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Excimer laser Photo-Refractive Keratectomy (PRK)

The excimer laser has been used experimentally in corneal surgery since 1985 and for the treatment of focusing errors since 1988. The laser uses ultraviolet light to cut the cornea tissue by breaking the molecular bonds within the cells. This allows removal of tissue with great accuracy and with minimal damage to surrounding tissue. The front surface of the cornea is reshaped by the laser to change its curvature and thus change the focusing power of the eye.

LASIK (Laser Assisted In-Situ Keratomileusis)

LASIK is an advanced method of refractive surgery that offers the possibility to correct a wide range of refractive errors. From 12 diopters of myopia to 4 diopters of hypermetropia and up to 6 diopters of astigmatism may be treatable.

For these higher degrees of focusing errors research has shown that PRK treatment on the surface of the cornea may be associated with somewhat less accurate vision correction, more haze and less rapid stabilisation of vision than for patients with low to moderate degrees of focusing error. For this group, LASIK eliminates most of the disadvantages of PRK.

In LASIK an automated corneal shaper (the microkeratomel) is first used to remove a very thin flap of corneal tissue. The excimer laser is then used as in PRK to reshape the new corneal surface that is revealed. Following this the thin flap of corneal tissue is replaced over the area of laser treatment. This leads to fast healing with virtually no discomfort, rapid stabilisation of vision and little haze or scarring.

How the eye functions

When light enters the eye it is focused primarily by the cornea which is the transparent front window of eye. The iris, which is the coloured part of the eye, is like a camera shutter opening and shutting to accommodate varying degrees of light intensity. The light rays pass through the opening in the iris - the pupil - and continue through the lens. In the final stage the light strikes a tissue in the back of the eye called the retina. The retina is a photosensitive membrane which transforms the light rays into nerve impulses which are sent into the brain via the optic nerve. The brain interprets the nerve impulses as visual images. The way light is focused by the eye is a major factor in determining the quality of vision. Three factors are important:

1. The curvature of the cornea
2. The power of the lens
3. The length of the eye

When these three elements are correctly balanced, light focuses on the retina giving clear vision. When not balanced correctly, as is the case for millions of people, a refractive problem occurs.

What are refractive errors?

Myopia

Myopia affects one in four people in New Zealand. People with this condition are nearsighted and can see near objects clearly while distant objects are blurred. When an eye is myopic it has too much focusing power and images are focused in front of the retina instead of exactly on the retina. This is caused either by excessive curvature of the cornea or the eyeball being too long. Laser vision correction can usually correct or significantly reduce myopia.

Hypermetropia

Insufficient focusing power is the cause of hypermetropia (longsightedness). The poor focusing is the result of an eyeball that is too short or a cornea that doesn't have enough curvature to refract light correctly. Too little focusing power causes images to fall behind the retina instead of hitting it directly. People who are hypermetropic have the ability to focus on distant objects but not on those close at hand. LASIK can correct up to 4 diopters of hypermetropia.

Astigmatism

With astigmatism the cornea is not equally curved in all directions like a soccer ball but has two different curvatures like a rugby ball. This results in an inability to clearly focus all of the image on the retina. Astigmatism may occur with myopia or with hypermetropia and the degree of astigmatism will determine suitability for laser vision correction.

Presbyopia

A normal part of the aging process is a gradual weakening of the eye's near-focusing ability. This is caused by changes in the lens and the muscles which move it. Presbyopia makes reading and other close visual work increasingly difficult with the consequent need for reading glasses or bifocals. Laser surgery can be used to create "monovision" where one eye is corrected for distance and the other eye is corrected for near. The brain takes its signals from the appropriate eye, often doing away with the need for any glasses. Patients try out the method with contact lenses before surgery to see whether they can tolerate monovision.

Available solutions for focusing errors:

Glasses are a proven safe method of accurately correcting vision, although they have some disadvantages.

Contact lenses provide more normal vision, however, with a small risk of complications such as infection. If you simply want to avoid glasses and have not tried contact lenses recently then you could see your optometrist to arrange a trial of contact lenses in the latest designs and materials.

Refractive surgery now offers a safe effective method of vision correction for people over the age of 20 who have a focusing error which is not actively changing and has been stable for at least a year.

Reasons for choosing refractive surgery:

• The inability to tolerate contact lenses.
• The desire to have more normal vision without dependence on glasses, especially in emergency situations or for contact or water sports.
• An occupation requiring good unaided vision: check entry criteria first with your prospective employer as some vocations specifically prohibit certain forms of refractive surgery.

Exclusion criteria

Certain general health or eye health conditions make PRK or LASIK inadvisable:

• Age less than 20.
• Pregnancy or breast feeding.
• Connective tissue disorders such as rheumatoid arthritis.
• Systemic medications likely to affect wound healing such as corticosteroids or antimetabolites.
• Keratoconus or other active cornea disease.

The pre-operative examinations

Following your initial enquiry about refractive surgery you will be booked in to see your surgeon or ophthalmic technician who will carry out a number of measurements on your eyes.

It is important for you to leave your contact lenses out prior to this examination. Soft lenses should be left out for 1 week and hard (rigid gas permeable) lenses should be left out for 1 month per decade of use.

This is because the hard lens distorts the cornea and it is essential to get measurements as accurate as possible. Even after this there may still be some delay in surgery if your refractive measurement is still changing. Once the measurements and initial consultation have taken place you may wear soft disposable lenses up to 2 days prior to surgery. Hard lens wearers may change to soft disposable lenses if necessary.

This pre-operative patient information is supplied to help you in your decision. If you feel that you wish to proceed, an appointment will then be arranged with the eye surgeon who will re-examine your eyes and review the results of the measurements already made. This second exam serves as a verification of the results of the initial screening exam and ensures that the surgeon has accurate information about your eyes.

You will then have an opportunity to discuss the procedure with the surgeon as it is essential that you understand fully the risks and benefits and have appropriate expectations before proceeding to surgery. A date for surgery can then be booked.

One eye is operated on at a time. For safety reasons we recommend an interval of 2 weeks between eyes. You will be given an informed consent form to sign once you have carefully read this information and your queries have all been answered.

On the day of surgery

Preparation

On arrival at the Laser and Surgical Centre the ophthalmic technician will greet you, check once again that all necessary measurements have been made and run through the operative procedure. You will have a chance to ask questions and will then sign an informed consent form. Eye drops will be administered which completely anaesthetize the cornea and you will then be walked through to the laser room.

In the laser room

You lie back on a firm bed while the surgeon and nurse explain what they are doing. The non-operated eye is covered to protect it from laser light. A lid speculum is placed on the eyelids to prevent blinking and a light sterile drape may be placed to cover the eyelashes. It is best if the other eye is kept open or gently relaxed shut. For a PRK procedure, the surgeon uses a blunt spatula to painlessly remove the epithelium, a thin cell layer which covers the part of the cornea being treated. The laser is positioned and during this phase you will be asked to look up into a small red fixation light. The entire procedure is painless. In most cases the laser treatment itself lasts between 1 and 2 minutes, although patients are in the surgery suite for 10-15 minutes.

For a LASIK procedure, the surgeon places a suction ring on the surface of the eye. The automatic corneal shaper is attached to this and is gently moved over the corneal surface to smoothly cut a thin flap of corneal tissue. The laser is then positioned and you will be asked to look up into a small red fixation light. The entire procedure is painless. The laser surgery itself usually lasts between 1 and 2 minutes and patients are in the surgery suite for about 20 minutes.

After PRK surgery

Immediately following the procedure a soft contact lens bandage is placed on your eye and this is worn for about three days while the epithelium heals and then it is removed. Patients walk away a few moments after the surgery and are encouraged to rest much of the next two days, when very significant healing occurs. Most have functional vision to resume normal activities and work within one to four days. However, it may take several weeks for the vision to fully stabilize.

You should carefully follow the instructions in the aftercare kit you will be given before you leave - the kit contains eye drops, analgesic tablets, sleeping tablets and a tablet to control nausea. Starting about 30 to 90 minutes after surgery, when the anaesthetic drops have worn off, some discomfort will be experienced for 24 - 48 hours. There is considerable variability but most patients report discomfort rather than pain and this usually passes within a few days.

Following LASIK

No contact lens is used but instead a shield or a pad is placed over the eye. This shield or pad is left in place until the next day when your eye will be checked by your surgeon. Patients walk away a few moments after the surgery and are encouraged to rest much of the next two days, when very significant healing occurs. Most have functional vision to resume normal activities and work within one to four days. However, it may take several weeks for the vision to fully stabilize. You should carefully follow the instructions in the aftercare kit you will be given before you leave - the kit contains eye drops, and analgesic tablets. Following a LASIK procedure there is usually minimal discomfort although once again there is considerable variability in what patients report regarding discomfort or pain.

Restrictions after surgery:

To minimise pain after surgery you should go directly home and rest in a darkened room

• Do not drive a car until the vision is adequate day and night. Do not swim in a pool or spa for one week.
• Do not use mascara for one week.
• Avoid gardening and other dirty or dusty activities for one week.
• Showers and baths are fine, but avoid getting soap or water into the eye while washing for one week.
• Do not use any eye drops or eye washes other than those prescribed or supplied by your doctor.
• If the contact lens falls out (following PRK) do not attempt to put it back in.

The post operative period:

For your post-operative care follow-up visits will be arranged. These are routinely at 1 day for LASIK and 3 days for PRK, then 1 week, 1 month and 3 months until the vision is stable. Over much of this period you will be using eye drops which will be supplied or prescribed by your surgeon. On these visits you will have various measurements of the eye made to monitor the healing process, and then you will be able to discuss any problems with your surgeon who will adjust your eye treatment as required. During the visual recovery period of 1-3 months the vision may fluctuate but from 3 months the vision is usually more stable.

Common symptoms

Pain or discomfort:

There is no pain or discomfort during the procedure itself. In the post-operative period, most patients experience only some irritation, like a feeling of sand under the eyelid, for a few days. However, some patients have moderate to severe pain in the few days after surgery, which is treated as necessary with medications supplied in the post-operative pack.

Glare and light sensitivity:

You will usually experience day and night glare for the first three to four days. After a few days you may continue to have a small amount of night glare which generally slowly clears. Most patients after six months will be back to their pre-procedural level of night glare.

Blurred vision, loss of contrast sensitivity:

You may have blurred vision for the first week or so after the procedure. This is due to the corneal surface epithelium healing and smoothing. Over the next few months the surface of the eye continues to heal and collagen protein that develops on the surface of the eye leads to varying amounts of haze which may be noticeable as a slight subjective reduction in crispness of vision. In most cases this settles completely over the first 3 - 6 months.

Imbalance between the two eyes:

Following treatment of the first eye until treatment of the second eye there will be a focusing difference between the two eyes. This can lead to symptoms such as apparent distortion of the vision, difficulty focusing or judging distances, or symptoms of eye strain such as headaches. These symptoms settle with treatment of the second eye.

Initial exaggerated effects of surgery:

Since small amounts of the removed tissue do heal back, the laser is programmed to remove enough tissue for the eye to stabilize at the desired correction. The initial effect may seem to be overcorrection but is an intended early result.

Possible complications

Over or undercorrection:

The amount of laser treatment applied to your eye is calculated by the computer from your pre-operative measurements. The final result depends on the amount of healing that the cornea carries out which varies between individuals. It is therefore possible to undercorrect an eye leaving it still with a degree of short sight or to overcorrect an eye leaving it with a degree of long sight.

Further treatment for undercorrection may be possible but some short sight may persist, especially with higher degrees of pre-operative short sight. Overcorrection cannot at this stage be as well treated with the laser. Both groups of patients may need to wear glasses or contact lenses either part time to see clearly or, if the degree of under or over correction is greater, full time.

Haze, scarring:

Almost everyone develops minor haze during the corneal healing process with minimal effect on vision. The estimated probability of more severe haze that can affect your vision is as follows:

• Mild myopia 0 - 3 diopters of short sight > 1 %
• Moderate myopia 3 - 6 diopters of short sight 1%
• Severe myopia 6 - 10 diopters of short sight 2 - 3%
• Extreme myopia more than 10 diopters of short sight 4-5%

Fortunately, the eye usually clears with time. If haze persists, additional laser may be required.

Reduced best-corrected vision:

Very rarely as a consequence of corneal distortion or severe haze the vision may not be able to be corrected with glasses or contact lenses to the level possible before surgery. Overall this occurs in about 1% of patients and usually is restricted to loss of the ability to read the bottom line or two of the eye chart.

Infection:

This potentially serious side effect is very rare. You will receive antibiotic eye drops after the procedure. The first 48-72 hours is the period of greatest risk. The risk of infection is lower than 1 in 500, and most infections are treated and eliminated. A severe enough infection can produce scarring which reduces sharpness of vision. Further laser may be required to eliminate or reduce the scar tissue.

Recurrent corneal erosions:

Delayed healing of the epithelium of the cornea can lead to the development of recurrent erosions causing foreign body sensation or pain. Generally these can be treated using lubricating drops or ointment.

Elevated intra-ocular pressure:

Some people respond to the use of steroid eyedrops with an increase in their intra-ocular pressure. This usually develops within a few weeks of starting drops and as the pressure will be monitored by your surgeon it can be appropriately treated. In rare cases the pressure may not return to normal levels after stopping the drops. This may lead to glaucoma needing long-term treatment.

Night vision difficulties, halos around lights:

Most patients find the symptoms of glare, light sensitivity, and halos settle by 3 months. A very few patients, however, continue to have increased light sensitivity which may be permanent, or problems with night vision which may prevent them from night driving.

Inability to wear contact lenses:

Because of the change in corneal shape following surgery there may be some who are unable to wear contact lenses and who, if they need corrective lenses, will therefore need to wear glasses.

False expectations:

Much of the publicity in the media on refractive surgery suggests that you can "throw away your glasses" which is misleading. While it is true that most patients having the excimer laser procedure will not require glasses or contacts after the procedure, many patients will require a weak pair of glasses for some activities such as night driving and most patients over the age of 45 will require reading glasses. It is best not to think of the procedure as eliminating your glasses, but as reducing dependence on them and improving your without glasses vision.

LASIK complications:

In addition to the complications listed for PRK, the LASIK procedure may rarely be associated with other complications. The procedure is very dependent on perfect equipment function and perfect surgical conditions. During the surgery, equipment failure or other surgical difficulties may mean the operation cannot be successfully completed and further surgery may have to be scheduled.

Complications related to production of the corneal flap:

• Complete detachment of the cap or free cap
• Holes in the flap, incomplete or excessively thin flap
• Loss of or maceration of the flap
• Decentered flap or folds within the flap
• Particulate debris under the flap
• Penetration of the anterior chamber, iris or lens damage, loss of intra-ocular contents
• Ketinal haemorrhage
• Central retinal artery occlusion

Post-operative complications:

• Interface infection, interface epithelial ingrowth
• Non adherent or lost flap
• Flap melt
• Progressive ectasia with loss of the refractive effect
• Irregular astigmatism

In general these complications may lead to a reduction in the best corrected visual acuity in the eye being treated. The incidence of loss of 2 or more lines of best corrected visual acuity is approximately 5%.

Other unknown complications:

Any list of complications however exhaustive must be incomplete. The more important and more common complications have been listed here. To date in extensive clinical trials there have been no reports of blindness caused by laser refractive surgery. Excimer laser PRK is a relatively new procedure and so possible long-term effects many years after surgery are unknown. The results since its first clinical use in 1989 have so far confirmed the safety of the procedure.

Frequently asked questions about laser eye surgery:

Does the surgery hurt?

The laser procedure itself does not hurt. A slight pressure may be felt during the procedure. There is the possibility of discomfort or pain after the procedure for approximately 2 to 3 days. With the LASIK procedure less post-operative pain is usually experienced.

Can you guarantee me perfect vision from PRK and LASIK?

No. We cannot absolutely guarantee perfect results from the surgery because each eye responds slightly differently. Experience from previously treated patients allows us to estimate the probability of your achieving perfect vision.

With low amounts of myopia and astigmatism (up to 3 diopters) 95% of patients will achieve perfect or near-perfect vision. With moderate amounts of myopia (3 to 6 diopters) 85% of people will achieve perfect or near-perfect vision. People with higher amounts of myopia and astigmatism also have a very good probability of achieving good vision, but with a significant chance that glasses or contact lenses will be needed at times. Further (enhancement) surgery may be necessary to achieve a full correction.

Is laser surgery completely safe?

As with any surgical procedure, laser eye surgery has possible risks that you must be aware of. Patients must remember that a specific final visual result cannot be guaranteed, although it can be accurately predicted from the results of thousands of previous cases. Serious complications are very rare and these are listed above.

How long will the correction last?

Results have shown that after the completion of the healing process the results gained will be permanent. There are rare cases of regression, which may be corrected with further surgery, but the vast majority of corrected eyes continue to stay in focus. The surgery does not however prevent other normal changes occurring within the eyes. One of these is increasing short sight so every effort is made to select only those patients whose focusing error is stable.

Can both eyes be done the same day?

For safety reasons we recommend an interval of 2 weeks between eyes. This allows the vision in the first operated eye to stabilise before the second eye is operated on. Occasionally, the surgeon may choose to adjust the amount of surgery based on the results of the first eye. In the extremely unlikely event of a complication such as intra-operative exposure to infection it would be a tragedy if both eyes were affected.

When can I drive?

A patient is legal to drive with one eye 6/12 or better, however, a patient should refrain from driving until they feel comfortable with the vision in their operated eye. Most people can drive after a few days although night driving may be difficult due to glare for a few weeks.

When can I return to work?

A minimum of 2 days off work is required during the early healing stage. Some patients however need up to 4 days.

How long will I be on medications?

Eyedrops are usually needed for 3-4 months following PRK. This depends on the amount of attempted correction and on your individual healing response. For LASIK, eyedrops are only used for 1 week.

Do I need to leave contact lenses out before surgery?

Because contact lenses can cause temporary changes in the shape of the cornea, they should be left out before pre-operative examinations and before surgery. Soft lenses should be left out for 7 days and hard (rigid gas permeable) lenses should be left out for 1 month per decade of use prior to the initial assessment. This is because the hard lens distorts the cornea and it is essential to get measurements as accurate as possible. Even after this there may still be some delay in surgery if your refractive measurement is still changing. Once the measurements and initial consultation have taken place you may wear soft disposable lenses up to 2 days prior to surgery.

Can I wear contact lenses after PRK and LASIK?

With a good surgical result, most patients do not need to use contact lenses postoperatively. If a patient was able to wear contact lenses comfortably prior to surgery, they will usually be able to wear them again after surgery. Because of the change in cornea shape following surgery there may be some who are unable to wear contact lenses and who, if they need corrective lenses, will therefore need to wear glasses.

Is there an upper age limit for PRK and LASIK?

After the age of 45 the ability of the eyes to do the extra focusing work needed to see for reading or close work gradually diminishes. Many people start to need reading glasses at this age and this requirement for reading glasses is not prevented by refractive surgery. This means that even if you have surgery, you will probably still need reading glasses from age 50 or 55.

Some people with a small to moderate amount of short sight are able to see for reading simply by taking off their distance spectacles. This ability will be removed following surgery and so it may appear that the need for reading glasses has been brought forward. Since many people develop some cataract between the ages of 65 and 85, if you are in this age group, we recommend that you consider waiting to have your vision corrected at the time of cataract surgery. The intra-ocular implant lens placed in your eye at the time of cataract surgery can correct either short sight or long sight. PRK can be used to correct focusing errors that occasionally remain after cataract surgery.

What is the worst possible complication?

The worst possible complication of PRK is a serious infection which could lead to a deterioration of the cornea. Infections very rarely occur, and when they do, they can usually be controlled well by medications with little or no loss of the quality of vision. The worst possible complication of LASIK is corneal damage from the keratome cutting too deeply. Proper care in instrument maintenance and assembly will prevent this. Even if the cornea does become seriously scarred because of an infection or other damage, a corneal transplant will usually restore clear vision again.