Refractive surgery update

A C K Cheng 鄭澤鈞, S K Rao，D S C Lam 林順潮

HK Pract 2005;27:15-19

Summary

There are a wide range of refractive procedures currently available. Of which, corneal refractive procedures with Laser-Assisted In Situ Keratomileusis (LASIK) is the most popular. With the advancement of technology, refractive procedures can now be performed with higher accuracy and safety. However, complications can still occur with any surgical procedure and proper communication with patients is of paramount importance for satisfactory results.

摘要

目前有多種手術可以治療屈光問題，其中LASIK（準分子激光原位角膜鑲術）是最常見的角膜矯正手術。 由於技術進步，現在手術比以往更安全更準確，但像任何手術一樣，仍有可能出現併發症，與病人之間良好的溝通， 是可否取得手術滿意效果的關鍵。

Introduction

Refractive error is an important problem in Hong Kong and many other Asian countries. It is estimated that over 36% of primary school¹ students in Hong Kong are myopic.

Traditionally, spectacles have been used to correct refractive errors and contact lenses have gained acceptance and became more popular in the past few decades. In recent years, refractive surgeries are also becoming an important alternative for this problem. In particular, Laser-Assisted In Situ Keratomileusis (LASIK) has become so popular that it is considered synonymous with refractive surgery although there are many other types of refractive procedures. In this article, the basic principles of refractive surgery will be explained and the most recent advances will be highlighted.

Basics of refractive surgery

To understand refractive surgery, one needs to understand the anatomy of the ocular structures. Light passes into the eye through two main refractive structures, the cornea and the crystalline lens. Normally, these two structures are responsible for focusing incoming light rays precisely on the retina. However, refractive errors occur when there is a mismatch between the refractive power of the eye and the focusing distance of the eye - the position of the retina.

In myopia, the refractive power of the eye is too strong so that light focuses in front of the retina. In hyperopia, the refractive power of the eye is too weak so that light focuses behind the retina. In astigmatism, the refractive power is different in different orientations, resulting in a blurred image with no single point of focus of the incoming light rays.

As mentioned before, there are two elements resulting in refractive errors - focusing distance (length of the globe) and refractive power of the system. Since the length of the globe cannot be easily changed, current refractive procedures attempt to alter the refractive power of the ocular system by modifying either the cornea, the crystalline lens or both structures.

In order to change the refractive power of the system, one option is to alter the focusing power of the existing structures. The normal cornea has a refractive power of 44D. Since it constitutes the major portion of the total refracting power of the eye, changing the corneal curvature is the most popular method of refractive correction used today. The crystalline lens has about 10D of refractive power and this can be altered by lens extraction and implanting an intraocular lens of a suitable power.

Apart from changing the existing structures, one can achieve the desired refractive outcome by placing an extra refractive element. This can be placed in front of the cornea and these are the spectacles and contact lens that we are familiar with. Alternatively, this can be placed in front or behind the iris, and these are termed phakic intraocular lenses.

Corneal refractive surgery

Corneal refractive surgery has been very popular in the past few years. For myopic correction, the central portion of the cornea is made flatter in order to reduce the refractive power. For hyperopic correction, the cornea is made more curved so that the refractive power of the cornea is increased. Corneal refractive surgery has gone through many different eras of development. Radial keratotomy was popularized in Russia.² In this procedure, radial incisions are made in the cornea with a sharp knife set to a particular depth. The number of incisions and their location is determined by the degree of myopia. These incisions allow the sides of the cornea to bulge outwards under normal intraocular pressure and thereby flatten the central portion of the cornea. This brings the focal point of the eye closer to the retina and improves distance vision. However, such incisions weaken the cornea significantly and making it more vulnerable to trauma. The outcome is also less predictable^3-8 and thus it is no longer a popular treatment option.

The advent of the excimer laser⁹ has improved the outcomes of refractive surgery. Using ultraviolet light with a wavelength of 193nm, the central part of the cornea is partially removed to produce a flatter corneal surface and reduce the total refractive power of the cornea to treat myopia. In hyperopia, the laser is used to remove the peripheral corneal tissue so that the central portion becomes relatively steeper after the ablation. Excimer laser was initially used to perform the photorefractive keratectomy (PRK) technique¹⁰ which involves removal of the surface epithelium of the cornea to expose the relatively inert stromal surface on which laser sculpting is performed. This is done using a local anaesthetic eyedrops and is painless. The procedure usually takes one minute or less. A protective contact lens is then placed on the eye to allow the surface of the eye to recover over a period of several days, and this also prevents most of the discomfort that is associated with the recovery period.

Usually vision improves almost immediately, but during the recovery period vision is generally not as good as it would be with the best possible glasses or contact lenses. Once the protective contact lens is removed after several days, vision continues to improve and may be at its best level within one week to one month after the surgery. Eyedrops are decreased rapidly in a few weeks, though in some cases patients may use eyedrops for several months after surgery.

The success of PRK in eliminating the need for glasses or contact lenses is excellent^11-15 However, the problem with PRK is the removal of surface epithelium which results in pain during the immediate postoperative period.^16,17 The possibility of developing postoperative corneal scarring and the high chance of a return of refractive error, especially among high myopia has made PRK a less popular choice now-a-days.

LASIK

The primary difference between LASIK and PRK is that prior to the use of the laser to change the shape of the cornea, a machine called a microkeratome is used to create a thin flap of cornea which is folded back, and the laser treatment is then performed on the exposed corneal stromal tissue. Because the corneal epithelium is not removed, a protective contact lens is not necessary after LASIK and there is virtually no postoperative discomfort. Visual recovery is faster and most patients can see quite well on the morning following the procedure. Most patients and surgeons prefer LASIK to PRK because of these advantages of less discomfort and more rapid visual recovery. Eyedrops are still used, but often for a shorter period of time.

A potential disadvantage with LASIK is the slightly increased risk of complications due to problems the process of cutting the corneal flap.^18-23 Should the flap be too shallow or too deep, or detached from the cornea, surgery may have to be discontinued, and in some cases such results may result in permanent scarring in the cornea. As the microkeratome technique has improved, these complications have become less common and for experienced surgeons, occur in less than 1% of surgeries. However, this kind of complication does not occur in PRK, because no flap is created.

LASIK offers significant advantages for those patients with high degrees of myopia,^24-26 because the risk of scarring with PRK in such eyes are quite significant. While LASIK does not eliminate this scarring entirely, it tends to be much less common when the laser treatment does not breach the Bowman's membrane of the cornea as in PRK.

Many people around the age of 40 years begin to have trouble reading material held close to the face, due to the natural weakening of their focusing muscles, which is known as "presbyopia". LASIK will not prevent the natural aging of the eyes or the need for reading glasses as one ages, even if one do not require them at a younger age. It is possible that nearsighted patients may need reading glasses sooner if both eyes are fully corrected. Monovision^27,28 may allow for improved reading ability in both nearsighted and farsighted patients after age 40. The monovision option is usually only selected by candidates over 40 years of age, and simply means that we leave one eye a little nearsighted after LASIK. For nearsighted patients the myopia is undercorrected in one eye, and for farsighted patients, the hyperopia is a little overcorrected to provide some reading ability. Monovision will not eliminate the need for reading glasses for fine print, but is useful for reading watch dials, opening mail or reading price tags. The disadvantage is that distance sharpness will not be as good and there will be more difficulty with activities such as driving at night or with sports such as golf or tennis. Night driving glasses may be needed in those with monovision to reduce night glare.

Wavefront LASIK

With the use of spectacles, contact lenses or LASIK, refractive errors such as myopia, hyperopia or astigmatism can be corrected. However, in the ocular system, there are other ocular irregularities like spherical aberrations and coma which can only be dealt with using customized laser correction. These elements are what we called higher order aberrations. No two eyes are the same even with the same amount of myopia and astigmatism.

If one considers traditional refractive surgery as an off-the-rack suit, wavefront LASIK can be considered as a tailor made suit, because each refractive treatment is based on exact imperfection present in the individual eyes.

Custom LASIK involves measuring the eye from front to back, using "wavefront" technology, to create a three-dimensional wavefront map. All of these visual irregularities are then displayed as a 3-D map, referred to as a wavefront map. This information is then electronically transferred to the laser, and computer-matched to the eye's position, enabling the surgeon to customize the LASIK procedure to each patient's unique visual requirements. Prior to the advent of wavefront technology, two people with the same prescription would receive the same glasses, contact lens or LASIK procedure.

Wavefront technology has the potential to improve not only the quantity of vision in terms of visual acuity measured by the standard 20/20 eye chart, but also the quality of vision in terms of contrast sensitivity and fine detail. This translates into a reduced risk of post-LASIK complications, such as glare, halos and difficulty with night vision. Early results are promising^29-33 but further studies need to be carried out to determine its full potential.

LASEK

Laser epithelial keratomileusis (LASEK) is a relatively new procedure. Like PRK, laser ablation is performed right below the epithelial flap. Since the flap includes epithelium only and can regenerate, the shape and the integrity of the flap is not as critical as in a LASIK flap. LASEK allows surgery to be performed in patients with corneas that are too thin for LASIK. However, LASEK procedure has a longer recovery time and may be associated with superficial haze with high corrections.

Intra-Stromal Corneal Ring

The Intra-Stromal Corneal Ring (ISCR) procedure,^34,35 involves inserting a ring in the stroma to increase the mid peripheral diameter of the cornea. This effectively flattens the front of the eye, decreasing myopia. Different sized rings are used to correct different amounts of myopia. However, at present it cannot be used for hyperopia, astigmatism or high degrees of myopia.

One possible advantage of intrastromal corneal rings is that they are "reversible" and can be removed,³⁶ returning the eye to virtually its natural state before surgery. This aspect, as well as the lack of any surgical effect on the center of the cornea (which does occur with laser treatment) may make this an attractive option for vision correction in eligible patients. Unlike most other refractive eye surgery procedures, however, the ICRS procedure corrects vision problems without removing any eye tissue. The best candidates for the corneal ring procedure are usually those patients with mild myopia who have minimum amount of astigmatism.

Phakic intraocular lens

Other than changing the existing refractive media of the eye, placing an extra refractive element can also achieve the desired refractive outcome. Phakic Intraocular Lens (IOL) Implantation,^37,38 is a surgical option for treating high refractive errors. It differs from laser refractive procedures because it involves implanting an IOL in the eye. The phakic IOL is implanted through a small incision in the peripheral cornea, similar to IOL insertion during standard cataract surgery. Unlike cataract surgery, the natural lens of the eye is not removed during the surgery. In some cases, peripheral iridotomy is needed to prevent pupil block glaucoma.

Implanting an intraocular lens is a very familiar and highly successful procedure in ophthalmology. However, unlike all other refractive or vision correction surgeries, it requires making an incision for entry into the eye rather than performing the corrective surgery on the surface of the eye. This potentially exposes the other internal structures of the eye to damage^39,40 and creates a small possibility of endophthalmitis, a much more serious problem than an external eye infection. Other complications like corneal decompensation, cataract formation and glaucoma can also occur. Currently, patients with extreme refractive errors are the candidates for phakic intraocular lens.

If the potential complications of surgery can be minimised, the excellent optical quality of intraocular lenses could make them a very popular choice for vision correction at some point in the future.

In summary, there are a wide range of refractive surgeries available for different degrees of myopia and astigmatism. While refractive surgeries can provide excellent vision to patients, patients should also be informed of the potential risks that exist with different procedures and benefits so that an informed decision can be made.

Key messages

1. LASIK is indicated for the treatment of myopia (short-sightedness), hyperopia (long-sightedness) and astigmatism.
2. Candidates must be over 18 years of age.
3. Candidates must have a stable refraction.
4. Candidates must be free of eye diseases including keratoconus, glaucoma, cataracts and certain retinal and optic nerve diseases.
5. Candidates must not be suffering from viral infection of the eye like herpes simplex and herpes zoster.
6. Candidates must not have uncontrolled diabetes, autoimmune or collagen vascular diseases or be on any medications which affect immune status.
7. Candidates must make their surgeon aware of certain eye problems including amblyopia (lazy eye), strabismus (muscle imbalance), severe dry eyes, or any recurrent, residual or active eye condition which may affect healing - Candidates must make their surgeon aware of certain general health conditions including keloid scarring with previous surgical healing, back problems, claustrophobia or other psychological problems, which may affect the surgery or recovery.

Appendix 1
Potential complications in LASIK surgeries

1. Post-operative side effects, adverse effects and complications - foreign body sensation, pain or discomfort, sensitivity to bright lights, blurred vision, dryness of the eyes, tearing and fluctuation in vision.
2. Corneal infection following LASIK is rare but very serious. Diffuse interface keratitis.
3. Refractive Complications - overcorrections and undercorrections.
4. Corneal Flap Complications - Epithelial defect, corneal flaps of inadequate size, corneal flaps of inadequate thickness, corneal flaps of inadequate quality or smoothness, free corneal cap, corneal perforation, corneal flap displacement, epithelial ingrowth.