If you haven’t already, read the Lone Dog’s very first Saturday catharsis, The Debate Over Pupil Size in Laser Vision Correction. The article you are about to read takes as its underlying assumption that pupil size is important to all patients because 1) if you are being treated on a wide-zone laser, your optical zone ideally should be 0.5 to 1.0 millimeter larger than your largest pupil size under dark conditions, or 2) if you are being treated on a VISX laser, no matter what they say, the LD thinks your risk of vision quality loss may increase if your dark-adapted pupils are larger than the chosen optical zone, and you should be counseled appropriately, or at least be knowledgeable enough to ask tough questions. Relax, I am about to define all these terms, and more.

What follows are excerpts from the LD’s upcoming Dictionary of Laser Vision Correction. This is ordered conceptually, not alphabetically.

Laser Vision Correction (LVC): changing the curvature of the front surface of your cornea using an excimer laser, in order to reduce or eliminate your dependency on appliances like glasses or contact lenses.

Excimer laser: a device that generates laser energy via a photochemical process known as an “EXCited dIMER” reaction. Clever name, huh?

Cornea: the clear surface tissue of the eye through which light passes. It’s like the glass on your watch. This is what gets cut by your surgeon and fried (excuse me, “ablated”) by the excimer laser during LVC. The cornea is curved like a magnifying glass, not flat like a window.

Ablation: the doctor-term used to describe the end result of the excimer laser vaporizing corneal tissue. What you want out of the laser is the perfect ablation.

The Perfect Ablation: the removal of the proper depth of corneal tissue at each and every location treated by the excimer laser, so that the curvature of the front surface of your cornea is changed in a specified fashion at each and every treated location, thereby providing you with 1) the uncorrected visual acuity that you desire, and 2) unchanged or improved vision quality.

The Uncorrected Visual Acuity That You Desire: not everyone wants the magic 20/20 number. Some individuals older than 40, or heading there, prefer to have one eye treated in such a fashion that it remains or becomes somewhat nearsighted. This allows them to avoid wearing reading glasses for awhile longer. Uncorrected means no appliances such as glasses, contact lenses, habitual squinting, or spouses who read road signs for you at night. For the sake of unimpeded conversation let’s assume your stated preference is for 20/20 vision without glasses or contact lenses — i.e., you desire full correction.

Visual Acuity (VA): “how low can you go” on the eye chart. Despite its common use in a thousand ways, formally visual acuity has no other meaning. For visual acuity measurements, the closer the fraction is to 1, the better. Although your acuity has a substantial contribution to your overall sensation that you see “well” or “not well”, it is not all four aces in the deck. After LVC, your visual acuity is primarily dependent on the curvature of the center of your cornea. Keep this in mind.

Vision Quality: everything else you take entirely for granted about your vision. Vision quality can be severely compromised by problems with the ablation of non-central cornea. Many LVC casualties are classified as successes by their surgeons, and told to quit whining or seek psychiatric help, because 1) a small area of central cornea got a reasonable ablation so that the uncorrected visual acuity is 20/40 or better, and 2) most or all of the non-central cornea got a lousy ablation. These people may no longer be wearing glasses but sure wish they could — the glasses haven’t been invented yet that can float them to the surface of their sea of vision distortion. Typical opinion of an LVC casualty in this circumstance: “The day I had my surgery, I wish I’d stayed in bed.”

On to the Anatomy of an Ablation . . .

Ablation Zone (AZ): the entire region of cornea that is zapped by the excimer laser. This does not equal your whole cornea.

Optical Zone (OZ): the central region of cornea that is zapped in such a fashion that the curvature of the cornea is changed so as to focus light 100% correctly.

Blend Zone (BZ): the ring around the central cornea that is zapped in such a fashion that the optical zone “blends” into the most peripheral untreated cornea smoothly, making a “ski slope” transition rather than a “stair step” transition. Also known as the transition zone. Light passing through this area of the cornea is not 100% correctly focused.

Here’s a little math: AZ = OZ + BZ

Effective Optical Zone (EOZ): the central area of cornea that ends up being “on target” in terms of corneal curvature, after all influences such as the corneal flap (if made — for LASIK, not PRK) and patient healing factors have had their chance to reduce it. The effective optical zone is always smaller than the originally programmed optical zone.

And the Pupil . . .

Pupil (1): the “black part” of the eye.

Pupil (2): the hole in the middle of the iris.

Iris: the “colored part” of the eye. The iris is not on the surface of the eye; it is covered by the cornea. You can’t scratch your iris.

Pupil (3): yeah, it’s a space, not a real thing.

Dark Pupil Diameter (DPD): the diameter that your pupil gets to after you’ve been sitting in the dark for a while.

Maximum DPD (MDPD) (1): what you want your surgeon to measure accurately.

Maximum DPD (2): the largest that your pupil will get when you have been in a darkened environment for a substantial period of time, without being on naughty drugs or eye drops. The MDPD is particularly important for 1) movies, and 2) night driving. The two are often a combined event. Some oral medications can influence the size of the MDPD (generally to increase it); if you are on these medications chronically, or intermittently but with frequency, it is “fair” to count their effect on your DPD as being relevant to your surgical planning.

The LD’s Recipe for maximizing the likelihood of your satisfaction after LVC:

1] OZ > MDPD

Or more specifically,

2] OZ minus MDPD > 0.5 mm

Or the very best recipe of all:

3] OZ minus MDPD > 1.00 mm

Remember, you want some room for remodeling (healing), which will reduce your programmed OZ down to your ultimate EOZ.

(Any docs using VISX lasers . . . chill, I’ll deal with you later. Ditto your defense attorneys.)

For all the right-brainers out there, we’re going to do it again with pictures. As you look at them, pay attention to the black rim outside the colored circle, which represents the OZ programmed by your surgeon, Joe, based on some inscrutable criteria of his own. Light that goes through cornea outside the central OZ, but gets in through the pupil, will be “seen” by you but it will not be in focus. It is a reasonable approximation of the rather complex optics of this situation to assume that all cornea overlying the pupil in these photographs will send light through the pupil. So black = bad.

[For geeks, the following images consist of the same infrared photograph of a brown-eyed subject after 10 minutes of dark adaptation at 1 lux. The colored circles correspond to OZs of different diameters which were measured against the internal photographic ruler.]

This is you, Lou “Big Pupil” Schmo. The median MDPD for people in their 20s and 30s is about 7.0 mm. So half are more and half are less. If you are 30-something, insist on being told your pupil measurement to one decimal place (‘eight-point-zero’, not ‘eight’). If it is less than 6.5 mm, insist on a repeat measurement on a separate occasion. If you are told 6.0 mm or less, either 1) you are an unusual specimen, although not quite a freak of nature, or 2) your surgeon doesn’t know what he’s doing. [See Accurate Pupil Measurement in Laser Vision Correction.]

Start with a 7.0 mm OZ, throw in a BZ out to 8.0 mm, and you’ve got decent coverage. As long as the laser delivers. [See Quality Control in Laser Vision Correction].

This is as far as the VISX laser gets. The newer VISX lasers can do a blend zone out to 8.0 mm. “Studies have shown” that patients with pupils larger than 6.5 mm treated on the VISX S4 apparently experience no worsening in a few indicators of vision quality (glare and halos around point light sources) at one month after surgery. That’s a little soon, given that the OZ has yet to shrink all the way down to the EOZ at one month. However, over time there is also some overall smoothing of the corneal surface which generally improves vision quality, so the two effects may neutralize.

The LD thinks that people with naturally whopping pupils (like you, Lou), especially those with higher prescriptions and a habit of wearing dirty soft contacts, backed up by scratched and out-of-date glasses, probably do experience the alleged “unchanged or improved” glare and/or halos after LVC. If you are a large-pupil, low-prescription, clean-glasses, “I don’t have any glare now” Schmo . . . don’t be overly reassured by what studies have shown. Not very many Schmos like you were in them. [See True Lies in LVC Clinical Research and Weasel Talk in LVC (coming soon)].

Nonetheless, VISX has the longest track record with the most patients under its corporate belt. It is a stand-alone LVC-only company, unlike most of (all?) the others. By virtue of being the most frequently-used laser platform, if you survey LVC casualties you find that the majority was treated with a VISX laser and wouldn’t stand near one again for a dollar (or pound or euro) a minute.

The LD is not giving medical advice or expert opinion. The LD is making an observation, which is that if you are Lou “Big Pupil” Schmo, and you get treated on a VISX S4 with a 6.5 mm OZ and a blend out to 8.0 mm that is a perfectly centered Perfect Ablation, studies have shown that your surgeon did the best he could with a VISX laser, and did not necessarily act recklessly by proceeding with surgery despite the fact that your MDPD was 1.5 mm larger than the OZ. It’s a personal decision (your decision, not his, since you are an informed consumer now). By the way, the patients in these studies that have shown this happy outcome were all operated on by very experienced, “top gun” types. Ask your surgeon hard questions about all the things you have learned in this article; if he evades them, dismisses them, or can’t understand them (really bad sign there), find someone else.

I don’t think this one requires much commentary. In fact, the same studies that have shown that a VISX treatment with a 6.5 mm OZ and a blend to 8.0 mm makes everybody happy (Lou, Lou’s surgeon, corporate stockholders) have also shown that a 6.0 mm OZ with a blend does not. This is not deeply buried in the fine print, but you wouldn’t know it from the title of the article [“Surgeons find no correlation between large pupil size and refractive complications.” EyeWorld: February 2003]. EyeWorld is not exactly the Magna Carta, Rosetta Stone, or Preamble to the Constitution of ophthalmology publications, either.

“But this is FDA-approved!”

Watch out for surgeons with such an incredibly limited understanding of how they can ruin lives. Every LVC study ever performed under the fine auspices of the FDA had to have tightly specified patient selection and treatment protocols. This led to several downstream, unacknowledged decision-making problems for someone like you.

One: you may not resemble the patients in those trials very well, but there is no way that you can determine this and your surgeon probably has little or no knowledge of the original trial enrollment criteria. He has generalized. They were nearsighted, you’re nearsighted. Sign here. The refractive surgery industry has encouraged this for all it is worth. Why spend a zillion bucks developing a laser and fancy software and jumping through the FDA’s hoops in order to be able to treat 10% of the nearsighted public?

Two: some of the original treatment protocols turned out to be lousy. This may have been discerned mid-protocol, but for whatever reason (NB) the trial was not modified or interrupted. For example, in the Alcon Summit Autonomous LADARvision trials the OZ was set at 5.5 mm for all patients with nearsightedness plus astigmatism, regardless of pupil size. Honest surgeons involved in this trial saw that the 5.5 mm OZ could be a vision quality disaster for all but the lowest prescriptions and smallest pupils, and they personally went “off label” with larger zones just as soon as they could (remember VISX, the front runner, offers only 6.0 mm and 6.5 mm OZ — they had already burned food in that kitchen). However the Alcon laser was FDA-approved for 5.5 mm since that was the trial protocol. This will be Lou’s surgeon’s defense. His lawyer will argue that the fact that the newer Alcon laser can make an OZ out to 8.0 mm, and is specifically utilized as the “large pupil” laser by surgeons who otherwise use the VISX laser, is irrelevant

Three: “Whatever reason”. The FDA was primarily tracking surgical outcomes like visual acuity and residual need for prescription correction. They were secondarily tracking outcomes related to other aspects of vision quality like glare, halos, and ghosting. These are typically more dependent on the ablation of non-central cornea. Specifically, they can be related to the kind of mismatch between OZ and MDPD illustrated above. Since these were secondary measures, the Alcon study wasn’t stopped because subjects had glare that they would not have had if they had been treated with a wider optical zone. [Coming to this web site soon: Ode to Rick Kwiecinski].

By the way, in most other surgical procedures “side effects” are referred to as complications, and repeat surgeries (because the first one didn’t work) are not termed “enhancements”.

Four: to VISX users and their med-mal golf buddies. The LD humbly points out that optical modeling (nothing involving the Theory of Relativity, just some diligent ray tracing) proves the intuitively sensible concept that a given mismatch between OZ and MDPD (e.g., OZ 0.5 mm smaller than MDPD) will be less symptomatic when it occurs more peripherally. To put it another way, if “black = bad” in the above photographs, the further out towards the edge of the cornea that the black starts, the less likely you are to have symptoms. This is why the 6.5 mm OZ works better than the 6.0 mm OZ. VISX didn’t really need to guinea-pig live humans to figure this out.

Five: to readers . . . don’t generalize the above VISX commentary to mean that a 6.5 mm OZ on the Alcon (or other wide-zone) laser is therefore OK if your MDPD is larger than 6.0 mm. VISX has concentrated on refining its hardware and software, working around two OZ diameters, to improve ablation accuracy. Alcon and similar platforms have concentrated on expanding the OZ diameter. Make sure you know what laser flavor you bought, and get what you pay for if it’s a wide-zone laser. Don’t be afraid to ask very explicit questions of good old Joe Surgeon before you grab the teddy bear and look up at the little red light.