Impaired Consciousness

Consciousness is a continuum with full alertness and awareness at one end of the spectrum and brain death at the other. Alteration of consciousness is a frequent admission diagnosis to critical care services.

Most patients require immediate and often extensive diagnostic workup as both time to diagnosis and treatment initiation are decisive factors for brain recovery.

The two basic components of consciousness, arousal and awareness, are clinically used to classify the state of consciousness, but, strictly speaking, the qualitative examination of arousal should be separated from the quantitative assessment of the patient’s self- and environmental awareness and cognitive function. These interdependent components are regulated by different neuroanatomic domains within the central nervous system.

Arousal and ensuing wakefulness are tightly dependent on the network function of the ascending reticular activating system (ARAS), namely,

• the midbrain reticular formation,
• messencephalic nucleus and tegmentum,
• thalamic intralaminar (centromedian) nucleus, and
• dorsal hypothalamus.

These entities build a network of brainstem and diencephalic centers with a strong connection to the cerebral cortices.

Self-awareness and the spectrum of cognitive interactions with the environment are the purview of the hemispheres.

Simplistically, the hemispheres are structured into three functional systems:

1. units that receive, process, and store sensory information;
2. units that generate and regulate motor activity;
3. and units responsible for programming, regulating, and verifying actions.

These three functional systems operate as an intercortical network; relate, as a whole, to the experience of awareness; and allow cognitive performance.

The partition of arousal network and intercortical (cognitive) network implies that the level of self-awareness and cognitive performance cannot be examined in a patient with lack of arousal.

In other words, in a person with a quantitatively reduced state of consciousness, the qualitative assessment of the content of consciousness is not possible.

Practically speaking, the assessment of content (i.e., testing a patient’s cognitive abilities and thinking) should be done after the best level of arousal has been achieved.

A fully aroused person can express completely normal or deficient awareness and cognitive performance. Conversely failure of arousal renders it impossible to test awareness and cognition.

When evaluating a patient’s mental status, the state of consciousness should be viewed as a continuum ranging from the patient who has full alertness and cognitive lucidity to the deeply comatose patient; it is not an all-or-nothing phenomenon.

An all-or-nothing approach limits the interpretation of remaining brain function and, hence, diagnostic certainty.

Furthermore, the duration and time development of coma Opens in new window are helpful diagnostic features and should complement the quantitative and qualitative assessment of consciousness.

Standard classification systems categorize consciousness based on systematic testing of arousal, awareness, and content. The following section delineates clinically important syndromes.

Concept and Terminology of Impaired Consciousness

1. Awake state

In the awake state, the examination allows the clinician to test a person for the degree of self-awareness and intactness of cognitive function. Drowsiness, sleepiness, and lethargy resemble reduced spontaneous physical and mental activities in a person who cannot sustain wakefulness without repeated external stimulation.

They are somewhat comparable to the experience of lighter sleep, though drowsy patients almost always have reduced attention and concentration and some degree of associated mild confusion.

2. Coma

Coma Opens in new window is a continuous state of unresponsiveness manifested by an inability to arouse to vigorous (noxious) external or internal stimuli. The degree of coma can differ:

• Whereas the deepest coma examination shows an absence of any response, including brainstem reflex responses (i.e., lack of oculo- and pupillomotoric responses). Some cyclic autonomic activity such as the sleep-wake cycle and changes in motor tone may coexist.
• Lighter stages (sometimes denoted as semicoma) can be identified by brief moaning to strong stimulation in association with changes in autonomic function.

3. Vegetative state

The vegetative state is characterized by the complete absence of behavioral evidence for self- or environmental awareness.

This state can follow coma and identifies a state in which brainstem and diencephalic (thalamic) activity is present to a degree that clinical signs of spontaneous or stimulus-induced arousal and sleep-wake cycles are observed.

Patients often show blink responses to light; intermittent eye movements (sometimes erroneously interpreted as following objects or looking at family members); stimulus-sensitive automatisms such as swallowing, bruxism, and moaning; or primitive motor responses. If this state lasts longer than 30 days, it is referred to as persistent vegetative state (PVS) and is used as a descriptive clinical syndrome rather than a disease-specific entity. The most common causes include cardiac arrest, head trauma, severe brain infections, and various causes of thalamic injury. Vegetative states can also be seen in the terminal phase of degenerative illnesses such as Alzheimer’s disease.

4. Stupor

Stupor Opens in new window refers to a state in which the patient can be roused only by vigorous and repeated stimuli but the state of arousal cannot be sustained without repeated external stimulation. In this state, the patient is unresponsive to stimuli unless their application is very strong and repeated.

In stupor, verbal output is unintelligible or nil and some purposeful movement to noxious stimulation may be noted. Restless or stereotyped motor activity is common and there is a reduction or elimination of the natural shifting of body positions. Although apparently alert, the stuporous patient will initiate no spontaneous movement or speech, stare blankly and seemingly take nothing in.

5. Minimal conscious state

Minimal conscious state can be diagnosed in patients displaying some but often inconsistent behavioral evidence of awareness of the environment, but they cannot communicate their content and are unable to follow instructions reliably.

This state describes a large group of patients who are different from vegetative patients in that they demonstrate some signs of awareness of themselves and their surroundings, albeit inconsistently.

The time progression and persistence of the impairment can be used to further classify abnormalities of consciousness.

6. Delirium

Delirium Opens in new window is classified as a mental disorder because it involves a fluctuating level of consciousness and pervasive impairment in mental, behavioral, and emotional functioning. It is commonly acute in onset and short in duration and is frequently correlated to a specific etiology such as medications, anesthesia, or sleep deprivation.

The criteria for delirium listed in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) include a disturbance of consciousness with reduced ability to sustain or shift attention and focus.

Attention and concentration deficits are always present in delirious patients and accompanied by a variable degree of cognitive dysfunction unattributed to pre-existing dementia. The dysfunction of this acute syndrome include memory deficits, disorientation Opens in new window, disturbance of language and situational perception, and disorganized behavior.

7. Dementia

Dementia Opens in new window is a chronic condition in which content of consciousness is affected initially, but in advanced stages reduced levels of arousal are seen.

Dementia is generally progressive and affects memory and at least two other cognitive domains such as language, executive function, planning motor tasks, and recognition. Dementing illnesses include a variety of intrinsic degenerative diseases of the cerebral hemispheres, but they also result from many other causes such as traumatic brain injury and hydrocephalus.

The underlying etiology can be reversible (i.e., after correction of a vitamin deficiency) or irreversible (Lewy body dementia). Additionally, disease progression can be highly variable, lasting weeks to years, often with fluctuating severity, or be static after the initial impact injury.

Anxiety Opens in new window, depression, and agitation may alternate and greatly complicate the disease course. The anatomically affected neurologic structures are commonly a result of bihemispheric cortical dysfunction; however, subcortical structures (i.e., vascular dementia from multifocal small vessel disease) can also represent the predominant pathology.

8. Akinetic mutism

Akinetic mutism is silent, alert-appearing immobility of a patient with injury to the hypothalamus or basal forebrain. It manifests as apparent depressed levels of consciousness in a patient with well-informed sleep-wake cycles and with little or no evidence of awareness or spontaneous motor activity.

Various etiologies can present or lead to an akinetic mute state, and it is imperative to have a rigorous neurologic examination and careful review of neuroimaging and electroencephalography (EEG).

Overview of Consciousness

To be conscious is to be aware, both of objects that are perceivable and of oneself as a subjective being. Consciousness may be normal, heightened, or lowered.

Heightened consciousness involves an enhanced sense of awareness or arousal: colors are brighter and more vivid, sounds seem louder and crisper, and there is a greater sense of alertness. Heightened consciousness may be induced by psychoactive stimulants (e.g., amphetamines) or hallucinogens (LSD) and may also be found in early mania.

Lowered consciousness is distinguished from reduced wakefulness or sleep. There is a qualitative difference between the pathway from clear consciousness to sleep and the pathway from clear consciousness to coma. The difference is that the individual who is asleep can always be aroused to a state of clear consciousness with stimulation, unlike the patient with lowered or clouded consciousness.