The most global issue in the study of human consciousness and its disorders is the nature of consciousness itself. Researchers and philosophers have been searching for an answer to it since ancient times, and it may be possible to approach it by combining the latest neuroscience data with philosophical theories.
From the point of view of physiology, consciousness includes two components: wakefulness (Eng. Arousal), that is, the ability to open eyes spontaneously and content of consciousness ( engliness awareness , contents of consciousness ), that is, the ability to consciously perceive information coming from purposefully respond to external incentives and arbitrarily perform any actions. The content of consciousness is determined by the functioning of the cerebral cortex. Wakefulness is ensured by the functioning of the ascending activating reticular system of the brain stem.
Types of impaired human consciousness
There are acute and chronic disturbances of human consciousness. Acute impairments of consciousness develop immediately after brain damage, which has resulted in the dysfunction of any of the components of the system that ensures the functioning of consciousness, primarily structures of the activating reticular system of the brain stem, as well as damage to the thalamus or diffuse damage of the white matter or cortex. These include stunning, stupor and coma, which represent varying degrees of oppression of wakefulness. In addition, a state of altered consciousness in waking patients (delirium) is isolated.
When stunned, the patient is awake, that is, spontaneously opens his eyes, but cannot perform actions associated with prolonged maintenance of attention. Sopor is a state of being an actor, reminiscent of a dream, from which the patient can be withdrawn for a short time only when using strong stimuli (loud sound or pain stimulus). Coma is also characterized by “undisturbability”: the patient is not able to open his eyes either spontaneously or in response to any stimulus, no matter how intense it may be. Both components of consciousness, both wakefulness and the content of consciousness, are absent in this case.
Depending on how severe the brain damage was, which led to the development of coma, the patient may experience other neurological disorders of varying severity (for example, paresis of the eye muscles, muscles of the face and limbs), as well as in varying degrees the functions of other body systems (for example, breathing control is often violated, and in all cases of coma, first of all, it is necessary to ensure adequate breathing and to start a ventilator). The duration of a coma is at least an hour and, as a rule, no more than four weeks (therefore, journalistic stories about people who have been in a coma for several years are incorrect, at least in terms of terminology).
Chronic disturbances of human consciousness are conditions that develop if the patient is “not completely” gone from the coma. For example, a person has a circulatory arrest lasting several minutes, which, of course, is accompanied by the development of coma; after resuscitation, the work of the heart was restored, over time, the activity of other body systems stabilized, and after a few days or weeks the patient opened his eyes – now this condition cannot be called a coma. He can recover spontaneous breathing, eyeball movements, blinking, and some limb movements. However, he does not react at all to the speech addressed to him, does not follow instructions, is not able to change body position on his own, and does not even fix his gaze and does not follow others with his eyes. Can say that restoration of wakefulness, in this case, is not accompanied by restoration of the content of consciousness, that is, dissociation occurs between the two components of consciousness. The conditions for this are the restoration of the function of the brain stem structures with the lost functions of the cerebral cortex.
Chronic impairment of consciousness includes the vegetative state / is active wakefulness syndrome and the state of minimal consciousness. A vegetative state is a clinical condition characterized by the complete absence of a patient (ie, a spontaneously opening eye) of a patient with signs of purposeful behavior, which indicate the patient’s awareness of his own personality or the surrounding reality. The state of minimal consciousness is a state accompanied by severe impairment of consciousness, in which, however, there are at least minimal and often unstable, but distinct signs of purposeful behavior, indicating that the patient is aware of his own personality or the surrounding reality. These signs include the localization of pain stimuli, tracking the eyes of others, targeted movements or emotions in response to appropriate external stimuli of the word, as well as at a higher level of awareness, following instructions, having an accessible understanding of speech and the ability to respond “yes” or “no” with gestures or words. These conditions are called chronic because patients may be in them for a long time – several months and years.
In patients who are in a vegetative state or a state of minimal consciousness, the functions of brain structures that control respiration, blood circulation, digestion, and so on, that is, autonomous (or vegetative, as reflected in the name of this syndrome) nervous system, are preserved. Patients are usually able to breathe without the aid of the ventilator and absorb food, they have chewing and swallowing movements, but such a complex motor act as swallowing food that gets into the mouth, which requires voluntary control of the muscles, is impossible. Therefore, they must be fed through a gastrostomy tube or a nasogastric tube, and to protect the lungs from draining the contents of the nasopharynx and for adequate rehabilitation of the tracheobronchial tree, they usually need a tracheostomy. In addition, patients have episodes similar to sleep, both externally and according to neurophysiological studies. There may be a similarity of lucid emotional reactions, such as crying or grimaces as in pain, and you need to carefully monitor when such reactions occur and if they are not related to any external stimulus that is significant for the patient, how this may be one of the signs of the beginning recovery of consciousness.
Also, speaking of disturbances of human consciousness, it is necessary to mention conditions that are clinically similar to them but do not apply to them. Locked-in syndrome most often occurs in infarction of the brain stem, resulting in tetraplegia (complete loss of movements of all extremities) and impaired function of cranial nerves, including the almost complete absence of eyeball movements, lack of articulation, and so on, but the consciousness is preserved. Since the patient is able to open his eyes, but because of the paralysis of almost all muscles, he cannot follow any instructions or in any other way make it clear that he is aware of the surrounding reality, this clinical picture may resemble a vegetative state. These syndromes can be distinguished with the help of a very thorough, repeated clinical examination for searching for conscious reactions, as well as with a neurophysiological study – an electroencephalogram (EEG) of patients with locked-in syndrome, as a rule, it will not differ from the EEG of a healthy and awake person.
Akinetic mutism can develop in some brain tumors and is characterized by the absence of voluntary movements and speech (in the absence of paresis, since the areas of the cortex controlling the corresponding muscles are not affected), while patients are conscious. As a rule, eye movements are preserved, patients can follow their eyes with those around them, and sometimes they can pronounce individual syllables and words. Often, after removal of the tumor, this syndrome regresses.
It is also necessary to mention brain death – a condition that develops with extremely severe brain damage and is an irreversible coma, which is determined by a number of reliable signs (in particular, the death of the respiratory center of the brain stem, which is diagnosed by the lack of response to a critical increase in carbon dioxide in the blood, or lack of blood flow in all intracranial vessels supplying the brain). Sometimes a “brain death” is mistakenly called a vegetative state (probably referring to the death of the cerebral cortex and recalling the outdated term “apallic syndrome”).
Causes of impaired human consciousness
The most common causes leading to the development of acute and chronic impairment of consciousness in adult patients are usually divided into traumatic (that is, the cause is severe traumatic brain injury) and non-traumatic. The latter include episodes of hypoxia due to circulatory arrest, a pronounced decrease in blood pressure for a long time, drowning and asphyxiation; severe cerebrovascular diseases (cerebral infarction and intracerebral hemorrhage or severe complication of subarachnoid hemorrhage), electrolyte disturbances, severe hypoglycemia, severe intoxication. In addition, the vegetative state may develop in the final stages of severe neurodegenerative diseases such as Alzheimer’s disease (in this case, the vegetative state is not preceded by a comma,
What happens when consciousness is disturbed?
In acute disorders of human consciousness, the lesion covers vast areas of the cerebral hemispheres, the thalamus, or the structures of the ascending activating system. In chronic impairments of consciousness from a morphological point of view, diffuse axonal damage is usually observed in combination with ischemic damage to the thalamus and zones of the adjacent blood supply. In case of nontraumatic disorders of consciousness, diffuse laminar necrosis of the cortex is most often detected, which is almost always combined with the death of neurons of the thalamus, hippocampus, tonsils. Multifocal coronary infarctions can also occur, often in areas of the adjacent blood supply that are combined with common ischemic foci in the thalamus. At the same time, stem structures as a whole remain intact.
When assessing the metabolic changes that develop in patients with chronic impairment of consciousness, PET showed a decrease in glucose metabolism, more pronounced in the vegetative state, and less so in the state of minimal consciousness. At the same time, there is no clear link between the level of metabolism and consciousness and the clinical outcome. Most likely, maintaining general cerebral metabolism at a certain level is a necessary but not sufficient condition for restoring awareness. The principal difference between the state of minimal consciousness and the vegetative state is the presence of small cortical “islands” with a healthy level of metabolism (usually they correspond to the sensory and motor zones and the frontoparietal cortex) and, on the contrary, homogeneous decrease in the activity of the cortex in patients in a vegetative state, without sites of normal metabolism. In addition, it is possible that there are critical areas whose metabolic rate is important for maintaining consciousness.
Prevalence of impaired consciousness
Information on the prevalence of chronic impairments of human consciousness in different countries is very fragmented. Epidemiological studies carried out so far only cover a small sample of the population. This is due both to the lack of diagnostic codes for the statistical classification of these syndromes and to the fact that after stabilization and discharge from the hospital, such patients often disappear from the doctors’ field of view and it is difficult to obtain reliable data on their condition. According to some data, in the USA the number of newly diagnosed patients with a vegetative state is about 4,200 cases per year. The prevalence of the vegetative state in the USA is 60–100 cases per million populations, in the Netherlands – 5 patients per million populations, in France – 483 per million populations, in Austria – 3.6 cases per 100,000 people.
Outcomes and prognosis for impaired consciousness
Despite the enormous progress in the intensive care of neurological and neurosurgical diseases, the chances of survival and good recovery in patients who are in a coma are still small. The prognosis for coma depends on many factors. One of the most important is etiology: with traumatic damage, the prognosis is usually better than with hypoxia. The longer the coma lasts, the less chance of a favorable outcome. In addition, many studies agree that the risk of death is highest with a low baseline Glasgow coma score, bilateral absence of pupil response to light, hypotension, as well as in elderly patients and in the presence of concomitant diseases.
With regard to chronic impairment of consciousness, dependence on the cause of the brain damage is preserved. According to the largest study, 33% of patients died during the first year after a coma of a traumatic etiology, whereas for non-traumatic reasons the mortality rate was 53%. After a twelve-month period, the average survival rate increases significantly and, with proper care, can approach the average life expectancy.
Regarding the prognosis regarding the recovery of human consciousness, the most important factors, according to the last systematic review (2018), are the presence or absence of signs of conscious activity (that is, the nature of the clinical syndrome – the vegetative state or the state of minimal consciousness) and damage etiology. It was established that the chances of achieving a less pronounced disability one year after a coma were greater in patients in a state of minimal consciousness and in patients who had a traumatic brain injury. Recovery data are very fragmented, and it is difficult to specify any reliable quantitative indicators. As an example, one can cite the results of meta-analyses, according to which, among patients in a vegetative state of traumatic etiology, recovery of signs of consciousness is observed in 78% of cases in a year;
Speaking about the prognosis, it is impossible to bypass the issue of the possibility of restoring human consciousness in patients for a long time, more than a year, which is in a state of chronic impairment of consciousness. The possibility of such a recovery is not questioned, but the chances of it are very small – up to 14% with a traumatic injury, and most often the patients remain severely disabled. It should also be noted that some of these cases are not associated with the return of consciousness as such, but with the use of better clinical and instrumental diagnostics, and the person, who was long considered unconscious, actually was able to hear, see and understand others, but not could demonstrate it.