Decreased Contrast Sensitivity, Night or Low-Light Vision

Contrast sensitivity deals with a patient's ability to discern images of varying shades from the background - in other words, how faded an image can be before it is indistinguishable from the background. Imagine driving in fog - the thicker the fog, the grayer all objects become and the more difficult to distinguish their features. Because contrast sensitivity is important to the quality of vision, several studies have been conducted over the last several years to determine the affect of LASIK on it.^2-8 All noted a decrease in contrast sensitivity which lasted from one to six months post-operatively. However, all but one also noted a return to pre-operative function by the six-month visit.

Subjective reporting of decreased night vision has been noted widely, with descriptions and illustrations of the potential effects of glare, halos and starbursts on a patient's vision. As this complication can occur and it can adversely impact a person's activities, we strongly suggest you discuss it with your ophthalmologist.

We have found two independent studies that note subjective reports of reduced night vision. First assessed 174 eyes that were treated between November 1997 and October 1998. Of them, 29% reported reduced night vision - however, it's important to note that 97% of patients indicated they would decided to have LASIK again.⁹ The second involved questionnaires sent to 50 patients; 8.8% reported difficulty with nighttime driving.¹⁰ Again, 98% noted overall satisfaction with their outcomes. The "Summary of Safety & Effectiveness Data" required by the US FDA tabulates the subjective reporting of glare, halos and difficulty with nighttime driving pre-operatively and post-operatively at various time intervals. Overall, two reports noted improvement of nighttime vision as compared to the pre-operative assessment and one noted a split, with between 5-9% of patients noting a significant improvement and the same number noting the symptoms were significantly worse.¹

Some studies suggest that by sloping the angle of ablation at the periphery of the ablation zone and by controlling the ablation depth, surgeons can significantly reduce the incidence of diminished night vision.^11-12 As more and more surgeons adopt the recommendations these studies suggest, we can expect the incidence of these complications to decrease.

However there are other factors that ophthalmologists believe may be salient. Many doctors believe that there may be a correlation between wide pupils and decreased night vision, particularly in cases where treatment of high myopia or high hyperopia is concerned.¹³ Some researchers contend that patients who have wide pupils may be more likely to experience decreased night vision following LASIK than the general population.¹⁴ Patients need to be aware that ablation zones do vary slightly from laser to laser and should discuss with their doctor whether this is an important consideration. Patients whose pupils widen to greater than the ablation zones used on their physician's laser in dim light need to carefully weigh the increased risk posed to their vision.

Yet not all ophthalmologists believe this possible correlation is the only determining factor. Not everyone with wide pupils and high corrections will experience decreased night vision. Conversely, some patients with small pupils and low corrections will describe experiencing glare and halos that are associated with decreased night vision. Currently, it is impossible to determine in advance who will experience the complication and who will not.

Those patients who experience persistent glare, halos or starbursts do have several treatment options, including prescription eye drops to help constrict the pupil at night to reduce the symptoms of decreased night vision. However, this pharmaceutical approach is not successful for all people, and it also can represent a significant expense over time. Similarly, enhancement procedures may be an option, but not everyone is eligible for an additional procedure. Eligibility for enhancements depends on a number of factors that a doctor must evaluate.

Patients should ask their doctors about the test they conduct for pupil diameter. Doctors measure the pupil in several ways. Most doctors will measure the pupil in both bright and dim light to simulate day-to-day lighting situations. Some doctors will utilize their experience and measure the pupil without using instrumentation, while others will put a pupil gauge up to the eye to determine the size.

Dry Eyes and Severe Dry Eye

Patients need to be aware that it is not uncommon to experience some symptoms of dry eyes post-operatively, especially in the first few weeks as the majority of corneal healing takes place.6, 11 Those patients who have a pre-existing dry eye condition need to be especially aware of the potential risks regarding LASIK and dry eyes.

As early as December 1999, ophthalmologists were paying attention to the possible connection between LASIK and dry eye. At the 1999 joint meeting of the American Academy of Ophthalmology and the Pan-American Association of Ophthalmology, Dr. Robert Maloney reported the results of a survey of 550 patients after LASIK. Forty to fifty percent noted experiencing dry eyes at some time of the day 3 months post-operatively. Dr. Richard Lindstrom noted that while the overall frequency of complications after LASIK is decreasing, the incidence of dry eye after LASIK is increasing. Dr. Roger Steinert supported Dr. Lindstrom's assertion and provided a possible explanation: patients seek LASIK precisely because they are contact lens intolerant, and such intolerance is often indicative of dry eye. Dr. Marguerite McDonald noted several other possible explanations for the increased frequency of this condition post-operatively:

• Use of post-operative steroids after LASIK may exacerbate pre-existing dry eye.
  
  
• Previous contact lens wear disrupts normal corneal physiology.
  
  
• Hormonal changes during menopause or birth control pills can cause dry eye.
  
  
• The surgery itself may cause dry eye by cutting the nerves of the central cornea or damaging cilia.

Since then, several studies ^{15, 18, 19} have demonstrated that some patients may experience a decrease in corneal sensation following LASIK for the first six months following the procedure as the nerves of the cornea regenerate. Corneal sensation is characterized by a feedback loop. When the eye is dry, a message is sent to the brain. The brain then stimulates the eye's lacrimal glands to produce tears. However, creating the flap cuts the nerves of the cornea. For some eyes, this process temporarily affects the cornea's ability to sense dryness. When corneal sensation decreases, the feedback loop is disrupted, and the eye will experience dry eye symptoms.

Patients need to be aware that they may have dry eye symptoms for several months following LASIK, even if they did not have symptoms or signs of dry eye prior to surgery.^{15, 18, 19}Those patients who have severe conditions pre-operatively need to be aware that, because surgery can exacerbate the symptoms, a doctor may deem them ineligible for surgery.