Phenobarbital in darkness can cause which ocular change?

Phenobarbital in darkness can cause which ocular change?

• A. Miosis
• B. Mydriasis
• C. Ptosis
• D. Anisocoria

Answer: (B)

Pupil size reflects the balance between sympathetic dilation and parasympathetic constriction. In darkness, dilation is the normal response because the sympathetic system dominates and the parasympathetic constrictor reflex is reduced. Phenobarbital, a central nervous system depressant, can blunt the parasympathetic pathways that mediate pupillary constriction. With constriction suppressed, the dilator actions of the sympathetic system predominate, leading to larger pupils in the dark—i.e., mydriasis. The other options describe constriction, eyelid droop, or unequal pupils, which aren’t the expected effect of this drug in darkness.

Pupil size reflects the balance between sympathetic dilation and parasympathetic constriction. In darkness, dilation is the normal response because the sympathetic system dominates and the parasympathetic constrictor reflex is reduced. Phenobarbital, a central nervous system depressant, can blunt the parasympathetic pathways that mediate pupillary constriction. With constriction suppressed, the dilator actions of the sympathetic system predominate, leading to larger pupils in the dark—i.e., mydriasis. The other options describe constriction, eyelid droop, or unequal pupils, which aren’t the expected effect of this drug in darkness.