Human Eye and an Explanation

¶ … human eye and an explanation of LASIK surgery

LASIK surgery and the structure of the human eye: A clear-sighted overview

Once upon a time, individuals who were nearsighted, farsighted, or suffered from astigmatism had a fairly limited choice of corrective technologies: they could use glasses or contact lenses. Now, LASIK and other types of ophthalmological surgical procedures have given many moderately visually impaired individuals a new outlook on life. No longer do LASIK recipients have to fumble for their glasses in the morning, or cope with complicated eye solutions -- they can even see the hands of their alarm clocks clearly, from the moment they wake up. However, the surgery has proven to be controversial in terms of the cost-benefit analysis of risk and expense entailed by LASIK patients.

Why are people nearsighted and farsighted: An overview of the structure of the human eye

The human eye consists of three layers of tissue. The first layer, known as the sclerotic coat, forms the 'white' of the eye along with the transparent cornea. The cornea, like the lens of a camera "admits light to the interior of the eye and bends the light rays" so that images can be brought into clear focus (Kimball 2010). Tear glands, when properly functioning, keep the cornea "moist and dust-free by secretions" (Kimball 2009). The middle choroid section of the eye is pigmented and gives the impression of eye color in the form of the iris. This section's primary function is to reduce the reflection of stray light. Another part of the middle section of the eye is the pupil. The size of the pupil automatically increases or decreases depending on the amount of light entering the eye. It helps the viewer focus on different aspects of a visual field. Finally, the inner layer of the eye, the retina, contains the actual light receptors of the eye, the rods and cones and interneurons that interact with the brain and produce the phenomenon of sight (Kimball 2009).

Of course, the human eye does not always function so smoothly. In a normal eye, the lens, located just behind the iris is "held in position by zonules extending from an encircling ring of muscle" (Kimball 2009). When this ciliary muscle is "relaxed, its diameter increases, the zonules are put under tension, and the lens is flattened; contracted, its diameter is reduced, the zonules relax, and the lens becomes more spherical" (Kimball 2009). These changes enable the eye to adjust its focus between far objects and near objects. The process of seeing properly has been analogized with taking a picture, but just as photographs can become distorted or blurry, so can the picture created by the eye.

If the eye is a camera, "the cornea and lens in your eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image" ("Facts," NIH, 2001). When the cornea is normally curved, it bends, or refracts, light on the retina but when irregular, the created image is distorted. "Farsightedness is caused when the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye -- particularly those from nearby objects -- will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. Farsightedness is called hypermetropia" (Kimball 2009). In contrast, if "the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye. Nearsightedness is called myopia" (Kimball 2009).

A final common condition that can be traced to cornea problems is that of astigmatism. "In people with astigmatism, either the corneal or lens shape is distorted, causing multiple images on the retina. This causes objects at all distances to appear blurry. Many people have a combination of either myopia or hyperopia with astigmatism" (Randleman & Payne 2010, p.1).

Some individuals suffer from vision problems all of their life, while for others, vision problems only arise with age: "As we age, the natural lens becomes stiffer and loses the ability to change shape. This is termed presbyopia, which is the loss of accommodation" (Randleman & Payne 2010, p.2). Farsightedness is the result of age-related corneal stiffening, which is why some cases of nearsightedness improve and correct themselves as the subject ages but farsightedness often worsens (Randleman & Payne 2010, p.2). Most individuals require reading glasses after the age of forty (Halem 2002, p.1).

What is LASIK? How is it used?

LASIK is an acronym for Laser in Situ Keratomileusis. LASIK uses a laser to reshape the cornea to enable the eye to function normally. "Glasses or contact lenses are used to compensate for the eye's refractive error by bending light rays in a way that complements the eye's specific refractive error. In contrast, LASIK and other forms of refractive surgery are intended to correct the eye's refractive error to reduce the need for other visual aids" (Randleman & Payne 2010, p.1). The acronym for the surgery means that the laser underneath a corneal flap is used in situ to reshape the cornea (keratomileusis). LASIK, through surgery, can correct refractive errors and ideally obviate the need for contacts or glasses by improving the patient's vision.

Just as there are different types of contact lenses, there are different types of laser eye surgery. Conventional LASIK directly bases its pattern upon the patient's glasses prescription. However, it has come under fire because it is more likely to result in visual aberrations such as glare, halos, and night vision issues than other forms of laser treatment. Wavefront-optimized LASIK is also based on the patient's glasses prescription, "but also takes into account corneal curvature and thickness, and applies laser energy in a unique fashion in the periphery of the cornea" (Randleman & Payne 2010, p.3). Another type of surgery using a Wavefront laser, Wavefront-guided LASIK, is deployed when alterations in the patient's eye must be surgically instituted beyond that of correcting basic refractive errors (Randleman & Payne 2010, p.3).

When considering what type of surgery is suitable for different candidates, the surgeon must consider the reason the patient is getting the surgery, the exact nature of the condition, and also the various risks and benefits of certain types of procedures. For example, the type of LASIK common amongst members of the U.S. Navy involves grinding rather than cutting the surface of the cornea in a technique known as photorefractive keratectomy, or PRK. The approach leaves the eye more stable, as "military doctors worry that the flap" created by LASIK surgery to reform the cornea "could come loose during combat, especially in a supersonic fighter" (Cloud 2006, p.1). The Air Force only allows its pilots to use PRK to improve their vision, but non-pilots can get either conventional LASIK or PRK, and Army personnel, including helicopter pilots and other aviators, are allowed to get either procedure (Cloud 2006, p.2).

It should be noted that, although LASIK is often used to refer to all forms of corrective visual surgery, technically PRK is not 'LASIK.' While PRK uses an excimer laser to reshape the cornea, "no corneal flap is created.prior to the corneal reshaping. Instead, the central portion of the thin outer layer of the cornea (the epithelium) is removed from the eye, usually after being loosened with a dilute alcohol solution. The excimer laser treatment is then applied to the underlying corneal tissue (the stroma) to reshape the eye. After the laser treatment, the cornea is covered with a bandage contact lens. Within days, new epithelial cells grow back and the bandage contact is removed" ("PRK," Refractive Surgery News, 2010). Because it is a more invasive form of surgery, there is a greater risk of eye infection after PRK surgery compared with LASIK. PRK also entails a longer recovery period, and a greater risk of the patient seeing 'haziness' due to damage to the cornea.

However, "PRK has one distinct advantage over LASIK: because no corneal flap is created, there is no risk of flap complications during or after the procedure," or dislodging the flap through accidental trauma ("PRK," Refractive Surgery News, 2010). It can also be performed on patients with thin corneas, as no 'flap' is created: "A person who has PRK surgery rather than LASIK will have a thicker residual 'stromal [corneal tissue] bed' after the excimer laser treatment. In LASIK, on the other hand, the corneal flap contains both epithelial and stromal tissues, so the underlying corneal stroma is reduced in thickness. The same amount of excimer laser reshaping with LASIK leaves less residual stroma under the corneal flap, which may affect the biomechanical strength and stability of the eye in some cases" ("PRK," Refractive Surgery News, 2010).

For candidates not considered good prospects for conventional LASIK because of extreme nearsightedness and who are leery of the long recovery period of PRK, phakic intraocular lenses can be implanted. "In these cases, a phakic intraocular lens may be used. This lens is implanted inside the eye and can effectively treat nearsightedness up to -20 diopters" (Randleman & Payne 2010, p.4). Intracorneal rings (thin plastic segments) implanted into the peripheral cornea to flatten the cornea can be used for individuals wary of the side effects of the surgery. While visual recovery is less predictable with the intracorneal rings, the procedure has the advantage of being reversible. However, it is only able to correct up to -3 diopters of myopia (Randleman & Payne 2010, p.4).

Because it is not considered necessary surgery, and because the conditions it is designed to treat can be corrective with less expensive glasses and contacts, most insurance plans do not cover LASIK. Having to pay $4,500-$5,500 is not uncommon, depending on the surgery or area of the nation where the patient resides (Ellin, 2008, p.1). Additionally, some candidates are not suitable for LASIK under any circumstances: younger or older individuals whose vision is still unstable; people who play traumatic contact sports involving blows to the face such as wrestling and boxing; people with autoimmune diseases (which can impede healing) and people with corneal diseases are not considered suitable candidates. Women who are pregnant or breast-feeding, people with insulin-dependant diabetes, and individuals with herpes simplex are also excluded (Halem 2002, p.2).

Patients with both near and far-sightedness may still need glasses after surgery due to age-related eye changes. It is typical that patients who were nearsighted for most of their lives and began to use reading glasses later in life will still require reading glasses after surgery. "In addition, LASIK results may not be as good if you either have very small amounts of astigmatism or very large refractive errors" (Halem 2008, p.1).

Patient controversies

According to some patients, the risks of the surgery, including seeing halos and the need for additional surgical adjustments for an over or under-correction of their vision, are often not made sufficiently clear. "It would have been nice if I'd known my advanced age (thirty-nine) might be problematic before I sat in the chair," said one woman, post-surgery, after complaining of chronic dry eye as a result of LASIK (Ellin 2008, p.2). "My consent form said: 'The patient understands that the benefit of the LASIK/PRK procedure is to have an improved uncorrected visual acuity.' I took that to mean that my eyesight would be 20/20. Most doctors, on the other hand, focus on the words 'improved uncorrected visual acuity'" (Ellin, 2008, p.2). "Within a month, something peculiar happened: The vision in my left eye started to change. At first, it only slipped to 20/20. A few months later, it was 20/25. After a year, my left eye had regressed to 20/40, the minimum needed to pass a driver's license test. Two years after I had LASIK surgery, my vision had deteriorated so dramatically that my doctor decided it was time to do a 'touch-up' (Ellin, 2008, p.2). Of the 835,000 patients who had LASIK in 2001, 5.8% needed additional surgery (Halem 2002, p.1).