Laser eye surgery works almost like magic to correct vision and get out of glasses and contact lenses. But not everyone will have eyes suitable for laser eye surgery.
Knowing whether or not laser eye surgery should be done is quite complex. There is a reason why it is important to visit a specialist prior to getting lasik. Having severe dry eye, abnormal corneas, too high or an unstable prescription or having unhealthy eyes can all exclude someone from having their vision corrected with laser eye surgery.
But let’s break down exactly what can prevent you from getting laser eye surgery.
Brace yourself, there is a lot to cover.
Note: While this is a VERY exhaustive list, it can't cover every possible situation. Therefore, seeing an eye doctor can help guide your own unique situation.
Dry Eye
It’s no secret that laser eye surgery will cause short term dry eye.
Within the cornea are tiny nerves. These nerves are responsible for sensing when the cornea is dry and telling the eye to produce more tears.
During laser eye surgery, these nerves get disrupted. Until these nerves regenerate, the eye is at an increased risk of drying out. This is the reason why preservative free artificial tear eye drops are used after lasik. (learn more about the Best Eye Drops After Lasik)
Not all dry eye is the same. Some dry eye can be controlled and treated with minimal treatment. Other causes of dry eye are much more severe.
For example, one common cause of dry eye is intolerance to contact lenses (and it is a very common reason people seek out lasik). Typically this dry eye can be readily and quickly resolved by starting treatment and going out of the contact lenses.
But other forms of severe dry eye are much more difficult or even almost impossible to fully control. Laser eye surgery should NOT be performed with uncontrolled dry eye.
- Sjogren’s syndrome is a classic example of a condition causing severe dry eye. This is an autoimmune condition where the tear-producing gland gets attacked by the body and the eye doesn’t produce enough tears. This commonly may develop from rheumatoid arthritis or other autoimmune conditions.
- The effect of medications - especially the use of Accutane (Isotretinoin) to treat acne. Accutane works to prevent pimples by reducing the oil production in facial pores. But the eyelids have oil glands too. And the oil from these glands are important to prevent tears from evaporating away too quickly. Accutane interferes with these glands to cause more dry eye. The effects of this medication are reversible after discontinuing and laser eye surgery should be postponed until some time after the course of the medication is completed and the eye is able to heal.
- Other types of medications that can cause dry eye include antihistamines, decongestants, stomach medications, hormone replacement therapy or birth control, antidepressants, chemo, lots of different meds as you can see… Being on these medications may not necessarily prevent you from getting lasik, but it is just more important to make sure the dry eye is under control.
While those are two specific conditions, many other things can lead to chronic difficult to control dry eye. This can range from having a trauma such as a chemical burn on the eye to having eyelid scarring that prevents proper closure of the eye.
If you have some condition that causes severe dry eye that is barely controlled with treatment, laser eye surgery is not for you.
Note: SMILE causes less dry eye than lasik or PRK. For those with mild or moderate dry eye, SMILE may be a treatment option.
Abnormal Cornea
You may already know whether or not you have dry eye. But odds are less that you are aware of having an abnormal cornea. And having an abnormal cornea is perhaps the biggest reason people are excluded from having laser eye surgery.
Laser eye surgery corrects vision by changing the cornea. Each pulse from the laser will actually remove a microscopic amount of cornea tissue. By placing these pulses in a particular pattern, laser eye surgery changes the curvature of the cornea to correct vision.
After laser eye surgery, the cornea is thinner than before.
It is important in life to have a cornea which stays the same shape. This allows the vision and prescription to remain stable. If the cornea becomes too thin, it can become weak and change shape.
By thinning the cornea, laser eye surgery can increase the risk of a cornea becoming weak.
But not everyone is going to be at risk of this.
Keratoconus
There is a condition in which the cornea weakens over time. As the cornea weakens, it changes shape. It becomes thin and blurs and distorts vision. This condition is called keratoconus.
The biggest risk for lasik weakening the cornea comes from those who already have keratoconus or have suspicious findings for keratoconus. During a laser eye surgery evaluation, detailed maps and images are taken of the cornea. These maps can identify a cornea at risk for developing keratoconus.
Note: Keratoconus causes the cornea to become irregular. The invention of topography-guided treatments have improved our ability to correct irregular corneas. While this hasn’t allowed us to fully correct keratoconus patients with laser eye surgery, PRK is being used in limited fashion to reduce the amount of distortions keratoconus patients have. But only after the keratoconus patient is treated with corneal cross linking beforehand to strengthen their cornea.
Thin Corneas
But also at risk are those who have a very large prescription or who already have thin corneas. If the surgery thins the cornea out too much, the cornea can become weak. As a general rule, most surgeons will require that at least 60% of the cornea remains untouched to prevent the cornea from becoming weak.
For thin corneas with high prescriptions (up to an extent), this can make PRK preferred over lasik as the treatment of choice.
To correct vision, laser eye surgery changes the shape of the rigid structural part of the cornea called stroma. But there is another layer called epithelium which rests on top.
Epithelium and stroma of the cornea; Image by StemBook (CC BY 3.0) / modified from original
Lasik creates a flap to bypass this epithelium; in PRK this epithelium is removed directly. This lasik flap is thicker than just the epithelium, so it creates a deeper treatment. Therefore in some cases, if there isn’t enough thickness of the cornea to safely do a lasik treatment (and leave 60% unchanged), PRK is able to work.
Finally, while uncommon, corneas can become too thin where surgeons just may not feel comfortable doing any laser eye surgery. How thin is too thin can also differ between surgeons but eventually the cornea can reach a value in which all surgeons agree is too thin.
Too High of Prescription
There are limits to how much prescription laser eye surgery can treat. We’ve already seen that thin corneas can limit how much prescription can be treated. But even with a normal or thick cornea, the laser has numerical limits to how much prescription it is capable of treating.
With modern lasers:
- For nearsighted prescriptions, if you have over -12.0 of prescription, you fall outside of the range of what the laser can treat.
- For farsighted prescriptions, having over +6.0 of prescription is too much.
- If you have astigmatism, up to 6.0 of that astigmatism can be treated.
Note: Different lasers have slightly different max treatment amounts. Different surgeons can also have different max limits they are comfortable treating.
However, having too much prescription isn’t a hard fast rule preventing you from getting laser eye surgery. It just means that laser eye surgery won’t be able to fully correct your prescription.
For nearsighted individuals, this means that afterwards there will still be some amount of a nearsighted prescription that requires glasses. But a much thinner pair of glasses.
For farsighted individuals, there will be some remaining farsighted prescription. But, when young, our natural lens is able to focus through small amounts of farsighted prescription just fine - keeping out of glasses.
Unstable Prescription
If the prescription of the eye is changing, then it’s impossible to do an accurate laser eye surgery correction. It may be accurate for a few months, but as the prescription continues to change, the vision will continue to become blurry.
When we are young, our eyes are developing and prescription changing. Gradually as we approach adulthood, our prescription stabilizes. This can be as early as 16 or can finish years later out of college and / or professional school in the early twenties.
So, it is important for the prescription to become stable before getting any laser eye surgery. (And you won’t find many surgeons that will treat individuals less than 18).
But being young isn’t the only time the prescription changes in our life.
- Medical conditions such as uncontrolled diabetes can cause the prescription of the eye to change. Any diabetes must be well controlled prior to correcting vision with laser eye surgery.
- It is possible that vision can change during pregnancy. For this and other reasons, this is not the time to have your vision corrected. It should be reserved for before pregnancy or after (possibly even if still breastfeeding).
- The natural lens inside our eye eventually becomes cloudy over time. For many people, this occurs between the ages of 60 and 80. As this lens changes, it can independently shift and change the prescription of the eye reducing the long-term accuracy of lasik. And eventually, this natural lens blurs our vision as well and becomes a cataract. Laser eye surgery can't fix that lens.
Unhealthy Eyes
Speaking of cataracts, there are other eye conditions in which laser eye surgery should be proceeded with caution or best avoided.
Poor Vision
Amblyopia (also known as lazy eye) occurs as a result of one eye not developing as good of vision as the other eye.
This isn't frequently bothersome for individuals with ambylopia. They see just fine since that's how their vision developed. But no matter how well the prescription is corrected with laser eye surgery, there will be limits to how well that amblyopic eye can see. It just won’t see as well as their good eye.
In addition, if the vision in the amblyopic eye is very poor, it may not be a good idea to take any risks of laser eye surgery with the good eye (no matter how small those risks may be).
But amyblopia isn’t the only condition causing poor vision. There are many. From retinal issues to cataracts and more. And laser eye surgery can’t improve the vision more than what glasses can do. If you can't see well out of glasses due to another eye condition, you still won't be able to see well after lasik or PRK.
Double Vision
If you have double vision that goes away with a pair of glasses, laser eye surgery won't be able to help you.
When the eyes don't line up perfectly straight, each eye can be looking in a slightly different direction. The eyes will be looking at different things and the brain won't be able to merge the picture together. This causes double vision.
Glasses fix double vision through special lenses called prisms. These lenses shift light so that both eyes are looking at the same thing.
Special prism lenses; image by Video2005, CC BY-SA 3.0, via Wikimedia Commons
These prisms can't be replicated with laser eye surgery. Lasers can correct your prescription but unless you wear glasses with prism afterwards, you will have double vision. And that may be more frustrating than having blurred vision.
Note: Sometimes symptomatic double vision can be prevented by correcting the eyes to monovision. One eye is focused for the distance while the other is focused up close for reading vision. But this is only really beneficial for those 40+ who are in the reading glasses stage of life.
Conditions that favor one procedure over another
The laser eye surgeries have slight differences. Because of this, one procedure may be preferred over another because of certain eye conditions. (but like above, these eyes must still have good enough vision to correct)
- When a bad corneal scar is present, it can interfere with the ability to create a lasik flap. PRK becomes the preferred procedure. In addition, PRK may thin the scar out a little more possibly leading to some vision improvements.
- Glaucoma is a condition in which high pressures inside the eye can lead to damage to vision over time. During lasik, the pressure of the eye increases when the lasik flap is created. This can accelerate vision loss due to glaucoma. PRK is the preferred procedure because it doesn’t involve any high pressures.
- After PRK there is an increase in the amount of inflammation on the surface of the eye due to the “injury" of the removed epithelium. Certain conditions can over-respond to this injury and PRK should be avoided. This includes individuals with certain autoimmune conditions such as rheumatoid arthritis. Lasik becomes the preferred option for those patients. And these conditions should be treated and under control prior.
- Like boxing? Or other martial art sports where you get hit repeatedly in the face? Then it's not a good idea to have a lasik flap present. While lasik flaps have gotten much more secure with today's treatments than in the past, avoiding the need for it altogether with PRK becomes the better choice.
Alternatives to Laser Eye Surgery
There are different ways to correct vision other than with laser eye surgery. These alternative procedures work differently and thus are able to avoid some of the issues that can exclude someone from laser eye surgery.
The main similarity between these alternative procedures is that instead of correcting vision by changing the shape of the cornea, these procedures use a lens to correct vision instead.
- ICL surgery. The ICL is a lens which is placed inside the eye in front of the natural lens. There is minimal dry eye after the procedure and ICL is a great procedure for high nearsighted prescriptions with thin corneas. Check out Learn Exactly What Is ICL Surgery for more information.
- Lens replacement surgery and cataract surgery. With these procedures, the natural lens or cataract is removed and replaced with a new artificial lens to correct vision. These procedures can correct all types of prescription, even ones that are too high for laser eye surgery. Because our natural lens allows us to focus up close when we are young, these procedures are typically reserved for individuals 45 and up. To learn more about cataract surgery, check out Your Comprehensive Handbook To Learn What Are Cataracts
Summary
Not everyone is going to be eligible to get their vision corrected with laser eye surgeries such as lasik, PRK and SMILE. There are certain conditions which prevent safe and accurate treatment with laser eye surgery. Before having any treatment, it is important to have a full evaluation of your own eyes.
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