Causes of Irregular Astigmatism
Irregular astigmatism may be present before keratorefractive surgery; it may be caused by the surgery; or it may develop postoperatively. Preoperative causes include keratoconus, pellucid marginal degeneration, contact lens warpage, significant dry eye, corneal injury, microbial keratitis, and epithelial basement membrane dystrophy (Fig 7-7). All these conditions should be identified before surgery. Common intraoperative causes include decentered ablations and central islands, and, less commonly, poor laser optics, nonuniform stromal bed hydration, and LASIK flap complications (a thin, torn, irregular, incomplete, or buttonhole flap; folds or striae of the flap; and epithelial defects). Postoperative causes of irregular astigmatism include flap displacement, diffuse lamellar keratitis and its sequelae, flap striae, posterior corneal ectasia, irregular wound healing, dry eye, and flap edema.
Figure 7-7 Irregular astigmatism in a corneal topographic map of the left eye of a patient with significant epithelial basement membrane dystrophy. The patient experienced glare and a general decline in quality of vision. Simulated K shows the flattest meridian at 39° and the steeper meridian at 129°. N = nasal; T = temporal.
(Courtesy of Ming Wang, MD.)
Schallhorn SC, Farjo AA, Huang D, et al. Wavefront-guided LASIK for the correction of primary myopia and astigmatism: a report by the American Academy of Ophthalmology. Ophthalmology. 2008;115(7):12491261.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.