenza or chickenpox. About 90% of cases in children are associated with aspirin (salicylate) use. Inborn errors of metabolism are also a risk factor. Changes on blood tests may include a high blood ammonia level, low blood sugar level, and prolonged prothrombin time. Often the liver is enlarged.

Prevention is typically by avoiding the use of aspirin in children. When aspirin was withdrawn for use in children a decrease of more than 90% in rates of Reye syndrome was seen. Early diagnosis improves outcomes. Treatment is supportive. Mannitol may be used to help with the brain swelling.

The first detailed description of Reye syndrome was in 1963 by Douglas Reye. Children are most commonly affected. It affects less than one in a million children a year. The general recommendation to use aspirin in children was withdrawn because of Reye syndrome, with use of aspirin only recommended in Kawasaki disease.

Neurological effects are believed to be more severe as the number of extra X chromosomes increases; a male with 48, XXXY is likely to have more severe symptoms than a male with Klinefelter syndrome. Developmental delays are common in infancy and childhood. Expected symptoms include speech delays, motor delays, and hypotonia (lack of muscle tone), also known as floppy baby syndrome.Individuals with XXXY syndrome exhibit cognitive and behavioral problems.

Patients typically show altered adaptive behavior, which is the ability of an individual to demonstrate essential living skills, including: social skills, community living, safety, functional use of academic skills and self-care. People with XXXY syndrome were found to score significantly less in the domains of daily living skills and communication compared to XXYY, and XXY individuals. This means that they typically demonstrate little ability in the domains of self-care, social skills, safety, application of academic skills, and responsibility.

Individuals with this syndrome also experience emotional symptoms such as anxiety symptoms, obsessive-compulsive behaviors, behavioral dysregulational and emotional immaturity. People with this syndrome typically have an IQ in the range of 40-60, where the average IQ range is 95-110. They also experience language-based learning disabilities that can affect their communication with others. Those with XXXY syndrome tend to display less externalizing and internalizing behaviors compared to those with 48, XXYY syndrome, which may have a positive effect on their social functioning. These individuals may also have increased vulnerability for autistic features. Changes in testosterone as well as androgen deficits may contribute to these individuals’ social behaviors that put them at increased risk for autistic features.

Of those fetuses that do survive to gestation and subsequent birth, common abnormalities may include:

- Nervous system

- Intellectual disability and motor disorder

- Microcephaly

- Holoprosencephaly (failure of the forebrain to divide properly).

- Structural eye defects, including microphthalmia, Peters' anomaly, cataract, iris or fundus (coloboma), retinal dysplasia or retinal detachment, sensory nystagmus, cortical visual loss, and optic nerve hypoplasia

- Meningomyelocele (a spinal defect)

- Musculoskeletal and cutaneous

- Polydactyly (extra digits)

- Cyclopia

- Proboscis

- Congenital trigger digits

- Low-set ears

- Prominent heel

- Deformed feet known as rocker-bottom feet

- Omphalocele (abdominal defect)

- Abnormal palm pattern

- Overlapping of fingers over thumb

- Cutis aplasia (missing portion of the skin/hair)

- Cleft palate

- Urogenital

- Abnormal genitalia

- Kidney defects

- Other

- Heart defects (ventricular septal defect) (Patent Ductus Arteriosus)

- Dextrocardia

- Single umbilical artery

Males with 48, XXXY can have average or tall stature, which becomes more prominent in adulthood. Facial dysmorphism is common in males with 48, XXXY and can include increased distance between the eyes (hypertelorism), skin folds of the upper eyelid (epicanthal folds), up-slanting opening between the eyelids (palpebral fissures) and hooded eyelids. Other physical features include the fifth finger or "pinky" to be bent inwards towards the fourth finger (clinodactyly), short nail beds, flat feet, double jointedness (hyperextensibility) and prominent elbows with cubitus varus where the arm rests closer to the body. Musculoskeletal features may include congentical elbow dislocation and the limited ability of the feet to roll inwards while walking and upon landing. Micropenis is another common symptom of this syndrome.

Individuals affected with XXXY are also prone to developing Taurodontism, which often presents early in life, and can be an early indicator of XXY syndrome. Those with this syndrome are also prone to hip dysplasia, and other joint abnormalities. An individual’s symptoms vary due to differing androgen deficiencies, and also with alter with age. Prepubescent boys with XXXY syndrome may not differ in physical appearance from a child without the syndrome. This is likely because androgen levels do not differ among pre-pubescent boys, but a difference does arise as puberty progresses. Those with XXXY syndrome may also experience feminine distribution of adipose tissue, and gynecomastia may also be present. Tall stature is more likely to appear in adolescence, when androgen levels begin to differ between those with XXXY syndrome and those that do not have it.

Timothy syndrome is a rare autosomal dominant disorder characterized by physical malformations, as well as neurological and developmental defects, including heart QT-prolongation, heart arrhythmias, structural heart defects, syndactyly (webbing of fingers and toes) and autism spectrum disorders.

Timothy syndrome often ends in early childhood death.

It is characterized by developmental defects including cryptophthalmos (where the eyelids fail to separate in each eye), and malformations in the genitals (such as micropenis, cryptorchidism or clitoromegaly). Congenital malformations of the nose, ears, larynx and renal system, as well as mental retardation, manifest occasionally. Syndactyly (fused fingers or toes) has also been noted.