Why intraocular pressure (IOP) decreases following cataract surgery has been an area of interest for some time, but a new study in the British Journal of Ophthalmology applied a novel approach—electronic Schiotz tonography—and found phacoemulsification increased trabecular outflow facility, which in turn fully accounted for reduction in IOP.

The study recruited 41 patients who were scheduled to undergo a standard clear corneal incision phacoemulsification with IOL implantation at St. Thomas’ Hospital. Researchers measured IOP using Goldmann applanation tonometry and electronic Schiotz tonography at baseline and again at three, six and 12 months postoperatively.

Data from 27 patients with reliable tonographic outflow tracings at baseline and all subsequent study visits were included in the final analysis, where only one eye from each patient was used. Sixteen individuals had cataract only, while 11 cases had an existing diagnosis of primary open-angle glaucoma (POAG). 

Investigators reported IOP reduced significantly at every time point postoperatively compared with baseline. Additionally, electronic Schiotz tonographic outflow facility (TOF) also improved significantly after cataract extraction at all time points (baseline of 0.14±0.06 vs. 0.18±0.09 at three months, 0.20±0.09 at six months and 0.17±0.07 µL/min mmHg at 12 months). As a point of comparison, the study correlated the results of five contralateral eyes of patients with cataracts only who did not have any intraocular surgery during the follow-up period. This group’s IOP and TOF did not change significantly at any postoperative visit.

Researchers noted the overall mean IOP reduction varied between 10% and 12% and corresponding positive enhancement of TOF at all time points.

This is the first study using electronic Schiotz tonography with documented anterior chamber depth and gonioscopy after modern cataract surgery with phacoemulsification and IOL implantation, researchers said. The study demonstrated that phaco increases TOF, and this fully accounts for the IOP reduction following cataract surgery, they added. 

Alaghband P, Beltran-Agulló L, Galvis EA, et al. Effect of phacoemulsification on facility of outflow. Br J Ophthalmol.2018 Nov;102(11):1520-26.