The eye is a remarkable organ, which transmits the visual world around us to the brain via a complex network neural network. Part of the system is optical, just like a camera. Images are focused upon the retina by the cornea, which has the most refractive power, but cannot change its focusing power. The lens, which lies behind the pupil and iris, does have variable power, allowing us to change gaze from a distant object to a near one. This process is called accommodation. This ability declines with years, as all too many of us are aware when we try and read the small print! – this is called presbyopia.
The image is transmitted from the retina to the brain by the optic nerves and visual tracts. The retina can be likened to the film in a camera.
I am not a refractive surgeon. I cannot give you the fine detail, but I hope that I can give you an idea of whether refractive surgery is something that you may like to pursue.
In myopia (left diag) the image is focused in front of the retina, this corrected with a concave (-) lens
(middle diag) and in hypermetropia a convex (+) lens brings the focal point back from behind the eye onto the retina.
• Hypermetropia, or long-sightedness. This occurs when the eye is too short (most commonly), or the lens inside the eye is not powerful enough. The result is that an image is brought to focus behind the retina, and some more positive lens power (+ convex lens) is required. In youth the accommodating power may be sufficient to get away with it, but as the years advance this will not be the case. Typically reading glasses are required at an earlier age in this group and distance glasses will follow. If there is enough long-sightedness glasses will be required both for distance and near at an early age.
• Myopia, or short-sightedness. This occurs when the eye is too long (most commonly) or the lens power is too strong. The result is that the image is in focus in front of the retina. A diverging lens (- power concave lens) is required to correct the vision.
• Astigmatism, with both refractive disorders there is an annoyance called astigmatism, which can prevent a focused image from appearing at the retina. Basically it occurs when the refractive system of the eye behaves in a non-spherical way. All good lenses are spherical. By adjusting the lens system so that the lens is aspherical (rugby or footy ball shaped) the vision can be corrected.
Refractive surgery aims to correct the disorders above. This is where we need to have a think about delivery Vs expectation. It is not unreasonable to wish to have perfect vision, both for distance as well as reading, just as we did when we were 15 yrs of age; this can be delivered so long as you are still young and have some of that lens accommodating power. The problem in terms of expectation begins when you are in your forties, since the accommodating power has weakened. More about that, later.
The three broad types of surgery available are:
- Laser ablation – PRK or LASIK or variants thereof
- Refractive lens exchange
- Intraocular contact lens (Phakic IOL)
Before considering these you should be aware that we are talking about surgery here. This has attendant risks, which, are very small but they are still greater than a pair of glasses ever will be. I always tell patients that nobody has ever been blinded by a pair of glasses – which is nearly true, but of course glasses can rarely shatter!
Contact lenses, on the other hand, do have attendant risks; particularly if guidelines are not followed.
What are the aims of surgery?
Ideally, the aim is to be spectacle or contact lens free.
The cornea can be re-shaped either to add refractive power (more convex +lens power) in the case of hyperopia, or reduce power (more concavity – lens power) in the case of myopia. This is achieved with an Excimer laser which vaporizes the corneal tissue with an extraordinary precision, whist preserving surrounding tissue.
The results with myopia are better, which is a relief, because myopia is the commonest refractive error. In general, refractive surgery is most predictable with lower degrees of refractive error. This is not to say that it is not helpful in the higher degrees, but the chances of achieving the desired refractive outcome first time around are less.
LASIK – (Laser In-Situ Keratomilusis):
A flap is raised, the surface of the cornea is treated with the laser, and the flap is replaced.
This reshaping can take place within the cornea, which is the case with LASIK, where a flap is raised, the tissue is ablated and the flap is replaced. The flap itself can be cut with the laser. The advantages of this method are that, by replacing the flap, the post-operative recovery period is almost pain-free. The healing or scarring response is minimized and the stability of the outcome is optimised.
PRK – PhotoRefractive Keratectomy
The same laser is used, but the surface layer (epithelial cells) is removed prior to the laser treatment. This method is used much less frequently, because the recovery is more uncomfortable and the healing and scarring response is less predictable. As with all things, however, there are some people for whom this method is more appropriate. Your refractive surgeon will discuss this with you.
The risks of this procedure will be discussed by your surgeon. There is a small risk of flap complications, infections, alterations to the corneal shape amongst others. It is important that you are fully informed before committing to the procedure. It would be reasonable to say that the order of risk is similar to a lifetime of contact lens wear, only the risk is taking place over a much shorter time. The vast majority of laser patients, particularly myopes, are very happy with the outcome.
It is common to have a dry eye sensation for the first few months after laser – lubricant eye drops are helpful.
Some people complain of haloes at night or light scatter effects. These tend to settle, and in most cases, whilst present, are not bothersome
In the first month or so the vision is not entirely stable, with a little overcorrection, subsequently settling at the spot where you need to be.
Regression occurs when after initial success there is some reversion back to the original refractive error. This is more common in PRK.
Retreatment: Due to issues above, or due to unexpected outcome a retreatmet is necessary.
Refractive lens exchange
This is exactly the same procedure as cataract extraction, only the clear natural lens is removed. The procedure is described on this site under cataract surgery. Risks and benefits are discussed. The lens power replaced in the eye can be selected following biometric assessment; it is then implanted so that refractive errors are corrected. Because the lens is plastic, there is no accommodation. There are two ways of dealing with this, namely the use of multifocal IOLs or Monovision. Again, this is described under cataract surgery. This surgery is particularly useful for those with high levels of refractive error, for whom laser is less predictable. It is also useful for those in late middle age who want to eliminate the requirement for glasses.
ICL – Intraocular Contact Lens
This procedure places a thin lens (similar to a contact lens) inside the eye in front of your natural lens. This procedure works well with high levels of refractive error and those not meeting certain criteria for laser surgery. There is a risk of ocular infection – Endophthalmitis – since the eye is being opened. Other problems can include some cataract formation.
I hope that this has given you an overview of what is available. There are some other techniques that are used, but this is the province of the refractive surgery experts. It goes to say that you should have this type of surgery performed by an expert refractive surgeon. I would be happy to make a referral should you wish.