What are refractive errors?
The eye functions like a camera. Light rays entering the eye are focused by the cornea and the lens of the eye onto the light sensitive retina (which is like the film of a camera) at the back of the eye. The retina then transmits a clear image, or photo, to the brain. In people with refractive error (or poor vision, such as near-sightedness or far-sightedness), the light rays do not get focused on to the retina and therefore, blurred images are formed. These can be measured as aberration patterns.
For perfect vision the lens should be clear so that light can pass through it. Light enters through the cornea, passes through your natural lens and is focused onto your retina, resulting in clear vision
Refractive errors happen when the shape of your eye keeps light from focusing correctly on your retina. A refractive error is a very common eye disorder. It occurs when the eye cannot clearly focus the images from the outside world.
Myopia and treatment options?
Myopia, commonly known as near-sightedness or short-sightedness, refers to a condition where the patient is unable to see distant objects. The word 'myopia' is derived from the Greek root myein - meaning 'to close', and ops - meaning 'eye', probably referring to the tendency of near-sighted individuals of partially closing the eye or squinting, to improve the sharpness of distant objects.
Myopia is rare before the school-going age, gradually increasing during school life and reaching its peak during the intense study years of college. Its etiology is multi-factorial, with both genes and the environment playing important roles. Though it is believed that prolonged reading harms the eyes, attempts to reduce accommodative fatigue (like introducing pauses during reading and teaching, eye exercises, etc.) have not helped to reduce myopia among children.
Myopia is classified into low (-3 diopters), moderate (-3 to -6 diopters), and high (more than -6 diopters). Persons with severe myopia are liable to have vision-threatening complications and should have a dilated eye examination at least once a year. (A diopter is the power of a lens for focussing light rays.)
The treatment options for myopia are spectacles, contact lenses, and refractive surgery. Spectacles are inexpensive and safe and offer clear vision, relief from eyestrain and headaches, and reduced risk of developing a squint. Patients with high power are advised to use high index glasses, which are not very thick. Plastic glasses are lighter, but get scratched easily.
Those who do not want to wear spectacles can opt for contact lenses, which are mainly of two types: soft and semi-soft or rigid gas permeable. Soft contact lenses may be conventional or disposable. Soft lenses do not allow enough oxygen to reach the eye and, on a long-term basis, may lead to complications like blood vessels on the cornea, inflammation and dry eyes. Semi-soft lenses or rigid gas permeable lenses are better, because they correct more astigmatism as compared to soft lenses. For high errors of astigmatism, surgeons may suggest toric contact lenses, or soft and semi-soft lenses.
Disposable lenses (daily or weekly) are discarded as indicated, while extended wear lenses can be worn while sleeping also, though this is not recommended. Sleeping or swimming with the lenses on increases the risk of corneal ulcers and infections, which are sight-threatening complications. All types of lenses should be removed at the end of the day, cleaned and soaked in the specified solution.
Hyperopia and treatment options?
Hyperopia refers to the condition known commonly as long-sightedness, wherein the patient has difficulty seeing objects or reading at close distances without the aid of glasses.
In general, LASIK is recommended for patients with up to +4 to +5 diopters spherical of hyperopia or far-sightedness. It is relatively easier to flatten the cornea for myopic patients, but it is more difficult to steepen the cornea in hyperopic patients. If the cornea is drastically steepened, there will be a gradual regression of the power and a risk of the patient losing the best corrected vision. If the error is more than +5 diopters, in young patients contact lenses are preferable to surgery. For patients above the age of 40, refractive lens exchange (along with the removal of the clear lens) with a regular monofocal/multifocal lens is recommended.
LASIK (or Laser-Assisted In-Situ Keratomileusis) is the most modern surgical procedure for correcting vision problems like myopia, hyperopia, and astigmatism. This is an advanced laser vision correction technique in which the curvature of the cornea is reshaped using a laser that is capable of removing tissues with precision up to a micron level. For this procedure patients should be at least 18 years of age and the refraction (spectacles power) should be stable (unchanged) for at least a year. Persons who typically opt for LASIK are those who find spectacles visually unacceptable, those who are intolerant to lenses, those who would like to participate in outdoor sports or opt for professions demanding excellent uncorrected vision.
However, though doctors strive to make the refractive error zero after LASIK, this may not always be possible. The main purpose of surgery is to offer sufficient good vision to patients so that they are not dependent upon glasses most of the time. Some of the possible side effects of LASIK are undercorrection, overcorrection, glare, halos, and reduced contrast sensitivity. Therefore patients must have a detailed eye examination before surgery, followed by a realistic discussion with the surgeon on the expected outcome of surgery.
Defects or aberrations that can be corrected with LASIK
Myopia or short-sightedness occurs when light rays are focused in front of the retina causing blurred vision, particularly when viewing distant objects. Objects that are near may be seen clearly, but not those that are far away. Myopia is often hereditary, usually due to an abnormally large eyeball or steeply curved cornea. Because the eyeball grows with age, myopia tends to progress, usually stabilizing by the time the person is 20 years old.
Hyperopia or far-sightedness is the opposite condition of myopia, where the light rays converge at a point beyond the retina. Initially objects that are near seem blurred, though distance vision remains clear. However, with age, objects at all distances become blurred.
Astigmatism is an irregularity in the shape of the normally spherical cornea. The cornea is shaped like an egg or the back of a spoon, causing distortion of both distant and near vision.
Why would you be interested in LASIK?
- You do not want to wear spectacles or contact lenses.
- You feel visually and socially restricted by spectacles or contact lenses.
- You are intolerant to contact lenses.
- You want to participate in certain outdoor sports where using spectacles or contact lenses may be a problem.
- You plan to join certain professions wherein excellent uncorrected visual acuity is a prerequisite.
Who can undergo a laser correction procedure?
- Who can undergo a laser correction procedure?
- You should not have had a significant change in your spectacle prescription for the past 12 months.
- You should not have any ocular surface abnormality such as dry eyes.
- Your cornea should have adequate thickness for it to be operated upon.
Before the laser procedure?
The patient undergoes a normal eye examination after which the ophthalmologist may suggest a vision corrective procedure. Thereafter, information about the patient's eye structure and function - including corneal thickness, corneal curvature, optical aberrations etc. - will be ascertained. All this is done at LVPEI using state-of-the-art diagnostic equipment. The eye test reports are studied by the ophthalmologist before making a final decision, in consultation with the patient.
What to expect during laser surgery?
At LVPEI laser surgery is done on an out-patient basis; this means you go home immediately after surgery. A relative or friend, who can take you home after the surgery, should accompany you. Usually, one eye is treated at a time. The procedure is performed under topical anesthesia.
You will be made to lie down on a couch and asked to look up at the microscope where you will see a blinking green light. When the suction ring is applied, your vision will fade out. You will start seeing the light again after the suction ring is removed. The whole procedure will be over in a few minutes. Usually there is no pain during the procedure.
After laser surgery
Once surgery is over your eye will be covered with a shield after the administration of some drops and ointment. You can return home immediately. You may experience pain for the first 24 to 36 hours, for which you will have to take oral analgesics. You will have to come for follow up visits from the very next day onwards. Your doctor will schedule the check-ups as necessary. The usual routine is:
- First check-up in the morning after surgery
- Seven days from the date of surgery
- One month from the date of surgery
- Three months from the date of surgery
- One year from the date of surgery
You will undergo special diagnostic tests at each visit to the Institute aimed at assessing your visual acuity. It is important that you visit the doctor as scheduled on every appointment. You will be advised to use eye drops or other medication during the post-operative period.
When will you know the result
Your vision will become clear within a few days. Sometimes you may require another round of treatment with laser, particularly with higher degrees of refractive error.
Although LASIK is an excellent procedure for low and moderate refractive errors, it may not totally remove the need for using glasses in everybody. However, it definitely decreases the dependence on glasses for day to day work.
Laser surgery is very safe and effective. But in some patients there could be side effects. Your doctor will be happy to discuss these with you and clear your doubts before surgery.
Undercorrection/overcorrection: Undercorrection may sometimes be planned intentionally or may occur as an unintentional effect. As a result, the eye remains short sighted even after the surgery. If the degree of residual myopia is significant, the eye may be retreated at a later date. Overcorrection can occur very rarely.
Glare/halo effect: You may feel some sensitivity to light at night or in bright sunlight. Sometimes in dim light, you may see a faded ghost image around the sharp bright image. This will pass after the first few days or weeks.
Decrease in contrast sensitivity: Some people find that their night time vision has become a bit dull. This happens because of a decrease in their ability to discriminate between different contrast levels.
Flap complications: Sometimes the anterior corneal flap that is made in LASIK may not be complete if the keratome stops mid-way because of suction loss. In this situation the flap is repositioned and ablation is deferred. The surgery is re-attempted after three months. In rare instances, the flap may tear or become detached.
Corneal ecstasia can occur if the corneal thickness is less to begin with, or if the cornea is thinned more than it can withstand with the lasers. Therefore, persons having inadequate corneal thickness are not suitable candidates for LASIK.
Other complications: Serious complications like corneal infections, corneal edema, corneal perforation etc., though possible, are extremely rare.
Alternatives to LASIK
For patients where the corneal thickness is not sufficient for doctors to perform LASIK, there are other alternatives. (Generally we do not do LASIK if the thickness is less than 470µm for spherical errors and less than 490µm for cylindrical errors). In such cases the options are:
1. Photorefractive kertectomy (PRK)
This was the most popular laser procedure for correcting refractive errors before the advent of LASIK. Here the laser is applied to the corneal surface. Since the epithelium (surface layer of the cornea) is removed, this leads to greater activation of inflammatory mediators and better healing. The problems of excessive healing or haze (scar) can decrease the clarity of vision, and regression or refractive error returning due to the addition of tissue. Haze and regressions are more if the error is high. Generally PRK is recommended for cases up to 6.0 diopters.
The problems encountered in the early post-operative period with PRK are more painful (because of epithelial defects), and delay visual rehabilitation as it takes 3-4 days for the epithelium to heal.
Early visual recovery, more comfort, practically no haze and very little regression (not in all cases) are the advantage of LASIK over PRK. PRK is preferred in cases with borderline corneal thickness where LASIK might be risky. Excessive ultraviolet exposure is a risk factor as it might cause haze after PRK; this is a problem faced predominantly by Indian subjects. After surgery, to minimize haze surgeons use ointments. L V Prasad Eye Institute is one of the few places in India that offers this procedure, and the results have been very encouraging.
2. Phakic intraocular lens
The Phakic IOL technique is recommended for patients with moderate to severe myopia, i.e., very high refractive powers (near-sightedness). It is used safely and effectively for the acutely near-sighted who are tired of wearing thick glasses and are not suited for the customized LASIK procedure, because they have low corneal thickness or flat corneas.
In this procedure an intraocular lens, (made of biocompatible material that has been tested and proven fit for implantation for over 50 years), is fixed in front of the natural clear lens — behind the cornea and on top of the iris. The word ‘phakic’ means that the natural crystalline lens is left in the eye. This is important because the natural lens plays an important role in helping the eye adjust between seeing objects that are near and far. This gives the eye another focusing lens that provides high-quality, high-definition vision like a normal eye.
Phakic IOL is performed as an outpatient procedure that takes 15 – 30 minutes. Usually one eye is treated at a time. The patient is administered eye drops to reduce the size of the pupil. The doctor uses an instrument to comfortably hold the eyelids open during the procedure. A local anesthetic is given to sedate the eye, so the procedure is virtually painless. A small incision is made in the cornea and the phakic IOL is centreed in front of the pupil, and is gently attached to the iris to hold the lens in place. The incision is closed with microscopic stitches that dissolve on their own.
Three types of lenses are used for this purpose: anterior chamber, iris fixated, and posterior chamber. The quality of vision is usually very good in patients after phakic IOL as compared to those with LASIK. The patient cannot feel the implanted lens.
Phakic IOL does not change the natural appearance of the face and does not require any special care or maintenance. Although it is intended to be permanent, the procedure is reversible if desired. The implanted lens can be removed any time, as the surgery does not affect the important natural structures of the eye.
3. Clear lens extraction (Refractive lens exchange) with negative intraocular lens implantation
This option is not considered for individuals with myopia as literature has suggested increased incidence of retinal detachment after removal of the lens. Though the incidence of retinal detachment is less in the case of hyperopia, this procedure is not recommended in young people with hyperopia since they stand the risk of posterior capsular opacification. Refractive lens exchange with multifocal or monofocal intraocular lens is recommended for high hyperopes over +5 D after the eye of 40 years.
In patients with high power, low corneal thickness, or a flat cornea, LASIK can cause complications or lead to reduced quality of vision. Phakic IOL is the recommended treatment here. Based on the position of the lenses, three types of these are used: anterior chamber, iris fixated, and posterior chamber.
As compared to LASIK, the quality of vision is better with phakic IOL and high power can be effectively corrected.
4) Astigmatic cataract surgery and multifocal lenses
Cataract surgery has become a refractive procedure now. Normally a monofocal intraocular lens (IOL) is fixed in the normal position of clear lens. The power of intraocular lens chosen is for distance or intermediate vision multifocal IOL is therefore an attractive option. In this procedure, the lens has multiple zones or rings which can focus light for distance, intermediate and near vision. There are two broad types of lens: the refractive bifocals (e.g. Ceeon bifocal) and the refractive multifocal (e.g. array). The purpose of this IOL is to give patients freedom from spectacles for near and distance vision. Patients do require a little additional power (plus lens) for reading print. The problems of this procedure include decrease in contrast (as all rays do not focus to a point) and glare while driving in highways. To derive maximum benefit, astigmatism must be corrected by applying cuts on the cornea (relaxing incision) or by laser treatment.