Does the increase in scleral permeability that gives prostaglandin analogs (PGAs) their efficacy in glaucoma also induce changes to ocular rigidity? A small study suggest this is so and documents changes to anterior chamber volume (ACV) that may result.

This prospective study included 33 eyes of 21 recently diagnosed open-angle glaucoma patients who had just started therapy with a topical PGA. The team measured ACV, corneal biomechanical parameters (shape, stiffness, thickness, hysteresis) and IOP at three visits over a four-month period.

As expected, the investigators reported a significant decrease in mean IOP with initiation of PGA, most notably during the first four weeks of treatment. They also observed a significant decrease in mean ACV at visits two and three compared with baseline—and a paradoxical increase in ACV in 37% of eyes at the second visit despite a significant mean reduction in IOP by tonometry. “As the scleral stiffness is reduced with PGA therapy, so is the resistance to deformation, allowing greater volume to accumulate simultaneously as IOP is lowered,” they wrote in their paper.

The study authors concluded that the IOP/ACV ratio at baseline significantly predicted the reduction in the respective measures of IOP and scleral stiffness. They suggest this ratio “could potentially serve as a new diagnostic parameter to determine the likelihood of PGA treatment success,” as It would be more independent of rigidity influences on tonometry.