Contact Lens and Anterior Eye (2001) 24, 153-160 © 2001 British Contact Lens Association W www,nature.com/clae Original Article VISION OF LOW ASTIGMATS THROUGH THICK AND THIN LATHE-CUT SOFT CONTACT LENSES* Pauline Cho~ and George C. Woo~ (Received 15 May 2001; in revised form lO July 2001) Abstract -- Distance and near visual acuity of 13 low astigmats were determined in a double-masked experiment through thick and thin (centre thickness 0.12 mm and 0.06 ram, respectively) spherical lathe-cut soft lenses. For each lens type, distance and near LogMAR VA and over-refraction were assessed with different IogMAR VA charts. For 70% of the subjects, the residual astigmatism was significantly lower than the refractive astigmatism with thicker lenses. No statistically significant differences in the distance and near logMAR VA was found between the two lens types using any of the charts used, though, in general, logMAR VA obtained through the thicker lens was better than IogMAR VA through the thinner lens. The variabilities in distance and near logMAR VA between the two lens types increased with decreased contrast. The variabilities in distance IogMAR VA were greater with Chinese charts than with English charts, and LogMAR VA with Chinese charts were significantly worse for both lens types. Based on the results of this study, we concluded that thicker spherical lathe-cut soft lenses provide better vision in low astigmats. The Snellen acuity test is inadequate for vision assessment of soft contact lens wearers. When a patient wearing thin soft contact lenses complains of poor vision in spite of 6/6 or 6/5 Snellen acuity, changing to thicker lenses may be considered. Contact Lens and Anterior Eye (2001) 24, 153-160. KEY WORDS: soft contact lenses, astigmats, IogMAR,vision, lens thickness, Chinese VA chart Introduction M any researchers have evaluated the ability of spherical soft contact lenses to mask small amounts of refractive or corneal astigmatism? -7 The results were by no means conclusive, although most reports favoured no significant masking. These studies were mainly concerned with the amount and efficacy of masking and contrast sensitivity performance of low to moderate refractive astigmatism using spherical soft contact lenses. Most of these investigators presumably used Snellen charts to assess vision. ~'2'~,s'° However, clinical observations have continued to indicate that, in a number of cases, low astigmats preferred vision with thicker soft lenses than with thinner ones, and most of these reports were also based on results obtained with high contrast Snellen charts. A number of reports have indicated that significant visual impairment can be caused by a decrease in contrast sensitivity due to mild residual astigmatism when wearing soft contact lenses. ~° Guillon et al. have evaluated the effect of target contrast and luminance on four different types of soft contact lenses (and specta- cles) and concluded that investigations of the compara- *The results of this study had been presented at 1st National Conference on Contact Lenses: 'Contact Lens Practice for the Future" Malaysia, April 2001. t PhD SOD, PhD, FAAO tive performance of visual corrections should include at least three different levels of contrasts, n Guillon and Sayer advised fitting a different lens type, preferably a standard thickness lens rather than a hyperthin lens, when a patient complained of unaccountable symp- toms. lz To date, it is uncertain how vision, both distance and near, and based on logMAR visual acuity charts, of low astigmats are affected by thicker and thinner soft contact lenses. Most of the studies of spherical soft lenses on low astigmats were concerned with the masking of corneal astigmatism.2,5; Very few of these investigators reported vision attained with soft lenses of different thickness, and for those who did, the Snellen Chart was used? ,8,~ As pointed out by a number of investigators, Snellen acuity is mainly a measure of a patient's ability to detect fine detail under high contrast condition and provides little information on what patients normally see in their everyday life?,13 Many investigators have reported on the change in contrast sensitivity when wearing soft contact lenses 8,u,14-~ but reports of visual acuity at different contrasts with soft contact lens wear are few? u7 Guillon et al. 1~ reported that conventional VA measurements do not detect significant differences in visual performance between different corrections (spec- tacles and soft contact lenses). They concluded that when evaluating the comparative performance of different visual corrections, logMAR VA charts of at least three contrasts (high (90%), medium (28%) and low