REFRACTIVE CORNEAL SURGERY AND OCULAR HYPERTENSION

 

INTRODUCTION:

There is a world-wide increase in the number of individuals opting for corneal refractive surgery procedures, such as photorefractive keratectomy (PRK) and Laser-Assisted In Situ Keratomileusis (LASIK). Since these patients are started on intense steroid eye-drops in the immediate post-operative period, it is practical to understand the implications of steroid-induced ocular hypertension in this scenario.

RISK OF GLAUCOMA IN MYOPIC PATIENTS :

Myopic patients have a high risk of glaucoma. This is attributed to the thinned out and stretched lamina cribrosa and scleral canal.

Intraocular pressure (IOP) exerts a force on the lamina situated at the scleral canal. The scleral canal, where the lamina is located, represents a relatively weak area in the wall of the globe.

IOP, as a force, can be considered to consist of two vector components. First, there is a posterior force vector compressing the laminar plates or pushing it outwards through the scleral canal. There is a second force vector contributed by the stress in the eye wall which pulls radially on the scleral insertion of the lamina. This latter component contributes, in large part, to the stress within the scleral wall.

LaPlace's equation for a spherical shell relates the pressure and radius to the wall stress:
s = (pi-pc)R/2h

In the equation:
s = stress
(pi-pc) = transmural pressure (or difference between internal and external pressure)
R = radius of sphere
h = thickness of sphere

It follows from LaPlace's equation that, for a given transmural pressure, the larger the radius of the globe (i.e., the greater tile axial length), the greater the wall stress and, hence, the greater the
potential distorting force on the optic nerve.

This may in part, explain why eyes with axial myopia may have increased risk for developing glaucoma.

STEROID INDUCED OCULAR HYPERTENSION FOLLOWING CORNEAL REFRACTIVE SURGERY:

Myopic patients usually undergo corneal refractive surgeries, which involves application of laser to the cornea in order to change the contour of the cornea. In this procedure high power laser energy is delivered to the cornea to ablate it. This causes an inflammatory reaction which needs to be controlled by intensive topical steroids, such as every two-hourly per day.

Looking at the risk of myopic patients to develop glaucoma, it is of practical importance to keenly follow-up these patients for any steroid-induced ocular hypertension.

REFERENCES:

In a study from Iran, myopic PRK was performed on 506 eyes of 269 patients. Preoperatively, spherical equivalent refractive error ranged from −1.00 to −5.00 diopters (D) and cylinder was less than 4 D. 

Ocular hypertension developed in 40 (7.9%) eyes overall, which occurred in 16 eyes (40%) 2–3 weeks postoperatively (mean IOP=23.5±3.0mmHg), in 20 eyes (50%) after 4–6 weeks (mean IOP=25.1±4.2 mmHg) and in 4 eyes (10%) 8–12 weeks following PRK (mean IOP=29.0±3.1 mmHg). There was no correlation between the level of IOP rise and preoperative spherical equivalent refractive error. IOP recovered to normal in all eyes after discontinuation of topical steroids and initiation of anti-glaucoma medications. Mean duration of IOP normalization was 28.5±27.7 (range 7–108) days and no instance of steroid-induced glaucoma was observed in any patient.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3589213/

In another study from Hungary, 43 paients developed ocular hypertension following PRK. They were divided into three groups. The first received only timolol, the second timolol and dorzolamide and the third only dorzolamaide. The second group treated with a combination of dorzolamide and timolol had the best ocular hypotensive response.

https://pubmed.ncbi.nlm.nih.gov/11489570/

Steroid induced IOP elevation has been noted by Seiler as well as Machat, occurring in 8 to 32% of treated eyes. Shimizu et al reported the incidence of post-PRK IOP rise (>21mmHg) to be 8.9%. Gartry has reported a post-PRK steroid response of 12% in their series.

Seiler T, Holschbach A, Derse M, Jean B, Genth U. Complications of myopic photorefractive keratectomy with excimer laser. Ophthalmology. 1994;101:153–160.

Machat JJ, Tayfour F. Photorefractive keratectomy for myopia: preliminary results in 147 eyes. Refract Corneal Surg. 1993;9(suppl):S16–S19.

Shimizu K, Amano S, Tanaka S. Photorefractive keratectomy for myopia: One-year follow-up in 97 eyes. Refract Corneal Surg. 1994;10(Suppl):S178–187.

Gartry DS, Kerr Muir MG, Marshall J. Excimer laser photorefractive keratectomy.18-month follow-up. Ophthalmology. 1992;99:1209–1219.

CONCLUSION:

In view of the significant number of patients developing elevated IOP in post-PRK patients, these individuals should be monitored no later than 2 weeks after initiation of corticosteroid treatment.

Javadi recommends substituting potent steroids such as betamethasone with weaker agents with lesser propensity for IOP elevation such as fluorometholone.