Common Questions About Suitability

What is 20/20 or 6/6 vision?

In the most common visual acuity test, an optometrist begins by placing an eye chart at a standard distance of twenty feet, or six metres away from you, depending on the customary unit of measure.

At this distance, the symbols on the line representing “normal” acuity on the eye chart, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet.

It’s possible to see better than the norm, though, which optometrists express as visual acuity of 20/16, 20/12.5, or better. For example, 97 percent of our short-sighted patients see 20/20 or better after surgery (which includes 20/16 or better, and 20/12.5 or better).

Three lines above, the letters are twice the dimensions of those on the 20/20 line. The chart is still at a distance of twenty feet, but a person with normal acuity can read these letters at a distance of forty feet.

This line is the ratio 20/40 (or 6/12). If this is the smallest line a person can read, the person’s acuity is 20/40, meaning, in a very rough kind of way, that this person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet.

To put this into perspective, 100 percent of our patients (-1.00D to -9.00D) see 20/32 or better after treatment, and 100 percent of patients (+1.00D to +6.00D) see 20/40 or better after treatment.

The biggest letter on an eye chart often represents acuity of “20/200”, the equivalent value of being “legally blind.” Many people with refractive errors have the misconception that they have “bad vision” because they cannot even read the E at the top of the chart without their glasses.

However, in most situations where acuity ratios are mentioned, they refer to best-corrected acuity. Many people with moderate myopia cannot read the E without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. A legally blind person is one who cannot read the E even with the best possible glasses.

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In Laser Eye Surgery, the surgeon’s goal is to get your vision without glasses after surgery to the same level as your best-corrected visual acuity (with glasses or contact lenses) before surgery, or better.

When optometrists or surgeons say “gaining or losing a line of vision”, they mean the change in your ability to read a line on the eye chart without glasses after surgery, as compared to the ability to read the line with glasses before surgery.

Check out this video in which expert laser eye surgeon Mr Glenn Carp explains 20/20 or 6/6 vision.

What is a prescription?

Optometrists and Ophthalmologists measure refractive errors of the eye, such as myopia, hyperopia, astigmatism, and presbyopia in units called dioptres.

These units show the amount of correction you need to see normally. One dioptre is the equivalent of a lens that can focus on an object one metre away. The more short-sighted, long-sighted, or astigmatic you are, the higher your prescription in dioptres. A typical prescription has three numbers, for example -5.00 / -1.50 x 180.

The first number (-5.00) identifies your degree of short-sightedness or long-sightedness. The minus sign in front of the number identifies you as short-sighted. A plus sign would show you were long-sighted.

The second number (-1.50) identifies your degree of astigmatism. This is written with either a plus sign or a minus sign.

The third number (180) indicates the axis or the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.

A prescription of -5.00 /-1.50 x 180 indicates that the patient is moderately short-sighted, with a moderate degree of astigmatism in a horizontal direction.

The table below illustrates the ranges from mild to severe short-sightedness and mild to severe long-sightedness:

Mild short-sightednessMyopia up to -3.00 dioptres
Low short-sightednessMyopia up to -3.25 to 6.00 dioptres
Moderate short-sightednessMyopia from -6.25 to -11.00 dioptres
Severe short-sightednessMyopia from -11.25 to -23.50 dioptres
Low long-sightednessHyperopia from +0.75 to +2.50 dioptres
Moderate long-sightednessHyperopia from +2.75 to +6.00 dioptres
Severe long-sightednessHyperopia from +6.25 to +12.00 dioptres

Watch a video in which Mr Glenn Carp answers the question of what is a prescription.

Why can London Vision Clinic treat higher Prescriptions than the norm?

Many patients will notice that here at London Vision Clinic we are able to treat prescriptions that are higher than those commonly treatable in other clinics.

Two of our values are conscientiousness and rigour. This means we spend as much time as possible with patients to assess their eyes and conduct their preoperative screening.

This extremely thorough evaluation is coupled with unique assessment technology (i.e. Artemis Insight 100) that assist us in approving cases that would be considered borderline at most clinics.

What’s more, we repeat many of the tests under extreme conditions (i.e. full darkness pupillometry, reduced light contrast sensitivity testing, dilated refractions). Again, all of this takes time, and if we needed to rush patients through a pipeline or had to hit certain quotas like high street chains, we would not have the ability to perform all of these tests.

On top of this, employing a full-time research manager enables us to efficiently offer individualised treatment plans and help patients with unique needs (i.e. reading vision, High Profile procedures, wavefront-guided treatments to treat higher-order aberrations).

High street surgeons that have considerably higher volume quotas may prefer to treat the easier cases at a faster rate. The fact that many clinics remunerate surgeons on a per-procedure basis also aids to create an incentive to treat more routine cases.

As a relatively small independent clinic, having only three expert surgeons perform surgery enables us to approve more patients for surgery. We do not have to worry about the differences in levels of experience or expertise of our surgeons, and because either Mr. Carp or Professor Reinstein see every case before surgery, we can be confident that our screening net is sufficiently stringent.

Accuracy decreases as operative prescription increases, and so many clinics may want to keep their results high by treating simpler prescriptions. Due to our expertise and technology, our results exceed the standard among all prescriptions.

Lastly, because of our commitment to aftercare, we are happy to approve patients who may have a greater likelihood of needing an enhancement. Clinics know that individuals with higher pre-operative prescriptions have a greater chance of needing an enhancement. Because of this, many clinics will limit their exposure with these patients by simply refusing to treat them.

Watch Mr. Glenn Carp talk about why our clinic can treat higher prescriptions than usual.

Do the results of Laser Eye Surgery differ between prescriptions?

In general, results (the term used in refractive surgery is efficacy) decrease as prescriptions increase. And so the short answer to this question is yes.

Our experience at London Vision Clinic, while not as dramatic as the norm, mirrors this general trend. Therefore, when reviewing a clinic’s results, it is important they reflect the vision patients achieve for specific prescriptions (as opposed to overall results across all prescriptions which are less relevant and often biased towards lower prescriptions).

One of the methods we use at mitigating this effect is by planning High Profile treatments with our patients who suffer from higher prescriptions.

High Profile treatments are akin to taking two attempts to reach a target instead of one. With one attempt, there is always the chance that we may overshoot or fall short of our target. Planning the treatment in two stages enables us to optimise safety and results for patients with higher prescriptions.

High Profile procedures cost more in terms of time and money, but the vast majority of our patients, when presented with this option, decide it is best for them.

Results also differ between short-sightedness and long-sightedness (i.e. 97 percent of short-sighted patients achieve “20/20” or better, while 90 percent of long-sighted patients achieve “20/20” or better).

At London Vision Clinic, however, 100 percent of patients, short-sighted or long-sighted, with or without astigmatism achieve “20/40” or better. Indeed, 100 percent of short-sighted patients up to -9.00D see “20/32” or better after treatment with at London Vision Clinic.

Watch Mr Glenn Carp answer the question “Do results differ between prescriptions?”.

Is there a greater chance of needing an enhancement if you have a higher prescription?

Because accuracy decreases as prescriptions increase, there is a greater chance of needing an enhancement if you have a higher prescription.

We find that enhancements are more typical amongst patients who have higher pre-operative prescriptions as well as long-sighted patients. Despite this, our enhancement rates are relatively low (between 5 and 8 percent).

Click here to watch a video in which Mr Glenn Carp discusses higher prescriptions and the chance of needing an enhancement.

Are Laser Eye Surgery prices higher for higher prescriptions?

Many clinics do charge people with higher prescriptions more than those with lower prescriptions.

We consider this upselling. The reasons for this are clear — patients with higher pre-operative prescriptions have a higher likelihood of needing an enhancement, and they build the cost of this potential enhancement into the total price at the outset (whether it’s actually needed or not).

Our approach at London Vision Clinic is to offer patients with higher prescriptions the option of planned High Profile treatments. We have found that High Profile treatments have a higher likelihood of reaching the targets we set out for the patient from the outset, meaning they’re also a safer way to treat higher prescriptions. We charge higher fees for planned High Profile treatments.

Watch Glenn Carp answer the question of high prescriptions and higher prices in this video.

Are there more risks associated with higher prescriptions?

There are two areas where higher risks are associated with higher prescriptions: night vision disturbances (halos and starbursts) and corneal haze.

One of the main focuses of Professor Dan Reinstein’s research over the last eight years has been the correction (and prevention) of night vision disturbances. Prof Reinstein has developed protocols for correcting and preventing halo and starburst effects even when treating very high prescriptions and patients with large pupils.

The leading optical company Zeiss incorporate many of his findings in the commercially available Carl Zeiss Meditec MEL 80 excimer laser system, in use today at the clinic. In our clinic, it is therefore extremely rare for patients to end up with night vision disturbances. We confidently believe that we have the most sophisticated systems in place for preventing or treating night vision disturbances.

Haze is extremely unusual in LASIK. In corneal surface procedures such as PRK/LASEK, however, the risk of haze increases with the prescription. It is also dependent on the smoothness of the surface created by the laser as well as the protocols for postoperative management of the cornea.

Corneal haze is part of the normal healing process of PRK/LASEK and gradually subsides with little or no permanent effect on vision.

If the haze is excessive or does not go away, the patient may need additional treatment either with medications to reduce the haze or further laser surgery to physically remove the haze or both.

In about three in every 1,000 cases, patients develop astigmatism after surgery. Contact lenses can usually correct this form of astigmatism (glasses will not). People who have very high prescriptions have a higher risk of this complication. Astigmatism can happen even if the surgery is perfect, but an inexperienced surgeon or one who does not use the best equipment increases this risk.

Watch Mr Glenn Carp discuss risks and higher prescriptions in this video.

What is myopia (short-sightedness)?

Myopia, or short-sightedness, is a common condition where the eye is too long and or the cornea is excessively curved resulting in too much focusing power.

This means the image falls in front of the retina. You can see objects up close clearly but objects in the distance appear blurry. Laser Eye Surgery is ideally suited to treat short-sighted patients.

Watch a video in which Mr Glenn Carp answers the question of what is myopia. 

What is hyperopia (long-sightedness)?

Hyperopia, or long-sightedness, is a condition where the eye is too short or the cornea is not curved enough, resulting in too little focusing power.

The image is focused behind the retina. Therefore, close objects appear blurry while objects in the distance are clear. Our clinic’s results with long-sighted patients exceed the average results published in the medical literature.

Watch Glenn Carp answer the question of what is hyperopia in a video.

What is astigmatism?

Astigmatism occurs when your cornea is not round and shaped like a football but more oval like a rugby ball.

In astigmatism, images appear blurred or ghost-like as light rays are refracted unequally. In extreme cases, images both up close and at a distance appear blurred. Many people who have myopia also have astigmatism. Astigmatism is treated using an oval laser beam, unlike myopia and hyperopia, which use circular beams.

Click here to watch Mr Glenn Carp answer the question of what is astigmatism.

Can Laser Eye Surgery treat the loss of reading vision?

The loss of reading vision that is evident around the age of 45 is known as presbyopia or ‘ageing eyes’. Unlike many Laser Eye Surgery clinics, we have been treating patients with presbyopia for years.

We call the technique used to correct presbyopia PRESBYOND® Laser Blended Vision. With this technique, one eye is treated to view objects mainly at distance, but a little up close, and the other is treated to view objects mainly up close, but a little at distance.

The brain puts the two images together and enables the individual to see at a distance and up close without effort. In most cases, the brain is able to compensate and you’ll experience an excellent depth of focus and overall visual acuity, without the need to wear glasses or contact lenses.

Watch Mr Glenn Carp discuss Laser Eye Surgery and loss of reading vision or presbyopia.

What conditions might make one unsuitable for Laser Eye Surgery?

Our laser eye surgeons are equipped to undertake some of the widest range of Laser Eye Surgery cases. However, there are still some people they cannot treat.

Obviously, this can come as a disappointment to some patients who are extremely motivated to have their vision corrected. Fortunately though, for some, certain conditions are only a temporary barrier, and with our help, some of these patients do eventually qualify.

If you have any doubts as to your suitability, check out our list of potentially disqualifying conditions. If you have or think you have any of these conditions, simply tell your Patient Care Coordinator during your first phone call.

Watch this video to learn about the conditions which might make you unsuitable for Laser Eye Surgery.

What are the suitability differences between LASIK, LASEK, and PRK?

The vast majority of patients undergoing Laser Eye Surgery worldwide are suitable for LASIK.

Roughly, however, 10 percent of patients have LASEK or PRK. The main consideration when recommending LASEK or PRK over LASIK is corneal thickness. Secondary considerations include the patient’s pre-operative prescription.

Watch a video about the suitability differences between the three procedures.

Dr Tim Archer
Dr Tim Archer

Dr Timothy Archer graduated from Oxford and Cambridge Universities with an undergraduate degree in Mathematics and a postgraduate degree in Computer Science. He joined the clinic in 2003, where he established his career specialising in laser refractive surgery research alongside Professor Reinstein. Today, he manages the in-house research team, of which achievements include 124 peer-reviewed papers, 32 book chapters, over 100 scientific articles and a published textbook. He also oversees and edits the content on London Vision Clinic’s website.

About Suitability