A persistent vegetative state (PVS) is a disorder of consciousness in which patients with severe brain damage are in a state of partial arousal rather than true awareness. After four weeks in a vegetative state (VS), the patient is classified as in a persistent vegetative state. This diagnosis is classified as a "permanent vegetative state" some months (3 in the US and 6 in the UK) after a non-traumatic brain injury or one year after a traumatic injury. Nowadays, more doctors and neuroscientists prefer to call the state of consciousness an "unresponsive wakefulness syndrome", primarily because of ethical questions about whether a patient can be called "vegetative" or not.

Generally, a person who is unable to voluntarily open the eyes, does not have a sleep-wake cycle, is unresponsive in spite of strong tactile (painful) or verbal stimuli, and who generally scores between 3 and 8 on the Glasgow Coma Scale is considered in a coma. Coma may have developed in humans as a response to injury to allow the body to pause bodily actions and heal the most immediate injuries before waking. It therefore could be a compensatory state in which the body's expenditure of energy is not superfluous. The severity and mode of onset of coma depends on the underlying cause. For instance, severe hypoglycemia (low blood sugar) or hypercapnia (increased carbon dioxide levels in the blood) initially cause mild agitation and confusion, but progress to obtundation, stupor, and finally, complete unconsciousness. In contrast, coma resulting from a severe traumatic brain injury or subarachnoid hemorrhage can be instantaneous. The mode of onset may therefore be indicative of the underlying cause.

Most PVS patients are unresponsive to external stimuli and their conditions are associated with different levels of consciousness. Some level of consciousness means a person can still respond, in varying degrees, to stimulation. A person in a coma, however, cannot. In addition, PVS patients often open their eyes in response to feeding, which has to be done by others; they are capable of swallowing, whereas patients in a coma subsist with their eyes closed (Emmett, 1989).

PVS patients' eyes might be in a relatively fixed position, or track moving objects, or move in a "disconjugate" (i.e., completely unsynchronized) manner. They may experience sleep-wake cycles, or be in a state of chronic wakefulness. They may exhibit some behaviors that can be construed as arising from partial consciousness, such as grinding their teeth, swallowing, smiling, shedding tears, grunting, moaning, or screaming without any apparent external stimulus.

Individuals in PVS are seldom on any life-sustaining equipment other than a feeding tube because the brainstem, the center of vegetative functions (such as heart rate and rhythm, respiration, and gastrointestinal activity) is relatively intact (Emmett, 1989).

Coma is a state of unconsciousness in which a person cannot be awakened; fails to respond normally to painful stimuli, light, or sound; lacks a normal wake-sleep cycle; and does not initiate voluntary actions. A person in a state of coma is described as being "comatose". A distinction is made in the medical community between a real coma and a medically induced coma, the former is a result of circumstances beyond the control of the medical community, while the latter is a means by which medical professionals may allow a patient's injuries to heal in a controlled environment.

A comatose person exhibits a complete absence of wakefulness and is unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function. The first is the cerebral cortex—the gray matter that forms the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system (RAS).

Injury to either or both of these components is sufficient to cause a patient to experience a coma. The cerebral cortex is a group of tight, dense, "gray matter" composed of the nuclei of the neurons whose axons then form the "white matter," and is responsible for perception, relay of the sensory input via the thalamic pathway, and many other neurological functions, including complex thinking.

RAS, on the other hand, is a more primitive structure in the brainstem which includes the reticular formation (RF). The RAS area of the brain has two tracts, the ascending and descending tract. Made up of a system of acetylcholine-producing neurons, the ascending track, or ascending reticular activating system (ARAS), works to arouse and wake up the brain, from the RF, through the thalamus, and then finally to the cerebral cortex. A failure in ARAS functioning may then lead to a coma. The word is from the Greek "koma", meaning "deep sleep."

If not stimulated externally, a patient with stupor will be in a sleepy state most of the time. In some extreme cases of severe depressive disorders the patient can become motionless, lose their appetite and become mute. Short periods of restricted responsivity can be achieved by intense stimulation (e.g. pain, bright light, loud noise, shock).

Like coma, chronic coma results mostly from cortical or white-matter damage after neuronal or axonal injury, or from focal brainstem lesions.Usually the metabolism in the grey matter decreases to 50-70% of the normal range. The patient lacks awareness and arousal. The patient lies with eyes closed and is not aware of self or surroundings. Stimulation cannot produce spontaneous periods of wakefulness and eye-opening, unlike patients in vegetative state. In medicine, a coma (from the Greek κῶμα koma, meaning deep sleep) is a state of unconsciousness, lasting more than six hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light, sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. Although, according to the Glasgow Coma Scale, a person with confusion is considered to be in the mildest coma. But cerebral metabolism has been shown to correlate poorly with the level of consciousness in patients with mild to severe injury within the first month after traumatic brain injury (TBI).

A person in a state of coma is described as comatose. In general patients surviving a coma recover gradually within 2–4 weeks. But recovery to full awareness and arousal is not always possible. Some patients do not progress further than vegetative state or minimally conscious state and sometimes this also results in prolonged stages before further recovery to complete consciousness.

Although a coma patient may appear to be awake, they are unable to consciously feel, speak, hear, or move. For a patient to maintain consciousness, two important neurological components must function impeccably. The first is the cerebral cortex which is the gray matter covering the outer layer of the brain. The other is a structure located in the brainstem, called reticular activating system (RAS or ARAS). Injury to either or both of these components is sufficient to cause a patient to experience a coma.

Disorders of consciousness are medical conditions that inhibit consciousness. Some define disorders of consciousness as any change from complete self-awareness to inhibited or absent self-awareness and arousal. This category generally includes minimally conscious state and persistent vegetative state, but sometimes also includes the less severe locked-in syndrome and more severe but rare chronic coma. Differential diagnosis of these disorders is an active area of biomedical research. Finally, brain death results in an irreversible disruption of consciousness. While other conditions may cause a moderate deterioration (e.g., dementia and delirium) or transient interruption (e.g., grand mal and petit mal seizures) of consciousness, they are not included in this category.

Lesions of the ascending reticular activation system on height of the pons and metencephalon have been shown to cause stupor. The incidence is higher after left-sided lesions.

Patients suffering traumatic brain injury experience profound disturbance of the basic functions of the cognitive, behavioral, emotional and intellectual systems. Such patients' ability to regulate interaction between the ego and the external world is greatly diminished and they typically exhibit inflexible, concrete and sometimes inappropriate behaviors.

Hyperprosexia, and paraprosexia are closely related medical and neuro-psychiatric phenomena associated with attention and concentration. They typically occur in patients suffering traumatic brain injuries.

- "Aprosexia" is an abnormal inability to pay attention, characterized by a near-complete indifference to everything.

- "Hyperprosexia" is the abnormal state in which a person concentrates on one thing to the exclusion of everything else.

- "Paraprosexia" is the inability to pay attention to any one thing (a state of constant distraction).

Lethargy is a state of tiredness, weariness, fatigue, or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overworking, stress, lack of exercise, improper nutrition, boredom, or a symptom of a disorder. It may also be a side-effect of medication or caused by an interaction between medications or medication(s) and alcohol. When part of a normal response, lethargy often resolves with rest, adequate sleep, decreased stress, physical exercise and good nutrition.

A minimally conscious state (MCS) is a disorder of consciousness distinct from persistent vegetative state and locked-in syndrome. Unlike persistent vegetative state, patients with MCS have partial preservation of conscious awareness. MCS is a relatively new category of disorders of consciousness. The natural history and longer term outcome of MCS have not yet been thoroughly studied. The prevalence of MCS was estimated to be 112,000 to 280,000 adult and pediatric cases.

Oneirophenia and schizophrenia are often confused although there are distinct differences between the conditions. Oneirophrenia has some of the characteristics of simple schizophrenia, such as a confusional state and clouding of consciousness, but without presenting the dissociative symptoms which are typical of that disorder. Oneiophrenia often begins with the inability to focus on things while schizophrenia frequently starts with a traumatic event. Persons affected by oneirophrenia have a feeling of dream-like derealization which, in its extreme form, may progress to delusions and hallucinations. Therefore, it is considered a schizophrenia-like acute form of psychosis which remits in about 60% of cases within a period of two years. It is estimated that 50% or more of schizophrenic patients present oneirophrenia at least once.

Oneirophrenia is often described as a dream-like state that can lead to hallucinations and confusion. Feelings and emotions are often disturbed but information from the senses is left intact separating it from true schizophrenia.

One of the defining characteristics of minimally conscious state is the more continuous improvement and significantly more favorable outcomes post injury when compared with vegetative state. One study looked at 100 patients with severe brain injury. At the beginning of the study, all the patients were unable to follow commands consistently or communicate reliably. These patients were diagnosed with either MCS or vegetative state based on performance on the JFK Coma Recovery Scale and the diagnostic criteria for MCS as recommended by the Aspen Consensus Conference Work-group. Both patient groups were further separated into those that suffered from traumatic brain injury and those that suffered from non-traumatic brain injures (anoxia, tumor, hydrocephalus, infection). The patients were assessed multiple times over a period of 12 months post injury using the Disability Rating Scale (DRS) which ranges from a score of 30=dead to 0=no disabilities. The results show that the DRS scores for the MCS subgroups showed the most improvement and predicted the most favorable outcomes 12 months post injury. Amongst those diagnosed with MCS, DRS scores were significantly lower for those with non-traumatic brain injuries in comparison to the vegetative state patients with traumatic brain injury. DRS scores were also significantly lower for the MCS non-traumatic brain injury group compared to the MCS traumatic brain injury group. Pairwise comparisons showed that DRS scores were significantly higher for those that suffered from non-tramuatic brain injuries than those with traumatic brain injuries. For the patients in vegetative states there were no significant differences between patients with non-traumatic brain injury and those with traumatic brain injuries. Out of the 100 patients studied, 3 patients fully recovered (had a DRS score of 0). These 3 patients were diagnosed with MCS and had suffered from traumatic brain injuries.

In summary, those with minimally conscious state and non-traumatic brain injuries will not progress as well as those with traumatic brain injuries while those in vegetative states have an all around lower to minimal chance of recovery.

Because of the major differences in prognosis described in this study, this makes it crucial that MCS be diagnosed correctly. Incorrectly diagnosing MCS as vegetative state may lead to serious repercussions related to clinical management.

In the UK, the formal rules for the diagnosis of brainstem death have undergone only minor modifications since they were first published in 1976. The most recent revision of the UK's Department of Health Code of Practice governing use of that procedure for the diagnosis of death reaffirms the preconditions for its consideration. These are:

1. There should be no doubt that the patient’s condition – deeply comatose, unresponsive and requiring artificial ventilation—is due to irreversible brain damage of known cause.

2. There should be no evidence that this state is due to depressant drugs.

3. Primary hypothermia as the cause of unconsciousness must have been excluded, and

4. Potentially reversible circulatory, metabolic and endocrine disturbances likewise.

5. Potentially reversible causes of apnoea (dependence on the ventilator), such as muscle relaxants and cervical cord injury, must be excluded.

With these pre-conditions satisfied, the definitive criteria are:

1. Fixed pupils which do not respond to sharp changes in the intensity of incident light.

2. No corneal reflex.

3. Absent oculovestibular reflexes – no eye movements following the slow injection of at least 50ml of ice-cold water into each ear in turn (the caloric reflex test).

4. No response to supraorbital pressure.

5. No cough reflex to bronchial stimulation or gagging response to pharyngeal stimulation.

6. No observed respiratory effort in response to disconnection of the ventilator for long enough (typically 5 minutes) to ensure elevation of the arterial partial pressure of carbon dioxide to at least 6.0 kPa (6.5 kPa in patients with chronic carbon dioxide retention). Adequate oxygenation is ensured by pre-oxygenation and diffusion oxygenation during the disconnection (so the brainstem respiratory centre is not challenged by the ultimate, anoxic, drive stimulus). This test—the apnoea test—is dangerous – and may prove lethal.

Two doctors, of specified status and experience, are required to act together to diagnose death on these criteria and the tests must be repeated after “a short period of time ... to allow return of the patient’s arterial blood gases and baseline parameters to the pre-test state”. These criteria for the diagnosis of death are not applicable to infants below the age of two months.

Polar T syndrome is a condition found in polar explorers, caused by a reduction in levels of the thyroid hormone T. Its effects include forgetfulness, cognitive impairment and mood disturbances. It can exhibit itself in a fugue state known as the "Antarctic stare".

It is regarded as one of the contributory causes of winter-over syndrome.

Brainstem death is a clinical syndrome defined by the absence of reflexes with pathways through the brainstem—the “stalk” of the brain, which connects the spinal cord to the mid-brain, cerebellum and cerebral hemispheres—in a deeply comatose, ventilator-dependent patient.

Identification of this state carries a very grave prognosis for survival; cessation of heartbeat often occurs within a few days although it may continue for weeks or even months if intensive support is maintained.

In the United Kingdom, the formal diagnosis of brainstem death by the procedure laid down in the official Code of Practice permits the diagnosis and certification of death on the premise that a person is dead when consciousness and the ability to breathe are permanently lost, regardless of continuing life in the body and parts of the brain, and that death of the brainstem alone is sufficient to produce this state.

This concept of brainstem death is also accepted as grounds for pronouncing death for legal purposes in India and Trinidad & Tobago. Elsewhere in the world the concept upon which the certification of death on neurological grounds is based is that of permanent cessation of all function in all parts of the brain—whole brain death—with which the reductionist United Kingdom concept should not be confused. The United States' President's Council on Bioethics made it clear, in its White Paper of December 2008, that the United Kingdom concept and clinical criteria are not considered sufficient for the diagnosis of death in the United States of America.

Symptoms of a dissociative fugue include mild confusion, and once the fugue ends, possible depression, grief, shame and discomfort. People have also experienced a post-fugue anger.

Solipsism syndrome refers to a psychological state in which a person feels that the world is not external to his or her mind. Periods of extended isolation may predispose people to this condition. In particular, the syndrome has been identified as a potential concern for individuals living in outer space for extended periods of time.

A brainstem stroke syndrome is a condition involving a stroke of the brainstem. Because of their location, they often involve impairment both of the cranial nuclei and of the long tracts.

A person may have vertigo, dizziness and severe imbalance without the hallmark of most strokes – weakness on one side of the body. The symptoms of vertigo, dizziness or imbalance usually occur together; dizziness alone is not a sign of stroke. Brainstem stroke can also cause diplopia, slurred speech and decreased level of consciousness. A more serious outcome is locked-in syndrome.

Jactitation is an archaic medical term (derived, perhaps as a corruption, from "jactation", meaning a restless tossing and turning of the body, and derived itself from Latin "jactare" or "jacere", both meaning "to throw or hurl") referring to the involuntary spasm of a limb, muscle, or muscle group. This is sometimes seen in fever patients or other situations of physical distress, but may occur in healthy individuals in a hypnogogic state. This hypnagogic jactitation often occurs in the legs, and may occasion a short explanatory dream about stumbling or missing the bottom stair.

Dissociative fugue, formerly fugue state or psychogenic fugue, is a dissociative disorder. It is a rare psychiatric disorder characterized by reversible amnesia for personal identity, including the memories, personality, and other identifying characteristics of individuality. The state can last days, months or longer. Dissociative fugue usually involves unplanned travel or wandering, and is sometimes accompanied by the establishment of a new identity. It is a facet of dissociative amnesia, according to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).

After recovery from fugue, previous memories usually return intact. Because of this, there is not normally any treatment necessary for people who have been in fugue states. Additionally, an episode of fugue is not characterized as attributable to a psychiatric disorder if it can be related to the ingestion of psychotropic substances, to physical trauma, to a general medical condition, or to dissociative identity disorder, delirium, or dementia. Fugues are precipitated by a series of long-term traumatic episodes. It is most commonly associated with childhood victims of sexual abuse who learn over time to dissociate memory of the abuse (dissociative amnesia).

Individuals experiencing solipsism syndrome feel that the world is not 'real' in the sense of being external to their own minds. The syndrome is characterized by feelings of loneliness, detachment and indifference to the outside world. Solipsism syndrome is not currently recognized as a psychiatric disorder by the American Psychiatric Association, though it shares similarities with depersonalization disorder, which is recognized. Solipsism syndrome is distinct from solipsism, which is not a psychological state but rather a philosophical position, namely that nothing exists or can be known to exist outside of one's own mind; advocates of this philosophy do not necessarily suffer from solipsism syndrome, and sufferers do not necessarily subscribe to solipsism as a school of intellectual thought.

Periods of extended isolation may predispose people to solipsism syndrome. In particular, the syndrome has been identified as a potential challenge for astronauts and cosmonauts on long-term missions,

and these concerns influence the design of artificial habitats.

Brain death is the complete loss of brain function (including involuntary activity necessary to sustain life). It differs from persistent vegetative state, in which the person is alive and some autonomic functions remain.

Brain death is used as an indicator of legal death in many jurisdictions, but it is defined inconsistently. Various parts of the brain may keep functioning when others do not anymore, and the term "brain death" has been used to refer to various combinations. For example, although a major medical dictionary says that "brain death" is synonymous with "cerebral death" (death of the cerebrum), the US National Library of Medicine Medical Subject Headings (MeSH) system defines brain death as including the brainstem. The distinctions can be important because, for example, in someone with a dead cerebrum but a living brainstem, the heartbeat and ventilation can continue unaided, whereas in whole-brain death (which includes brain stem death), only life support equipment would keep those functions going. Patients classified as brain-dead can have their organs surgically removed for organ donation.