Which mechanism explains evaporative dry eye in Parkinson's disease?

• A. Reduced blink rate
• B. Increased tear production
• C. Decreased tear evaporation
• D. Normal eyelid function

Answer: (A)

Evaporative dry eye happens when the tear film is lost to evaporation faster than it can be replenished, usually because the tear film is unstable and the eyes are not being lubricated evenly. In Parkinson's disease, reduced spontaneous blinking means the eyes are exposed longer between blinks. This longer exposure allows the tear film to evaporate more, leading to instability and quicker breakup of the film, which manifests as evaporative dry eye. Even if tear production is normal, evaporation outpaces replacement due to less frequent blinking.

Increased tear production wouldn’t cause dryness; decreased tear evaporation would reduce dryness; normal eyelid function wouldn’t account for the increased evaporation seen with reduced blink rate.