WHAT CAN THE LASER TREAT?
WHAT THE EXCIMER LASER CAN TREAT
The clear cornea, arching above the coloured iris, is curved. As the first air-solid barrier to light rays entering the eye, it therefore functions as an important lens, bending the rays to focus onto the retina at the back of the eyeball.
Treatable refractive errors
Myopia
The short-sighted cornea is too curved, i.e. too powerful a lens for the length of the eye. The laser reshapes the curve, flattening it centrally to reduce the focus power of the eye.
Astigmatism
The corneal curves are different when measured at different directions. The same effect is seen on examining the curves on an egg vertically compared to horizontally. Modern Excimer Lasers can be programmed to vary their laser treatments according to the corneal directions.
Hypermetropia
The long-sighted cornea is not sufficiently curved, i.e. it is too weak a lens. The laser reshapes the curve, flattening it around the periphery to steepen the centre, thereby increasing the focus power of the eye. Currently, the range of treatable hypermetropic errors is smaller than the range of treatable myopic errors.
NON-TREATABLE REFRACTIVE ERRORS
Presbyopia
Presbyopia refers to the reduction in the range of focus from far to near that occurs in normal eyes after the mid forties. These normal eyes continue to focus well in the distance but increasingly need reading glasses to manage near focus. It arises from changes in the lens and focus muscle inside the eye. The cornea is not involved so Excimer Laser therapy does not correct presbyopia.
The loss of uncorrected reading vision caused by presbyopia can be partially overcome by choosing to leave one eye slightly short-sighted for reading while its fellow is fully corrected for distance. This is called "Mono-vision", where only one eye is optimised for each focus distance.
Extreme myopia
For myopia and combined myopic astigmatism, there are practical safety limits to the cornea's ability to achieve normal uncorrected vision and to maintain it with stability over time. Currently, beyond the range between Ð12D and +5D, the healing process is not sufficiently stable despite the laser's ability to cut up to -30D. Future intra-ocular options, presently experimental, hold great promise for treating eyes beyond -12D.
OTHER OPTIONS for VISUAL CORRECTION
Glasses and contact lenses remain safe and reliable means of correcting vision.
Phakic intra-ocular lenses. Implantation of a second corrective lens within the eye to correct extreme refractive errors is an expanding and promising experimental area.
Astigmatic Keratotomy is the surgical incision of the peripheral cornea. It may be an option for some types of astigmatism.
Radial Keratotomy, "the Russian operation", consists of multiple deep radial surgical cuts placed in the cornea to relax its curvature and reduce myopia. It has been superseded by Excimer Laser.
Clear lensectomy refers to using cataract extraction and implant lens techniques to remove the clear natural lens from within a very long or short sighted eye. Normal vision is achieved by choosing the power of an implant lens. Sight threatening retinal detachment is already a significant risk in these eyes, a risk increased by intra-ocular surgery.
RISKS AND CONCERNS
Although the vast majority of excimer laser refractive treatments do very well, no surgical procedure can be risk free. Many variables exist for the individual patient and procedure including the patient's eye, the mechanics of the surgery and the patient's healing response.
The main risks and considerations about excimer laser refractive treatment which you should understand are listed below. This list cannot be all inclusive. If other concerns arise, please discuss these with your surgeon prior to the surgery.
Discomfort
The treated eye may be uncomfortable, but rarely painful, for one day following Lasik and two to three days following PRK. Drops, ointment and bandage contact lenses are used to minimize these symptoms, which may include light sensitivity.
Variably blurred vision
Initially, blurring occurs from disruption to the corneal epithelium and excess tearing. Later less severe blurring is due to gradual stabilisation of the new corneal shape. This is most obvious in reading vision and lasts longer in PRK cases. The duration is extremely variable across individual patients and depends on the amount and type of error corrected and the healing response.
Corneal haze and halo effects
Haze occurs more often with PRK than Lasik treatment. It has little effect of vision overall but may be evident in dim light where haloes around single lights might be seen. In a few cases, haze can persist and may be associated with regression of the visual result. Rarely, it may cause permanent scarring.
Undercorrection and Overcorrection
Excimer Laser typically corrects the eye's focus to normal with an accuracy of within 10% of the amount of correction attempted. If a significant under or over correction occurs, an additional enhancement procedure can be done. This possibility increases with the higher the level of refractive error treated.
A particular desired result cannot be guaranteed in a surgical procedure that depends on individualised treatment and a patient's own healing response.
Regression
Sometimes the corrective effect of a treatment may partially or completely reverse even many months after the procedure. The reasons for this are many and not well understood. Regression is more frequently seen in the correction of higher refractive errors.
Enhancement procedures
To achieve a desired result, it is sometimes necessary to perform additional "touch-up" or enhancement laser procedures. This is necessary more often for combined treatments involving astigmatism and for larger refractive errors. An enhancement is a means of optimizing vision. It is usually done 3 or more months after the original laser surgery.
Visual inconvenience between treatments
When one eye has been corrected and the untreated fellow eye remains considerably long or short sighted, the difference in the two visions may be troublesome and tiring. Using a contact lens instead of glasses reduces this effect.
Loss of Best Corrected Visual Acuity - BCVA
It is possible to lose lines of vision (as measured on the vision testing charts) as a result of the Excimer Laser refractive surgery. With PRK treatment, this is caused by irregular corneal healing causing prolonged haze and distortion. With Lasik, flap complications may reduce BCVA, either temporarily or permanently.
Decentration
Decentration is a rare but serious complication which may cause a loss of BCVA. It is difficult to treat.
It may arise from significant eye movement during the procedure. Our "autotracking" flying spot laser minimises this risk by automatically following any eye movements during its delivery of the laser treatment. Additionally, a small fixation device is used to hold the eye in the correct central position during treatment.
Infection
The Excimer Laser treatment beam is self sterilising because of the high energy it delivers to the target tissue. Infection can possibly develop under the Lasik flap or in the regrowth epithelium after surface PRK. It is important that post-operative antibiotic drops are used as requested.
LASIK FLAP COMPLICATIONS
The Lasik procedure has attractive benefits including less discomfort, less haze and quicker visual recovery. However, significant complications can arise from its inherently greater technical complexity.
A hinged flap is cut and raised using a microkeratome instrument. It is possible that the flap may be inadequately hinged or completely cut free. The flap may be twisted or misaligned after replacement.
Wrinkling of the flap, from compressive movements of the eyelids, can develop in the early post-operative period. This can produce variable visual distortion and loss of BCVA.
Debris and epithelial cells can be found beneath the flap necessitating re-lifting and cleaning even months later if growth of epithelial cells is evident. This can occur with apparently stable vision and is a major reason for prolonged follow-up.
If a flap cut is technically unsatisfactory, it may be necessary to defer the laser, replacing the imperfect flap to allow the cornea to heal. Usually no loss of vision or significant scarring occurs. The treatment can be rescheduled.
Rare late flap problems include corneal and flap melting.
In many cases, flap complications are treatable but the potential for irreversible damage to the transparency of the cornea, with reduction in vision, must be appreciated.
Increased intra-ocular pressure
In susceptible individuals, the use of post-operative steroid eye drops can cause painless elevations in intra-ocular pressure (IOP) which are associated with glaucoma. The pressure typically normalises with cessation of the drops.
PRESBYOPIA
The need for reading glasses in adults with good distance vision is due to changes in the internal lens and focussing muscles inside the eye. Excimer Laser refractive surgery, which reshapes the corneal surface of the eye, does not treat presbyopia. In fact, short-sighted adults, used to reading easily without glasses, will often need reading glasses when their distance vision is corrected by the laser.
This inconvenience can be partially overcome by choosing "Mono-vision", where only one eye is optimised for each near and distant focus. However, some depth perception and stereovision is lost when glasses are not worn with "mono-vision".
CONTRAINDICATIONS AND EXCLUSIONS
If you expect or require perfect vision, you should not undertake Excimer Laser surgery.
You should be over 20 years old and have had a stable refractive error for at least 12, and preferably 24, months.
Other eye and general conditions preclude Excimer Laser treatment. These include substantial keratoconus, any active inflammatory eye disease, a history of corneal Herpes Simplex, cataracts, Rheumatoid Arthritis or Collagen vascular diseases (eg "Lupus"), pregnancy and the presence of a cardiac pacemaker.
Amblyopia or a "lazy eye" is a reason for caution in choosing refractive surgery. Unless the lazy eye can read comfortably with best correction, the refractive surgery on the better eye has to be considered as surgery on "your only eye". The significance of the normal risks of the surgery is very greatly increased.
PRESENCE OF CATARACTS
When cataract is present, Excimer laser surgery for the cornea is not recommended. The normal treatment of cataract includes replacing the cataractous internal lens of the eye with an intra-ocular implant lens (IOL). The choice of the power of the IOL to be implanted enables us to correct an existing refractive error. Multifocal IOLs are now available for some cases.
Remote and long term risks
As surgical procedures, both LASIK and PRK involve risks to, including reduction and possible loss of, eyesight in the treated eye. As relatively new procedures, rare unforeseen consequences or side effects may arise in the future that are not known at this time.
