Stages of Consciousness
Consciousness exists on a continuum from full alertness to complete unresponsiveness, with four primary stages defined by the NIH Stroke Scale: Alert (0), Drowsy (1), Obtunded (2), and Coma/Unresponsive (3). 1
Primary Classification System
The NIH Stroke Scale provides the most clinically relevant staging system for consciousness assessment 2, 1:
- Alert (Score 0): Patient is fully awake, attentive, and responds immediately without need for stimulation 1
- Drowsy (Score 1): Patient appears sleepy but opens eyes and becomes appropriately responsive when the examiner speaks or lightly touches their arm; may drift back to sleep when not actively engaged but can be easily awakened with verbal stimuli or light touch 1
- Obtunded (Score 2): Patient requires repeated or painful stimulation to arouse and maintain attention 2, 1
- Coma/Unresponsive (Score 3): Patient shows no response or only reflexive responses to stimulation 2, 1
Alternative Classification Systems
Traditional Clinical Staging
A more granular six-stage system is recognized in clinical practice 3:
- Hyperalertness: Heightened state of arousal beyond normal wakefulness 3
- Alertness: Normal state of wakefulness 3
- Somnolence or lethargy: Mild reduction in alertness 3
- Obtundation: Marked tendency to fall asleep 3
- Stupor: Requires vigorous stimulation for arousal 3
- Coma: Complete unresponsiveness with various subtypes including akinetic mutism, persistent vegetative state, and locked-in syndrome 3
Glasgow Coma Scale Framework
The American Academy of Neurology recommends the Glasgow Coma Scale (GCS) as providing an operational and robust description of consciousness 4. The GCS evaluates three components: eye opening (1-4 points), verbal response (1-5 points), and motor response (1-6 points), with total scores ranging from 3-15 4.
Critical Assessment Considerations
The FOUR Score should be prioritized over the GCS in severely affected, intubated, or suspected brainstem injury patients 4. The FOUR Score evaluates eye response, motor response, brainstem reflexes, and respiratory pattern, with 0-4 points per component 4.
Common Pitfalls in Assessment
- Do not confuse drowsiness with aphasia: A drowsy patient has altered arousal, while an aphasic patient may be fully alert but unable to communicate 1
- Avoid mistaking drowsiness for confusion: Drowsiness refers specifically to arousal level, not cognitive function 1
- Ensure the patient is not sedated from medications, which could artificially affect the consciousness score 1
Bidimensional Model of Consciousness
Consciousness involves two distinct dimensions that must be assessed separately 5:
- Level of arousal: The degree of wakefulness and responsiveness to stimuli 5
- Subjective awareness: Recognition of the contents of consciousness and the environment 5
These dimensions are positively correlated in normal physiological states with the exception of REM sleep, while a disturbance in this relationship characterizes vegetative state, minimally conscious state, complex partial seizures, and sleepwalking 5.
Neuroanatomical Basis
The ascending reticular activating system (ARAS) in the mesencephalic formatio reticularis serves as the motor of consciousness 3. The ARAS projects via three pathways: (1) through the reticular thalamus diffusely to the cortex, (2) via hypothalamus to the basal forebrain and limbic system, and (3) to the medial raphe of the brainstem and locus coeruleus with their diffuse cortical projections 3. Stimulation of the ARAS produces an arousal reaction; its destruction causes coma and related states 3.
Clinical Significance
The consciousness level assessment contributes to the overall NIHSS score, which helps identify the possible location of vessel occlusion and provides early prognosis in stroke patients 1. Early recognition of altered consciousness levels helps guide appropriate interventions and predict potential complications 1.