Induced prismatic effects due to poorly fitting spectacle frames

Abstract

To provide patients with the best vision possible practitioners undertake comprehensive visual examinations and optical dispensing procedures.  Ensuring proper frame alignment is an important part of the spectacle dispensing process.  Ideally,

for maximum optical benefit, patients should look through the optical centers of their lenses.  Rays entering a lens at any point other than the optical center will be deviated as long as the lens has power, thus inducing prismatic effects.  These prismatic effects may lead to visual complaints such as asthenopia,blurry vision and headaches.  The aim of the study was to investigate the prevalence of induced prismatic effects due to poorly fitting spectacle frames.  A sample of 100 spectacle wearing subjects was selected as participants.  Questionnaires were completed and any symptoms experienced recorded.  The habitual spectacle position in the primary position of gaze was noted, pupil and optical centers marked and where relevant, induced prism was calculated.  Data was analysed using the ANOVA Kruskal-Wallis and Mann-Whitney tests.  On initial observation of frame position, about45% of subjects were wearing their spectacles incorrectly.  A comparison of the marked pupil centers in primary gaze and optical centers revealed that 100% of participants were found to not be looking through the optical centers of their lenses.  Fifty one percent were within horizontal and 3.12% within vertical ANSI tolerances.  Symp-
toms were reported by 50% of subjects experiencing base-out, 0% base-in and 47% vertical induced prismatic effects.  However, no correlation existed between those subjects experiencing induced prism and symptoms reported.  Prism adaptation may account for some participants being asymptomatic.  Sixty three percent of subjects were not aware of the importance of properly fitted spectacle frames.  Proper optical dispensing with associated patient education is necessary to achieve optimal optical benefits of spectacles and careful attention should be given to this aspect by practitioners.  It is recommended that patients return periodically to their optometrists to have the frame alignment assessed and the fit modified if necessary. (S Afr Optom 2011 70(4) 168-174)