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Pre-op screening

LASIK Screening

This is the most important part of the specs removal surgery. At Indovision Cataract &n Laser Eye Centre, we make sure screening is done as per standard international protocols. The screening process may take around 2 hr while the LASIK surgery takes less than 13 mins.If you wear contact lenses, you need stop wearing them before your baseline evaluation. Contact lenses can lead to a change in shape of the cornea. Depending on the type of your contact lenses you need to stop wearing them. If you wear:Soft contact lenses: it is recommended that you stop wearing them 2 weeks before your baseline evaluation Rigid gas permeable lenses: Stop wearing at least 3 weeks before evaluation Hard lenses: These lenses affect the cornea the most. You should stop wearing at least 4 weeks before evaluation

THE TESTS INCLUDE

Auto refractometer test

Dilated refraction

Post mydriatic test (PMT)

REFRACTION

It is the most important part of LASIK screening. Even the best surgery may not give accurate results if the correct power of the eye has not been determined.

To ensure correct spectacle power, following tests are done:

Auto refractometer test
Commonly knowns as computerized eye testing. The patient is asked to rest his chin on chin rest & head on the head rest. The machine focuses the eye & patient has to look at a target in the machine. The machine captures the reading of the reference spectacle power of the eye. This reading gives a fairly accurate measure of spectacle power. This is used as a guide to subjective refraction.
Dilated refraction
In this test, pupil dilating eyedrops are put in the eyes. Patient is asked to wait for 20-30 mins with his eyes closed until the pupil fully dilates. A retinoscopy is then performed to determine the spectacle power of the eye. The dilating drops relax the ciliary muscle of the eye so the true refractive power of the eye can be determined..
Post mydriatic test (PMT)
This is also known as the true refraction or final refraction for vision correction. Trial lenses are placed in front of the eye & the best lens power is noted. Auto refractometer reading & dilated refraction are taken as guide for PMT. However, dilating drops are not mandatory in myopia.

INTRAOCULAR PRESSURE (IOP)

Like blood pressure in the body, the fluid in the eye contributes to intraocular pressure (IOP). A balance is maintained in the in flow & outflow of fluids in the eye. The pressure created by this fluid or aqueous humor is called IOP. The normal IOP ranges from 11 to 21 mm of Hg. The intraocular pressure can be measured by non-contact tonometer (NCT).

  1. NCT – In this method, an air puff is directed on the cornea & compression of the cornea is determined. This helps to calculate the IOP of the eye. This is a screening method. Reliability of this test is slightly low.
  2. Applanation tonometer (AT) – This is considered as gold standard in measuring the IOP. Anesthetic drops are put in the eye. Eye is stained with fluorescein dye. Then a device is brought in contact with the cornea that helps to applanate (flatten) it. Reading is taken to measure IOP.

Importance of IOP measurement

 

Steroid drops are used after LASIK surgery. These steroid drops may lead to transient rise in IOP. For those who have Glaucoma (raised IOP), the LASIK surgery may not be suitable. Some people tend to develop a rise in IOP after use of steroid. These are known as steroid responders. For such patients’ adequate measures need to be taken.

THICKNESS OF THE CORNEA

Normal corneal thickness is around 530 microns. A minimum corneal thickness of 450 microns is needed to undergo LASIK. In a cornea which is less than 495 microns, should be treated with caution. Entire cornea is evaluated. The anterior & posterior curvatures of the cornea are determined. Any abnormal thinning or weakness of the cornea will eliminate you from any spec’s removal procedure.

SCREENING OF ANY PATHOLOGY IN THE EYE

LASIK can be done only in healthy eyes. LASIK should be avoided in eyes with any corneal pathology, glaucoma, retinal diseases & keratoconus. It is also contraindicated in systemic disorders like Lupus & rheumatoid arthritis.

RETINAL EVALUATION

A dilated retinal evaluation is mandatory for LASIK screening. In myopia, the length of the eyeball is more than normal length. This leads to a more stretched & thinned out retina. It is quite common to have fine breaks or holes in retina. Such holes need to be sealed before LASIK is done. Retinal holes when left untreated can lead to retinal detachment & permanent loss of vision.

DRY EYE TEST

LASIK involves making a flap in the cornea. While making a flap, the corneal nerves are also cut. This lead to dryness. The dryness usually improves over 3-6 months. During this time, lubricating drops will be prescribed to you. However, if you are suffering from dry eyes, you may not be a good candidate to undergo LASIK, as LASIK may aggravate dry eyes. Dry eye test to be done before LASIK.

  • Shirmer’s test
  • Tear film breakup time

Shirmer’s test

This test measures the amount of tear secretions. A bend filter paper with markings is placed in the lower lid. After 5 mins, wetting of the paper is noted. The amount of wetting grades the severity of dryness.

Tear film breakup time

The eye is numbed with anesthetic drops & stained with fluorescent dye. Appearance of a first random dry spot is noted. Normally dry spot appears after 15-35 seconds. If dry spot appears in less than 10 seconds, there is dryness.