Refractive surgery and how the eye works

The best patients are well-informed patients. I love it when a patient comes to the decision to have refractive surgery based on a good understanding of how the eye works, how refractive surgery works, and the risks and benefits of refractive surgery. I want all my patients to have a thorough understanding of all that the surgery entails. Towards that end, I am starting a series of blog posts that will provide as much detail as I can in a way that any patient can understand. I like the “explain it like I’m five” mindset, but we don’t do refractive surgery on 5 year-olds, so I am going to tailor these blog posts to a slightly older audience. My oldest son, Sam, is 11 years old and is pretty sharp. Prior to posting, I’ll make sure Sam reads through and understands what I’m trying to say. I figure if I can get him to understand how an eye works, and how refractive surgery works, then any patient I’m likely to have will be able to understand it too.

“But eyes are icky and I’m afraid of refractive surgery – I don’t want to know any details!”

First of all, eyes aren’t icky – they are amazing! But I totally understand people’s squeamishness when it comes to eyes. They seem so mysterious and delicate.

They may seem mysterious, but modern science has provided answers to all but the most esoteric questions regarding how human eyes function. We know in incredible detail how eyes work, including not only how they focus light, but how that light is turned into information, encoded, sent to the brain, and decoded.

And they aren’t delicate either. As a former Army ophthalmologist, I’ve seen eyes with horrific battlefield injuries repaired and sight restored. Eyes can be incredibly resilient.

But I’ve also seen colleagues during medical school who could surgically assist during cases with gunshot wounds to the belly, with blood and guts everywhere, without breaking a sweat, who, upon seeing a tiny bloodless surgical incision into an eye, turn white and pass out. There is something about eyes that naturally makes people squeamish. This stems from the vital role that eyes play – providing us our most precious sense. Prior to modern times, with microscopes and surgical instruments, eye injuries often could not be fixed. Thus our natural fear of anything getting close to our eyes. A small scratch on the eye can be incredibly painful when that same scratch on the skin would barely be noticeable. Why? Because eyes injuries could have such a serious impact on our ability to survive, we evolved incredibly sensitive pain fibers in the cornea, so any injury would provide immediate feedback on the importance of protecting our eyes.

So, I get it. Some people don’t want to know all the gory details. So in this series, I’m going to focus just on how the eye works, because I think if you understand how the eye works, understanding refractive surgery, and its risks and benefits, just falls out from your understanding of the eye. So let’s get started.

Basic optics

The eye is amazingly complex in its details, but beautifully simple in form. And one doesn’t need to understand the biology of how light activates retinal cells to send information back to your brain and how that information is encoded and decoded, in order to understand how an eye works. As long as an eye is healthy, with no disease or damage, it has one function: to focus light onto the retina. 

The retina is the part of the eye that turns light into information and sends it back to the brain. Light must be perfectly focused on the retina for an image to be clear. If light is focused in front of the retina or behind the retina, the image you see will be blurry.

When you look at an object, say a chair across the room, you are seeing the light rays that are bouncing off that chair and into your eye. Those light rays are diverging (moving farther apart from each other). When you look at something far away the light rays are hardly diverging at all, and are essentially parallel, but when something is closer, like a phone in your hand, the light is diverging more. In order for that chair or phone, to be seen clearly, your eye must change those diverging light rays into converging light rays, in order to bring them back together to meet on the retina. So that’s what the front of the eye is designed to do: to bend the light rays so that they converge onto your retina. The eye has 2 main light-bending structures: the cornea and the crystalline lens.

The cornea and crystalline lens are positive lenses, meaning they both act to bend light towards a focal point, just like a magnifying glass. Imagine using a magnifying glass to focus the sunlight onto a piece of paper to burn the paper. This is essentially what your cornea and crystalline lens are doing: taking all the light that enters the eye and focusing it on one point.  Of the 2, the cornea is actually the stronger lens, accounting for a large portion of the focusing power of the eye. In fact, in some eyes, the cornea is such a strong lens that the crystalline lens isn’t even needed to focus the light onto the retina – the cornea alone is sufficient. So, if the cornea is potentially strong enough, why do we need the crystalline lens at all?

The cornea is static, meaning it does not move or change from moment to moment. If we only had a cornea and no crystalline lens then our vision would be static – we would not be able to change our focal point. The crystalline lens on the other hand can move and change shape. This movement of the crystalline lens is what gives us the ability to change our focal point. So we can look at our phone and see a clear image, then look back at the chair across the room and see a clear image. Without the crystalline lens, the focal point of our eye is static and we can only see clearly at once distance. If our eye is focused so that the chair is clear, the phone will be blurry. If the phone is clear the chair will be blurry. But with the crystalline lens’s ability to change shape, we can look across the room and see the chair clearly, then look at our phone, and the crystalline lens instantly becomes more curved, and we can see our phone clearly.

Then why do some people need reading glasses?

In children, the crystalline lens is very soft. It’s about the consistency of a warm gummy bear, thus it is easy to bend. But as we get older the lens gets hard and stiff. Sometime between ages 40 and 50 the lens gets so stiff that it can no longer bend enough to change your focus from distance to near. This is called presbyopia, and it is a normal part of aging. This aging of the lens is the same process that eventually results in a cataract, usually between age 60-70. 

So, why do I need glasses to see clearly?

If you are nearsighted (myopic) your cornea is too curved for the length of your eye, causing the light to be focused in front of your retina. If you are farsighted (hyperopic) your cornea is too flat for the length of your eye, causing the light to be focused behind the retina. 

If you have astigmatism, your cornea is not round, like a baseball, it is shaped more like an American football, with one direction more curved than another. This causes the light to be bent in different amounts depending on where the light hits the cornea, resulting in the image being blurred.

For people who are nearsighted (myopes) glasses are a negative lens that spreads out the light, so that the light is focused further back, onto the retina.  For people who are farsighted (hyperopes) a positive lens bends the light together, so it is focused closer in, onto the retina. 

How does refractive surgery work?

Refractive surgery is any eye surgery that is designed to reduce or eliminate your need for glasses or contacts. To allow you to see clearly without glasses or contacts we need to change the natural lenses of the eye (the cornea or crystalline lens) to focus the light onto the retina.

With LASIK or PRK we accomplish this by changing the curvature of your cornea, so it is the exact right shape to focus the light precisely onto your retina. With ICLs we accomplish this by placing a lens inside your eye. And with Refractive Lens Exchange (RLE) and Cataract surgery, we accomplish this by exchanging your natural lens for a new lens. 

Will I need reading glasses after surgery?

This depends on your age, and what type of surgery we decide on. If you are under the age of 40, we will almost certainly be doing LASIK, PRK, or ICL surgery. With these surgeries, you retain your natural lens and thus you retain your ability to change your focal point – so you shouldn’t need reading glasses.

After age 40, if you are getting LASIK or PRK, whether or not you will need reading glasses afterward depends on the status of your crystalline lens. If it is still soft and bendy enough, you will retain the ability to see up close, and won’t need reading glasses. If it has become too stiff (what we call presbyopia) you will need reading glasses to see up close. For most people presbyopia sets in during the early 40s, and pretty much everyone can count on feeling the effects of presbyopia by their early 50s.

So, if you are over age 40, and you have LASIK or PRK, you may need reading glasses. It is vital that we explore this prior to any refractive surgery so that you understand how LASIK or PRK will affect your near vision. During your consultation, we will discuss in detail how refractive surgery will affect your near vision and we will give you options. Some people will elect not to correct presbyopia and will have LASIK or PRK in order to have great distance vision, knowing they will need reading glasses. Some people elect to treat their presbyopia with LASIK or PRK with monovision (AKA: blended vision.) And some people will elect to treat their presbyopia with Refractive Lens Exchange (RLE) with a multifocal lens, which allows them to see clearly in the distance, and up close.

Tell me more about these multifocal lenses with RLE…

Intraocular lenses (IOLs) have been around since the 50s when a British surgeon noticed that plastic windshield shrapnel that penetrated world war 2 pilot’s eyes was well tolerated in the eye. Those eyes which were repaired but retained plastic shrapnel didn’t have any reaction to the plastic at all. Cataracts have been occurring since the beginning of recorded history (and before) but up until this time we could remove cataracts, but we didn’t replace them with anything except ridiculously thick glasses. With the development of plastic intraocular lenses, by the 1960’s we could remove a cataract and replace it with a plastic lens. The science still wasn’t very exact, and you probably were still going to need glasses afterward even to see in the distance clearly, with bifocals to see up close, but at least now the glasses weren’t so ridiculously thick. Then the science and manufacturing of IOLs got better, and we could reasonably expect to remove a cataract and replace it with a plastic lens that would allow you to to see in the distance clearly without glasses (as long as you didn’t have a lot of astigmatism), though you would still need reading glasses to see up close. Then, IOLs were developed that fix astigmatism too. And in the last few years IOLs have been developed that treat astigmatism AND presbyopia so that for most patients with healthy eyes, we can reasonably expect to remove a cataract and replace it with a lens that allows you to see clearly at distance, intermediate, and near without the need for glasses.

These new multifocal IOLs have significant advantages for correction of presbyopia over monovision, especially in certain patients (hyperopic presbyopes), but they aren’t perfect. Some people will still need glasses in certain situation, such as trying to read in dim lighting, or driving at night, but most patients find that they don’t need glasses for most of their activities.

How do multifocal lenses work?

Different models of multifocal lenses work differently. Some have concentric rings that bend the light in differing amounts. Some take advantage of the fact that different colors of light are bent in differing amounts. Some slightly move inside your eye, like your crystalline lens did (though, up until this point, those lenses have failed show a sufficient amount of movement to reliably predict someone won’t need reading glasses.) Each model of multifocal lens has its unique pros and cons, which will be discussed in detail prior to your surgery.

What is the difference between a multifocal IOL for correction of presbyopia vs LASIK or PRK with monovision?

With multifocal IOLs we can get both eyes seeing well at distance and up close, thus both eyes are working together, which can give greater depth perception, and more natural feeling vision. 

But with multifocal IOLs, at night, lights will likely have a starburst effect or halo effect, and in low light situations, like an evening at a restaurant, you may need more light or glasses to see up close and read that menu.

With monovision, you likely won’t have near vision issues in dim lighting. But, when driving at night, your near eye will see large blurred lights instead of the point of light coming from oncoming car headlights, or streetlamps. Maybe your brain is able to ignore that blurred light, and you don’t really notice it, but many people find they need some night driving glasses in order to feel comfortable.

What is monovision or blended vision?

Monovision (AKA: blended vision) is a treatment for presbyopia. LASIK or PRK is used to correct one eye so it sees clearly in the distance, and the other eye so it sees clearly up close. When you look at something far away, the eye set for distance vision sees clearly, but the eye set for near vision is blurry. And when you look up close, the eye set for distance vision is blurry, and the eye set for near vision is clear. This might sound like it would be disorienting, and for some people it is. Some people do not feel comfortable with monovision. But for most, after a period of adjusting that can take anywhere from a few days to a few weeks, the brain becomes accustomed to monovision, and when you look out in the distance the brain just ignores the near eye, and when you look up close the brain ignores the distance eye, and you find yourself able to see clearly up close and at a distance without needing glasses.

Once you adjust to blended vision you won’t need glasses most of the time, though you still may find you need them for certain activities, such as reading for a prolonged period of time or driving at night. If you are going to sit down and enjoy a good book you will probably find you are more comfortable with a pair of glasses that correct your distance eye so it sees well up close, so you are reading with both eyes. It just feels more comfortable when both eyes are working together. And driving at night with oncoming headlight causing enlarged blurred lights and halos in your near eye, you may feel more comfortable with glasses that correct your near eye to see well at distance. But most people with monovision don’t need glasses for the majority of their day.

Will monovision affect my depth perception?

Yes. Part of our ability to perceive depth relies on both our eyes working together, and with monovision, they aren’t. Thus, with monovision, you may find it harder to judge distances. But our ability to perceive depth and judge distances relies on more than just our eyes working together and those abilities remain intact with monovision, thus most people with monovision are still able to golf, drive, and interact with the world normally.

What is “refractive error?”

Refractive error is another word for your glasses prescription. Your glasses prescription is written down with 3 numbers, for example, -2.25 – 1.50 x 150. The first number is your spherical error and represents the strength of a spherical lens needed to focus an image on the retina. If your spherical error is negative (-2.25) it means that without glasses, when you look at something the image is focused in front of your retina, but if you place a -2.25 Diopter lens in front of your eye, it will push that image back, onto your retina, and into focus. If your spherical error is positive (+3.00) it means that, without glasses, when you look at something the image is focused behind your retina, but if you place a +3.00 D lens in front of your eye it will bend the light more strongly and the image will land on the retina, thus in focus.

-Dr. Matthew Sharpe