LASIK defense experts will attempt to talk over plantiff attorneys' heads. They'll say things like "the patient's problem is not his pupil size... the problem is spherical aberrations". The defense is counting on the fact that a plantiff attorney does not have a thorough understanding of higher order aberrations. Well, I'm going to attempt to explain higher order aberrations, particularly spherical aberration, so you can see how this is all pupil-size related/dependent. The diagrams above should help you understand the concepts. Keep in mind that vision is never as precise in a human eye as depicted in the diagrams. All eyes have some aberrations. It has been said that about 10% of the unfocused light in a normal, unoperated eye is due to higher order aberrations. This usually causes no problems for the patient. In eyes after corneal refractive surgery, the aberrations may be render the patient's vision horribly distorted and the patient may no longer be able to function normally in dim light.
The first concept to understand is that "irregular astigmatism" is generally interchangeable with "higher order aberrations". Corneal astigmatism is measured, obviously, on the front surface of the eye, the cornea. Regular astigmatism is correctable with glasses or soft toric contact lenses. Irregular astigmatism cannot be corrected with glasses or soft toric contact lenses. Aberrations can be described as unfocused light rays striking the retina at the back of the eye (what the patient actually sees). Keep this in mind -- astigmatism/front/cornea, aberrations/back/retina.
The terms "wavefront", "visual aberrations", and "aberrometry" are really not so hard to understand as long as a lying defense expert is not doing the explaining. Lying defense experts don't want you to understand because once you fully understand, the light bulb turns on in your head and suddenly you understand how all of this is pupil size related. And that's when you can control the lying expert.
When a LASIK surgeon takes the measurement of a patient's visual aberrations, he/she uses a wavefront aberrometer which maps how the eye sees. The deviation from a perfectly focused image is expressed mathematically in root mean square error (RMS). Total RMS is a summary of all optical errors of the eye. Lower order RMS measures just the simple shapes of defocus which are correctable with glasses or soft toric contact lenses (myopia, hyperopia, astigmatism). Higher order RMS measures the complex shapes of defocus (irregular astigmatism) which cannot be corrected with glasses or soft toric contact lenses. The two most common forms of higher order aberrations are coma and spherical aberrations.
Aberrations get their name from the shape of the distorted image. Imagine a star in the sky at night. If the patient has an off-centered LASIK treatment, the star will have the appearance of a comet with a tail. In other words the light will be smeared in one direction. The name for this visual aberration is coma.
Many patients have spherical aberrations after LASIK (all of them would if their pupils dilated beyond the LASIK treatment). Just like coma gets its name from its shape, spherical aberrations gets its name from the fact that the unfocused light has the shape of a ball. The headlight from a car streaks out from the source in all directions. The fourth image above shows how spherical aberrations happen.
Most LASIK patients have both, coma and spherical aberration. But to what degree? How "bad" is it? That's where the RMS values are important. When you're looking at a patient's medical records and you get to the post-op wavefront scan, look for the RMS value of higher order aberrations. The LASIK industry rule-of-thumb is that anything less than .3 microns (all higher order aberrations combined) should not cause the patient too much of a problem. Maybe their "brain will adapt" and they'll get used to it and quit complaining.
If the aberrations are severe, the patient's quality of life is likely compromised.
Now, please pay attention because this next concept is key. AT WHAT DIAMETER WAS THE SCAN TAKEN? If you want to know what the patient sees at night, the scan diameter must be as large as the patient's naturally dilated pupil size in the dark. DO NOT FALL FOR A LASIK SURGEON'S TRICK OF THE TRADE -- MEASURING THE ABERRATIONS SMALLER THAN THE PATIENT'S PUPIL SIZE. Taking a wavefront scan smaller than the patient's pupil is like weighing a 250 lb. woman on a scale that only goes up to 150 lbs. It's meaningless to night vision.
The aberrations are induced on the corneal surface when the laser changes the shape of the cornea. Let's consider a simple example of a patient with pre-op myopia. The laser flattens an area of the cornea to change the focus of the eye (see the diagrams above). It sounds great in theory, but it creates a huge problem for patients with large pupils. The reason it creates a huge problem for patients with large pupils is very simple. Most lasers only correct a 6 mm or 6.5 mm area in the center of the cornea (known as the "optical zone"). Some patients have pupils as large as 8 mm at night. OK, common sense is going to tell you that if the image was unfocused before LASIK, it's going to be unfocused after LASIK if you didn't fully correct it. After LASIK a patient with 8 mm pupils will have a "multi-focal" cornea. This is a huge problem, and one that glasses and soft toric contact lenses cannot correct. The greater the mismatch, the more severe the problem.
Now I'm going to throw you a curve ball. The greater the degree of pre-op myopia, the more likely the patient will experience night vision disturbances. And the reason for this is, again, related to pupil size. You need to understand the concept of "effective optical zone". Just because a LASIK surgeon used a 6 mm optical zone doesn't mean that 6 mm was fully corrected. Believe it or not, the geniuses that designed lasers for refractive surgery did not take into account that the cornea is not flat! Lasers were designed to treat flat surfaces. What happens is that the laser loses efficiency on the slope of the cornea. The very center/top of the cornea receives the full effect of the laser but the periphery doesn't. The end result is an effective optical zone smaller than intended. Whenever the patient's pupils are larger than, say, 4.5 or 5 mm, light will pass through the periphery of the treatment zone that wasn't fully corrected. You can actually see and measure this on the topography. The yellow ring is the border of the effective optical zone.
The first thing that the industry did in response to these problems (and associated problems) is that they added a "blend zone". The blend zone was designed to prevent a sharp transition between the treated area of the cornea and the untreated area of the cornea. It does not receive the refractive correction. Light that passes through the blend zone will be unfocused. If a LASIK surgeon says his laser has an 8 mm ablation zone, he's counting the blend zone. The blend zone doesn't count!
The second thing the industry did (because the first thing wasn't good enough) was to release a new, improved form of LASIK called "custom LASIK" or "Wavefront LASIK". Sure, they "attempt" to measure and "attempt" to treat that ~10% of defocus that glasses can't correct. And they also added more laser energy to the periphery of the treatment to compensate for the loss of efficiency on the slope of the cornea. But this still doesn't cut it when the patient has a large pupil. And oh by the way, wavefront custom LASIK actually increases the higher order aberrations -- it doesn't reduce them in normal eyes.
Unfortunately for patients with large pupils, most aberrometers can only measure up to a 6 or 6.5 mm scan diameter. There are some aberrometers that can take a larger scan. If you find one that can measure larger, you'll need to also find a surgeon who is willing to take the scan larger. Good luck!