Ans. d (All of the above) (Ref. Khurana's ophthalmology, 2nd ed., p 77; Parson's 20th/pg. 76)EXCIMER LASER IN SITU KERATOMILEUSIS (LASER)# Principle:- Myopia is corrected with preservation of Bowman's layer.- A superficial corneal flap (approx. 160 pm) is created with a microkeratome.- The keratome is withdrawn, the flap is reflected, and the exposed underlying corneal stroma is ablated with an excimer laser to correct the myopia.- Then the flap is repositioned on the corneal bed and fixed in place by force of its own adhesion.# Indication:- Severe myopia (up to 10-12 diopters).- Photoablation:* Excimer laser produces photoablation.* It produces UV light of very short wavelength, which breaks chemical bonds of biologic materials, converting them into small molecules that diffuse away.- Photorefractive keratectomy:* LASIK for correcting refractive errors and* LASIK for phototherapeutic keratectomy for corneal disease like "band-shaped" keratopathy.# LASIK is superior to photorefractive keratectomy in that it:- Less painful procedure* As LASIK does not expose raw corneal surface at the end of surgery it is less painful.- Can be repeated by lifting the flap created if further refinement is necessary- Gives a faster visual rehabilitation- Has less myopic regression- Causes less scarring than photorefractive keratectomy- Can Rx higher myopia* It can also treat a higher myopia, causes less scarring and has less myopic regression. However, because a flap needed to be created, it has a higher potential risk.* Involves the use of microtome* Is contraindicated in patients with thin cornea# Side effects of LASIK:- Reduced corneal sensation- Inaccurate biometry if the patient requires cataract surgery in later life- Tear film abnormality- Erroneously high intraocular pressure with applanation tonometer.