Glaucoma After Refractive Surgery
The force required for applanation of a Goldmann tonometer is proportional to the central corneal thickness. As a result, an eye that has a thin central cornea may have an artifactually low IOP as measured by Goldmann applanation tonometry (GAT). Patients with normal-tension glaucoma have significantly thinner corneas than do patients with primary open-angle glaucoma. When a correction factor based on corneal thickness is applied, more than 30% of glaucoma patients demonstrate abnormally high IOP. The correction factor needed may be lower for measurements taken with the Tono-Pen (Reichert Technologies, Depew, NY) and the pneumotonometer.
For IOP measured with GAT, an artifactual IOP reduction occurs following surface ablation and LASIK for myopia, both of which reduce central corneal thickness. Similar inaccuracies in IOP measurement can occur after surface ablation and LASIK for hyperopia. After excimer laser refractive surgery, the mean reduction in IOP measurement is 0.63 mm Hg per diopter of correction, with a wide standard deviation. Postoperatively, some patients may demonstrate no change in IOP measurement, whereas others may exhibit an increase. In general, the reduction of measured IOP is greater after LASIK than after surface ablation. Surface ablation patients with a preoperative refractive error of less than 5.00 D may have a negligible decrease in IOP measurements.
Topical corticosteroids that are used after refractive surgery pose a serious risk of corticosteroid-induced IOP elevation, particularly because an accurate IOP measurement is difficult to obtain. By 3 months postoperatively, up to 15% of surface ablation patients may have IOP above 22 mm Hg. If the elevated IOP is not recognized early enough, optic nerve damage and visual field loss can occur.
If topical corticosteroids are used postoperatively for an extended time, periodic, careful disc evaluation is essential. Optic nerve and nerve fiber layer imaging may facilitate the evaluation. Periodic visual field assessment may be more effective than IOP measurement for identifying at-risk patients before severe visual field loss occurs (see Chapter 10, Fig 10-4).
Refractive surgery patients who develop glaucoma are initially treated with IOP-lowering medications, and their IOP is carefully measured. If medication or laser treatment does not adequately reduce the IOP, glaucoma surgery may be recommended. Patients who have had refractive surgery should be warned prior to glaucoma surgery of the potential for transient vision loss from inflammation, hypotony, or change in refractive error. The glaucoma surgeon should be made aware of the patient’s previous LASIK in order to avoid trauma to the corneal flap. For additional information on glaucoma management, see BCSC Section 10, Glaucoma.
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Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.