Although researchers know intraocular pressure (IOP) fluctuates constantly depending on everything from the time of day to a patient’s body position, it’s challenging to know just how much those factors affect peak pressure. Using a primate model, researchers at the University of Alabama at Birmingham were able to put a number on one of those factors: eye rubbing.

The researchers instilled antibiotic ointment in both eyes of three rhesus macaques and continuously recorded the IOP—with an implanted wireless telemetry system—as the monkeys rubbed their eyes to remove the ointment. After repeating the experiment four times, they found IOP increased by an average of 80mm Hg to 150mmHg above baseline for three to four seconds, and by as much as 310 mmHg depending on the individual and type of rubbing. The monkeys who used the back of their hand or wrist, rather than their fingers or knuckles, had the largest increase in IOP.

While more research is necessary in a human population, the findings help support the notion that patients—especially those with glaucoma—should avoid eye rubbing whenever possible.