Laser eye surgery suitability: who is a good candidate?

Whether you can have laser eye surgery is decided not by how much you want it, but by the measurements of your eyes. The US Food and Drug Administration (FDA), the American Academy of Ophthalmology (AAO) and the Refractive Surgery Commission (KRC) of the German ophthalmological associations set out specific criteria for this.

Stable prescription: the past year matters

The laser corrects the refractive error measured on the day of treatment. If it keeps changing afterwards, the result will soon be out of date. If your glasses or contact lens prescription had to be changed in the past year, the FDA says you are therefore probably not a good candidate. According to the FDA, fluctuations are more likely with hormonal changes caused by conditions such as diabetes, during pregnancy and breastfeeding, or with certain medicines. Age matters too: the KRC lists treatment before the age of 18 as a contraindication, and according to the FDA, people in their early 20s or younger are more likely to have a prescription that is still changing.

The cornea: thick enough and healthy

In LASIK and femto-LASIK, a corneal layer (the flap) about 0.1 to 0.15 mm thick is cut and folded back first; the excimer laser then removes tissue underneath. The KRC advises against LASIK if the cornea is thinner than 480 µm (flap cut with a femtosecond laser) or 500 µm (mechanical microkeratome) – roughly half a millimetre – or if less than 250 µm would remain under the flap, including after any re-treatment.

However, according to the AAO, no single value can reliably rule out later bulging of the cornea (ectasia). An abnormal corneal topography is considered the main risk factor; another risk factor is when the flap and the ablation together account for 40 per cent or more of the original corneal thickness.

If your cornea is on the thin side, a surface treatment such as PRK, LASEK or Trans-PRK may be an option. Without a flap, more of the deeper corneal tissue is preserved, according to the AAO; the trade-off is that the first few days are more uncomfortable and vision recovers more slowly.

Ruling out keratoconus

In keratoconus, the cornea becomes thinner and bulges forward into a cone shape. The AAO lists it as a contraindication because the outcome of laser treatment can then become unpredictable, and for LASIK the KRC also excludes early forms (‘forme fruste’). That is why the cornea is mapped by computer before any procedure (topography or tomography). Signs you may notice yourself, according to the AAO, include distorted vision (straight lines look wavy), greater sensitivity to glare and a prescription that keeps changing. Around one in ten people with keratoconus has a parent with the condition – so mention it if it runs in your family.

How many dioptres? The treatment ranges

The KRC distinguishes between an ‘application range’ (procedure suitable, side effects rare) and a ‘borderline range’ (increasingly poorer results, more frequent side effects). Outside these ranges, it does not recommend the procedure. Minus stands for short-sightedness, plus for long-sightedness, and astigmatism for a cornea that is curved more steeply in one direction than the other:

With astigmatism, the eye refracts light more strongly in one direction than in the other. The KRC therefore does not use the average but the stronger direction: −5 D of short-sightedness plus a −2 D cylinder (the term used for astigmatism on a prescription) counts as −7 D – already in the borderline range for PRK and Trans-PRK, but still within the application range for LASIK.

Contact lenses: why you take a break before the measurements

Contact lenses can change the shape of the cornea – according to the FDA, for up to several weeks after you stop wearing them. If measurements are taken too soon, the FDA warns of inaccurate readings, a poor surgical plan and poorer vision afterwards. Before the first examination, the FDA therefore recommends a break of 2 weeks for soft lenses, at least 3 weeks for toric soft or rigid gas-permeable lenses and at least 4 weeks for hard lenses. According to the AAO, toric and rigid lenses are more likely to warp the cornea and for longer, so it can take several visits before the readings are shown to be stable. Your ophthalmologist will therefore decide exactly how long your break should be.

Dry eyes, pregnancy and other reasons for caution

• Dry eyes: According to the FDA, LASIK tends to make dry eye worse. In the two US PROWL studies (published in 2017), 34 to 35 per cent had dry eye symptoms three months after LASIK – fewer than before surgery, but just over a quarter (27 to 28 per cent) of those with no symptoms beforehand reported new ones. According to the AAO, dry eye should be adequately treated first (more on dry eyes).
• Pregnancy and breastfeeding: Your refraction can fluctuate during this time (FDA); the AAO lists both as a relative contraindication.
• Medical conditions and medicines: The AAO’s relative contraindications include diabetes, autoimmune diseases, previous herpes infections of the cornea and glaucoma, as well as medicines such as isotretinoin (for acne) or amiodarone (for heart rhythm). A cataract that affects vision or uncontrolled glaucoma rules out corneal surgery. The FDA advises telling your doctor about all your medicines, including over-the-counter ones.
• Expectations: The AAO also lists unrealistic expectations as a contraindication; it recommends discussing presbyopia, which will still develop later on, in advance.

When an ICL or lens exchange may be a better fit

There are three clinical reasons that can favour a lens inside the eye:

• High prescriptions: In high short-sightedness, laser results can be less predictable, according to the AAO, whereas phakic lenses have optical and structural advantages here; in the US they are approved for up to −20 D.
• Thin or irregularly shaped cornea: In this case, the AAO says surgery inside the eye may be considered as an alternative to laser treatment.
• Existing presbyopia: Lens exchange then falls within the KRC’s application range (with restrictions for multifocal lenses – see the table).

ICL: The implantable contact lens sits behind the iris, in front of your own lens, which is preserved together with its ability to focus up close. A 2014 Cochrane review (three trials, 228 eyes, −6 to −20 D; only one of them used an ICL, two an iris-fixated lens) found that, one year after surgery, eyes with a phakic lens were less likely than laser-treated eyes to lose best-corrected visual acuity, and had better contrast sensitivity and higher satisfaction – with caveats: the methodological quality of the trials is partly unclear, and both lasers and lenses have improved since. Because the eye is opened, the KRC also lists rare risks such as a rise in eye pressure, clouding of your natural lens (cataract), damage to the cell layer on the back surface of the cornea (endothelium) or infection. There must be enough room in the eye: the KRC requires an anterior chamber depth of at least 2.8 mm for short-sightedness and at least 2,000 endothelial cells per mm². These cells on the back surface of the cornea are then checked at least once a year. More about the ICL.

Lens exchange: Your own lens is replaced with an artificial one, as in cataract surgery; whatever ability to focus up close remains is lost. With monofocal lenses you will usually need reading glasses for close work (unless monovision is used, with one eye set for near vision). Multifocal lenses (MIOL) can improve intermediate and near vision but, according to the KRC, may worsen vision in dim light and cause haloes and glare. For short-sighted eyes, the AAO cites a risk of retinal detachment after lens exchange of 1.5 to 8.1 per cent, which builds up over the years. More about lens exchange.

Warning signs: according to the US National Eye Institute, a sudden shower of new dark spots (floaters), flashes of light or a curtain-like shadow in your field of vision can indicate a retinal detachment. This is an emergency – go to an eye doctor or an emergency department straight away (more on this).

What is checked before surgery

According to the KRC, the assessment before surgery includes at least corneal topography and thickness, visual acuity, eye pressure, pupil size in dim light and an examination of the front and back of the eye with dilated pupils. After corneal surgery, the AAO recommends that you receive a written record of your values from before and after – they can matter later on, for example if you need cataract surgery. After LASIK, the FDA advises: if you are in severe pain, or if your sight or other symptoms are getting worse rather than better, contact your doctor straight away – do not wait for your next scheduled check-up.

At My Laser Vision in Wallisellen, you can book a free pre-assessment of around 30 minutes for refractive procedures – a first step towards finding out whether laser eye surgery, an ICL or lens exchange could in principle be suitable for you. The full assessment before surgery can, as described above, involve further measurements and sometimes several visits.