Association between total astigmatism and myopia

Purpose: To determine whether there is an association between degree of myopia and total astigmatism. Setting: St. Paul’s Eye Unit, Royal Liverpool University Hospitals, Liverpool, United Kingdom. Methods: Refractive and keratometric data for 105 eyes of 105 patients with myopia and astigmatism who attended a photorefractive keratectomy (PRK) assessment clinic were analyzed. The refractive data of the patients having PRK were also analyzed before and after treatment. Results: There was a strong correlation between total astigmatism (A) and degree of myopia. This could be represented by a linear regression line in the form A = −0.13 × myopia + 0.86 (at the corneal plane). Although there were trends, there was no significant correlation between the degree of corneal (keratometric) astigmatism and myopia. Data of 35 patients who had PRK were analyzed to determine whether the observed effect on astigmatism could have been predicted from the relationship between myopia and astigmatism. Forty‐nine percent of patients with myopia and astigmatism were overcorrected by 0.6 diopter (D) (0.2 to 2.0 D), resulting in a preoperative to postoperative axis shift of 73 degrees; 40% were undercorrected by 1.2 D (0.5 to 3.1 D). Both over‐ and undercorrections could have been predicted from the relationship between myopia and astigmatism. Conclusions: The total amount of astigmatism in the myopic eye is proportional to the degree of myopia. The amount that one intends to treat should, therefore, be modified according to the amount of myopia to be treated, as determined by the relationship between the degree of astigmatism and myopia.