LASIK (Laser In Situ Keratomileusis) is a rapidly evolving of ophthalmic surgical procedure used to correct focusing errors of the eye. A sophisticated microkeratome shaves a thin flap from the corneal surface of the eye then a computer-controlled excimer laser reshapes the underlying tissue in order to correct the focusing error. As with any surgical procedure, complications are bound to occur. A thorough understanding of the potential complications of LASIK and the various strategies to manage them is essential for surgeons performing the procedure. This helps in improving surgical outcomes and in offering patients comprehensive informed consent.
|
Study |
Thin flap |
Irreg flap |
Button holed flap |
Dislodged flap |
Vision loss of 2 lines or more |
Incomplete flap |
Flap folds |
Epithelial ingrowth |
Diffuse LamellarKeratitis |
Infectious keratitis |
|
Gimbel 1998 |
0.3% |
NR |
0.3% |
1.2% |
1.6% |
1.2% |
1.5% |
NR (1.0% intervention) |
NR |
NR |
|
Lin 1999 |
0.49% |
0.2% |
0.2% |
2.0% |
0% |
0.3% |
1.1% |
NR (2.2% intervention) |
3.2% |
NR |
|
Stutling 1999 |
0.75% |
0.09% |
0.56% |
1.1% |
4.7% |
0.75% |
0.2% |
9.1% (1.3% intervention) |
0.2% |
0.1% |
I. Structural Complications:
· Cause: occurs during flap construction with the microkeratome and contributing factors are the corneal curvature, eye-socket shape, conjunctival swelling, low eye pressure due to inadequate suction ring attachment, keratome synchronization or a damaged blade.
· Management: reposition flap & abort procedure to return with a deeper/wider flap 10-12 weeks later is the safest course but some advocate PRK (except in high myopes) in order to prevent irregular astigmatism/buttonhole-related scar.
· Cause: microkeratome blade comes to a premature halt due to jamming or an obstacle. This may produce scarring near the visual axis.
· Management: reposition flap immediately and postponing procedure. Cut deeper and wider flap next time.
This is an emergency that requires prompt repositioning in order to prevent infection, fixed folds and epithelial ingrowth.
· Cause: In the early postoperative period eyelid movement over a dry eye is the main factor. Eyelid rubbing or squeezing and trauma are later causes. Histological studies show minimal healing at the stromal interface after LASIK.
· Management: the flap should be reflected and the interface carefully examined and cleared of debris. A bandage contact lens should be applied. Dislocation is less likely to occur with superiorly hinged flaps. Contact sports should be avoided and an eye shield worn at night for 1-3 weeks after surgery.
A free cap results from unintended complete dissection of the corneal flap and is more common with flat corneas. If the cap cannot be recovered, the epithelium is allowed to grow centrally but a significant hyperopic shift will occur.
· Cause: often a free flap results from the cutting of a thinner flap than intended or the flap becomes caught in the microkeratome
· Management: reposition the flap using preplaced fiducial corneal markets – with laser treatment if the optical zone is adequate. Apply a bandage contact lens. Suturing is rarely necessary. If the flap cannot be retrieved, laser ablation should be postponed until the epithelium has healed and the refraction stabilised.
Flap folds may induce irregular astigmatism with resulting loss of best-corrected visual acuity. Macro and micro folds may upturn with micro folds being wrinkles of Bowmans Layer. Some folds do not adversely affect vision.
· Cause: Flap folds result from uneven alignment of the flap edge with the peripheral epithelial ring and uneven smoothing. Thinner, larger flaps tend to shift more readily with resultant surface tenting.
· Management: ranges from stroking the flap with a moist sponge at the slip-lamp to lifting and refloating of the flap to the placement of sutures to stretch the recalcitrant flap into position.
Epithelial implantation and ingrowth
Implantation of epithelial cells and the interface occurs either due to seeding during surgery or migration under the flap. The cells appear as tongues or pearls and connection to the outside epithelium might be conspicuous or undetectable at the slip-lamp. Most isolated nests of cells will disappear without consequences outsells continuous with contiguous surface epithelium made progress to involve the visual axis with irregular astigmatism and may interfere with nutrition of the overlying flap leading to melting. In growth is more common after enhancement procedures.
· Cause: mechanical seeding from instruments such as the keratome, backflow during irrigation, outgrowth from epithelial plugs in eyes with previous incisional keratotomy and in- growth at the junction of the epithelium and keratotomy. In-growth is more common after buttonholing of the flap.
· Management: If the epithelial cells progress towards the visual axis or induce stromal melting, the flap should be lifted, the stromal bed and flap under-surface should be thoroughly scraped and irrigated (+/- 50% ethanol) and the flap repositioned. Non-progress service isolated epithelial cells should be monitored. A hyperopic shift is an early indicator of possible underlying stromal melt. This may result in irregular astigmatism and loss of best-corrected visual acuity.
Interface Debris
Debris occurs commonly and is usually inert so it needs to be distinguished from infection and inflammation.
· Cause: metal, & oil from keratome, gland secretions, glove powder, air bubbles, lint fibres, methyl-cellulose sponges, etc.
· Management: left flap and irrigated after checking patients 20 minutes following the procedure.
Epithelial Defect
5% of LASIK patients show an epithelial defect at the edge of the flap on the first postoperative day and so are at risk of epithelial ingrowth and local melting.
· Cause: these are more common and patients with recurrent corneal aberrations and anterior basement membrane dystrophy where PRK may be a better choice of treatment.
· Management: either reposition the epithelium or debride it and place a bandage contact lens.
This devastating complication usually results from faulty keratome assembly. It should be managed like any traumatic globe rupture.
If the remaining stromal bed thickness after treatment is less than 250 microns, the weakened cornea may stretch and distort producing a condition like keratoconus. The cornea should be closely examined and measured with pachymetry and in order to exclude keratoconus and to ensure a safe corneal bed thickness. Safeguarding 250 microns of stromal bed is the current standard of care but Barraquer has recommended a minimal thickness of 300 microns of stress bearing corneal stroma. Calculations made prior to surgery may need to be rechecked after the flap has been raised and before laser ablation is carried out in high refractive error cases as the actual thickness may vary from the expected one.
II. Refractive Complications:
Uneven laser ablation produces topographic central islands that lead to visual fluctuations, ghost images and monocular diplopia.
· Cause: Uncertain. Possibly from shielding of the central stroma by the pulverised tissue plume or central collection of fluid as the ablation is performed. Degradation of the laser options has also been suggested.
· Management: Re-treatment based on the last topography map or a hard contact lens. Improved central ablation and scanning or flying spot lasers reduce the risk.
Eccentric ablation may produce astigmatism that causes clear, monocular diplopia and halos is very difficult to correct. Decentration exceeding 0.3 mm is visually significant. Low illumination during ablation improves patient fixation. Miotics shift the pupil supero-nasally so should be avoided. Patients should be encouraged to maintain fixation throughout the ablation. Pupil tracking lasers should prevent decentred ablation is due to drift.
· Cause: patient movement or surgeon error.
· Management: retreatment in the opposite direction using a wide optical zone, laser ablation worth masking agents, miotic drops or a hard contact lens.
· Cause: inaccurate preoperative refraction; not rechecking the contact lens wearer’s refraction; not removing hard contact lenses for five weeks before preoperative refraction; data input error.
· Management: lift flap and retreat - even months later. Beware corneal ectasia disguised as myopic regression and undertreat plano-cylindrical errors.
· Cause: inaccurate refraction, eye rotation or shift/drift during the laser ablation.
· Management: accurate refraction, mark cardinal points before ablation, retreatment.
Regression is more likely after high myopia correction and after hyperopic LASIK but needs to be differentiated from natural progression of refractive error by checking corneal topography.
· Cause: corneal steepening due to epithelial/sub-epithelial/stromal hyperplasia.
· Management: there is no data to show that topical steroids reduce corneal steepening. Additional laser ablation must be preceded by careful and conservative calculation of residual stromal bed thickness.
· Cause: subclinical decentration (< 1 mm) and/or wide-area ablation; more likely after astigmatism treatment; irregular astigmatism due to flap folds, topographic abnormalities, dry eye or simple residual myopia.
· Management: widen ablation zone using wavefront- or topography-guided lasers; mild miotics; cabin light with night driving; tinted lenses; dry eye treatment
If pupils >6mm there is significant risk of halos and glare and patients should be warned accordingly. Patients should be made aware of pre-existing glare, halos and starburst effects at night before LASIK is undertaken.
The effect of LASIK on contract sensitivity is unclear.
III. Loss of Best Spectacle Corrected
Visual Acuity
Loss of two or more lines of best spectacle corrected visual acuity after LASIK is reported in 4.8%. It is more frequent with correction of larger refractive errors and with correction of compound astigmatism. The complications of LASIK potentially affect BSCVA.
IV Dry Eyes
Most LASIK patients develop dry eye symptoms, some show surface roughening (PEK), 40 percent develop corneal iron pigmentation and, occasionally, sterile infiltrates are seen on the flap edge.
· Cause: reduced corneal sensation with subsequent reduction in blink rate; another theory implicates suction ring damage to limbal goblet cells.
· Management: gradual improvement in symptoms over the few weeks following the procedure should be expected. Surface lubrication will ameliorate symptoms. Temporary collagen plugs or long-lasting silicon lacrimal punctum plugs may produce symptomatic relief.
The reported incidence of bacterial infection after LASIK is 0.1% so, although the risk of infection is small, a sterile surgical technique should be used. Blepharitis, Rosacea, dry eyes, NSAIDs and undiagnosed connected tissue diseases may predispose to the formation of sterile infiltrates at the edge of the corneal flap. A history of Herpes Simplex infection is a contraindication to LASIK because of the risk of reactivation
Proliferation of inflammatory cells at the LASIK interface occurs in 0.2 - 3.2% of cases and can lead to stromal melting, induced hyperopia /astigmatism & reduced vision. It may be associated wit epithelial ingrowth, epithelial defects, micropannus haemorrhage & blepharitis. It may appear on day one or two after LASIK, there may be pain & redness. The cause is not known. If DLK does not resolve by Day 5, central coalescence leads to corneal melt & central scarring.
· Management: raise flap, scrape & irrigate if hourly prednisolone 1% & antibiotic drops do not settle DLK by 4th day.
Increased risk of cataract, mesothelial cell damage, macular haemorrhage, difficult contact lens fitting, and difficulty predicting intraocular lens power for eyes requiring cataract surgery. Vitreo-retinal complications are rare (0.06%).
Summary:
LASIK refractive surgery is a relatively new technique with a very high success rate. Very high standards of care are required as healthy eyes are placed at risk with every LASIK procedure performed. The tools and techniques of LASIK continue to evolve.
Survey of Ophthalmol Vol 46 (2) Sept-Oct 2001