By Dr. David Sawatzky
In the last column I reviewed common refractive errors and the effects they have on our vision both in and out of the water. In this column I will continue that discussion by reviewing reading glasses, contact lenses and how they interact with diving.
Presbyopia (far-sightedness associated with age)
During normal distant vision the power of the glasses (if required), cornea and relaxed lens combined cause light to be focused on the retina. When you look at a near object, light from the object will diverge at the eye, requiring greater refractive power in order to see the object clearly. This is accomplished by contracting the ciliary muscle to take the strain off the lens in the eye. The natural elasticity of the lens causes it to become ‘rounder’, thereby increasing its refractive power. The closer the object is to the eye, the greater the refractive power required to focus the image on the retina.
As we get older, the lens capsule becomes less elastic. For most people, the closest point at which we can focus moves away from the eye about one cm per year (some children can actually bring the tip of their nose into clear focus). Most people will have trouble reading sometime after age 40. The nearest point that they can focus becomes so far from their eyes that either their arms become too short to hold the object, or by the time the object is far enough away from the eye to be in focus, the print is too small to read! The only solution is to use a plus lens to increase the refractive power of the system (reading glasses).
If the person already needed glasses to see clearly at distance, they will need two pairs of glasses, one for distance and one for close in (reading). These two different lenses are often combined into one pair of glasses with the correction for distance in the top part of the lens and the correction for close in the bottom (bifocals).
In even older people whose lens capsule has become so stiff that the lens has very little ability to change shape, three pairs of glasses may be required: one for far distance, one for middle distance (computer) and one for close (reading). If all three corrections are ground into one pair of glasses, they are called ‘trifocals’. Another option is to have the top of the lens correct for reading a computer screen and the bottom correct for reading paper (closer). While working on a computer this allows you to look down at paper on the desk in front of the computer screen and see it clearly through the bottom of the glasses. You will have to take the glasses off to see clearly at distance.
A common alternative to bifocals and trifocals is to gradually change the correction in the lens from distance at the top to close at the bottom (‘progressive’ or ‘varifocal’ lenses). With these lenses you can see clearly at any distance by finding the area of the lens with the correct refractive power for the distance and looking through that area. Initially you will be tipping your head up and down trying to find the correct area of the lens to see clearly.
Many other potential solutions for presbyopia exist. These include adjustable-focus eyeglasses (nuisance), exercises (don’t work), surgery (experimental) and brain image processing enhancement (experimental) but none of these are good options yet. Correcting one eye for distance and one for close (LASIK) does work but has some problems.
The main problem with presbyopia and diving is the difficulty involved in reading our gauges! Most divers wear their gauges on their arms and the forearm cannot be moved very far away from the eyes, so in an older, presbyopic diver (or a diver with hyperopia/farsightedness) the gauges will be blurry.
One solution is to attach the gauges to a retractor. The retractor holds the gauges near the body so that they do not dangle. When you want to read them you grab them and hold them at arms length in front of your eyes so that they are far enough away to see clearly. When you let go they are pulled back in tight to the front of your chest. I have used this trick for many years with the consoles on my rebreathers (I am now 60 years old). I have also used Cochran dive computers for several years as they have large easy to read numbers and I can still read them on my forearm (barely).
Corrective dive masks are a solution that I will discuss in my next column.
Contact Lenses and Diving
One obvious solution to the problem of visual correction and diving is to wear contacts under your dive mask. The discussion here will be limited to the three most common types of contacts: hard, gas permeable and soft.
Hard contact lenses are relatively small (smaller than the colored part of the eye) and do not allow gas to penetrate through them. The cornea has no blood supply, so the only way for it to obtain oxygen and get rid of its waste products is through the tears and direct exchange with the air. A hard contact lens prevents the part of the cornea covered by the lens from exchanging oxygen and carbon dioxide with the air. This is not normally a problem because in a well fitted hard contact, every time you blink approximately 20% of the tear film between the lens and the cornea is exchanged. Nevertheless, the underlying cornea slowly becomes hypoxic. This is one of the reasons why many individuals cannot wear hard contacts all day long and everyone has to take them out when they sleep (you do not blink while you are sleeping). A hard contact corrects for some astigmatism by forcing the cornea to change shape.
Soft contact lenses are much larger (covering more than the colored part of the eye) and allow gas to permeate through the lens. With soft contact lenses, the cornea under the lens does not become hypoxic and the contact can be worn longer. In addition, soft contacts have very little tear exchange when you blink. This means you are less likely to get an object (dust, etc.) between the contact and the cornea, a common problem for all hard contact lens wearers.
A gas permeable contact is between a hard and soft contact in size, stiffness and gas permeability.
Hard contacts are small and will wash out of the eye easily if you flood or lose your mask
Hard contacts are small and they will wash out of the eye relatively easily if you flood or lose your mask while diving. Gas permeable contacts are larger and soft contacts are so large that simply keeping the eyelids partially closed should keep them in the eye, even if you lose your mask.
If soft contacts are exposed to a solution with a higher (seawater) or lower (fresh water) osmolality than tears, they stick to the cornea. If your mask comes off while diving and you keep your eyes partially closed for 30 seconds, soft contacts will stick to the cornea and be less likely to wash out. Swimmers and water polo players who use this technique seldom lose a soft contact. Unfortunately, this does not work as well for gas permeable contacts and does not work at all for hard contacts. To loosen the contact before taking it out, the eye should be irrigated with an isotonic saline solution (any contact solution) and allowed two to three minutes to come to equilibrium.
Soft contacts can absorb the defogging chemicals some divers use in the mask and irritate the eye. These chemicals should be avoided if you are diving with soft contacts (spit works just fine) or the mask should be rinsed very carefully after applying the defogging solution. Soft contacts can also become infected if exposed to contaminated water (especially fresh water) and it is important to clean and disinfect them after diving.
You may have blurred vision after diving with hard or gas permeable contacts. During the dive, tears and the surface of the cornea absorb gas from the air in the mask. They are very fast tissues and a normal ascent will allow time to unload the extra inert gas. Soft contacts allow gas to diffuse through the contact relatively quickly. With gas permeable and especially hard contacts, bubbles can form between the contact and the cornea. If the bubbles become larger in diameter than the thickness of the tear film, they will form indentations on the surface of the cornea. This stippling of the cornea will cause blurred vision. After surfacing the bubbles will quickly disappear. Over the next 30 – 120 minutes the cornea will return to its normal shape and the diver’s vision will clear.
Divers who wear hard contact lenses must ensure that the lens is fitted correctly. If it is too tight there will be reduced tear exchange when you blink and bubbles will be more likely to accumulate. Also, the tear film between the contact and the cornea will be thinner and smaller bubbles will indent the cornea. Hard and gas permeable contact lens wearers must reduce their rates of ascent and consciously blink more often, especially near the surface.
Contacts can be worn while diving and have several advantages. They provide a wider field of corrected vision than corrective lenses mounted inside the dive mask; you do not have to worry about finding a safe place to put your glasses while diving; and when you surface and take your mask off, you have corrected vision immediately.
Soft contacts are preferable. If you lose or flood your mask, keep your eyes closed or squint to reduce the risk of losing a contact. Be careful using defogging solutions and take special care to clean and disinfect contacts after diving. Everyone should ascend slowly for the last 10 msw (30 fsw) and contact wearers should consciously blink more often while ascending to the surface. In the next column I will discuss corrective dive masks.
DIVER MAGAZINE. NORTH AMERICA'S LONGEST ESTABLISHED DIVE MAGAZINE. SUBSCRIBE TODAY.