Modified Goldmann Prism Shows Suitability for Measuring IOP Following LASIK, PRK

The modified Goldmann prism may measure significantly less intraocular pressure (IOP) reduction following myopia-correcting corneal refractive surgery compared with the standard Goldmann prism, according to a retrospective, cross-sectional study published in BMC Ophthalmology. The reduction in IOP with both prisms is associated with the degree of myopia correction, according to the report.  

Researchers obtained IOP utilizing both Goldmann applanation tonometry (GAT) and modified-GAT prisms preoperatively and at the 3-month postoperative visit in 120 eyes of 64 patients who underwent  laser in situ keratomileusis (LASIK; n=58; mean age, 30 years; 57% women) or photorefractive keratectomy (PRK; n=62, mean age, 30 years; 61% women) between May 1, 2020 and June 1, 2021. The team collected demographics, central corneal thickness (CCT), manifest refraction and corneal curvature (CC) data at both visits in addition to surgical parameters, including maximum ablation depth.

The mean paired IOP after LASIK decreased by 3.28 mm Hg measured with a standard Goldmann applanation tonometer prism (IOPg) compared with 1.93 mm Hg when measured with a modified correcting applanation tonometer surface Goldmann prism (IOPc; P ≤.0001). The mean paired IOP following PRK decreased by 1.92 mm Hg as measured by IOPg and 1.06 mm Hg as measured by IOPc (P ≤.0001). 

Increased LASIK ablation depth and postoperative change in CCT trended towards a significant decrease in IOPg (P =.07 and P =.12, respectively), but not IOPc (P =.18 and  P =.32, respectively), whereas the PRK ablation depth was not associated with IOP decreases using either tonometry method, according to the report. 

The researchers note that the IOP reduction with both prisms was associated with the degree of myopia correction.

“The modified correcting prism demonstrated significantly less IOP reduction following LASIK and PRK compared to the standard GAT prism. The IOP reduction following both LASIK and PRK is likely an error due to CCT thinning and corneal biomechanically-induced errors in Goldmann pressure measurement […]” according to the researchers. “A newer, modified Goldmann prism may help detect glaucoma and ocular hypertension at an earlier stage in patients which have undergone LASIK or PRK.” 

Study limitations include a retrospective nature, single center design, lack of direct intracameral IOP measurements for comparison, and the narrow manifest refraction spherical equivalent characteristics among the study participants.