Loss of brainstem function is incompatible with life. The brainstem reflexes that are assessed include pupillary reaction to light, corneal reflex, gag reflex, and oculovestibular reflex. All reflexes should be absent in brain death. There should not be a light response to a bright light introduced to the pupil. Most pupils are midposition (4 to 6 mm), although there may be variations from 4 to 9 mm, all of which are compatible with brain death. In addition, pupils may be round, ovoid, or irregularly shaped. The corneal reflex is tested with a wisp of cotton or saline drops, and the reflex should be absent. With the application of deep pain, observe for any grimacing in response to the deep pain. The gag reflex is tested with a tongue blade. Along with an absent gag reflex, the cough reflex should also be absent. Ocular movements, as tested by head turning and caloric testing, are absent in brain death. (See
Chapter 7 for a description of caloric testing and
Chapter 8 for assessment of ocular movement, including precautions to be taken before testing.) Note that sedatives, aminoglycosides, tricyclic antidepressants, anticholinergics,
anticonvulsants, and chemotherapeutic agents can diminish or completely abolish the caloric response.
5,
8 Loss of brainstem function also results in loss of breathing and vasomotor control, which results in apnea and hypotension to loss of blood pressure.