See the sea without glasses

In the December '94 and February '95 issues of Diver Magazine I discussed the eye and diving in some detail. I also briefly discussed the various types of corneal surgery and their significance in diving. There have been several important advances in the area recently and so many divers are considering the procedures that I have decided to do an entire column on them. In addition, you might have noticed my new photo in this issue of the magazine, sans glasses. I had corneal surgery in December and so far have been very pleased with the results.

There are currently more than 17 different types of surgery being done on eyes to try to correct them so that a person does not have to wear glasses and more procedures are constantly being researched in the labs. However, there are only a few corneal surgeries that are commonly available in Canada and I will limit my discussion to them. If you have had or are planning any other type of surgery on your eyes, you would be well advised to carefully discuss with both the eye specialist and a diving medical specialist the implications on your ability to dive safely.

The eye is really just a living camera. The cornea is a clear lens on the front of the eye and it accounts for about 75% of the refraction (bending of the light) that occurs in the eye. The lens inside the eye accounts for the other 25%. When the eye is focused at a distance, ideally the lens should be relaxed and the image focused on the retina. When the eye is focused on a closer image, the muscles around the lens contract and the lens changes shape to become more round (a stronger lens) and bend the light more so that it is again focused on the retina. If the cornea is too flat (the eye is too short for the shape of the cornea), it will not bend the light enough and the lens will have to help bend the light for the image to be focused on the retina, even for objects at a distance. When this person tries to focus on a closer image, there will be a distance where the lens is as round as it can be and any object closer than that distance will appear out of focus (blurry). People with these kinds of eyes are said to be farsighted because they can see clearly at a distance but not close up. They need glasses for near vision. There are several procedures that attempt to correct this problem but the only one that I would consider is the excimer laser using the LASIK procedure. I will discus it in more detail later.

If the cornea is too round (the eye too long for the shape of the cornea), it will bend the light too much and the image on the retina will be blurry when the person looks far away but clear when they look at items up close. If the lens helps to bend the light, these individuals can often focus on the end of their nose (assuming that they have a sufficiently protuberant proboscis)! They are therefore said to be near-sighted because they can see clearly up close but at a distance everything is blurry. They need glasses for far vision. There are several corneal surgical procedures to correct this problem.

So far we have assumed that the cornea is perfectly spherical. In reality this is seldom true. If you imagine holding a contact lens between two fingers and applying a gentle pressure so that the lens is slightly bent, a cornea with this shape is said to be astigmatic. To describe the astigmatism, we need two numbers. The first is the amount of non-spherical bending of the cornea and is measured in diopters (a measurement of the amount a lens bends the light). The second number tells us the direction of the bend and can be either the ridge (the direction of minimal deviation) or perpendicular to that line (the direction of maximal deviation). Again there are several corneal surgical procedures that attempt to correct this problem.

The above tables illustrate a sample refraction (actually mine before I had the surgery). The sphere measures the spherical curvature of the cornea and a negative number means that the cornea is too round and the person near-sighted. The greater the number, the more near-sighted the person is. The cylinder is the measurement of the astigmatic curvature of the cornea and again the larger the number, the more the cornea deviates from a perfect sphere. The axis is the direction of the astigmatic curvature. As mentioned above, astigmatism can be described in two directions and therefore the two refractions in tables 1 and two simply represent two ways to describe the same eyes. The spherical equivalent is a way to describe the average error in curvature of the cornea and is simply the sphere plus 1/2 the cylinder.

The last concept that must be understood in this column is that the ability of the lens to become rounder and thereby focus the light from closer objects, declines with age. Therefore, the minimum distance from the eye that we can clearly focus on an object increases by approximately one cm per year and in most individuals, sometime between age 40 and 50, the distance is so great that they need reading glasses. There is so far, no way to correct this problem. With this basic understanding of the eye, we can now move on to discuss the common corneal surgical procedures that try to correct the problems of near-sightedness, farsightedness and astigmatism.

The three common corneal surgical procedures are radial keratotomy, surface photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). The last two procedures use the excimer laser and are therefore sometimes called laser surgery.

In radial keratotomy, the surgeon attempts to make a series of radial cuts 90% of the way through the corneal to weaken the edges of the cornea and allow the centre of the cornea to flatten. This only works in near-sighted eyes where the power of the cornea is too strong (cornea too round). This surgery has been around for many years and hundreds of thousands of operations have been done, mostly in Russia, but many have also been done in Canada and the USA. There are serious problems with this operation. The list of visual complications includes a change in vision during different parts of the day, increased susceptibility to glare, and reduced night vision. In addition, a change in barometric pressure will cause a change in visual acuity (for example, moving from a city at one altitude to a city at another altitude, flying, mountain climbing, etc.). In addition, the cornea has a tendency to continue to flatten with time so that several years after the surgery the person is often farsighted. The main problem from a diving perspective is that the cuts in the cornea do not heal with anywhere near the original strength of the cornea. This leaves the cornea permanently weakened and a blow to the eye or a significant pressure differential applied to the eye (such as forgetting to equalize your mask during descent) can result in rupture of the eye. I do not know of any cases where this has happened during diving but it is certainly possible and has happened with a direct blow to the eye. The effect that constant changes in pressure experienced during diving might have on the cornea after this surgery is unknown. The Canadian Forces has decided that any person who has had this type of surgery is unfit for military diving, pilot, air traffic control, tactical helicopter observer and search and rescue technician duties, for life. I would strongly recommend the same policy be used for recreational divers. Recent developments are so much better than this crude surgery that radial keratotomy should no longer be considered.

There are now three commonly available procedures where the excimer laser is used to change the shape of the cornea to correct near-sightedness, astigmatism and most recently, farsightedness. The most readily available and older procedure is called surface photorefractive keratectomy (surface PRK). In this operation the front of the cornea is removed by the laser, flattening the front of the cornea and thereby reducing its refractive power and/or correcting for small amounts of astigmatism. The small amount of corneal tissue removed does not weaken the cornea, thus the eye is no more susceptible to rupture from a blow than an eye that has not be operated on. The cornea also does not change shape with changes in the barometric pressure. The first laser surface PRK was performed in 1987 and there have now been over one million procedures done worldwide. Some of the possible complications include clouding and scarring of the cornea but this is rare. The basic problem with this procedure is that the epithelium (skin) on the surface of the cornea is removed by the laser (it takes three days to heal) and it takes several months for the vision to stabilize. Another problem is that the cornea tries to repair the defect and the more successful the healing process, the worse the visual result! In many cases the procedure must be repeated to remove the repair tissue. Nevertheless, this procedure usually provides good correction for small amounts of simple near-sightedness (maximum -3.00 diopters) and small amounts of astigmatism (maximum 1.00 diopter). Higher degrees of near-sightedness and astigmatism can be corrected with this procedure but the eye takes progressively longer to heal and the results become more unpredictable. The Canadian Forces have decided to allow individuals to dive after this procedure, but demand that they wait at least six months after the last surgery to allow the vision to stabilize and to ensure that complications do not occur. This procedure is not covered by provincial health care plans and costs approximately $2100 per eye in Toronto.

The third procedure is laser in situ keratomileusis (LASIK). This is a modification of surface PRK and is the procedure that I had done on my eyes. In this procedure, a powered knife (automated microkeratome) is used to cut a three sided flap on the surface of the cornea. The flap is folded out of the way and the laser is used to carve the centre of the cornea to change the shape to correct the refraction. The flap is then unfolded back to cover the surgical site. The major advantages of this procedure is that the skin is not removed from the surface of the cornea and the amount of healing that occurs in the centre of the cornea is minimal. There should also be less chance of scarring and blurring of the cornea. Although the use of the excimer laser to perform this kind of surgery is more recent, similar surgery has been performed for many years with the centre of the cornea being sculpted by several different procedures. The major advantages of this procedure are that up to 15.00 diopters of near-sightedness and 6.00 diopters of astigmatism can be corrected (most eyes can be fixed), the healing time is very quick and a second procedure to fine-tune the correction is less common than with surface PRK. For example, I could see 20/200 before the surgery but after the surgery the vision in my worst eye was 20/30 at 24 hours, 20/25 at three weeks and 20/20 by about six weeks. To put those numbers in perspective, you can legally drive without glasses if your best eye is 20/40, my best vision with glasses was 20/25 and normal vision is 20/20. The Canadian Forces do not yet have a formal policy for this procedure, but I am confident that they will decide to allow individuals to dive after it, probably in much less than six months. This procedure is also not covered by provincial health care plans and costs approximately $2300 per eye in Toronto. There are now three generations of excimer laser machines in use and I would strongly recommend finding a location that is using a third generation machine for this procedure.

The most recent generation of lasers can also be used to carve the edges of the centre of the cornea under the flap so that when the flap is placed back over the surgical site, the centre of the cornea is actually more rounded. This allows for the correction of up to 6.00 diopters of farsightedness and astigmatism.

In conclusion, radial keratotomy should not be done and would make you permanently unfit for diving. Surface photorefractive keratectomy (PRK) is a good procedure for simple near-sightedness of up to -3.00 diopters and up to 1.00 diopter of astigmatism but it takes several months to heal after each procedure and often requires two procedures for each eye. Individuals often have one eye done at a time because of the long healing time, extending the process over one to two years. Laser in situ keratomileusis (LASIK) is an excellent procedure for up to 15.00 diopters of near-sightedness, 6.00 diopters of astigmatism and 6.00 diopters of farsightedness. The cost is slightly higher but the visual correction is almost immediate, the results far more predictable and the requirement for a second procedure uncommon.

After surface PRK, you should not swim until the surface of the cornea has healed (approximately four weeks) and you should not dive until vision has stabilized (several months after each procedure). After the LASIK procedure, you should not swim for approximately one week and again not dive until vision has stabilized (approximately four weeks). Unfortunately, I am over 40 and will probably require glasses for reading and close work in the next few years. Nevertheless, the advantages of swimming, boating, sailing, skiing, climbing and caving. without glasses must be experienced to be believed. I am admittedly slightly biased on this topic but I trust I have made my perspective clear.

(These opinions and observations are for general information only and in no way intended to persuade anyone to have eye surgery. If you are considering any vision correcting surgery you should consult your own family physician for advice and details.)