Every operation, including LASIK, can have complications:
About one in 1000 have some complication, most minor and often not noticeable to the patient. All operations have complications; however, in refractive surgery we are operating on "healthy" eyes rather than diseased ones. Hence there is a higher responsibility on the doctors to be very open as to these risk factors so that the patient can make an informed choice for this elective surgery. (Don't forget that one complication for any operation involving a general anaesthetic is death. No one has yet died from a Lasik operation!)
The most notable complications are:
a) Epithelial slide - In some patients the epithelium is more loosely attached to the underlying "stroma" or body of the rest of the cornea. When the microkeratome passes over the eye to cut the flap, some scuffing or sliding of this epithelial layer can occur. This can lead to delayed visual recovery and more postoperative pain than normal. There is also a higher risk of "Sands of the Sahara" syndrome with large epithelial slides. Slight epithelial disturbance is not important but in larger ones it is sometimes more prudent to delay doing the other eye to another day. This is less common these days using femtosecond lasers or modern microkeratomes to make the flap.
b) Free cap - incidence about 0.05%. Lasering can often go ahead despite the free cap and the result is not affected. I have had 2 free caps in 18,000
c) Incomplete flap - In this case it is often better to replace the flap and not attempt lasering as the area is usually too small. A new cap can be cut again in a few months. I have had less than 4 of these in 18,000
a) Blindness: True blindness, defined by ophthalmologists as "no perception of light", has not occurred with Lasik. However, there have been 5 cases of severe visual loss reported to date, all in the USA. In these patients, sight was so damaged that the eyes were functionally blind, seeing only vague movements. One of these was a fungal infection in a diabetic patient. The other four were due to optic neuropathy. This latter complication is presumably related to the marked increase in intraocular pressure caused by the suction ring during the cutting of the flap leading to optic nerve ischaemia. It is not recorded how long the suction ring was on the eye for these cases. In my own hands this time is around 5 seconds, during which time the blood supply to the eye is cut off. This is equivalent to pushing hard on your eyeball with your finger for this time. Statistically this complication may be very rare, but it is probably unwise to have Lasik if you have an already compromised optic disc such in in glaucoma. Similarly, if you have significant ischaemic risk factors such as hypertension, hyperlidaemia or diabetes, then you should talk this through with your ophthalmologist.
A couple of cases of macular haemorrhages have also been reported following Lasik. This is again caused by the suction on the eye and is a bigger risk in a very high myope. This does not mean blindness but does result in severe loss of central vision.
b) Infection: Presents within the first 12-24 hours with a painful red eye and blurred vision. Incidence is less than 0.1%. Treated with antibiotic drops.
c) Diffuse Lamellar Keratitis (DLK) or "Sands of the Sahara Syndrome": A sterile infiltrate under the flap. Presents in the first 12-48 hours with blurred vision but with no pain. Unusual complication and treated with steroid drops.
d) Striae in the flap: Incidence less than 0.25%. Can degrade the vision. If significant it is best to lift the flap early to straighten them out.
e) Epithelial Ingrowth: 0.5 to 1.0 % incidence. Occurs at 1-4 weeks Postop. A few epithelial cells at the edge of the flap do not matter and can be left (the majority). If they grow further in, then the flap has to be lifted and the cells scraped off as they can interfere with vision.
f) Dry eyes: Some degree of dry eye is almost universal for up to 12 weeks, as the corneal nerves have been cut or lasered. They recover over 2-3 months. Some people notice this more than others and it also depends on the environment in which you work. Artificial tears can be bought from any chemist and can be used as often as necessary. There are about 6 makes and you can use the one which you find suits you best. Preservative free tears are better as they are less toxic. As you "see with your tear film", any degradation of the tears can lead to fluctuating vision.
There is a recent article of interest on this problem:
"SAN FRANCISCO — The problem of dry eye after LASIK is most likely the result of a neurotrophic epitheliopathy induced by the severing of corneal nerves when the flap is made, rather than diminished tear production. This is the conclusion of a study by Steven E. Wilson, MD, as reported in the June 2001 issue of Ophthalmology.
In this retrospective case control study, individual eyes of 19 patients with moderate to severe erosions of the corneal epithelium at 1 to 3 months following LASIK were compared to eyes of 19 patients who did not develop epithelial erosions on the corneal flap. No patients who had significant signs of dry eye prior to surgery were included in the study.
The comparison of the two groups of patients revealed no difference in tear production at 1, 3, or 6 months and no significant difference in corneal irregularity or refractive correction, though some patients had a temporary decrease in visual acuity.
What was found, according to Dr. Wilson, of the department of ophthalmology at the University of Washington in Seattle, was that "the signs and symptoms of LASIK-induced neurotrophic epitheliopathy (LNE) tend to resolve at approximately 6 months after surgery." Other studies have shown that on average this is when corneal nerves complete regeneration into the flap.
Dr. Wilson pointed out that approximately 4% of patients who have LASIK develop the LNE-associated epithelial erosions, which "may interfere with vision in some patients." Patients who have dry eye disease prior to LASIK are more likely to develop LNE and have more severe outcomes. He also said "it is unknown whether LNE is attributable to diminished neurotrophic factors released from the nerves or some other factor such as a decrease in the frequency of blinking." He called for further study "to clarify the mechanism and the association with the return in corneal sensation."
Dr. Wilson emphasized the importance of warning LNE-affected patients that "LASIK enhancement will likely be associated with a return of the symptoms and signs of LASIK-induced neurotrophic epitheliopathy." He advised in these cases that "enhancement be performed in one eye at a time, separated by at least 6 months so the patient's visual function is maintained."
g) Regression: Some regression can occur, especially in higher myopes and longsighted treatments. There is most change in refraction in the first week. There is a slight regression averaging 1/4 Dioptre between 1 and 3 months post op, but no significant change after this time. Re-treatment or "enhancement" is best done at about 2-4 months and our rate is 4% in the higher groups (over -6 Dioptres) and less often in lower treatments. Hyperopes have a higher re-treatment rate of about 5%.
h) Night vision problems: Night vision problems: These are more likely to occur in the higher myopic corrections for the following reasons:
As the surgery is limited by depth, in the higher corrections it may be necessary to save depth by making the optical zone smaller. Usually I don't go smaller than a 6.00mm optical zone and a 7.6 mm transition or "blend" zone. (In the lower corrections we would use a 6.5mm optical zone with a 7.5 to 9mm blend zone). Older people have smaller pupils and hence are less likely to have these problems then younger patients. There is potentially a bigger "prolate" to "oblate" change in the higher corrections. The human cornea is flatter in the periphery then the centre (prolate) to minimise spherical aberration (like that suffered by the Hubble space telescope before it was fixed). Having PRK or LASIK for myopia used to flatten the centre of the cornea more than the periphery and left the periphery steeper than the centre (oblate ). This led to night vision problems such as loss of acuity in poor light.. See the page on spherical aberration on this web site . All the modern lasers have corrected this by having an ablation profile that leaves the cornea prolate. This is called "wavefront optimised" and works by putting more laser shots in the periphery.
Jack Holladay (www.docholladay.com), an American ophthalmologist, says that "predatory animals have prolate corneas and prey animals have oblate corneas. Prolate corneas have better central vision and oblate corneas have better peripheral vision. This is important in a prey animal as it needs good peripheral vision to see who's going to be having it for lunch. A predator, however, need good central vision to catch the prey"
One has to differentiate between a "blur circle" and a "night halo". A blur circle occurs because the eye is not exactly zero and is corrected by wearing a spectacle lens, whereas a night halo is not corrected by wearing a lens. In general any small refractive error will be more noticeable in dim light because the pupil is bigger. Hence a patient with a low myopia (-1 Dioptre or better) will have excellent vision in good light but will notice the refractive error more at night.
For the particular problems of hyperopia, go to this page.
Subconjunctival haemorrhages caused by the suction ring are common and harmless. However, be aware that you may have red patches on the whites of the eyes for some days after surgery.
3. Re-treatments or "enhancements":
Normally done between 1-3 months after the first treatment by lifting the flap.
There is no use of the microkeratome, so there are no complications due to this machine.
All re-treatments have the same limitation of corneal thickness as primary treatments. i.e.: we have to leave 250 microns below the flap for long term stability and strength of the cornea and this is often the limiting factor in how much laser an eye can have.
Does lasik have any long term problems?
1. Corneal Ectasia: This means a bulging outwards of the central cornea which has been thinned too much by myopic Lasik. (it is not a problem with hyperopes). The international standard is to leave 250 microns behind to maintain corneal strength (see Lasik contraindications). This problem is obviously more likely with the higher treatments and most cases have occurred with myopia over -8 Dioptres and again it is more likely with an unwise re-treatment. The effect of corneal ectasia is that the cornea becomes steeper and hence the eye becomes short sighted again. This may be corrected by glasses, but in more severe cases, a gas permeable contact lens may be needed or even a corneal transplant (corneal graft). Corneal ectasia is similar to the corneal disease of Keratoconus or 'conical cornea'. Despite following all the guidelines, I personally have had one patient who had a moderate corneal ectasia who needs a gas permeable lens and since then I have been more cautious and conservative. I normally leave at least 300 microns untouched for extra safety. Most cases of corneal ectasia will occur within 6 months after surgery, but we cannot rule out milder cases taking years to develop. Corneal ectasia is not really a problem with longsight as much less tissue is taken off and even this is in mostly in the mid-periphery rather than the centre.
2. Flap Stability: The edges of the flap are sealed by epithelium by one day after surgery and over the next few months the flap itself adheres more and more to the underlying cornea. We do re-treatments at about 3 months, when the flap can be physically lifted, rather like lifting off the sticky flap on an envelope.. It would take a hard oblique blow directly onto the flap to actually move it and actual reports of this happening seem to be very rare. One case was with the airbag of a car accidentally going off, another was a soldier running through some bushes and being caught in the eye by a thorn.
3. Dry Eyes: Long term dry eye problems can occasionally occur, especially in older people with an initial poor tear film. This can be usually cured by putting in silicone plugs in the lower punctae of the eyelids; rather like putting a plug in the plughole of a bath - what tears there are stay around for longer before evaporating.