Affected individuals believe that they are in the process of transforming into an animal or have already transformed into an animal. It has been associated with the altered states of mind that accompany psychosis (the that typically involves delusions and hallucinations) with the transformation only seeming to happen in the mind and behavior of the affected person.

A study on lycanthropy from the McLean Hospital reported on a series of cases and proposed some diagnostic criteria by which lycanthropy could be recognised:

- A patient reports in a moment of lucidity or reminiscence that they sometimes feel as an animal or have felt like one.

- A patient behaves in a manner that resembles animal behavior, for example howling, growling, or crawling.

According to this criteria, either a delusional belief in current or past transformation or behavior that suggests a person thinks of themselves as transformed is considered evidence of clinical lycanthropy. The authors note that, although the condition seems to be an expression of psychosis, there is no specific diagnosis of mental or neurological illness associated with its behavioral consequences.

DSM-IV Criteria

Clinical Lycanthropy is thought to be a cultural manifestation of schizophrenia due to the first 4 symptomatic criteria. The first criteria are delusions, and this fits clinical lycanthropy because a person believing that he or she turns into an animal is a delusion. The second symptom is hallucination, and people with clinical lycanthropy have vivid hallucinations of being an animal, and having traits that animal has, whether it be claws, fur, fangs, or whatever that particular animal has. The next symptom is disorganized speech, from a certain human, cultural perspective. The people who have the diagnosis of clinical lycanthropy often emit the sounds of the animal which they believe they become. So, if a person believes that he or she is a werewolf, they may begin to howl under the moon or sometimes even in the daylight. The last symptom that matches schizophrenia is grossly disorganized behavior. This is appropriate because individuals with clinical lycanthropy often act like the animal they believe they have become, including living outside and changing their diet.

It also seems that lycanthropy is not specific to an experience of human-to-wolf transformation; a wide variety of creatures have been reported as part of the shape-shifting experience. A review of the medical literature from early 2004 lists over thirty published cases of lycanthropy, only the minority of which have wolf or dog themes. Canines are certainly not uncommon, although the experience of being transformed into a hyena, cat, horse, bird or tiger has been reported on more than one occasion. Transformation into frogs, and even bees, has been reported in some instances. In Japan, transformation into foxes and dogs was usual (, ). A 1989 case study described how one individual reported a serial transformation, experiencing a change from human to dog, to horse, and then finally cat, before returning to the reality of human existence after treatment. There are also reports of people who experienced transformation into an animal only listed as "unspecified".

There is a case study of a psychiatric patient who had both clinical lycanthropy and Cotard delusion. The term "ophidianthropy" refers to the delusion that one has been transformed into a snake. Two case studies have been reported.

Clinical lycanthropy is defined as a rare psychiatric syndrome that involves a delusion that the affected person can transform into, has transformed into, or is a non-human animal. Its name is associated with the mythical condition of lycanthropy, a supernatural affliction in which humans are said to physically shapeshift into wolves. It is purported to be a rare disorder.

Species dysphoria is the experience of dysphoria, sometimes including clinical lycanthropy (delusion or hallucination of one's self as an animal) and dysmorphia (excessive concern over one's body image), associated with the feeling that one's body is of the wrong species. Earls and Lalumière (2009) describe it as "the sense of being in the wrong (species) body... a desire to be an animal". Outside of psychological literature, the term is common within the otherkin and therian communities. The phenomenon is sometimes experienced in the context of sexual arousal to the image of one's self as an animal.

Clinical vampirism, more commonly called Renfield's syndrome or Renfield syndrome, is an obsession with drinking blood. The earliest formal presentation of clinical vampirism to appear in the psychiatric literature, with the psychoanalytic interpretation of two cases, was contributed by Richard L. Vanden Bergh and John F. Kelley in 1964. As the authors point out, brief and sporadic reports of blood-drinking behaviors associated with sexual pleasure have appeared in the psychiatric literature at least since 1892 with the work of Austrian forensic psychiatrist Richard von Krafft-Ebing. Many medical publications concerning clinical vampirism can be found in the literature of forensic psychiatry, with the unusual behavior reported as one of many aspects of extraordinary violent crimes. The behavior has never gained official recognition by the psychiatric profession and is not found in any edition of the "International Classification of Diseases" or the "Diagnostic and Statistical Manual of Mental Disorders".

Involutional melancholy's 'course was chronic, with agitation, depersonalization and delusions of bodily change and guilt' featuring strongly, but 'without manic features'. Symptoms of fear are also considered to occur, as well as despondency and hypochondriacal delusions. The late onset of the disorder was matched with a prolonged course with poor prognosis and/or deterioration, in the absence of treatment.

"Species dysphoria" is informally used mainly in psychological literature to compare the experiences of some individuals to those in the transgender community. Otherkin and therian communities have also used it to describe their experiences.

In a 2008 study by Gerbasi "et al.", 46% of people surveyed who identified as being in the furry fandom, (usually defined as a person who enjoys anthropomorphic animals, occasionally to an almost obsessive degree), answered "yes" to the question "Do you consider yourself to be less than 100% human?" and 41% answered "yes" to the question "If you could become 0% human, would you?" Questions that Gerbasi states as being deliberately designed to draw parallels with gender dysphoria, specifying "a persistent feeling of discomfort" about the human body and the feeling that the person was the "non-human species trapped in a human body", were answered "yes" by 24% and 29% of respondents, respectively. Likewise, these studies support the fact that the therianthropic, otherkin and furry communities are very similar in nature and are often interconnected.

As described by those who experience it, species dysphoria may include sensations of supernumerary phantom limbs associated with the species, such as phantom wings or claws. Species dysphoria involves feelings of being an animal or other creatures "trapped in" a human body and so, is considered different from the traditional definition of clinical lycanthropy, in which the patient believes they have actually been transformed into an animal or have the ability to physically shapeshift. However, some cases that have been labeled as "clinical lycanthropy" actually seem to be cases of species dysphoria, involving persons who have no delusion of transformation but instead have feelings of being in some way a non-human animal, while still acknowledging they possess a human form. Keck "et al." propose a redefinition for clinical lycanthropy that covers species dysphoric behaviours observed in several patients, including verbal reports, "during intervals of lucidity or retrospectively, that he or she was a particular animal" and behaving "in the manner of a particular animal, i.e. howling, growling, crawling on all fours". Keck "et al." describe one patient as a depressed individual who "had always suspected he was a cat" and "laments his lack of fur, stripes and a tail". Except for the persistent feeling of being feline, the patient's "thought processes and perception" were "usually logical".

Involutional melancholia or involutional depression is a traditional name for a psychiatric disorder affecting mainly elderly or late middle-aged people, usually accompanied with paranoia. It is classically defined as "depression of gradual onset occurring during the involutional years (40-55 in women and 50-65 in men), with symptoms of marked anxiety, agitation, restlessness, somatic concerns, hypochondriasis, occasional somatic or nihilistic delusions, insomnia, anorexia, and weight loss." Involutional melancholia is not recognized as a psychiatric disorder by the DSM-5, the American Psychiatric Association's (APA) classification and diagnostic tool.

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.

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.

Cynanthropy (sometimes spelled "kynanthropy") is, in medicine, the pathological delusion of real persons that they are dogs and in anthropology and folklore, the supposed magical practice of shape-shifting alternately between canine and human form, or the possession of combined canine and human anatomical features, a form of therianthropy.

The Greeks spoke of cynanthropy ("kyon", dog). The term existed by at least 1901, when it was applied to myths from China about humans turning into dogs, dogs becoming people, and sexual relations between humans and canines (De Groot, 184). After lycanthropy, cynanthropy is the best known term for a specific variety of therianthropy.

Anthropologist David Gordon White called Central Asia the "vortex of cynanthropy" because races of dog-men were habitually placed there by ancient writers. Hindu mythology puts races of "Dog Cookers" to the far north of India, the Chinese placed the "Dog Jung" and other human/canine barbarians to the extreme west, and European legends frequently put the dog men called Cynocephali in unmapped regions to the east. Some of these races were described as humans with dog heads, others as canine shapeshifters (White, 114-15).

The weredog or cynanthrope is also known in Timor. It is described as a human/canine shapeshifter who is also capable of transforming other people into animals against their wills. These transformations are usually into prey animals such as goats, so that the cynanthrope can devour them without discovery of the crime (Rose, 390).

Very few cases of the syndrome have been described, and the published reports that do exist describe clinical vampirism as behaviors that are subsumed under more conventional psychiatric diagnostic categories such as schizophrenia or paraphilia. A case of vampirism in Turkey reported in 2012 was discussed as an unusual feature of a patient diagnosed with dissociative identity disorder and post-traumatic stress disorder. While not referencing the literature on Renfield's syndrome, two Irish psychiatrists surveyed the psychiatric literature on vampirism as evidence of a changing discourse in psychiatry from the narrative of case studies to the depersonalized discourse of checklist diagnostic criteria.

A number of murderers have performed seemingly vampiric rituals upon their victims. Serial killers Peter Kürten and Richard Trenton Chase were both called "vampires" in the tabloids after they were discovered drinking the blood of the people they murdered. Similarly, in 1932, an unsolved murder case in Stockholm, Sweden was nicknamed the "Vampire murder", due to the circumstances of the victim's death.Clinical vampirism in the context of criminal acts of violence, as well as "consensual" vampirism as a social ritual, have been extensively documented in the many works of Katharine Ramsland. Others have commented upon the psychiatric implications of "vampire cults" among adolescents.

Sleep state misperception (SSM) is a term in the International Classification of Sleep Disorders (ICSD) most commonly used for people who mistakenly perceive their sleep as wakefulness, though it has been proposed that it be applied to those who severely overestimate their sleep time as well ("positive" sleep state misperception). While most sleepers with this condition will report not having slept in the previous night at all or having slept very little, clinical recordings generally show normal sleep patterns. Though the sleep patterns found in those with SSM have long been considered indistinguishable from those without, some preliminary research suggest there may be subtle differences (see Symptoms and diagnosis: Spectral analysis).

Patients are otherwise generally in good health, and any ills—such as depression—appear to be more associated with fear of negative consequences of insomnia ("insomnia phobia") than from any actual loss of sleep.

Sleep state misperception was adopted by the ICSD to replace two previous diagnostic terminologies: "subjective insomnia complaint without objective findings" and "subjective sleepiness complaint without objective findings."

Other synonyms of the term include: paradoxical insomnia, pseudo-insomnia, subjective insomnia, subjective sleepiness, and sleep hypochondriasis.

Sleep state misperception is classified as an intrinsic dyssomnia. While SSM is regarded a sub-type of insomnia, it is also established as a separate sleep-condition, with distinct pathophysiology. Nonetheless, the value of distinguishing this type of insomnia from other types is debatable due to the relatively low frequency of SSM being reported.

Sleep state misperception can also be further broken down into several types, by patients who—

- report short sleep (subjective insomnia complaint without objective findings)

- or no sleep at all (subjective "total" insomnia)

- report excessive daytime sleepiness (subjective sleepiness complaint without objective findings)

- report sleeping too much (subjective hypersomnia without objective findings)

Heterogeneous medical condition in medicine are those medical conditions which have several etiologies, like hepatitis or diabetes. Medical conditions are normally defined pathologically (liver inflammation) or clinically (excessive urination) and not etiologically, and therefore it is normal to have more than one cause for them. The word is used as an opposition to homogeneous, meaning that given a group of patients, the disease is the same for all of them.

When a condition is heterogeneous, it is normally divided in endotypes.

Mild cognitive impairment (MCI), also known as incipient dementia and isolated memory impairment, is a neurological disorder that occurs in older adults which involves cognitive impairments with minimal impairment in instrumental activities of daily living. MCI involves the onset and evolution of cognitive impairments beyond those expected based on the age and education of the individual, but which are not significant enough to interfere with their daily activities. It may occur as a transitional stage between normal aging and dementia. Causation of the syndrome in and of itself remains unknown, as, therefore, do prevention and treatment.

The initial affected individual described in 1986 had a complex phenotype, and was later found to have both beta-mannosidosis and Sanfilippo syndrome. People have been described with a wide spectrum of clinical presentations from infants and children with intellectual disability to adults who present with isolated skin findings (angiokeratomas). Most cases are identified in the first year of life with respiratory infections, hearing loss and intellectual disability. Because of its rarity, and non-specific clinical findings, beta-mannosidosis can go undiagnosed until adulthood, where it can present with intellectual disability and behavioral problems, including aggression.

An endotype is a subtype of a condition, which is defined by a distinct functional or pathobiological mechanism. This is distinct from a phenotype, which is any observable characteristic or trait of a disease, such as morphology, development, biochemical or physiological properties, or behavior, without any implication of a mechanism. It is envisaged that patients with a specific endotype present themselves within phenotypic clusters of diseases.

One example is asthma, which is considered to be a syndrome, consisting of a series of endotypes. This is related to the concept of disease entity

Clinical:

Patients often present with a history of fever of unknown origin, muscular weakness, poor development, abnormal dentition, normal serum calcium, phosphorus, and alkaline phosphatase levels. Associated clinical findings also include glaucoma, photosensitivity, heart block, foot deformities, and chronic psoriasiform skin lesions.

Radiological:

Classic radiologic findings were first described by Edward B. Singleton and David Merten in 1973.

Typical radiographic appearances include skeletal demineralization, expanded shafts of the metacarpals and phalanges with widenend medullary cavities, cardiomegaly, and intramural calcification of the proximal aorta with occasional extension into the aortic or mitral valves.

Other commonly seen radiographic findings include shallow acetabular fossa, subluxation of the femoral head, coxa valga, hypoplastic radial epiphysis, soft tissue calcifications between the radius and ulna, constriction of the proximal radial shaft, acro-osteolysis, and equinovarus foot deformities.

Although MCI can present with a variety of symptoms, when memory loss is the predominant symptom it is termed "amnestic MCI" and is frequently seen as a prodromal stage of Alzheimer's disease. Studies suggest that these individuals tend to progress to probable Alzheimer's disease at a rate of approximately 10% to 15% per year.

When individuals have impairments in domains other than memory it is classified as nonamnestic single- or multiple-domain MCI and these individuals are believed to be more likely to convert to other dementias (e.g., dementia with Lewy bodies).

The physical symptoms of FXTAS include an intention tremor, cerebellar ataxia, and parkinsonism. This includes small, shuffling steps, muscle rigidity and slowed speech, as well as neuropathic symptoms. As the disease progresses to the more advanced stages, an individual with FXTAS is also at risk of autonomic dysfunction: hypertension, bowel and bladder dysfunction, and impotence.

An individual with FXTAS may also exhibit the following symptoms: a decrease in cognition, which includes diminishing short-term memory and executive function skills, declining math and spelling abilities and decision-making abilities. FXTAS may also result in changes in personality, due to alterations of the limbic area in the brain. This includes increased irritability, angry outbursts, and impulsive behaviour

Opioid-induced hyperalgesia or opioid-induced abnormal pain sensitivity, also called paradoxical hyperalgesia is a phenomenon associated with the long-term use of opioids such as morphine, hydrocodone, oxycodone, and methadone. Over time, individuals taking opioids can develop an increasing sensitivity to noxious stimuli, even evolving a painful response to previously non-noxious stimuli (allodynia). Some studies on animals have also demonstrated this effect occurring after only a single high dose of opioids.

Tolerance, another condition that can arise from prolonged exposure to opioids, can often be mistaken for opioid-induced hyperalgesia and vice-versa, as the clinical presentation can appear similar. Although tolerance and opioid-induced hyperalgesia both result in a similar need for dose escalation to receive the same level of effect to treat pain, they are nevertheless caused by two distinct mechanisms. The similar net effect makes the two phenomena difficult to distinguish in a clinical setting. Under chronic opioid treatment, a particular individual's requirement for dose escalation may be due to tolerance, opioid-induced hyperalgesia, or a combination of both. In tolerance, there is a lower sensitivity to opioids, which occurs via two major theories: decreased receptor activation (desensitization of antinociceptive mechanisms), and opioid receptor down-regulation (internalization of membrane receptors). In opioid-induced hyperalgesia, sensitization of pronociceptive mechanisms occurs, resulting in a decrease in the pain threshold, or allodyna. Identifying the development of hyperalgesia is of great clinical importance since patients receiving opioids to relieve pain may paradoxically experience more pain as a result of treatment. Whereas increasing the dose of opioid can be an effective way to overcome tolerance, doing so to compensate for opioid-induced hyperalgesia may worsen the patient's condition by increasing sensitivity to pain while escalating physical dependence.

The phenomenon is common among palliative care patients following a too rapid escalation of opioid dosage.

Singleton Merten Syndrome is an autosomal dominate genetic disorder with variable expression with an onset of symptoms during childhood.

The syndromes within CAPS overlap clinically, and patients may have features of more than one disorder. In a retrospective cohort of 136 CAPS patients from 16 countries, the most prevalent clinical features were fever (84% of cases, often with concurrent constitutional symptoms such as fatigue, malaise, mood disorders or failure to thrive), skin rash (either urticarial or maculopapular rash; 97% of cases) especially after cold exposure, and musculoskeletal involvement (myalgia, arthralgia, and/or arthritis, or less commonly joint contracture, patellar overgrowth, bone deformity, bone erosion and/or osteolytic lesion; 86% of cases). Less common features included ophthalmological involvement (conjunctivitis and/or uveitis, or less commonly optic nerve atrophy, cataract, glaucoma or impaired vision; 71% of cases), neurosensory hearing loss (42% of cases), neurological involvement (morning headache, papilloedema, and/or meningitis, or less commonly seizure, hydrocephalus or mental retardation; 40% of cases), and AA amyloidosis (4% of cases). Age of onset is typically in infancy or early childhood. In 57% of cases, CAPS had a chronic phenotype with symptoms present almost daily, whereas the remaining 43% of patients experienced only acute episodes. Up to 56% of patients reported a family history of CAPS. Previous studies confirm these symptoms, although the exact reported rates vary.

There are three main disorders caused by Hermansky–Pudlak syndrome, which result in these symptoms:

- Albinism and eye problems: Individuals will have varying amounts of skin pigment (melanin). Because of the albinism there are eye problems such as light sensitivity (photophobia), strabismus (crossed eyes), and nystagmus (involuntary eye movements). Hermansky–Pudlak syndrome also impairs vision.

- Bleeding disorders: Individuals with the syndrome have platelet dysfunction. Since platelets are necessary for blood clotting, individuals will bruise and bleed easily.

- Cellular storage disorders: The syndrome causes a wax-like substance (ceroid) to accumulate in the body tissues and cause damage, especially in the lungs and kidneys.

It is also associated with granulomatous colitis, an inflammation of the colon, and with pulmonary fibrosis, a potentially fatal lung disease.

The Baggio–Yoshinari syndrome (BYS), formerly known as the Brazilian Lyme-like disease and Brazilian human borreliosis, is a disease transmitted by the "Amblyomma cajennense" tick, but the organism that causes the infection is still unknown. Clinical features resemble those of Lyme disease (LD).