Morphologic basis of the refraction shift after penetrating glaucoma surgery

PURPOSE: To study the morphological basis of a refraction shift that occurs after penetrating glaucoma surgery.

METHODS: The study included 45 patients with progressive primary open-angle glaucoma, who underwent trabeculectomy. We examined patients prior to surgery, one week, one month and three months after the surgery. The patients underwent visual acuity check, tonometry, measurements of biomechanical properties of the fibrous tunic of the eye, its axial length (AL), anterior chamber depth (ACD), lens thickness (LT) and pupil width. Refractometry and keratometry were also performed, including corneal curvature in steep and flat meridians (R1, R2), its average curvature (Ave), and the corneal cylinder.

RESULTS: Most of the significant changes occurred during the first week after surgery. Myopic shift continued to develop during the whole observation period: sphere increased from -0.2 [-1.19; 1.25] diopters to -0.68 [-2.61; -0.32] diopters by the third month. During the first week the cylinder increased from -0.54 diopters to -1.13 diopters, returning to previous condition by month 3 (-0.62 [-1.6; -0.28] diopters). Corneal cylinder value correlated with total astigmatism up to the end of the first month, then it increased again up to -1.13 [-1.81; -0.94] diopters by month 3. During all the observation period regular astigmatism prevailed, R1 and R2 changed in tandem, which complicated the analysis of corneal cylinder genesis. Intraocular pressure (IOP) decreased from 24.8 [21.2; 29.0] to 9.2 [4.95; 16.2] mm Hg during the first week, then it remained within the range of 10-17 mm Hg. Change of biomechanical properties (corneal hysteresis, corneal resistance factor) showed the decrease of the fibrous tunic inner tension, consequent to IOP compensation after surgery. AL decreased by 0.11 [0.06; 0.22] mm during the first week, then it showed an insignificant opposite trend. Simultaneously ACD decreased from 2.45 [2.23; 2.64] to 2.39 [1.95; 2.65] mm, also followed by a subsequent insignificant opposite trend. Pupil width during the first week increased from 3.03 [2.69; 3.38] to 3.63 [2.20; 4.06] mm; then the pupil resumed its diameter by half. The spherical component had a negative correlation with AL and ACD. Total astigmatism correlated with the corneal one. Its delta correlated with the delta of the corneal cylinder power and, more weakly, with the preoperative cylinder power.

CONCLUSION: A myopic shift with temporary astigmatism occurs after trabeculectomy. The refraction shift depends mostly on the biometric parameters of the eye. Induced astigmatism is mostly corneal in nature.

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