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Neurological Assessment in the ICU: Coma and Consciousness
Introduction
Neurological assessment in the intensive care unit demands a systematic and reproducible approach to evaluating patients with impaired consciousness. The neurologist must distinguish between disorders of arousal and disorders of awareness, as these distinctions have profound implications for diagnosis, management, and prognosis.
Defining Consciousness
The Two Dimensions of Consciousness
Consciousness has two distinct dimensions. Arousal (wakefulness) is mediated by the ascending reticular activating system (ARAS) in the brainstem tegmentum, hypothalamus, and thalamus. Awareness (content) is mediated by widespread thalamocortical and corticocortical networks. Consciousness requires both intact arousal and awareness functioning together, and lesions disrupting either dimension produce clinically distinct syndromes.
Spectrum of Impaired Consciousness
Coma is eyes-closed unresponsiveness with no evidence of arousal or awareness. The vegetative state (unresponsive wakefulness syndrome) involves return of sleep-wake cycles without reproducible awareness. The minimally conscious state (MCS) shows inconsistent but reproducible evidence of awareness such as visual tracking, command following, or intelligible verbalization. The confusional state or delirium has preserved arousal with fluctuating and disorganized awareness.
The Structured Neurological Examination in the ICU
Glasgow Coma Scale (GCS)
The GCS evaluates eye opening (E1-E4), verbal response (V1-V5), and motor response (M1-M6) with a total score range of 3 to 15. Its limitations include inability to assess verbal response in intubated patients and failure to evaluate brainstem reflexes.
FOUR Score (Full Outline of UnResponsiveness)
| Domain | Score 4 | Score 3 | Score 2 | Score 1 | Score 0 |
|---|---|---|---|---|---|
| Eye (E) | Eyelids open, tracking or blinking to command | Eyelids open, not tracking | Eyelids closed, open to loud voice | Eyelids closed, open to pain | Eyelids remain closed with pain |
| Motor (M) | Thumbs up, fist, peace sign to command | Localizing to pain | Flexion response to pain | Extension response to pain | No response or generalized myoclonus |
| Brainstem (B) | Pupil and corneal reflexes present | One pupil wide and fixed | Pupil OR corneal reflexes absent | Pupil AND corneal reflexes absent | Absent pupil, corneal, and cough reflexes |
| Respiration (R) | Not intubated, regular breathing | Not intubated, Cheyne-Stokes pattern | Not intubated, irregular breathing | Intubated, breathes above ventilator rate | Intubated, breathes at ventilator rate or apnea |
The FOUR score evaluates four domains: eye response, motor response, brainstem reflexes, and respiration, each scored 0 to 4 (total 0-16). Its advantages over GCS include testability in intubated patients, capture of brainstem function, and ability to detect locked-in syndrome.
Brainstem Reflex Assessment
The pupillary light reflex tests the afferent CN II and efferent CN III pathways; clinicians assess pupil size, symmetry, and reactivity. The corneal reflex tests the afferent CN V1 and efferent CN VII. The oculocephalic reflex (doll's eyes) demonstrates intact brainstem function when conjugate contralateral eye deviation occurs with head rotation. The oculovestibular reflex (cold calorics) produces tonic eye deviation toward the irrigated ear in comatose patients with an intact brainstem when ice water is used. The gag and cough reflexes assess CN IX and X integrity.
Motor Response Patterns
Localizing refers to purposeful movement toward a noxious stimulus that crosses the midline. Flexion withdrawal is a stereotyped flexion without localizing. Decorticate posturing (abnormal flexion) produces upper extremity flexion and lower extremity extension, suggesting hemispheric or internal capsule dysfunction. Decerebrate posturing (extension) produces extension and internal rotation of all extremities, suggesting midbrain or pontine dysfunction. No response suggests extensive brainstem injury.
Ancillary Studies
Electroencephalography (EEG)
Continuous EEG (cEEG) is the gold standard for detecting nonconvulsive seizures and nonconvulsive status epilepticus. Background patterns correlate with depth of encephalopathy, ranging from alpha through theta, delta, burst-suppression, to electrocerebral silence. Reactivity of EEG to stimulation is a favorable prognostic indicator.
Neuroimaging
CT head without contrast provides emergent evaluation of structural causes including hemorrhage, herniation, and hydrocephalus. MRI with diffusion-weighted imaging (DWI) detects ischemic injury, diffuse axonal injury, and posterior reversible encephalopathy.
Evoked Potentials
Somatosensory evoked potentials (SSEPs) are particularly valuable: bilateral absence of cortical N20 responses strongly predicts poor outcome after cardiac arrest. Brainstem auditory evoked potentials (BAEPs) assess brainstem integrity from the cochlear nerve to the midbrain.
Etiological Approach to Coma
Structural Causes
Structural causes include supratentorial mass lesions causing transtentorial herniation, infratentorial lesions directly compressing or destroying the ARAS, and bilateral thalamic lesions (as in top-of-the-basilar syndrome or deep cerebral venous thrombosis).
Metabolic and Toxic Causes
These include hypoglycemia, hepatic encephalopathy, uremia, electrolyte derangements, drug intoxication (opioids, benzodiazepines, barbiturates), septic encephalopathy, thyroid storm, and myxedema coma.
Clinical Pearls
Reversible causes of coma must always be ruled out first: hypoglycemia, opioid overdose (naloxone trial), and nonconvulsive status epilepticus (urgent EEG). The FOUR score is superior to the GCS in the ICU because it can be applied to intubated patients and captures brainstem function. Asymmetric pupils in a comatose patient should prompt emergent imaging to evaluate for uncal herniation. A comatose patient who yawns, coughs, or swallows spontaneously has intact lower brainstem function, which is prognostically important. The neurological examination should be documented serially and precisely because trend changes are more informative than any single assessment.
References
- Posner JB, Saper CB, Schiff ND, Claassen J. Plum and Posner's Diagnosis and Treatment of Stupor and Coma. 5th ed. Oxford University Press; 2019.
- Wijdicks EFM, Bamlet WR, Maramattom BV, Manno EM, McClelland RL. Validation of a new coma scale: the FOUR score. Ann Neurol. 2005;58(4):585-593.
- Claassen J, Taccone FS, Horn P, et al. Recommendations on the use of EEG monitoring in critically ill patients. Clin Neurophysiol. 2013;124(10):1948-1957.
- Edlow JA, Rabinstein A, Traub SJ, Wijdicks EFM. Diagnosis of reversible causes of coma. Lancet. 2014;384(9959):2064-2076.