<prompt> LASIK Laser assisted in situ keratomileusis is useful in treatment ofMyopiaAstigmatismHypermetropiaAll of the above  <response>: Ans. d All of the above Ref. Khuranas ophthalmology, 2nd ed., p 77; Parsons 20thpg. 76EXCIMER LASER IN SITU KERATOMILEUSIS LASER Principle:- Myopia is corrected with preservation of Bowmans layer.- A superficial corneal flap approx. 160 pm is created with a microkeratome.- The keratome is withdrawn, the flap is reflected, and the exposed underlying corneal stroma is ablated with an excimer laser to correct the myopia.- Then the flap is repositioned on the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. 