Consciousness: definition, structure and characteristics

In clinical neurology, consciousness is conveniently described as two interconnected axes: wakefulness (the ability to be "on") and awareness (the ability to perceive oneself and the world, to create meaningful experiences). This division helps explain why states are possible in which the eyes are open but there is no conscious response, and conversely, when awareness is partially preserved but external communication is impossible. [1]

Consciousness is not reducible to "intelligence" or "memory." Intelligence may be high, but in delirium, reasoning breaks down due to impaired attention and orientation. Memory may be impaired, but the person remains fully conscious and communicative. Therefore, in medicine, the state of consciousness and attention are assessed first, and then individual cognitive domains. [2]

Another important clinical principle: consciousness is assessed not only by verbal expressions. Evidence of awareness can include purposeful movements on command, sustained gaze tracking, meaningful responses to stimuli, and attempts at communication, not simply eye opening or groaning. This is why current guidelines emphasize the need for standardized behavioral scales and repeated assessments. [3]

Finally, the term "impaired consciousness" in emergency medicine often encompasses a broad spectrum, from mild confusion to coma. Causes can be neurological, infectious, metabolic, toxic, and sometimes mixed, so the approach always begins with ruling out life-threatening and reversible causes. [4]

Table 1. Terms that are most often confused

Term	What does it mean in simple words?	Why is it important to distinguish?
Wakefulness	the ability to wake up, open eyes, and maintain a level of activation	may be without awareness in severe brain damage
Mindfulness	the presence of meaningful experience and targeted responses	key criterion for diagnosing disorders of consciousness
Confusion	decreased clarity of thinking, attention, and orientation	often reversible, but requires searching for the cause
Delirium	acute fluctuating disturbance of attention and consciousness plus cognitive symptoms	associated with complications and increased mortality
Coma	lack of wakefulness and awareness, eyes closed, no target responses	a high-threat condition that requires urgent diagnosis

[5]

Neural systems that support wakefulness and awareness

Wakefulness is maintained by ascending activation systems in the brainstem and their connections with the thalamus, basal structures, and cortex. These networks regulate the overall level of activation and the cortex's "readiness" to process information, which is why lesions in the brainstem, diencephalic structures, and their pathways often lead to severe impairment of consciousness. [6]

The thalamus is considered an important node involved in both maintaining consciousness and organizing the content of experience through connections with the cortex. Recent reviews emphasize that the contribution of the thalamic nuclei is heterogeneous: different nuclei are connected to different cortical networks, meaning that clinical manifestations depend on which circuits are involved. [7]

Awareness is closely linked to the coordinated functioning of distributed cortical networks and their interactions with subcortical structures. Clinically, this is evident from the fact that diffuse white matter damage, severe hypoxic lesions, and major bilateral cortical dysfunctions can disrupt the integrity of experience and goal-directed behavior even in the presence of partially preserved wakefulness. [8]

An important practical conclusion: "consciousness" is not located in a single point in the brain. It depends on the connectivity and dynamics of networks, which is why diagnostics utilize not only physical examination but also methods that reflect the functional state of the brain: electroencephalography, neuroimaging, and assessment of brainstem reflexes in a clinical context. [9]

Table 2. Structures and functions most often discussed in connection with consciousness

Level	Key elements	What happens when there is dysfunction?
Brain stem	ascending activating systems, brainstem reflexes	depression of wakefulness, risk of coma
Thalamus	communication nodes between the subcortex and the cortex	disruption of integration and "assembly" of experience
Frontoparietal networks	attention, control, target responses	disintegration of target behavior, decreased awareness
Diffuse connections	white matter and interstitial circuits	"communication breakdown" between areas of the brain

[10]

Normal and transitional states: sleep, anesthesia, post-sleep stupor

Sleep and wakefulness are not "on or off." Sleep consists of stages that differ in physiology and reactivity, and upon awakening, sleep inertia may persist for a while, with thought speed and attention temporarily reduced. In everyday life, this manifests as a brief stupor and slowness upon awakening, especially with sleep deprivation. [11]

Sedation and general anesthesia also produce controlled changes in consciousness, but it is clinically important to remember that drug effects can mask the true neurological status. Therefore, in intensive care, when assessing consciousness, care is taken to consider the dosage of sedatives, the timing of their administration, and the possibility of "awakening pauses," if safe. [12]

A separate category is acute reversible conditions with fluctuations in consciousness and attention. The most typical example is delirium: it often occurs in hospitalized patients, especially the elderly, and is associated with an increased risk of complications, prolonged hospitalization, and increased mortality. Therefore, delirium is considered an emergency requiring identification of the cause and correction of risk factors. [13]

Seizures can also impair consciousness, sometimes without noticeable major seizures, making recognition difficult. In such situations, electroencephalography plays a significant role because it helps identify subtle epileptic activity and distinguish it from metabolic or toxic depression of consciousness. [14]

Table 3. How to distinguish common “similar” conditions according to clinical logic

State	Typical dynamics	What most often helps to distinguish
Post-sleep stupor	minutes, less often tens of minutes	connection with awakening and rapid recovery
Sedation	depends on the drug and dosage	medication history and temporal relationship to administration
Delirium	hours and days, often fluctuating	severe impairment of attention and orientation
Convulsive disturbance of consciousness	minutes and hours, sometimes in waves	electroencephalography and clinical signs

[15]

Disorders of consciousness in brain damage

In severe brain damage, several stable clinical states are distinguished, differing in the level of wakefulness, signs of awareness, and prognosis. Coma is characterized by the absence of wakefulness: the eyes are closed, there are no sleep-wake cycles, and no goal-directed responses. This may then lead to a state with open eyes and cycles, but without signs of conscious behavior, which in modern terms is often referred to as unresponsive wakefulness syndrome. [16]

A minimally conscious state is characterized by the appearance of repetitive signs of awareness: goal-directed responses, following simple commands, sustained visual tracking, and attempts at communication. This diagnosis is crucial for prognosis and rehabilitation planning, so current recommendations emphasize the need for careful differentiation and repeated examinations. [17]

A classic practical problem is frequent misclassification. Simple summary assessments, such as the Glasgow Coma Scale total score, may not reflect qualitative differences between diagnoses, especially in the intermediate ranges, and can be misleading in cases of intubation, aphasia, or motor limitations. Therefore, standardized behavioral scales and examinations focused on specific signs of awareness are considered more informative for the differential diagnosis of disorders of consciousness. [18]

Another important condition to remember is locked-in syndrome: consciousness is preserved, but external communication is nearly impossible due to paralysis. Misrecognition is particularly dangerous here, so in doubtful cases, eye movements and blinking are assessed as potential communication channels, and neurophysiological methods are used as indicated. [19]

Table 4. Key differences in conditions after severe brain injury

State	Wakefulness	Signs of awareness	Typical approach to assessment
Coma	No	No	clinical examination, brainstem reflexes, causes
Unresponsive wakefulness syndrome	There is	No	serial standardized assessment of behavior
Minimally conscious state	There is	there are, but they are inconstant	Coma Recovery Scale Revised and observation
Locked-in syndrome	There is	There is	search for communication channels, assessment of eye movements

[20]

How Doctors Assess Consciousness at the Bedside

The examination begins with the basics: response to voice and pain, orientation, speech, ability to follow commands, movement patterns, pupillary responses, and other brainstem reflexes. An important goal is to distinguish decreased alertness from communication impairments, when a person may be conscious but unable to speak or move. [21]

The Glasgow Coma Scale is widely used due to its simplicity and historical significance, but it has limitations: the total score does not always correspond to a specific diagnosis of disorder of consciousness, and identical scores can mask different clinical presentations. This is especially true in the moderate severity ranges, where variability is greatest. [22]

Therefore, the Full Outline of Unresponsiveness scale, which includes assessment of brainstem reflexes and respiratory pattern and may be more convenient for intubated patients, is increasingly being used in intensive care practice. Validation studies demonstrate good reproducibility and prognostic value of this scale, and modern reviews comparing scales note its advantages in intensive care and emergency departments. [23]

For long-term disorders of consciousness during the rehabilitation phase, the Coma Recovery Scale Revised is considered a key tool. Guidelines for disorders of consciousness emphasize that serial standardized assessments improve diagnostic accuracy and help monitor recovery, as well as reduce the risk of missing minimal signs of awareness. [24]

Table 5. Consciousness Rating Scales: Strengths and Limitations

Scale	What does it evaluate?	Where it is especially useful	Main limitations
Glasgow Coma Scale	eyes, speech, motor skills	initial assessment, trauma	difficulties with intubation, poor differentiation of diagnoses by the sum
Full Outline of Unresponsiveness	eyes, motor skills, brainstem reflexes, breathing	intensive care, intubation	requires skill in assessing reflexes and breathing
Coma Recovery Scale Revised	detailed behavioral assessment of mindfulness	rehabilitation and long-term conditions	staff training and serial production are needed

[25]

Diagnosis of the causes of decreased consciousness and general principles of treatment

In acute impairment of consciousness, the algorithm typically begins with ensuring the airway, breathing, and circulation while simultaneously searching for reversible causes. Common causes include infections, metabolic disorders, hypoxia, stroke, traumatic brain injury, intoxication, and drug effects. The approach is designed to avoid overlooking treatable causes that quickly lead to irreversible brain damage. [26]

Diagnostic steps are selected based on the clinical presentation, but typically include glucose measurement, oxygenation assessment, basic laboratory tests, electrolytes, inflammation markers as indicated, and toxicology screening as appropriate. Neuroimaging is used when stroke, hemorrhage, space-occupying lesion, or trauma is suspected, and electroencephalography is helpful when seizure activity is suspected or to clarify the cause of impaired consciousness. [27]

If delirium is the primary problem, the mainstay of treatment is identifying and correcting the underlying cause, normalizing the environment and sleep, controlling pain, eliminating triggering medications, ensuring hydration, and correcting sensory deficits. Guidelines emphasize the need for early recognition, as delirium is associated with adverse outcomes. [28]

For prolonged disturbances of consciousness following severe brain injury, current guidelines recommend eliminating confounding factors, optimizing wakefulness, and conducting repeated standardized assessments. For adults with traumatic brain injury 4-16 weeks post-injury, guidelines recommend prescribing amantadine at a dose of 100-200 mg twice daily to accelerate functional recovery in the early stages. [29]

Table 6. Common causes of acute loss of consciousness and initial diagnostic guidelines

Category of reasons	Examples	What is usually checked first?
Metabolic	hypoglycemia, electrolyte shifts, liver and kidney failure	glucose, electrolytes, liver and kidney function
Infectious	sepsis, meningitis, encephalitis	temperature, inflammatory markers, assessment of the lesion, puncture as indicated
Neurological	stroke, hemorrhage, trauma	neuroimaging, neurological examination
Toxic and medicinal	alcohol, opioids, sedatives, combined effects	anamnesis, toxicology, respiratory assessment
Convulsive	nonconvulsive status epilepticus	electroencephalography as indicated

[30]

Table 7. Signs requiring immediate emergency care

Sign	Why is it dangerous?
sudden loss of consciousness or seizures	risk of hypoxia and acute brain injury
severe drowsiness with the inability to wake up	risk of respiratory depression and coma
facial asymmetry, limb weakness, speech impairment	suspected stroke
high fever plus confusion and stiff neck	risk of meningitis or encephalitis
respiratory depression after medication or alcohol	risk of toxic damage and respiratory arrest

[31]

Result

Consciousness in medicine is most conveniently understood as a combination of wakefulness and awareness, supported by networks of the brainstem, thalamus, and cortex. [32]

Three things are key for clinical practice: standardized assessment of the condition, search for reversible causes in acute loss of consciousness, and serial examinations in long-term disorders of consciousness, because a single examination is often wrong. [33]