Contact Lens Practice Pearls
Contact Lens Practice Pearls
Tight-Fitting Lenses: Sometimes You Have to Loosen up
BY GREGORY J. NIXON, OD, FAAO
Patients commonly complain of a decrease in the visual performance of their contact lenses at the end of the day. Because most soft lenses experience some degree of dehydration throughout the day, blurred vision after a number of hours of wear is often simply attributed to this. While lens dehydration itself certainly plays a role, I have encountered an increasing number of tight-fitting soft lenses that contribute to end-of-day blur as well.
How Does a Tight Lens Cause Blurred Vision?
When a lens tightens down on the ocular surface, it can lose its natural shape, and its curvature may be altered. Unlike a GP lens that undergoes flexure, a tight-fitting soft lens can undergo a buckling effect, resulting in an irregular optical surface that can blur and distort an image.
The disrupted optics of a tight-fitting lens can be detected by performing keratometry or topography over the lens. The actual measurement is not of concern; rather, the qualitative assessment of the mire pattern is diagnostic itself.
For patients, this visual effect doesn’t always degrade acuity to a large degree, but it can diminish the crispness and overall quality of vision. Worse yet, this visual compromise often isn’t aided by over-refraction.
How to Detect a Tight-Fitting Soft Lens
Initially, a tight-fitting lens is quite comfortable. But in addition to blurred vision, a tight lens can induce other symptoms after hours of wear, such as discomfort and difficulty removing the lens. Therefore, the best time to detect a tight-fitting lens is toward the end of the day.
The lens fit evaluation often shows an impingement of the conjunctiva at the edge of the lens. The lens may or may not be completely adhered to the ocular surface, but it will lack independent movement as it drags the conjunctiva with it upon blinking. It can even cause the appearance that the conjunctiva under the lens—between the limbus and lens edge—has a slight blanched appearance, while the exposed conjunctiva at the lens edge will be slightly raised and discolored. Fluorescein staining after lens removal will often reveal a distinct circumlimbal imprint ring in the paralimbal conjunctiva.
What’s the Remedy?
To combat the effects of a tight fit, using a flatter base curve lens (usually 8.8mm or 9.0mm) or a smaller-diameter lens (such as 13.8mm) can lessen the harmful interaction of the peripheral portion of the lens with the conjunctiva. This cannot only prevent the visual disturbance created from lens buckling, but will allow independent movement of the lens to promote better circulation of oxygen and tears. This can further prevent lens tightening by diminishing both lens dehydration and buildup of posterior surface debris from corneal epithelial metabolic byproducts that can become trapped beneath a lens.
Tighter Isn’t Always Better
When faced with a choice of parameters, many practitioners have traditionally chosen to fit the steeper of two base curves to maximize lens centration. While a tighter-fitting lens almost always provides great initial comfort and centration, dehydration effects can cause the lens to become too tight later in the day.
That is why I’ve started to change my mindset to fitting the loosest lens that still provides adequate coverage and initial comfort to minimize the risk of end-of-day adherence issues. CLS
Contact Lens Spectrum, Volume: 29 , Issue: April 2014, page(s): 48