Spectacle measurements versus contact lens measurements of the angle of deviation in myopic patients with strabismus

Abstract

Purpose To compare the measurements of the angle of deviation through spectacles versus contact lenses and with the theoretically calculated angle. Patients and methods A total of 20 patients with comitant strabismus, more than 20 PD with spectacles, associated with bilateral myopia (spherical equivalent >1 D in both eyes) were included. Angle of deviation was measured using prism and cover test with spectacles and with contact lenses fitted according to the cycloplegic refraction and refined by subjective refraction. Agreements between the measurements obtained with contact lenses and those obtained with spectacles as well as the theoretically calculated angle were analyzed. Results The mean age was 15.6±10.9 years. Average spherical equivalent was −9.1±4.2 D. Measurements obtained with contact lenses were significantly lower than those with spectacles (P<0.001) with coefficient of variation of 27.7%, indicating poor agreement of both measurements. Limits of agreement in Bland–Altman plots were more than 9 PD (d ± 2 s, −8.8 to 6 PD). The agreement was lower with larger angles. Measurements obtained with contact lenses were significantly lower than theoretical angles (P=0.002) but coefficient of variation between both measurements was 8.8%, indicating good agreement of measurements. There was little systematic or proportionate bias between the contact lens measurements and the theoretical angle. Conclusions Contact lens measurements were significantly lower than spectacle measurements in myopic patients. The contact lens measurements were in good agreement with the theoretically calculated angle. This overestimation of the angle with spectacles might be responsible for overcorrection of myopic patients with exotropia after surgery.