Large pupils and LASIK
Starbursts, halos and pupil size
Night vision after LASIK with large pupils
Patients with large pupils are not good candidates for LASIK. After LASIK, patients with large pupils may suffer from permanent, debilitating visual aberrations (starbursts, halos, multiples images) and loss of contrast sensitivity (inability to see fine detail) at night. The LASIK industry uses the term "glare" to describe starbursts seen by LASIK patients as shown in the upper right image.
The FDA placed warnings on its LASIK site concerning large pupils: LASIK Surgery Checklist and When is LASIK not for me?
There is abundant evidence in the medical literature demonstrating the importance of pupil size in refractive surgery. Some key medical studies which establish the importance of pupil size in LASIK can be found at this link:
The American Academy of Ophthalmology (AAO), American Society of Cataract and Refractive Surgery (ASCRS), Federal Trade Commission (FTC), Food and Drug Administration (FDA), and Ophthalmic Mutual Insurance Company (OMIC) have issued statements regarding pupil size in refractive surgery.
Industry & Regulatory Guidance on Pupil Size
The truth about pupil size and LASIK
One argument that patients hear from the LASIK industry is that pupil size alone does not predict who will experience night vision disturbances. There are actually two factors involved – pupil size and laser optical zone. Pupil size/optical zone mismatch, termed “negative clearance”, is a result of an effective optical zone smaller than the maximum pupil diameter at night. A patient should never consent to surgery where the fully treated area is smaller than the dark-adapted pupil. LASIK surgeons frequently state that the cause of night vision disturbances is multifactorial. Factors involved in night vision disturbances include quality and centration of the ablation, residual refractive error, and negative clearance. High myopia results in smaller effective optical zones which create greater negative clearance. It is well known that all corneal aberrations increase with increasing pupil size. To say that pupil size is not a factor in night vision disturbances is incorrect and absurd. Negative clearance is a preventable cause of night vision disturbances with proper patient screening.
LASIK providers and industry supporters have a financial incentive to coverup the importance of pupil size. If you see information that minimizes or downplays the importance of pupil size, consider the source. Is the information provided by a LASIK surgeon who has a financial interest in patients' decision to have LASIK? Is the information published on a website that promotes LASIK or refers patients to LASIK surgeons? Do not be deceived by reckless surgeons or fake patient education sites that downplay the importance of pupil size.
Why does pupil size matter?
The pupil controls the amount of light entering the eye similar to the aperture of a camera. The diameter of the cornea that receives the full intended correction by the laser should be at least as large as the patient's pupils in dim light.
The standard optical zone for LASIK surgery is 6.0 - 6.5 millimeters. Use of larger optical zones place patients at higher risk for development of the vision-threatening complication post-LASIK ectasia. Most lasers are not FDA-approved for optical zones larger than 6.5 millimeters. The ablation zone may include an additional blend zone which should not be considered when determining the size of the effective optical zone. In 2004, Netto et al found dark-adapted pupil sizes of candidates for refractive surgery range from 4.3 to 8.9 millimeters with an average of 6.5 millimeters. Based on these findings, a large percentage of patients should be disqualified for LASIK.
The effective optical zone (also called "functional optical zone") is the fully corrected area of the cornea, after healing, as determined by topography. The effective optical zone of the laser treatment should cover the entire diameter of the entrance pupil in dim light. If the pupil dilates larger than the effective optical zone, unfocused light rays will pass through uncorrected cornea and the resulting image will be distorted. The greater the disparity between the effective optical zone and pupil size, the more severe the visual disturbances.
The risk for night-time visual disturbances is further increased for patients with high myopia due to smaller effective optical zones associated with deeper ablations. If the ablation zone of the laser treatment is decentered, even an adequate effective optical zone for the pupil size may not cover the entire pupil diameter.
More information about the correlation between pupil size and night vision disturbances (spherical aberrations) can be found at the following link:
What about Stiles-Crawford effect?
The light-sensitive tissue lining the back of the eye is the retina. The retina contains photoreceptors called rods and cones. Cones work only in bright light and are less numerous in the periphery of the retina. Rods are dominant in dim light and are more numerous in the periphery of the retina.
Stiles-Crawford effect, published in 1933, states that light rays entering the eye through the center of the pupil have a greater effect on vision than light rays entering through the pupil margin. This principle applies to cones (day vision), not rods (night vision).
Many LASIK surgeons misstate the Stiles-Crawford effect by falsely claiming that it minimizes night vision disturbances after LASIK.
Leo J. Maguire, MD, Mayo Clinic ophthalmologist and former FDA consultant: "The problems with pupil-related aberration are further magnified by the reality that the Stiles-Crawford effect is negated in night vision."
Pupil Size Lawsuits
There have been several LASIK medical malpractice lawsuits based on pupil size. Unfortunately, these cases are challenging for the plantiff due to phony "expert" witnesses -- hired guns for the defense who are paid to give bogus testimony about pupil size. A true "expert" knows that pupil size is a critical factor in quality of vision after LASIK. When a pupil size case makes it to trial, and a jury sees all the evidence, these cases can be won. Here are two pupil size cases that caught the attention of the LASIK industry:
$4 Million Award in Pupil Size Case
$3 Million Judgment in Pupil Size Case
LASIK surgeons use junk science in their defense against patients with large pupils who file lawsuits. It is important to understand flaws in the methodology of medical studies published by phony expert witnesses. Read more:
Importance of Accurate Pupil Measurement
Watch a video of LASIK surgeons discussing the importance of accurate pupillometry.
Pupils should be measured in a dark room after allowing the patient's eyes to become accustomed to the darkness. This is known as "dark-adaption". For more about accurate pupil measurements, see:
www.lasermyeye.org/keratoscoop/columns/lonedog/lonedog11jun2003.html
Jack Holladay, MD: "Accurate pupillometry is an essential part of the evaluation for refractive surgery. With reports of halos and glare following refractive surgery on many of the prime-time news shows, pupillometry has become one of the preoperative tests that patients expect. It is very clear from the published and anecdotal reports of nighttime glare and halos that a large pupil is the predominant factor leading to these problems."
Source: Review of Ophthalmology, Vol. No: 9:03 Issue: 3/15/02, The High Cost of Inaccurate Pupillometry
Effective optical zone (EOZ), also known as functional optical zone (FOZ)
The goal of LASIK is to change the refractive power of the eye by removing corneal tissue with a laser. Due to the risk of post-LASIK ectasia, the diameter of the cornea that is fully treated by the laser (called the optical zone) is generally limited to 6 - 6.5 millimeters. If the cornea were a flat piece of plastic, a 6 millimeter LASIK treatment would be sufficient for a patient whose pupils dilate in the dark to 6 millimeters. But the cornea is not flat, and it's not made of plastic. Factors that play a role in the effective optical zone after LASIK include the healing response of the cornea and the "cosine effect" or "radial compensation function". It is important to be aware of these factors and how they relate to night vision disturbances after LASIK. Learn more: