Night Vision Disturbances (NVDs) and Loss of Contrast (LOC)

Night Vision Disturbances (NVDs)

Night driving problems, also called night vision disturbances (NVDs), are common after refractive surgery. The causes of problems with vision at night range from residual refractive errors to a small or misplaced optical zone resulting in GASH, to an uneven corneal surface (irregular astigmatism). Night vision problems after refractive surgery range from mildly annoying, but fully functional vision, to an inability to function at night.

Most commonly, night vision problems (GASH) appear when the lasered optical zone is either too small, or off center. This doesn’t usually cause a problem during the day because when there is a lot of light the pupil stays small. But at night, the pupil gets bigger to allow more light to enter the eye. When the pupil becomes bigger than the optical zone, GASH is the result.

If the night vision problems are caused by simple refractive error or regular astigmatism, they can be reduced or eliminated with glasses, contact lenses and/or additional surgery. Additional surgery usually comes with a risk level similar to the original surgery.

If the night vision problems are caused by a too-small or misplaced optical zone, glasses and/or soft contacts cannot usually mask the GASH. Rigid gas permeable contact lenses (“RGPs,” aka “hard lenses”) are the only option to mask GASH from some surgical results. Fitting RGPs after refractive surgery can be challenging (due to the changed curvature of the cornea) and not everyone is able to wear these lenses successfully. Surgical options to fix these problems are not yet reliable and must be approached with caution, although technology continues to advance. Additional surgery usually comes with a risk level similar to the original surgery.

Another approach that works for some is the use of prescription eye drops that constrict the pupil. This method works when the constricted pupil is the same size or smaller than the optical zone created by refractive surgery. Brimonidine tartrate and pilocarpine are drops that constrict the pupil.

Loss of Contrast Sensitivity (LOCS)

Loss of contrast sensitivity (LOCS) occurs in almost all eyes after refractive surgery. Luckily, for most people the loss is minor and not noticeable, especially when both eyes are operated on at the same time and the point of reference is removed.

The objective of glasses, contact lenses or refractive surgery is to allow the light entering the eye to focus uniformly on a single part of the retina. When this happens, the vision is crisp and clear. When there is a loss of contrast sensitivity, the light entering the eye doesn’t focus uniformly. Instead, some of the light scatters and the vision in that eye can appear darker then it used to be, washed out and/or foggy. Loss of contrast sensitivity can range from not noticeable to extremely annoying. At night, or when it’s dark indoors, those with severe loss of contrast sensitivity can’t see very well when compared to those with normal levels of contrast sensitivity. Often, they crave light. Sometimes they even have problems discerning distance and walking up and down stairs. During the daytime, vision can appear washed out or bland, as if someone turned down the color (think of a faded photo). It can sometimes be difficult to see and identify objects, and driving can be a challenge.

Loss of contrast sensitivity can occur for many reasons following refractive surgery, ranging from residual refractive error to astigmatism (regular or irregular) to haze on the corneal surface.

Treatment of loss of contrast sensitivity is dependent upon the cause. Loss of contrast sensitivity due to residual refractive error can be reduced with glasses, contact lenses and sometimes additional refractive surgery. Additional surgery usually comes with a risk level similar to the original surgery. But these type cases are usually the mild ones and the residual refractive errors are usually more noticeable then the loss of contrast sensitivity.

Loss of contrast sensitivity due to haze (especially haze formed after PRK) can often be treated, usually with success. It is important to notify the surgeon of the formation of haze immediately. Loss of contrast sensitivity due to a too-small, or misplaced optical zone, or an irregular corneal surface after refractive surgery cannot currently be fixed with additional surgery, although technology continues to improve.