Predictors of Unwanted Prismatic Effect Among Bespectacled Symptomatic Ammetropes (Refractive Error Less Than 4D) With Displaced Optical Centre

Aim and objectives:- This study tries to estimate the relationship between alignment of spectacle, decentration, spherical equivalent and type of lens with induced prism in spectacles. Design:- Descriptive cross sectional study. Materials and Methods:- Symptomatic cooperative ammetropes, using spectacles and between the age of 10 and 70 years were included in this study. Induced prism in spectacles were calculated by the Prentice formula. Results:- The study group included 29 ammetropic patients having asthenopic symptoms while using spectacles. 65.52% (n=19) were females. The mean age of this group was 48.33 years, with a SD of ± 17.45. This study showed a statistically significant positive correlation between spherical equivalent and induced prism and there is no significant correlation between decentration and induced prism. Conclusion:- Spherical equivalent is considered as an important factor affecting the induced prism rather than decentration and alignment problem. refractive error. Decentration can be due to faulty fit or secondary to mal-alignment of glasses due to misuse. The role of spherical and cylindrical errors, type of lens used (convex or concave) and the alignment of glasses in inducing prismatic effect among spectacle corrected anisometropics with asthenopia is studied here. A descriptive cross sectional study was conducted after getting approval from institutional ethics committee. The study period was six months. Subjects having spectacles corrected anisometropia with persistent asthenopic symptoms were included. For this study asthenopia was defined as headache with or without eye pain, watering and irritation on constant near work. All consecutive cases with one or more symptoms in either or both eyes during sustained near work with constant and regular ( a minimum of 8 hours per day) use of spectacles for a period of atleast four months were selected. Spherical equivalent was calculated from the absolute retinoscopy reading. Spectacles with centered optical centre, history of headache Abstract not related to constant near work, symptoms less than four months duration, irregular spectacle wearers, associated accomodation-convergence anomalies and latent squints were excluded. Age, gender, socioeconomic status and occupation of the patients were noted. The visual acuity, duration of spectacle wear and power of spectacles, alignment of spectacles, inter pupillary distance and decentration of optical centre in millimetres were recorded. Inter pupillary distance was measured with millimetre ruler. The optical centre of spectacle was found out by lensmeter. The prism induced was calculated by using Prentice formula P= CF [C = the distance of the image from optic axis in centimetres (decentration); F= the focal power of the lens]. The measure of horizontal and vertical prism were calculated separately. In spectacles having both horizontal and vertical prisms, the residual prism was calculated by the formula Pr= 2 (Pr=