Postoperative Anisometropia and Its Impact on Binocular Vision Outcomes Following Conventional Monocular IOL Power Calculation in Bilateral Ametropic Patients

Abstract

Purpose: To evaluate the frequency and severity of postoperative anisometropia and its associated binocular visual consequences following unilateral cataract surgery, using conventional monocular intraocular lens (IOL) power calculation, in patients with bilateral ametropia. Methods: This prospective descriptive study included 132 bilateral-ametropic adults (mean age 46.9 ± 5.1 years; range 30–55 years) who underwent unilateral phacoemulsification with monofocal IOL implantation at Al-Walidian Specialized Eye Hospital, Khartoum State, Sudan, between January 2019 and December 2023. Written informed consent was obtained from all participants, and the study was conducted under an official permission letter issued by the Graduate Studies Administration of Al-Neelain University to the administration of Al-Walidian Specialized Eye Hospital. Preoperative assessment included visual acuity, refraction, ocular biometry, binocular vision screening, and full ocular health evaluation; the fellow eye was confirmed free of visually significant cataract in all cases, and IOL power was calculated for all patients using the same SRK/T formula embedded in the biometry device, targeting emmetropia in the operated eye. At 3 months postoperatively, anisometropia was calculated as the absolute difference in spherical equivalent between the operated and fellow eyes. Binocular vision was assessed using the Worth Four Dot (W4D) test and the synoptophore. Subjective visual symptoms (headache, asthenopia, diplopia) and spectacle dependence were recorded using the standardised routine eye examination form applied to all patients. Anisometropia was categorised as mild (<1.50 D), moderate (1.50–3.00 D), or severe (>3.00 D). Data were analysed using descriptive statistics and Pearson's chi-square test (SPSS version 25.0). Results: Severe postoperative anisometropia was observed in 68.2% of patients, moderate anisometropia in 29.5%, and mild anisometropia in 2.3% (χ² = 86.86, p < 0.001). Increasing anisometropia severity was significantly associated with deterioration in binocular visual function: on the W4D test, 66.7% of severe cases maintained normal fusion, whereas 24.4% showed suppression and 8.9% showed diplopia (χ² = 10.99, p = 0.027). Synoptophore findings worsened similarly with severity, with stereopsis absent in 33.3% of severe cases. Multiple visual symptoms increased with anisometropia severity (χ² = 9.70, p = 0.008), and spectacle dependence was strongly associated with anisometropia severity, with all severe cases classified as totally dependent on spectacles (χ² = 264.0, p < 0.001). Baseline (preoperative) interocular anisometropia was small and did not differ meaningfully across the eventual postoperative severity groups (means 0.42, 0.28, and 0.29 D in the mild, moderate, and severe groups respectively), arguing against pre-existing refractive imbalance as an explanation for the severe postoperative anisometropia observed. Mean postoperative UDVA in the operated eye was 0.78 ± 0.20 (decimal Snellen equivalent), confirming a generally good isolated surgical outcome. Conclusions: Conventional monocular IOL power calculation in unilateral cataract surgery is associated with a high frequency of postoperative anisometropia in patients with bilateral ametropia, and this refractive imbalance is associated with reduced binocular function, greater symptom burden, and increased spectacle dependence. As an observational, single-centre study without a comparator group, these findings should be interpreted as hypothesis-generating; they support, but do not by themselves establish, the need for IOL planning strategies that consider the refractive status of the fellow eye. Limitations: Single-centre design, absence of a comparison group, and use of the hospital's routine clinical examination form for symptom and spectacle-dependence assessment rather than a separately validated psychometric questionnaire.