Neurology · Altered Mental Status
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The initial management of any patient with altered mental status or coma requires stabilization of the ABCs (Airway, Breathing, Circulation) followed by the administration of dextrose, thiamine, and naloxone if the etiology is unknown.
A fixed and dilated pupil in a comatose patient is a classic sign of uncal herniation causing oculomotor nerve (CN III) compression.
Decorticate posturing (flexion of the upper extremities) indicates damage above the red nucleus in the diencephalon, while decerebrate posturing (extension of the upper extremities) indicates damage below the red nucleus in the midbrain or pons.
The Glasgow Coma Scale (GCS) is the standard clinical tool for assessing the depth of coma, with a score of 8 or less generally necessitating endotracheal intubation for airway protection.
Metabolic coma typically presents with symmetric neurological findings and preserved brainstem reflexes, whereas structural coma often presents with asymmetric findings and absent brainstem reflexes.
Doll’s eyes (oculocephalic reflex) are absent in patients with brainstem damage, indicating a poor prognosis and potential brain death.
Non-convulsive status epilepticus is a critical diagnostic consideration in patients with unexplained coma and should be evaluated via urgent EEG.
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A 62-year-old male is brought to the emergency department after being found unresponsive at home. On physical examination, the patient is intubated for airway protection due to a GCS of 7. Neurological exam reveals asymmetric pupils with the right pupil being fixed and dilated, and the patient exhibits decerebrate posturing to painful stimuli. His blood glucose is 110 mg/dL and a non-contrast head CT shows a large right-sided epidural hematoma with midline shift.
What is the most likely underlying mechanism for the patient's fixed and dilated pupil?
Uncal herniation
The patient's fixed and dilated pupil is a hallmark of uncal herniation, which compresses the ipsilateral oculomotor nerve (CN III), a concept covered in the second bet.
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Etiology / Epidemiology
Coma results from bilateral hemispheric dysfunction or brainstem reticular activating system (RAS) failure. Common causes include trauma, stroke, hypoglycemia, and drug overdose.
Clinical Manifestations
Defined by Glasgow Coma Scale (GCS) ≤ 8. Look for decorticate or decerebrate posturing and fixed, dilated pupils indicating herniation.
Diagnosis
Perform non-contrast head CT immediately to rule out hemorrhage. Check capillary blood glucose and ABG for metabolic derangements.
Treatment
Administer thiamine before/with dextrose, plus naloxone (the 'coma cocktail'). Do not delay imaging for labs.
Prognosis
Outcome depends on etiology; hypoxic-ischemic injury has poor recovery. Brain death is a clinical diagnosis.
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Epidemiology & Etiology
Coma is a state of profound unconsciousness lasting >1 hour. Etiologies are categorized as structural (mass, hemorrhage, stroke) or metabolic (hypoglycemia, hypoxia, uremia, toxins). Metabolic causes are statistically more common than structural lesions in the ED.
Pertinent Anatomy
The ascending reticular activating system (ARAS) in the midbrain and thalamus is essential for arousal. Coma requires either bilateral cortical damage or a focal lesion affecting the brainstem RAS.
Pathophysiology
Disruption of the thalamocortical projections prevents cortical activation. Structural lesions cause coma via mass effect and subsequent herniation syndromes. Metabolic insults cause global neuronal dysfunction through ATP depletion or neurotransmitter imbalance.
Clinical Manifestations
Assess GCS (Eye, Verbal, Motor). Decorticate posturing (flexion) suggests diencephalic injury; decerebrate posturing (extension) suggests midbrain/pons injury. Fixed, dilated pupils suggest uncal herniation and imminent brainstem compression.
Diagnosis
The non-contrast head CT is the gold standard for excluding intracranial hemorrhage or mass. Serum glucose <40 mg/dL is a reversible metabolic emergency. Toxicology screen is adjunctive but rarely changes immediate management.
Treatment
Secure the airway if GCS ≤ 8. Administer thiamine before glucose to prevent Wernicke encephalopathy. Use naloxone for suspected opioid overdose. Avoid hyperventilation unless there is acute evidence of herniation.
Prognosis
Prognosis is dictated by the underlying cause and duration of coma. Persistent vegetative state may occur after 1 month. Brain death requires absent brainstem reflexes and a positive apnea test.
Differential Diagnosis
Locked-in syndrome: preserved vertical eye movements
Psychogenic unresponsiveness: normal pupillary light reflex
Status epilepticus: rhythmic muscle twitching
Hypoglycemia: diaphoresis and tachycardia
Opioid overdose: pinpoint pupils and respiratory depression