Comparison of outflow facility before and after the microhook ab interno trabeculotomy

To elucidate the mechanism of intraocular pressure (IOP) reduction by microhook ab interno trabeculotomy (μLOT), the aqueous humour outflow facility was compared preoperatively and post-operatively.

Fifty-one eyes (37 patients; mean age, 67.2 ± 11.8 years) were included. The IOP, number of medications and outflow facility coefficient (C) estimated by pneumatonography were compared preoperatively and post-operatively using the paired t test. Linear regression analysis was performed to identify possible correlations between the C value and IOP or number of medications. To adjust for biases from including both eyes of a patient and differences in background, the preoperative and post-operative C values were compared using a mixed effects regression model.

The mean preoperative IOP (18.2 mmHg) and mean number of medications (2.8) decreased significantly post-operatively by 26% and 18%, respectively, to 13.5 mmHg and 2.3 (p < 0.0001, for both comparisons). The preoperative C value of 0.27 µl/min/mmHg increased significantly (p < 0.0001) by 89% to 0.51 µl/min/mmHg post-operatively. Linear regression analysis indicated that higher IOP was associated with lower C values (estimate, -0.01/mmHg, p = 0.0107); medication numbers were not associated with the C value (estimate, -0.04/medication, p = 0.1739). Mixed effects regression analysis showed that the post-operative measurement (estimate, 0.11/preoperative measurement, p < 0.0001) was associated with a higher C value, while age, sex, µLOT procedure, IOP and medication numbers were not.

Outflow facility assessed by the tonographic C value increased significantly after µLOT. Increased conventional outflow by elimination of the outflow resistance at the trabecular meshwork is the main mechanism of IOP reduction after µLOT.