1. The pupillary light reflex is an important sign for determining the level of a brain stem lesion. In patients with depressed consciousness caused by metabolic encephalopathy, drug ingestion, or diffuse pressure on the diencephalon, the pupils are slightly smaller than normal but respond vigorously to light (1). Pressure on the pretectal area (eg, from a pineal tumor) prevents light from causing pupillary constriction and results in large, unreactive pupils (2). Injury to the oculomotor nerve (N. III) can occur because of swelling in the ipsilateral cerebrum (eg, from a brain tumor), which causes the uncus (the medial edge of the temporal lobe) to herniate through the tentorial opening and crush the oculomotor nerve. This first leads to a large, unreactive pupil in the affected eye (3). It can also later cause the affected eye to deviate laterally. This blown pupil is an ominous sign that the brain stem is about to be compressed from above. Damage to the midbrain tegmentum causes complete loss of pupillary response to light (4), but the pupils may dilate if a painful stimulus (eg, pinching the neck) is applied because of a sympathetic response (the ciliospinal response). Injury to the pons results in pinpoint pupils (5), which can be seen with a magnifying lens to respond slightly to light. Pontine injury not only disrupts the descending hypothalamic pupillodilator pathway but also interrupts ascending inputs to the Edinger-Westphal nucleus that inhibit its tone.
Source: The Modulatory Functions of the Brain Stem, Principles of Neural Science, Fifth Editon
Citation: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, Mack S. Principles of Neural Science, Fifth Editon; 2012 Available at: Accessed: September 26, 2017
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