LASIK complications can be categorized to intraoperative and postoperative complications:
The intraoperative complication rate of LASIK has been reported between 0.7-6.6%, most commonly flap-related, either mechanical (microkeratome) or femtosecond (FS) Laser.
Buttonhole creation in a flap is typically caused by buckling of the cornea during flap creation. This occurs predominantly in steep corneas. Other risk factors include suction loss, defective blade, abnormal advancement of blade.
Management: Do not perform laser ablation. Either perform PRK or recut the flap and ablate a minimum of 3 months later.
Free caps are caused by inadequacy of captured tissue in the suction ring. They occur predominantly in flat cornea. To prevent free cap, use larger ring and choose larger flap hinge.
Management: Perform laser ablation, orient the cap properly and replace it on the bed, allow air-drying for 3-5 minutes.
Incomplete, short, or irregular flaps
These abnormal flaps are caused by inadequate suction or microkeratome malfunction.
Management: Do not manipulate the flap, do not perform laser ablation, place a bandage contact lens, and perform PRK or recut the flap and ablate at least 3 months later.
Perforation through the cornea is a rare but devastating complication. It has been reported in older model of mechanical microkeratomes when it was not properly assembled or the depth plate was not properly placed.  It can also occur during laser ablation on extremely thin cornea.
Management: Immediately de-activate the suction, remove the microkeratome, and repair the perforation in sterile manner.
Vertical gas breakthrough
Vertical gas breakthrough can occur during FS laser-assissted flap creation, resulting in escape of gas bubbles from the dissection plane into the subepithelial space. The cause is unknown but a thin flap or a focal break in the Bowman's layer may be causative.
Management: Lift the flap cautiously and perform laser ablation.
Anterior chamber gas bubbles
Gas bubbles in the anterior chamber can occur during FS laser-assissted flap creation, resulting in escape of gas bubbles from the dissection plane into the trabecular meshwork then to the anterior chamber. The anterior chamber bubbles can interfere with pupillary tracking, but usually are self limiting and resolve over a short period of time.
Management: Lift the flap cautiously and perform laser ablation.
Corneal Epithelial Defect
The risk factors of epithelial erosion during LASIK include older age, previous corneal trauma, diabetes mellitus, epithelial basement membrane dystrophy (EBMD), and type of microkeratome. It can also occur in FS laser microkeratome with a reduced incidence. Epithelial defect can predispose to delayed healing, diffuse lamellar keratitis (DLK), epithelial ingrowth, flap striae, and infectious keratitis.
Prevention: limiting toxic topical medications, minimizing use of topical anesthetics, frequent use of lubricating drops, preoperative inspection of blade and meticulous microkeratome maintenance.
Management: Use bandage soft contact lens for larger defect (>1 mm), topical lubricants until re-epithelialization establishes.
It occurs in two occasions: presence of corneal pannus as is common in contact lens wearers, using inappropriate size or position of the suction ring resulting in passage of the blade over limbal or conjunctival vessels.
Management: Apply gentle pressure on the oozing vessels with a dry sponge directly or pushing a fold of conjunctiva over the limbal feeders with a dry sponge. Remove any blood in the ablation zone. After replacing the flap, phenylephrine 2.5% may be used to constrict the blood vessels.
The sources include meibomian gland secretions, particles from sponge, talk from gloves, metallic fragment from blade, red blood cells, epithelial cells, and debris from tear film. 
Prevention: Use an aspirating speculum, operate in a lint-free environment, drape the lashes and eyelids.
Management: Only if an inflammatory reaction elicited by debris, then lift the flap, irrigate and remove the debris manually. Otherwise they are well tolerated.
Overcorrection and Undercorrection
Undercorrection is the most common complication after primary LASIK. Overcorrection is mostly seen after retreatment. Both are related to the ablation algorithm, nomogram, age, and the amount of refractive error.
20% of patients will report some form of visual change. Some patients may suffer from visual changes such as glare, halo, or star-bursting patterns around lights, haze, and decreased contrast sensitivity. The FDA reports that visual disturbances tend to stabilize three to six months after the procedure.
Flap Fold or Striae
The risk factors for flap folds include excessive irrigation of flap during LASIK, poor repositioning of the flap at the end of procedure, thin flaps, deep and highly myopic ablation with flap-bed mismatch. Flap folds may be classified into macro- and microstriae.
Macrostriae are full thickness, rolling stromal folds, occur because of flap malposition or slippage.
Management: Perform immediate refloating and repositioning. After 24 hours, macrostriae may require refloating, de-epithelialization, hydration, stroking, and suturing.
Microstriae are fine folds in Bowman’s layer, occur because of mismatch of flap to new bed and often visually insignificant.
Management: Observation with aggressive lubrication, if visually significant perform refloating, stroking, and suturing.
The risk factors are excessive lid squeezing, eye rubbing, excessive dry eye, presence of epithelial abrasion, poor intraoperative repositioning, excessive irrigation of flap, and trauma.
Prevention: Check adhesion of flap at the end of procedure, Remind the patient not to squeeze or rub the eyes and wear the shield for the first 24 hours and every night for the first week.
Management: Reposition the flap, suture the flap in the event of persistent fold, and use lubricants.
Dry Eye and Corneal Sensation
Dry eye is one of the most common side effects of LASIK in 60-70% of all patients with varying degrees due to corneal denervation.
Prevention: Perform a thorough exam preoperatively to discover dry eye syndrome and treat aggressively with topical lubrication, cyclosporine A, and systemic treatment with oral tetracyclines, and oral omega-3 fatty acids.
Management: mild dry eye syndrome: frequent use of non-preserved artificial tears, and gels. Severe dry eye syndrome: topical cyclosporine A, topical corticosteroid, oral tetracyclines, oral omega-3 fatty acids, and punctual occlusion. 
Diffuse Lamellar Keratitis (DLK)
See the discussion in Diffuse Lamellar Keratitis section.
Pressure-induced Stromal Keratitis (PISK)
PISK is a late-onset interface opacity similar to DLK with a visible fluid cleft in the interface as a result of elevated IOP because of prolonged corticosteroid treatment.
Management: rapid tapering or cessation of the corticosteroids and use of anti-glaucoma medication to lower IOP. 
Central Toxic Keratitis (CTK)
See the discussion in Central Toxic Keratitis section.
Infection under a LASIK flap is one of the most vision threatening complications. The most common organisms are gram-positive bacteria followed by atypical mycobacteria.
Symptoms: decreased vision, pain, photophobia, and redness
Differential Diagnosis: DLK that usually seen within first 24 hours and typically begins at flap periphery
Prevention: Adequate sterilization of the instruments, preoperative treatment of blepharitis, use of sterile surgical technique, postoperative antibiotic prophylaxis.
Management: Lift the flap, culture the interface, and irrigate with antibiotics. Start empirical fortified treatment including vancomycin (50 mg/ml), tobramycin (14 mg/ml), or gatifloxacin, moxifloxacin.
Atypical mycobacteria: topical clarithromycin (10 mg/ml), oral clarithromycin (500 mg bid), and topical amikacin (8 mg/ml).
Fungal keratitis: Natamycin (50 mg/ml), amphotericin (1.5 mg/ml).
The risk factors are poor adhesion of the flap edges, epithelial abrasions at the flap margin, buttonhole flap, free cap, ablation at the edge of stromal bed, epithelial irregularity at the edge of flap, introduction of epithelial cells during the cut, or insertion of instruments, inadequate irrigation, previous RK, reoperation.
Two types of epithelial ingrowth recognized: isolated epithelial pearls in the interface, and epithelial sheet growing into the interface from the periphery.
Symptoms: reduced vision, irregular astigmatism, and risk of stromal melting.
Prevention: Avoid epithelial defects, remove epithelial cells and debris from the interface, and avoid wide ablation zone.
Management: No treatment for asymptomatic isolated nests, otherwise lift the flap and scrape both the underside and the stromal bed then reposition the flap.
See the discussion in Post Lasik Ectasia section.
Ischemic optic neuropathy, retinal detachment, vitreous hemorrhage, and posterior vitreous detachment, are potential but very rare complications of LASIK, occurring in less than 0.1% of patients.
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