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Coma: Pathophysiology, Structural Basis, and Emergency Approach

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1. Definitions and Spectrum of Consciousness

• Plum and Posner's Diagnosis and Treatment of Stupor and Coma, pp. 20-30

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2. Neuroanatomical Basis of Consciousness: The Ascending Arousal System

The Ascending Arousal System (AAS)

• Cholinergic neurons - pedunculopontine nucleus (PPN) and laterodorsal tegmental nucleus (LDT): project to the thalamic relay nuclei (via the dorsal pathway), and to the lateral hypothalamus and basal forebrain. Fire fastest during REM sleep and wakefulness. Their activation depolarizes thalamic relay neurons allowing thalamocortical transmission.
• The thalamic reticular nucleus (GABAergic) acts as a gate - it hyperpolarizes relay neurons to suppress transmission during sleep; cholinergic input inhibits the reticular nucleus, releasing thalamocortical "gating."

• Noradrenergic - locus coeruleus (LC): diffuse cortical projections; promotes arousal
• Serotonergic - dorsal raphe nuclei
• Dopaminergic - ventral periaqueductal grey matter
• Histaminergic - tuberomammillary nucleus of posterior hypothalamus (a key "wake-promoting" nucleus; antihistamines cause sedation by blocking this)

• Orexin/Hypocretin neurons (lateral hypothalamus): stabilize the wake state; loss causes narcolepsy
• Basal forebrain cholinergic neurons: project to entire cortex

• Plum and Posner's, pp. 40-60

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3. Structural Lesions That Cause Coma

A. Bilateral Cerebral Hemisphere Injury

• Requires bilateral diffuse damage - unilateral hemisphere lesions do not cause coma (except when causing herniation)
• Causes: bilateral strokes, diffuse traumatic brain injury, global anoxia/ischemia, bilateral subdural hematomas, hypertensive encephalopathy, severe hypoglycemia
• Clinical: no focal signs initially; bilateral motor abnormalities; pupils usually preserved until late

B. Diencephalic (Thalamic/Hypothalamic) Injury

• The thalamus is the "relay hub" - bilateral thalamic damage mimics bilateral cortical injury
• Paramedian thalamic infarcts (from top-of-basilar syndrome) cause acute coma with characteristic small pupils and upward gaze palsy
• Posterior hypothalamic lesions disrupt orexin and histamine arousal pathways

C. Brainstem (Rostral Pons and Midbrain) Injury

• The most critical zone for arousal is the rostral pontine and midbrain tegmentum where the AAS is concentrated
• Lesions here (hemorrhage, infarct, demyelination, trauma) cause coma by directly damaging the AAS
• Clinical signs: abnormal eye movements, loss of brainstem reflexes, respiratory pattern changes, decerebrate posturing

Herniation Syndromes (Supratentorial Mass --> Brainstem Compression)

• Unilateral temporal lobe mass pushes uncus over the free edge of the tentorium
• Compresses CN III first: ipsilateral pupil dilates and becomes non-reactive ("blown pupil") - earliest sign
• Then compresses ipsilateral posterior cerebral artery: contralateral occipital infarct
• Progresses to compress midbrain: contralateral hemiparesis (ipsilateral Kernohan's notch phenomenon also possible)
• Rostrocaudal deterioration: midbrain --> pons --> medulla

• Bilateral or midline masses cause symmetric downward displacement of diencephalon through tentorium
• Early: small reactive pupils, Cheyne-Stokes breathing, decorticate posturing
• Middle stage: mid-position fixed pupils, hyperventilation, decerebrate posturing
• Late: pontine --> medullary failure, apnea, death

• Cerebellar tonsils herniate through foramen magnum
• Compresses medulla: sudden apnea, cardiovascular collapse
• Can be precipitated by lumbar puncture in the setting of posterior fossa mass

• Posterior fossa mass pushes upward through the tentorium
• Compresses dorsal midbrain: Parinaud syndrome (upgaze palsy, convergence-retraction nystagmus)

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4. Metabolic and Toxic Causes of Coma

• A - Alcohol, Anoxia (cardiac arrest)
• E - Epilepsy (postictal state, nonconvulsive status)
• I - Insulin (hypoglycemia)
• O - Opiate/Overdose
• U - Uremia
• T - Trauma, Temperature (hypothermia/hyperthermia)
• I - Infection (septic encephalopathy, meningitis/encephalitis)
• P - Psychiatric (rare - diagnosis of exclusion), Poisoning
• S - Stroke, Structural, Syncope

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5. Clinical Examination of the Comatose Patient

A. Respiratory Pattern

B. Pupils

C. Eye Movements

• Resting eye position: Conjugate horizontal deviation toward a hemisphere lesion (away from hemiplegia); conjugate deviation away from a pontine lesion (toward hemiplegia)
• Doll's eye reflex (oculocephalic): Turn head rapidly side to side - intact eyes move contralaterally (positive = brainstem intact); absent in brainstem lesion or deep metabolic coma
• Caloric testing (oculovestibular): Cold water irrigated into one ear - tonic deviation toward that side if brainstem intact; absent in brainstem failure. More sensitive than doll's eyes.
• Vertical gaze palsy (upward): Pretectal/dorsal midbrain (Parinaud), top-of-basilar syndrome
• Ocular bobbing: Spontaneous fast downward then slow return - pontine destruction

D. Motor Responses

• Decorticate (flexor) posturing: Arm flexion, leg extension - lesion above red nucleus (diencephalon/internal capsule)
• Decerebrate (extensor) posturing: Arm and leg extension, pronation - lesion at midbrain/upper pons level; indicates more severe dysfunction
• Flaccid: Pontine-medullary level or severe metabolic

E. Glasgow Coma Scale and FOUR Score

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6. Approach to the Unconscious Patient in the ED

STEP 1: Simultaneous Resuscitation (First 1-2 Minutes)

• Airway: Assess and protect - GCS ≤8 typically warrants intubation (RSI); assume full stomach; C-spine precautions if trauma
• Breathing: Oxygenation (SpO2 >94%), capnography
• Circulation: IV access x2, BP, HR, cardiac monitoring, 12-lead ECG
• Glucose (STAT fingerstick): Give dextrose 50% 50mL IV if hypoglycemic (or if glucose unknown and cannot be checked promptly)
• Temperature: Treat hyperthermia/hypothermia

STEP 2: The "Coma Cocktail" (Empiric Treatment While Awaiting Results)

1. Dextrose 50% (D50W): 50mL IV if glucose <60 mg/dL or unknown - reversal of hypoglycemic coma is rapid and complete
2. Thiamine 100mg IV: Give before or with dextrose in any malnourished/alcoholic patient (prevent precipitating Wernicke's)
3. Naloxone 0.4-2mg IV/IM/IN: If opioid toxicity suspected (pinpoint pupils, respiratory depression, bradycardia); titrate to respiratory effort rather than full reversal (avoid acute withdrawal/agitation)
4. Flumazenil: Use cautiously - contraindicated if benzodiazepine dependence or mixed TCA/BZD ingestion; risk of seizures

STEP 3: Rapid History (Parallel with Examination)

• Tempo: abrupt (structural - SAH, ICH, cardiac arrest) vs. gradual (metabolic)
• Preceding symptoms: headache (SAH, meningitis, ICH), fever (infection, serotonin syndrome), seizure (postictal), chest pain, palpitations
• Medications and drug use: insulin, anticoagulants, opioids, TCAs, antiepileptics
• Past medical history: diabetes, epilepsy, atrial fibrillation, liver disease, renal disease, prior stroke/neurosurgery
• Trauma: even minor head trauma can cause epidural/subdural hematoma
• Alcohol/substance use

STEP 4: Targeted Physical Examination

• Fever: infection (meningitis, septic encephalopathy), heat stroke, neuroleptic malignant syndrome, anticholinergic toxidrome
• Hypothermia: hypothyroidism, hypoglycemia, sedative overdose, environmental
• Hypertension: hypertensive encephalopathy, ICH, Cushing's response (late sign of raised ICP: HTN + bradycardia + irregular breathing)
• Hypotension: shock, Addisonian crisis, sepsis

• Head: scalp lacerations, raccoon eyes (anterior fossa fracture), Battle's sign (mastoid bruising = posterior fossa fracture), CSF rhinorrhea/otorrhea
• Neck: nuchal rigidity (meningitis; do not forcibly flex if trauma suspected)
• Skin: jaundice (hepatic), cyanosis, cherry-red (CO poisoning), needle tracks, petechiae/purpura (meningococcemia), diaphoresis (hypoglycemia, sympathomimetics)
• Breath: ketones (DKA), fetor hepaticus (liver failure), uremic fetor, alcohol

• Pupils (size, symmetry, light reflex)
• Eye position and movements (doll's eyes, caloric if needed)
• Corneal reflexes
• Gag/cough reflex
• Respiratory pattern
• Motor responses to pain (nail bed pressure, supraorbital ridge)
• Reflexes (hyperreflexia + extensor plantars = UMN, hyporeflexia = metabolic/toxic)
• Fundoscopy: papilledema (raised ICP), subhyaloid hemorrhage (SAH), hypertensive retinopathy

STEP 5: Emergency Investigations (Parallel with Exam)

• Fingerstick glucose
• 12-lead ECG (MI, arrhythmia, Brugada, TCA toxicity wide QRS, Osborn J waves in hypothermia)
• Pulse oximetry

• CBC, CMP (Na, K, Cl, HCO3, BUN, Cr, glucose, Ca, Mg, PO4)
• LFTs, ammonia
• ABG (pH, pCO2, pO2, lactate)
• Troponin
• Coagulation (PT/INR, aPTT)
• Thyroid function (TSH)
• Cortisol (if adrenal crisis possible)
• Serum and urine toxicology screen
• Blood cultures x2 if sepsis/meningitis suspected
• Thiamine level, B12

• Non-contrast CT head (URGENT): First-line; detects ICH, SAH, large ischemic stroke, hydrocephalus, mass lesion, midline shift
• CT angiography: if basilar artery occlusion suspected (sudden coma, pinpoint pupils, locked-in), or aneurysmal SAH
• MRI brain (if available): superior for brainstem lesions, diffuse axonal injury, herpes encephalitis, posterior reversible encephalopathy syndrome (PRES), venous thrombosis

• Do NOT perform before CT if papilledema, focal deficits, or concern for mass/herniation
• Indicated after CT if: meningitis suspected, SAH suspected with negative CT (xanthochromia), encephalitis
• If bacterial meningitis strongly suspected and CT will delay LP: draw blood cultures, start empiric antibiotics (ceftriaxone 2g + vancomycin + dexamethasone), then perform LP

• Early EEG if nonconvulsive status epilepticus (NCSE) suspected - can mimic structural coma with no motor activity
• Especially after clinical seizures; unexplained persistent coma; subtle eye deviations or nystagmus

STEP 6: Disease-Specific Treatment

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7. Specific Warning Signs Requiring Immediate Action

• Unilateral fixed dilated pupil = uncal herniation - immediate CT, neurosurgery, ICP management
• Bilateral pinpoint pupils = opioid toxicity (reversible) or pontine hemorrhage
• Absent brainstem reflexes = severe structural or metabolic coma; brain death protocol if applicable
• Hypoglycemia = most treatable cause - check and treat immediately in every case
• Cushing's reflex (HTN + bradycardia + irregular respirations) = impending herniation
• Fever + meningismus + coma = bacterial meningitis; start antibiotics before CT if LP delay anticipated

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8. Key Prognostic Factors

• Cause is the strongest predictor: toxic/metabolic causes (especially hypoglycemia, sedatives) have the best prognosis; anoxic coma after cardiac arrest carries a poor prognosis
• Duration: Longer coma = worse outcome
• GCS motor score at 72 hours and absence of brainstem reflexes are key prognostic markers post-cardiac arrest
• Myoclonic status epilepticus after global anoxia is generally a sign of severe cortical damage and poor prognosis

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Sources

• Plum and Posner's Diagnosis and Treatment of Stupor and Coma (9780190208875), pp. 19-268 - definitive reference on structural basis and physiology
• Goldman-Cecil Medicine, 26th ed. (9780323930345) - clinical clues table and neurological examination
• Tintinalli's Emergency Medicine, 9th ed. (9781260019933) - ED approach, FOUR score, laboratory workup
• Bradley and Daroff's Neurology in Clinical Practice (9780323642613) - spectrum of consciousness disorders