Tolerance to prism induced by readymade spectacles: setting and using a standard.
OBJECTIVE
To determine first how much induced horizontal and vertical prism could be comfortably tolerated with readymade spectacle wear. Ultimately, the purpose is to develop a guide for dispensing decisions for blindness prevention programs in low-resource countries.


METHODS
Nine participants each wore plano spectacles with differing prism power for 8 h (plano control). If visual discomfort could not be tolerated, participants removed the spectacles, noting duration of wear and reason for discontinuation. Distance and near visual comfort were rated, and participants asked if they would be able to adapt to wearing the spectacles.


RESULTS
On average, half of the wearing time was spent on concentrated near viewing tasks. The highest prism powers (1 Delta BU, 2 Delta BO, 2 Delta BI) could not be worn for 8 h by the majority of participants. Comfort ratings at near (similar to those at distance) were statistically significantly different when the highest prism power was compared with each of the lower powers (vertical prism: both the control and 0.5 Delta differed from 1 Delta; horizontal prism: the control, 0.5 Delta and 1 Delta all differed from 2 Delta).


CONCLUSIONS
Most spectacle wearers would likely comfortably tolerate <or=0.5 Delta vertical, <or=1.0 Delta base out, or <or=1.0 Delta base in induced prism. A guide to the maximum interpupillary distance/optical center distance disparities likely to be comfortably tolerated with varying spectacle powers was formulated. Powers up to +/-1.50 DS are unlikely to have sufficient lens decentration to cause discomfort. This small study supports the feasibility of using readymade spectacles in low-resource settings even though optical center distance may be different to the interpupillary distance of the wearer. It would seem that fear of visual discomfort because of induced prism need not preclude the use of readymade spectacles, potentially correcting about 75% of refractive error and presbyopia, mostly with powers +3.00 to -3.00 DS. Further study is needed to more fully address these issues.
du Toit
Rènée
R
The International Centre for Eyecare Education, Sydney, Australia. rdutoit@bigpond.com
Ramke
Jacqueline
J
Brian
Garry
G
D000328:Adult; D001766:Blindness / Q000517:prevention & control; D003906:Developing Countries; D004867:Equipment Design; D005139:Eyeglasses / Q000009:adverse effects* / Q000592:standards; D006801:Humans; D013285:Strabismus / Q000175:diagnosis / Q000503:physiopathology / Q000534:rehabilitation*; D013997:Time Factors