If you wear soft contact lenses for astigmatism, don’t be surprised if your vision gets blurry when you watch television while lying on the couch. The problem? Your contacts are rotating out of position.
In order to correct astigmatism, toric soft contacts have different lens powers in different parts of the lens, with a design feature that enables the lens rotate and stabilize in a position so the correct lens powers are aligned in front of the appropriate parts of your eye.
Design features to maintain rotational stability include a thicker, heavier portion at one point in the lens periphery, “thin zones” in specific portions of the lens or a combination of both these features.
Researchers in the UK recently conducted a study of the effect of gravity and gaze direction on toric contact lens orientation and eye chart visual acuity.
Four types of toric soft contacts were assessed: Acuvue Oasys for Astigmatism (Johnson & Johnson / Vistakon), Purevision Toric (Bausch & Lomb), Air Optix for Astigmatism (CIBA Vision) and Proclear Toric (CooperVision).
The study evaluated toric lens orientation and visual acuity when the subjects were positioned on their side and when they were sitting upright and looking in different directions of gaze.
The researchers found that all toric lenses in the study rotated as a result of change in the lens wearers’ posture and head position. Mean rotation ranged from 11 to 29 degrees, causing varying degrees of reduced visual acuity. Lens rotation and reduced visual acuity also occurred when the subjects look far up, down and to the side.
The take-home lesson: If you wear toric soft contact lenses for astigmatism, be aware that unusual posture and extreme gaze directions will cause these contacts to rotate, affecting the sharpness of your vision.
To restore clear vision, return to a normal upright position and blink a few times while looking straight ahead: This will help the lens return to its proper position.
SOURCE: Toric lens orientation and visual acuity in non-standard conditions. Contact Lens & Anterior Eye. February 2010.
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