Night Vision and LASIK
It is somewhat ironic that the greatest advancements toward eliminating the problem of night glare after LASIK have occurred at the same time that the media and the general public have become most aware of the problem. With the advent of lasers that allow the doctor to create what is known as a blend zone, the incidence of night glare is dropping significantly.
The most important preoperative factor in preventing night glare following LASIK is identify whether a patient is at risk so that the appropriate adjustments can be made. Preoperative patient education and informed consent should address night glare in all patients. That said, night glare can be minimized or prevented after small-zone LASIK with several simple but useful methods.
Pupil Size: A Critical Factor
Colvard pupillometry is the most important screening test for pupil size because it standardizes measurement of the pupils and gives reproducible results. Patients with larger pupils are asked about their degree of night glare prior to surgery to determine the severity as well as to bring it to their attention. This is done so that patients with large pupils do not incorrectly blame postoperative night glare on their doctor, since these patients are at increased risk for postoperative night glare. In addition, patients with high degrees of myopia and astigmatism will typically have smaller, more oval treatment zones, which can increase this risk of night glare following surgery; as such, these patients are advised of the risks.
Patients with large pupils (pupillary diameters greater than 6.5 mm), higher degrees of myopia, or astigmatism will sign an informed consent document stating that they are aware that they have a greater chance of experiencing postoperative night glare. These patients are told that night glare usually improves with time but some degree of night glare can be permanent. Patients should be counseled very carefully, especially if their lifestyles/occupation involve a significant amount of night vision demands. Those patients with several of the risk factors listed above may not be good candidates for LASIK.
The importance of pupil size cannot be understated. There are other factors, however, that often play a part in night glare. I have found that the typical patient who exhibits night glare after LASIK is male, young (less than 30 years of age), highly nearsighted (greater than 6.0 D), highly astigmatic (greater than 3.0 D), with large pupils (greater than 6.0 mm in dim light), and suffers from dry eye.
Lasers for Large Pupils
Once the risk of postoperative night glare has been identified, the treatment should be "customized" to minimize the postoperative symptoms. Each excimer laser differs in the functionality of its own specific tools, which include ablation patterns, optical zones, and blend zones. This makes it difficult to generalize about the treatment parameters that should be used.
However, a few generalizations can be made. For example, if your eye dilates to 7mm but the laser being used can only correct out to 6.5mm, that's .5mm of corneal tissue that isn't being ablated. This un-ablated 'ring' is what causes halos. This makes some lasers more likely to cause halos than others. Specifically, LASIK procedures performed with the VISX S2 laser, which has a maximum spherical ablation zone of 6.5 mm, can be expected to cause some degree of postoperative night glare in the form of halos, if the pupils are larger than these measurements in dim light.
Some lasers, however, succeed in preventing halos to an extent. Scanning lasers such as the Bausch and Lomb 217, the Alcon-Summit Autonomous LADARVision, and the LaserSight LSX allow programmable treatment zones with 'blend zones.' These blend zones reduce the risk of halos. Following ablation, an abrupt division between treated cornea and untreated cornea is left at the outer edge of the laser's treatment zone. Without blend zones, which effectively create a smooth transition between the laser's treatment zone and the untreated area of the cornea, this abrupt change in angle can create visual aberration in the form of a "halo." The smoother the transition, the less likely halos are.
For patients with larger pupils, the ablation should be centered on the center of the pupil to avoid any 'edge glare' from a de-centered ablation. Surgeons need to carefully monitor the corneal thickness, as the increase in the zone sizes will dramatically increase the ablation depth. It is very difficult to preserve an adequate amount of corneal tissue (250 microns) when LASIK is performed with an optical zone greater than 7.0 mm.<>
It should be noted that night glare is not a common problem when two important rules are followed; patients must be appropriately screened prior to surgery, and a scanning laser with blend zones should be used. The closely monitored FDA LASIK trials that were performed in Canada last year for the Bausch and Lomb 217 excimer laser found that 96.7% of the eyes studied had no night glare and 98.7% had the same or better night vision compared to preoperative testing. This is because the laser has a 6.0 mm treatment zone with an 8.5 mm blend zone. Our experience with the treatment of patients with pupils greater than 6.0 mm using the Bausch and Lomb 217 laser has yielded excellent results in over 300 eyes.
Patients that do experience postoperative night glare should be seen immediately, as the loss of ability to drive at night can be a very stressful event. These symptoms generally subside after a period of between 3 to 6 months. During the recovery phase, the treatment of dry eyes and any residual refractive error or monovision often improves the symptoms. For those patients that still have difficulties, there are still treatment options to help patients manage during the recovery phase. These include night driving glasses, turning the car dome light on when driving, and, although rare, the use of special drops just before night driving. For those rare patients with persistent night glare, custom/ wavefront LASIK should offer a refined method of enlarging the treatment zones in the next 12 months.
Louis Probst, MD, is Medical Director for TLC Chicagoland, TLC Madison, and TLC Windsor.
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