How are brainstem reflexes evaluated during a bedside neurological examination?

Brainstem Reflexes Evaluation at the Bedside

Brainstem reflexes are systematically evaluated by testing pupillary light reflex, corneal reflex, oculocephalic (doll's eyes) reflex, oculovestibular (caloric) reflex, and gag/cough reflexes, with each reflex localizing to specific brainstem levels and requiring removal of confounders like sedation before interpretation.

Core Brainstem Reflexes and Testing Technique

Pupillary Light Reflex (Midbrain)

• Shine a bright light directly into each pupil and observe for constriction 1
• Normal pupils are 3-5 mm; in brain death they are typically midposition or dilated (4-9 mm) 1
• Use a magnifying glass when uncertainty exists about pupillary response 1
• Automated pupillometry is superior to penlight examination when available, providing objective quantification and reducing interrater variability, particularly for small pupils 2
• Specificity for poor neurological outcome increases with time: 0% false positive rate achieved after 96 hours post-cardiac arrest 2

Corneal Reflex (Pons)

• Touch the cornea with tissue paper, cotton swab, or squirts of water 1
• Observe for any eyelid movement/blink response 1
• Take care not to damage the cornea during testing 1
• Bilateral absence at ≥72 hours post-cardiac arrest predicts poor outcome with high specificity 3, 2

Oculocephalic Reflex (Pons/Midbrain)

• Rapidly rotate the head horizontally while holding eyelids open
• Normal response: eyes move conjugately in the opposite direction to head movement
• Only perform when cervical spine injury is excluded
• Absence indicates pontine/midbrain dysfunction 4

Oculovestibular Reflex (Pons/Medulla) - Caloric Testing

• Confirm external auditory canal patency first 1
• Elevate head to 30 degrees 1
• Irrigate each ear separately with 10-50 mL of ice water 1
• Observe for 1 minute for eye movement 1
• Wait several minutes between testing each side 1
• Absence of eye movement indicates severe brainstem dysfunction 1

Gag and Cough Reflexes (Medulla)

• Gag reflex: Stimulate posterior pharynx with tongue blade or suction device 1
• Cough reflex: Insert suction catheter into trachea, advance to the carina, perform 1-2 suctioning passes 1
• Absence indicates medullary dysfunction 1

Facial Grimace Response (Pons)

• Apply deep pressure on condyles at temporomandibular joints 1
• Apply deep pressure at supraorbital ridge 1
• Observe for any grimacing or facial muscle movement 1

Critical Confounders That Must Be Excluded

Testing is unreliable and should be delayed when confounders are present 2, 5:

• Sedatives/analgesics: Stop for at least 5 elimination half-lives of the longest-acting drug 2
  • Even propofol (short-acting) requires ≥24 hours washout 2
  • Consider antidotes (use flumazenil cautiously due to seizure risk) 2
• Neuromuscular blockade: Must be completely reversed 2, 5
• Hypothermia: Requires 24-hour observation after rewarming 6
• Severe hypotension: Maintain age-appropriate blood pressure 1, 4
• Metabolic derangements: Correct before testing 2, 5

Timing and Prognostic Implications

Post-Cardiac Arrest Context

• No clinical signs reliably predict poor outcome at 24 hours 3, 5
• At ≥72 hours, in the absence of confounders, bilateral absence of both pupillary AND corneal reflexes predicts poor outcome with near-zero false positive rate 3, 2, 5
• Absent vestibulo-ocular reflexes at 24 hours are less reliable (0-14% false positive rate) 3
• Motor response ≤3 on Glasgow Coma Scale at 72 hours has 5% false positive rate 3

Pediatric Brain Death Determination

All brainstem reflexes must be completely absent on two separate examinations 1:

• Pupils fixed and unreactive
• Absent corneal reflexes bilaterally
• Absent oculovestibular reflexes bilaterally
• Absent gag, cough, sucking, and rooting reflexes
• No facial movement to noxious stimuli
• Must be combined with apnea testing 1

Common Pitfalls to Avoid

1. Testing too early: Sedation effects persist longer than clinicians estimate; wait the full washout period 2
2. Relying on single reflex: Always use multimodal assessment—no single test has sufficient specificity 2
3. Inadequate stimulus: Use adequate pressure/intensity for each reflex 1
4. Misinterpreting spinal reflexes: Spinal-mediated movements can occur despite brain death; requires expertise to differentiate 1
5. Ignoring pupil size: Small pupils may appear unreactive with penlight; use pupillometer when available 2
6. Testing with cervical spine instability: Never perform oculocephalic reflex until spine cleared 4

Examination Frequency

• Baseline assessment before and immediately after critical events (e.g., ECMO cannulation) 4
• Daily assessment by neurologist/neurointensivist when available 4
• Every 1-4 hours by nursing staff based on acute brain injury risk 4
• Serial examinations are essential—single assessments are insufficient 4, 5