A 65-year-old man is brought to the emergency department after he suddenly became unresponsive at home. When the paramedics arrived at his residence, they found him pulseless with ventricular fibrillation. After 10 minutes of cardiopulmonary resuscitation, the patient regained spontaneous circulation. In the hospital, he is intubated and placed on mechanical ventilation. Vital signs remain stable. A therapeutic hypothermia protocol is initiated for the next 72 hours. Following rewarming, he remains comatose with fixed and dilated pupils. There is no direct or consensual pupillary response to light. MRI of the brain reveals diffuse loss of gray-white matter differentiation with sulcal effacement. This patient's pupillary findings are most likely caused by damage to which of the following brain regions?
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This patient's presentation and MRI findings (eg, loss of gray-white matter differentiation with sulcal effacement) are consistent with anoxic brain injury due to cardiac arrest. The presence of nonreactive pupils to light stimulation following cardiac arrest carries a poor prognosis and indicates anoxic damage to the brainstem at the level of the upper midbrain.
During the normal pupillary reflex, the optic nerve and tract transmit the light stimulus to the midbrain at the level of the superior colliculus, where it is received by the pretectal nucleus and subsequently relayed to the bilateral Edinger-Westphal nuclei. These nuclei subsequently project preganglionic parasympathetic fibers through the oculomotor nerve (CN III) to the ciliary ganglion, which then projects postganglionic fibers that innervate the sphincter pupillae muscle (constricts the pupil). When light is shone in one eye, both the ipsilateral pupil (direct response) and contralateral pupil (consensual response) constrict.
(Choice A) The medulla contains the glossopharyngeal (CN IX) and vagus (CN X) nerves, which provide the afferent and efferent limbs of the gag reflex, respectively.
(Choice C) The occipital lobe contains the primary visual cortex. Unilateral occipital lobe damage can cause contralateral homonymous hemianopia (± sparing of the macula), whereas bilateral occipital lesions may result in cortical blindness.
(Choice D) The superior optic radiations (which carry information from the superior retina/inferior visual field) travel through the parietal lobe. Lesions can lead to contralateral homonymous inferior quadrantanopia.
(Choice E) The pons contains the horizontal gaze center, which helps mediate the oculocephalic (doll's eye) reflex. It also contains the trigeminal (CN V) and facial (CN VII) nerves, which mediate the afferent and efferent limb of the corneal reflex, respectively.
(Choice F) The inferior optic radiations (which carry information from the inferior retina/superior visual field) travel through the temporal lobe (ie, Meyer's loop). Lesions classically cause contralateral homonymous superior quadrantanopia.
(Choice G) After passing through the optic tract, fibers containing information from the contralateral visual fields synapse in the lateral geniculate nucleus of the thalamus before fanning out into the optic radiations; lesions impacting this region lead to contralateral homonymous hemianopia. In contrast, the fibers that mediate the pupillary reflex synapse at the pretectal nucleus.
Educational objective:
The upper midbrain contains the neural structures (eg, optic nerve, pretectal nuclei, Edinger-Westphal nuclei, oculomotor nerve) that mediate the direct and consensual pupillary light reflex.