Five brain stem reflexes are tested, each assessing a specific arc through the brain stem. The tests are performed by the examining doctors (not by nurses or other team members), and both doctors must independently perform and verify each reflex. No reflex should be detectable if the brain stem is irreversibly non-functional.

1. Pupillary Light Reflex

The pupils are assessed for size, equality, and response to a bright light source. In brain stem death, the pupils are typically fixed and dilated (though size may vary), and there is no response to direct bright light in either eye. This tests the afferent arc (CN II, optic nerve) and the efferent arc (CN III, oculomotor nerve, via the Edinger-Westphal nucleus). It is important to exclude pharmacological pupil dilatation (atropine, topical mydriatics) and direct eye injury before concluding that the reflex is absent.

2. Corneal Reflex

A wisp of cotton wool or a fine cotton-tipped swab is gently applied to the cornea (not the sclera or conjunctiva). The normal response is bilateral blinking. The afferent arc is the ophthalmic division of CN V (trigeminal nerve) and the efferent arc is CN VII (facial nerve). In brain stem death, there is no blinking response in either eye.

3. Oculo-Vestibular Reflex (Caloric Testing)

This test assesses the integrity of the vestibulo-ocular arc. The patient’s head is elevated to 30 degrees (to bring the horizontal semicircular canal into the vertical plane). After confirming the external auditory canal is clear of obstruction, 50 mL of ice-cold water is instilled into each ear canal over approximately 1 minute via a fine catheter. In an intact brain stem, this causes a slow tonic deviation of the eyes toward the cold ear (and nystagmus with the fast phase away in a conscious patient). In brain stem death, there is no eye movement whatsoever. The two ears are tested separately with a minimum 5-minute interval between them.

4. Motor Response to Supraorbital Pressure

Firm pressure is applied to the supraorbital notch (above the eye, where the supraorbital nerve exits the skull) using the examiner’s thumb. This provides a central painful stimulus that bypasses spinal cord pathways. In brain stem death, there is no facial grimacing or purposeful motor response to this stimulus. Limb movements in response to peripheral stimuli may still occur due to intact spinal cord reflex arcs — these are not brain stem responses and should not be misinterpreted as evidence of brain stem function. The so-called Lazarus sign (spontaneous arm movements including crossed arm extension) is a well-recognised spinal cord reflex that can occur after brain stem death.

5. Gag and Cough Reflex

The gag reflex is tested by stimulating the posterior oropharynx with a tongue depressor or suction catheter. The cough reflex is tested by introducing a suction catheter into the trachea through the endotracheal tube and advancing it to the carina. In brain stem death, there is no gag or cough response to either stimulus. The gag reflex tests CN IX (glossopharyngeal nerve, afferent) and CN X (vagus nerve, efferent); the cough reflex tests a similar arc via tracheal and laryngeal afferents.

6. The Apnoea Test

The apnoea test is the definitive test for the absence of the respiratory drive originating from the brain stem respiratory centres. Before the test, the patient is pre-oxygenated with 100% oxygen for at least 10 minutes to maximise oxygen stores and reduce the risk of hypoxia during the test. The PaO2 and PaCO2 at baseline are documented by arterial blood gas.

The patient is then disconnected from the ventilator. Oxygen is insufflated via a suction catheter placed into the endotracheal tube at the carina at a rate of 6 L/min to maintain oxygenation during the apnoea period. The patient is observed for any respiratory effort — even a single breath, a single diaphragmatic movement, or any abdominal excursion counts as respiratory effort and renders the test negative.

The test continues until the PaCO2 rises to at least 6.65 kPa (50 mmHg), confirmed by arterial blood gas, and there has been no respiratory effort. The significant rise in PaCO2 above the normal threshold for respiratory drive (approximately 5.3 kPa) provides a maximal stimulus to the brain stem respiratory centres — if there is no response at PaCO2 6.65 kPa, the respiratory drive is absent. If at any point SpO2 falls below 85%, the test must be abandoned, the patient reconnected to the ventilator, and an alternative approach (e.g. using a test lung, or combining with CPAP) considered.

CLINICAL PEARL Pre-existing COPD or chronic CO2 retention does not prevent brain stem death testing but does require modification. If the patient has a chronically elevated PaCO2, the apnoea test requires the PaCO2 to rise to 6.65 kPa OR to rise by at least 0.5 kPa above the pre-test baseline — whichever is higher. Document the baseline PaCO2 carefully.