Photorefractive keratectomy (PRK) is a type of eye surgery that uses laser to correct refractive disorders. The laser alters the curvature of the cornea to bring about the correction in vision.
The cornea is the transparent disc in front of the eye. Light enters the eye through the cornea, passes through the pupil and the lens, and forms an image on the light-sensitive layer inside the eye called the retina. Refraction of light takes place at the cornea and lens so that a sharp image is formed on the retina. Changes in curvature of the cornea could result in the image forming in front of the eye, as in short-sightedness or myopia
, or behind the eye, resulting in long-sightedness or hyperopia
. While lenses are commonly used to treat refractive errors of the eye, corrective eye surgeries are commonly performed nowadays.
The cornea consists of three layers separated by two membranes. From outside to inside, these include:
- The outer epithelial layer
- Bowmanís membrane
- Stroma, which is the thickest layer consisting of collagen and water
- Descemetís membrane
- The inner endothelial layer
Excimer laser refractive surgery uses laser to vaporise the corneal tissue. It is of two main types, laser in situ
) and photorefractive keratectomy.
- In LASIK, a flap of the cornea is made with the help of a microtome, the stroma is modified and the flap is repositioned.
- In PRK, the corneal epithelium is removed, and a part of the stroma is vaporized, so that the curvature of the cornea is altered.
Why is PRK done?
People who suffer from refractive errors like short-sightedness, long-sightedness, astigmatism and presbyopia have to wear corrective lenses in the form of spectacles or contact lenses to correct their vision. Some professions do not allow the use of spectacles. Some people cannot tolerate contact lenses. There are still others who prefer a permanent solution in the form of laser surgery. LASIK is the most-commonly used laser procedure to correct refractive errors. PRK is, however, preferred in particular conditions
. These include:
- Retreatment following LASIK surgery- If the desired correction following LASIK is not obtained, a repeat procedure may be needed. A repeat LASIK procedure puts the patient at risk of ectasia, that is, bulging out of the cornea. Therefore, a PRK procedure is preferred in these patients.
- Thin cornea- PRK can be used for short-sightedness, long-sightedness, astigmatism and presbyopia up to particular limits in patients with thin corneas
PRK vs LASIK: Advantages and Disadvantages of PRK over LASIKWhat are the Advantages of PRK?
Advantages of PRK over LASIK include the following:
What are the Disadvantages of PRK?
- PRK can be used in some conditions where LASIK is not considered safe
- Risk of ectasia is reduced by preserving the corneal stroma and preventing the formation of another flap when used as a revision procedure following incorrect LASIK correction
- Dislocation of the flap which could occur with LASIK does not occur
The disadvantages of PRK as compared to LASIK include the following:
- The pain is more following the PRK surgery
- The time for the vision to come back to normal is longer with PRK. The patient may take around a week to get back to work or to drive, but may require 6 to 8 weeks for complete recovery of vision following PRK
- Corneal haze may occur in the post-operative period with PRK. The use of mitomycin C soon after the procedure reduces the chances of haze, but can increase the chances of dry eye
Which Patients are not Eligible for PRK?
PRK should not be done in patients with the following conditions:
- Uncontrolled inflammatory conditions like rheumatoid arthritis and systemic lupus erythematosus
- Dry eye due to conditions like Sjogrenís syndrome
How is the Patient Selected and Prepared for PRK Surgery?
- Lack of sensations on the cornea i.e. corneal anesthesia
- Lagophthalmus, which is an inability to close the eyelids completely
Proper selection of the patient for PRK surgery is necessary to ensure that the patient gets the best benefit of the procedure.
A careful history of the patient should be taken to understand the type of refractive disorder, prior use of contact lenses or spectacles, presence of a corneal or other eye disorder, as well as the presence of general health disorders. The patient should be above the ages of 18 years. Disorders like diabetes should be under control. The refractive error should be stable prior to the procedure.
The history should be followed by a careful and complete eye examination. It should include the examination of the orbit, eye movements, pupil size, corneal thickness, lens, tear film production and a fundus examination to rule out retinal changes.
The patientís expectations of the final result should be set prior to the procedure. They should also be told what to expect during the procedure.
The patient should be advised not to use eye make up for at least 3 days prior to the surgery. Perfume should not be used on the day of the surgery. Eye drops may be prescribed for use prior to the surgery.
What is the Procedure for PRK?
The PRK procedure is performed on an outpatient basis. The whole procedure takes around 15 minutes, while the actual laser treatment takes only around a minute. It is done under local anesthesia, using anesthetic eye drops. The patient may be given an oral sedative to remain calm during the procedure.
During the procedure, the area around the eye is cleaned with povidone iodine or spirit, and a drape is put to expose the eye on which the surgery is being conducted. Anesthetic eye drops are put into the eye to numb it. It is kept open with a lid speculum.
The epithelial layer of the cornea is removed using a sterile brush, sharp blade, blunt
spatula, alcohol or with excimer laser. The patient is asked to look at a fixation point. The stroma is then vaporized as necessary with the laser.
Following the procedure, mitomycin C is sometimes applied to the eye to reduce post-operative haze. Painkiller, anti-inflammatory and antibiotic eye drops are put into the eye and a bandage contact eye lens is kept in place.
The patient is then shifted to a recovery room, where he/she will have to wait for around half an hour, after which he/she may be allowed to go home. He /she will be prescribed eye drops, and will be instructed as to when and how to use them. Vitamin C may be prescribed one week before and 2 weeks following the procedure to reduce corneal haze. The patient will have to follow-up with the doctor on a regular basis as instructed.What are the Post-procedure Instructions to the Patient?
Following the procedure, the patient is given instructions to protect the eye and prevent infection:
- Rubbing the eye should be avoided for at least 2 weeks
- Sunglasses may be needed for around a week following the surgery
- Contact with contaminated water should be avoided for at least one week
- Strenuous exercise should be avoided or 1 week
What are the Complications of PRK?
Complications of PRK include the following:
- Pain- Patients are prescribed painkiller to reduce post-operative pain
- Corneal haze- The use of mitomycin C soon after the procedure reduces the chances of haze, but can increase the chances of dry eye
- Problems with vision like glare, ghost images and halos
- Over correction or under-correction
- Dry eye
- Increased intraocular pressure
- Rarely, formation of central island, that is elevation of the central cornea