Coma: Assessment and Management for MCCQE1

Introduction

Coma is a state of unarousable unresponsiveness in which the patient lies with eyes closed and shows no meaningful response to environmental stimuli. For the MCCQE1, understanding the systematic approach to a comatose patient is a high-yield topic falling under the CanMEDS Medical Expert role. You must be able to differentiate between structural and metabolic causes, perform a targeted neurological exam, and initiate life-saving management.

Pathophysiology and Anatomy

Consciousness requires two intact components:

1. Arousal (Wakefulness): Dependent on the Ascending Reticular Activating System (ARAS) in the brainstem.
2. Awareness (Content): Dependent on the cerebral cortex (bilateral hemispheres).

Coma results from:

• Bilateral hemispheric dysfunction OR
• Brainstem (ARAS) dysfunction.

Etiology: Structural vs. Metabolic

Distinguishing between structural and metabolic causes is the first diagnostic pivot point.

The “AEIOU TIPS” Mnemonic

A useful mnemonic for MCCQE1 preparation to recall causes of altered level of consciousness:

• Alcohol / Acidosis
• Epilepsy / Endocrine / Electrolytes
• Insulin (Hypoglycemia)
• Opiates / Oxygen (Hypoxia)
• Uremia
• Trauma / Temperature (Hypothermia)
• Infection (Meningitis/Encephalitis)
• Psychogenic / Poisoning
• Stroke / Seizure / Shock

Clinical Assessment: The Canadian Approach

Time is brain. The assessment must be rapid and concurrent with stabilization.

Glasgow Coma Scale (GCS)

Memorize this table for the MCCQE1.

Herniation Syndromes

Understanding herniation is critical for identifying surgical emergencies.

Diagnostic Investigations

Laboratory

• Bedside: Glucose (Fingerstick).
• Blood: CBC, Electrolytes (Na, Ca, Mg), BUN/Cr, LFTs, INR/PTT, ABG, Ammonia, Osmolality.
• Toxicology: Serum ethanol, acetaminophen, salicylates; Urine drug screen.

Imaging

• CT Head (Non-contrast): The gold standard initial imaging to rule out hemorrhage, mass effect, or hydrocephalus.
• CTA (CT Angiography): If basilar artery thrombosis is suspected.
• MRI Brain: Superior for posterior fossa lesions, diffuse axonal injury, or early ischemia (DWI sequences).

Other

• Lumbar Puncture: ONLY after CT excludes mass effect. Indicated for suspected meningitis or subarachnoid hemorrhage (if CT negative).
• EEG: To rule out non-convulsive status epilepticus.

Canadian Guidelines: Neurological Determination of Death (NDD)

In Canada, “Brain Death” is legally defined as the irreversible loss of the capacity for consciousness combined with the irreversible loss of all brainstem functions, including the capacity to breathe.

Key Criteria (Summary):

1. Etiology: Established cause capable of causing NDD.
2. Deep Coma: No motor response to pain (excluding spinal reflexes).
3. Absent Brainstem Reflexes:
   • Fixed pupils.
   • Absent corneal reflex.
   • Absent oculocephalic/oculovestibular reflexes.
   • Absent gag/cough reflex.
4. Apnea Test: No respiratory effort despite PaCO2 ≥ 60 mmHg (and ≥ 20 mmHg above baseline).

Key Points to Remember for MCCQE1

• Glucose first: Always check capillary glucose in any patient with altered mental status.
• Thiamine before Glucose: In suspected malnutrition/alcohol use disorder, give thiamine before glucose to prevent precipitating Wernicke’s encephalopathy.
• Pupils: In metabolic coma, pupils are usually small but reactive. In structural coma, pupils are often asymmetric or unreactive.
• Basilar Thrombosis: A “locked-in” patient may look comatose but has intact awareness. Look for vertical eye movements (often the only preserved motor function).
• Naloxone: Has a short half-life. Patients may require repeated doses or an infusion.

Sample Question

Clinical Scenario

A 68-year-old male with a history of hypertension and atrial fibrillation is brought to the Emergency Department after collapsing at home. On arrival, he is comatose. His blood pressure is 190/110 mmHg, heart rate is 90 bpm, and respiration is irregular.

On neurological examination:

• GCS is 4 (E1, V1, M2).
• The right pupil is 7 mm and fixed; the left pupil is 3 mm and reactive.
• There is decerebrate posturing on the left side.
• Doll’s eye reflex is absent.

Which of the following is the most likely diagnosis?

Options

• A. Pontine hemorrhage
• B. Right-sided Uncal herniation
• C. Opioid overdose
• D. Metabolic encephalopathy
• E. Left-sided Subdural hematoma

Explanation

The correct answer is:

• B. Right-sided Uncal herniation

Detailed Explanation: This clinical presentation is classic for Uncal Herniation.

• Ipsilateral Blown Pupil: The right pupil is dilated (7 mm) and fixed. This is caused by the herniating uncus (temporal lobe) compressing the ipsilateral Oculomotor nerve (CN III) and its parasympathetic fibers.
• Contralateral Motor Deficit: The compression of the ipsilateral cerebral peduncle (specifically the corticospinal tract) results in contralateral hemiparesis or posturing (in this case, left-sided decerebrate posturing).
• Coma: Compression of the midbrain reticular activating system leads to a low GCS.
• History: The history of HTN and Afib suggests a catastrophic intracranial event (hemorrhage or massive embolic stroke) leading to increased ICP.

Why other options are incorrect:

• A. Pontine hemorrhage: Typically presents with pinpoint pupils (damage to descending sympathetic fibers), not a unilateral blown pupil.
• C. Opioid overdose: typically presents with bilateral pinpoint pupils and respiratory depression, not asymmetric pupils and posturing.
• D. Metabolic encephalopathy: Usually presents with symmetric deficits and reactive pupils.
• E. Left-sided Subdural hematoma: A left-sided lesion would typically cause a left blown pupil (ipsilateral) and right-sided motor symptoms.

References

1. Medical Council of Canada. (n.d.). Objectives for the Qualifying Examination Part I. Retrieved from mcc.ca.
2. Shemie, S. D., et al. (2006). Severe brain injury to neurological determination of death: Canadian forum recommendations. CMAJ, 174(6), S1-S13.
3. Toronto Notes. (2023). Neurology: Coma and Impaired Consciousness. Toronto Notes for Medical Students, Inc.
4. Young, G. B. (2023). Stupor and coma in adults. In: UpToDate, Post, T.W. (Ed.), UpToDate, Waltham, MA.