When diagnosing any neurological condition, history and examination are fundamental. History is obtained by family, friends or EMS. The Glasgow Coma Scale is a helpful system used to examine and determine the depth of coma, track patients progress and predict outcome as best as possible. In general a correct diagnosis can be achieved by combining findings from physical exam, imaging, and history components and directs the appropriate therapy.

Imaging basically encompasses computed tomography (CAT or CT) scan of the brain, or MRI for example, and is performed to identify specific causes of the coma, such as hemorrhage in the brain or herniation of the brain structures. Special tests such as an EEG can also show a lot about the activity level of the cortex such as semantic processing, presence of seizures, and are important available tools not only for the assessment of the cortical activity but also for predicting the likelihood of the patient's awakening. The autonomous responses such as the skin conductance response may also provide further insight on the patient's emotional processing.

Metabolic studies are useful, but they are not able identify neural activity within a specific region to specific cognitive processes. Functionality can only be identified at the most general level: Metabolism in cortical and subcortical regions that may contribute to cognitive processes.

At present, there is no established relation between cerebral metabolic rates of glucose or oxygen as measured by PET and patient outcome. The decrease of cerebral metabolism occurs also when patients are treated with anesthetics to the point of unresponsiveness. Lowest value (28% of normal range) have been reported during propofol anesthesia. Also deep sleep represents a phase of decreased metabolism (down to 40% of the normal range)

In general, quantitative PET studies and the assessment of cerebral metabolic rates depends on many assumptions.

PET for example requires a correction factor, the lumped constant, which is stable in healthy brains. There are reports, that a global decrease of this constant emerges after a traumatic brain injury.

But not only the correction factors change due to TBI.

Another issue is the possibility of anaerobic glycolysis that could occur after TBI. In such a case the glucose levels measured by the PET are not tightly connected to the oxygen consumption of the patient's brain.

Third point regarding PET scans is the overall measurement per unit volume of brain tissue. The imaging can be affected by the inclusion of metabolically inactive spaces e.g. cerebrospinal fluidin the case of gross hydrocephalus, which artificially lowers the calculated metabolism.

Also the issue of radiation exposure must be considered in patients with already severely damaged brains and preclude longitudinal or follow-up studies.

Despite converging agreement about the definition of persistent vegetative state, recent reports have raised concerns about the accuracy of diagnosis in some patients, and the extent to which, in a selection of cases, residual cognitive functions may remain undetected and patients are diagnosed as being in a persistent vegetative state. Objective assessment of residual cognitive function can be extremely difficult as motor responses may be minimal, inconsistent, and difficult to document in many patients, or may be undetectable in others because no cognitive output is possible (Owen et al., 2002). In recent years, a number of studies have demonstrated an important role for functional neuroimaging in the identification of residual cognitive function in persistent vegetative state; this technology is providing new insights into cerebral activity in patients with severe brain damage. Such studies, when successful, may be particularly useful where there is concern about the accuracy of the diagnosis and the possibility that residual cognitive function has remained undetected.

With due regard for the cause of the coma, and the rapidity of its onset, testing for the purpose of diagnosing death on brainstem death grounds may be delayed beyond the stage where brainstem reflexes may be absent only temporarily – because the cerebral blood flow is inadequate to support synaptic function although there is still sufficient blood flow to keep brain cells alive and capable of recovery. There has recently been renewed interest in the possibility of neuronal protection during this phase by use of moderate hypothermia and by correction of the neuroendocrine abnormalities commonly seen in this early stage.

Published studies of patients meeting the criteria for brainstem death or whole brain death – the American standard which includes brainstem death diagnosed by similar means – record that even if ventilation is continued after diagnosis, the heart stops beating within only a few hours or days. However, there have been some very long-term survivals and it is noteworthy that expert management can maintain the bodily functions of pregnant brain dead women for long enough to bring them to term.

The management of patients pronounced dead on meeting the brainstem death criteria depends upon the reason for diagnosing death on that basis. If the intent is to take organs from the body for transplantation, the ventilator is reconnected and life-support measures are continued, perhaps intensified, with the addition of procedures designed to protect the wanted organs until they can be removed. Otherwise, the ventilator is left disconnected on confirmation of the lack of respiratory centre response.

Disorders of consciousness present a variety of ethical concerns.

Most obvious is the lack of consent in any treatment decisions. Patients in PVS or MCS are not able to decide for the possibility of withdrawal of life-support. It is also a general question whether they should receive life-sustaining therapy and, if so, for how long? The problems regarding a patient's consent also account for neuroimaging studies. Without patient's consent, such studies are perceived as unethical.

Additionally only few patients have created advance directives before losing decision-making capacity.

Typically approval must be obtained from family or legal representatives depending on governmental and hospital guidelines.

But even with the consent of representatives, researchers have been refused grants, ethics committee approval and publication.

Social issues arise from the enormous costs that are caused by people with disorders of consciousness. Especially chronic comatose and vegetative patients, when recovery is highly unlikely and treatment in the ICU is considered futile by clinicians.

In addition to the aforementioned problems, the question rises why medical resources were being used not for the broader public good but for patients who seemed to have only little to gain from them.

Still research is everything but sure about the irreversibility of these conditions. Some studies demonstrated that some patients suffering from disorders of consciousness may be aware despite clinical unresponsiveness. These recent findings could have a major impact on ethical and social issues.

While the diagnosis of brain death has become accepted as a basis for the certification of death for legal purposes, it should be clearly understood that it is a very different state from biological death - the state universally recognized and understood as death. The continuing function of vital organs in the bodies of those diagnosed brain dead, if mechanical ventilation and other life-support measures are continued, provides optimal opportunities for their transplantation.

When mechanical ventilation is used to support the body of a brain dead organ donor pending a transplant into an organ recipient, the donor's date of death is listed as the date that brain death was diagnosed.

In some countries (for instance, Spain, Finland, Poland, Wales, Portugal, and France), everyone is automatically an organ donor after diagnosis of death on legally accepted criteria, although some jurisdictions (such as Singapore, Spain, Wales, France, Czech Republic and Portugal) allow opting out of the system. Elsewhere, consent from family members or next-of-kin may be required for organ donation. In New Zealand, Australia, the United Kingdom (excluding Wales) and most states in the United States, drivers are asked upon application if they wish to be registered as an organ donor.

In the United States, if the patient is at or near death, the hospital must notify a transplant organization of the person's details and maintain the patient while the patient is being evaluated for suitability as a donor. The patient is kept on ventilator support until the organs have been surgically removed. If the patient has indicated in an advance health care directive that they do not wish to receive mechanical ventilation or has specified a do not resuscitate order and the patient has also indicated that they wish to donate their organs, some vital organs such as the heart and lungs may not be able to be recovered.

Misdiagnosis of PVS is not uncommon. One study of 40 patients in the United Kingdom reported that 43% of those patients classified as in a PVS were misdiagnosed and another 33% able to recover whilst the study was underway. Some cases of PVS may actually be cases of patients being in an undiagnosed minimally conscious state. Since the exact diagnostic criteria of the minimally conscious state were formulated only in 2002, there may be chronic patients diagnosed as PVS before the notion of the minimally conscious state became known.

Whether or not there is conscious awareness in vegetative state is a prominent issue. Three completely different aspects of this issue should be distinguished. First, some patients can be conscious simply because they are misdiagnosed (see above). In fact, they are not in vegetative state. Second, sometimes a patient was correctly diagnosed but is then examined during the early stages of recovery. Third, perhaps some day the very notion of the vegetative state will change so as to include elements of conscious awareness. Inability to disentangle these three cases leads to confusion. An example of such confusion is the response to a recent experiment using functional magnetic resonance imaging which revealed that a woman diagnosed with PVS was able to activate predictable portions of her brain in response to the tester's requests that she imagine herself playing tennis or moving from room to room in her house. The brain activity in response to these instructions was indistinguishable from those of healthy patients.

In 2010, Martin Monti and fellow researchers, working at the MRC Cognition and Brain Sciences Unit at the University of Cambridge, reported in an article in the "New England Journal of Medicine" that some patients in persistent vegetative states responded to verbal instructions by displaying different patterns of brain activity on fMRI scans. Five out of a total of 54 diagnosed patients were apparently able to respond when instructed to think about one of two different physical activities. One of these five was also able to "answer" yes or no questions, again by imagining one of these two activities. It is unclear, however, whether the fact that portions of the patients' brains light up on fMRI could help these patients assume their own medical decision making.

In November 2011, a publication in "The Lancet" presented bedside EEG apparatus and indicated that its signal could be used to detect awareness in three of 16 patients diagnosed in the vegetative state.

It is very important for family members and health care professionals to be aware of natural movements also known as Lazarus sign or Lazarus reflex that can occur on a brain-dead person whose organs have been kept functioning by life support. The living cells that can cause these movements are not living cells from the brain or brain stem, these cells come from the spinal cord. Sometimes these body movements can cause false hope for the family members.

A brain-dead individual has no clinical evidence of brain function upon physical examination. This includes no response to pain and no cranial nerve reflexes. Reflexes include pupillary response (fixed pupils), oculocephalic reflex, corneal reflex, no response to the caloric reflex test, and no spontaneous respirations.

It is important to distinguish between brain death and states that may be difficult to differentiate from brain death, (such as barbiturate overdose, alcohol intoxication, sedative overdose, hypothermia, hypoglycemia, coma, and chronic vegetative states). Some comatose patients can recover to pre-coma or near pre-coma level of functioning, and some patients with severe irreversible neurological dysfunction will nonetheless retain some lower brain functions, such as spontaneous respiration, despite the losses of both cortex and brain stem functionality. Such is the case with anencephaly.

Note that brain electrical activity can stop completely, or drop to such a low level as to be undetectable with most equipment. An EEG will therefore be flat, though this is sometimes also observed during deep anesthesia or cardiac arrest. Although in the United States a flat EEG test is not required to certify death, it is considered to have confirmatory value. In the UK it is not considered to be of value because any continuing activity it might reveal in parts of the brain above the brain stem is held to be irrelevant to the diagnosis of death on the Code of Practice criteria.

The diagnosis of brain death needs to be rigorous, in order to be certain that the condition is irreversible. Legal criteria vary, but in general they require neurological examinations by two independent physicians. The exams must show complete and irreversible absence of brain function (brain stem function in UK), and may include two isoelectric (flat-line) EEGs 24 hours apart (less in other countries where it is accepted that if the cause of the dysfunction is a clear physical trauma there is no need to wait that long to establish irreversibility). The patient should have a normal temperature and be free of drugs that can suppress brain activity if the diagnosis is to be made on EEG criteria.

Also, a radionuclide cerebral blood flow scan that shows complete absence of intracranial blood flow must be considered with other exams – temporary swelling of the brain, particularly within the first 72 hours, can lead to a false positive test on a patient that may recover with more time.

CT angiography is neither required nor sufficient test to make the diagnosis.

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.

Although MCS patients are able to demonstrate cognitively mediated behaviors, they occur inconsistently. They are, however, reproducible or can be sustained long enough to be differentiated from reflexive behavior. Because of this inconsistency, extended assessment may be required to determine if a simple response (e.g. a finger movement or a blink) occurred because of a specific environmental event (e.g. a command to move the finger or to blink) or was merely a coincidental behavior. Distinguishing between VS and MCS is often difficult because the diagnosis is dependent on observation of behavior that show self or environmental awareness and because those behavioral responses are markedly reduced. One of the more common diagnostic errors involving disorders of consciousness is mistaking MCS for VS which may lead to serious repercussions related to clinical management.

Giacino et al. have suggested demonstration of the following behaviors in order to make the diagnosis of MCS.

- Following simple commands.

- Gestural or verbal yes/no responses (regardless of accuracy).

- Intelligible verbalization.

- Purposeful behavior such as those that are contingent due to appropriate environmental stimuli and are not reflexive. Some examples of purposeful behavior include:

- appropriate smiling or crying in response to the linguistic or visual content of emotional but not to neutral topics or stimuli.

- vocalizations or gestures that occur in direct response to the linguistic content of questions.

- reaching for objects that demonstrates a clear relationship between object location and direction of reach.

- touching or holding objects in a manner that accommodates the size and shape of the object.

- pursuit eye movement or sustained fixation that occurs in direct response to moving or salient stimuli.

Oneirophrenic patients are resistant to insulin and when injected with glucose, these patients take 30 to 50% longer to return to normal glycemia. The meaning of this finding is not known, but it has been hypothesized that it may be due to an insulin antagonist present in the blood during psychosis. However, There is currently no known treatment for oneiophrenia.

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.

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).

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.

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.

According to a May 2014 article published in NewScientist, spectral analysis may help clinicians find objective evidence for sleep state misperception:

The DSM-IV-TR states that the fugue may have a duration from days to months, and recovery is usually rapid. However, some cases may be refractory. An individual usually has only one episode.

Locked-in syndrome can be difficult to diagnose. In a 2002 survey of 44 LIS patients, it took almost 3 months to recognize and diagnose LIS after the patient had suffered the incident (i.e., a stroke or an injury) that had caused his/her LIS. Locked-in syndrome may mimic loss of consciousness in patients, or, in the case that respiratory control is lost, may even resemble death. Patients are also unable to actuate standard motor responses such as withdrawal from pain; as a result, testing often requires making requests of the patient such as blinking or vertical eye movement.

Brain imaging may provide additional indicators of locked-in syndrome, as brain imaging provides clues as to whether or not brain function has been lost. Additionally, an EEG can allow the observation of sleep-wake patterns indicating that the patient is not unconscious but simply unable to move.

Since hypermetabolism itself is a symptom and not an independent disease, treatment first and foremost requires attention to the underlying disease. Usually once the underlying cause is remedied, the symptoms will subside. The duration of symptoms depends upon the severity of the illness or trauma. Although hypermetabolism is a potentially dangerous condition that usually signals an underlying issue, it is one of the body’s strongest defenses against illness and injury.

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.

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.

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.