Learn more about vision correction
Part 1 – Prescription, general health, drugs and eye conditions
In the first part of this guide, we review your prescription, what the numbers mean and which treatments might apply to correct your vision. We also look at a number of medical health conditions, eye conditions and medications that may affect your suitability for laser eye surgery and could be a relative or absolute block to having laser vision correction.
In the second part we will look at some common factors that play a role in your suitability for having laser eye surgery, your age and which type of surgery may be most relevant.
This article should be taken as a guide and not absolute medical advice. You should always have a full consultation with a clinician if you are considering laser eye surgery. Your laser surgeon may refer you to another specialist to gain additional information or examinations, before deciding if you are suitable to have laser vision correction.
Laser eye surgery – public demand
The public demand for laser eye surgery has risen steadily over the last few years, with an estimated 30-50 million procedures performed worldwide. Each year approximately 900,000 Americans and 100,000 Britons have laser eye procedures.
The satisfaction rate is very high due to a technology that has evolved greatly over the past 30 years, together with a greater understanding of who makes a good candidate for treatment.
Improving technology and technique have made an enormous difference to complication rates, e.g. epithelial ingrowth (surface cells growing under the LASIK flap) appears to have been eliminated by the use of femtosecond laser in primary LASIK surgeries and the incidence of infection has fallen from 1 in every 3,000 cases to less than 1 in 20,000.
Increasingly, patients are considering laser eye surgery as an alternative way of correcting their ammetropia (a focusing error requiring the use of glasses, including short-sight, long-sight, astigmatism and presbyopia, the need for reading glasses in middle-age), with contact lens wearers twice as likely to consider surgery than spectacle wearers.
Can I have laser eye surgery?
Table 1 – Prescription ranges for laser eye surgery
A quick guide to prescription plus and minus signs
Prescriptions may look confusing at first but here is a quick guide to understanding what the plus and minus means before the numbers:
- A dash or minus sign before a number means it is a short sighted prescription. 85% of short-sighted glasses have a prescription up to -6.00 diopters.
- A plus sign before a prescription means it refers to a long-sighted correction, which may be either for distance vision or reading glasses.
- Reading glasses may be bought over the counter and typically range from +0.50 up to +3.50.
- Astigmatism corrections can have either a plus sign or a minus sign depending on the format in which it is written—it doesn’t affect the severity, just the mathematical way it is recorded.
What your prescription numbers mean
Your prescription can have three or four numbers. It will usually have three if you are under 45 years of age. If you are over 45, then it will likely have four numbers.
- The first number is for the sphere, sometimes written as SPH, and gives the basic prescription type, with a negative number for short-sight and a positive number for long-sight.
- The second number is for the cylinder, often written as CYL, and is a measure of the amount of astigmatism within the prescription. This sounds complicated or serious but is a normal part of almost every pair of glasses. Astigmatism numbers can be written with either a plus or minus format but that does not change the severity of the astigmatism itself.
- The third number is called the AXIS and just describes the angle at which the astigmatism lies within the glasses. It does not affect the severity of the prescription.
- For patients who are middle-aged and older, the prescription will usually have a fourth number called the ADD. This is short for ‘addition’ and measures the amount of additional lens power needed to correct for clear reading vision. This additional power may be incorporated into the above numbers as part of varifocal or bifocal glasses, or may be used as a stand-alone pair of reading glasses.
TABLE 2: Example prescription format
You should bring any eye examination reports from the past 5 years to your consultation, including your prescription measurements, as well as your current glasses. It will help your doctor understand how stable your prescription is and if you need to delay surgery until your eyes have stopped deteriorating.
Quick guide to suitability for eye treatment
If you meet the following criteria, you are 90% likely to be suitable for eye treatment either with a laser procedure or possibly a lens implant.
QUICK CHECKLIST: Suitability criteria for laser eye surgery
▢ Am I above 21 years of age?
▢ Has my prescription been stable over the last 12 months? (for distance vision; this does not apply to reading glasses)
▢ Can you see well with each eye when wearing glasses?
▢ Are your eyes healthy?
If you would like more information and to run through a 10-minute telephone screening consultation, contact Focus Clinic.
Contraindications to having LASIK, PRK or LASEK
You will need a full consultation and eye examination to determine more accurately if you are eligible for laser eye or lens implant surgery.
As a guide, patients in the following categories will generally not be considered for LASIK, PRK or LASEK treatment, either for issues with their general health or from previous eye problems.
You are probably not suitable if you suffer from or have:
- Auto-immune disease e.g. rheumatoid arthritis, systemic lupus erythematosus—there may be an increased risk of inflammatory complications and possibly irregular healing.
- Inflammatory bowel disease e.g. Crohn’s disease or colitis patients have an increased risk of complications affecting the cornea.
- Immune suppression: conditions such as HIV or if the patient is taking immune-suppressing drugs (such as methotrexate, azathioprine, mercaptopurine) means that there is an increased risk of infection or irregular healing.
- Pregnancy: during pregnancy and lactation the refraction may vary and there is an altered wound healing response. The patient must wait 3 months after giving birth or cessation of breastfeeding.
- Oral steroids e.g prednisolone tablets—patients that have a condition requiring significant steroid use may also be at a higher risk of developing infections or other complications related to the condition requiring steroid use.
- Roaccutane, isotretinoin: this drug causes a significantly decreased tear production. As above, the patient can be considered if the drug is stopped or substituted with the GP’s consent for 3-6 months before treatment.
Suitability for laser eye surgery is normally diminished if you suffer from:
- Severe dry eyes—patients with severe dry eyes should not have laser refractive surgery because the treatment can exacerbate the disease and make it extremely symptomatic. Patients with mild to moderate amounts of dryness may still be suitable with appropriate pre-treatment of the dry eye problems in order to meet the threshold required for safe laser eye surgery.
- Diabetic retinopathy—this is an absolute contra-indication as it can accelerate the progression of diabetic retinopathy. It has also been reported that pronounced aggravation of proliferative retinopathy has occurred after LASIK. Diabetes that is well controlled and with healthy retinas may still be candidates for LASIK (but not PRK).
- Glaucoma—in eyes which suffer from glaucoma, the optic disc is already compromised. During LASIK treatment, the intraocular pressure (IOP) is raised to above 90 mmHg which may cause further damage to the optic disc. The topical steroids used postoperatively may also affect IOP management in these patients.
- Keratoconus and corneal thinning dystrophies—in dystrophies where the cornea is abnormally thin, LASIK would reduce the corneal thickness even more and may cause keratectasia. Signs of subclinical keratoconus such as inferior steepening of the cornea beyond a certain degree, even where corneal thickness is adequate, are a contraindication to LASIK.
- History of ocular inflammatory disease—eyes that have recurrent inflammatory conditions, e.g. iritis or uveitis will be more susceptible to an inflammatory attack after surgery which may be difficult to control. A prior occurrence of iritis that has not recurred and which is not recent may not be a block to having treatment. Your clinic can advise based on your individual history.
- Herpetic ocular disease—there is evidence that corneal infection by the herpes or zoster viruses can be reactivated by laser eye surgery.
- Sjögren’s syndrome—these patients suffer from an auto-immune based dry mouth and dry eyes. LASIK or PRK/LASEK treatment will exacerbate their symptoms.
- Fuch’s endothelial dystrophy—this condition affects the cell layer on the inside surface of the cornea, which is necessary to keep the cornea clear. In Fuch’s dystrophy, the number of cells is reduced. While laser treatment has been shown to not affect the number of cells, the LASIK flap may not adhere as well during the healing phase. PRK remains a possible treatment for such patients.
- Unstable refractive error—the prescription must be fairly stable before treatment is considered. A change of more than 0.50 D equivalent in 12 months or less is deemed unstable.
- Visually significant cataract—in cases where there is a significant lens opacity indicating the development of a cataract (a whitening of the lens inside the eye), it is in the patient’s best interest that this be treated first with cataract surgery. Further refractive surgery afterwards may or may not be necessary. Modern premium lens implants can be used to correct both distance and reading vision. Some patients over the age of 55 may opt for a refractive lens exchange (RLE) as a way to correct vision, instead of laser eye surgery.
- A squint or needing a prism which is not corrected by contact lenses.
- Blindness in one eye or partial sightedness—patients with significantly reduced vision of 20/40 or less in one or both eyes, e.g from optic nerve damage, macular degeneration, lazy eye or other causes, should not have refractive surgery.
- Acute depression or obsessive-compulsive disorder (OCD)
- Difficulty in getting correct glasses—patients who have a lot of difficulty finding a glasses prescription that is suitable may not be eligible for treatment, due to the natural variation in healing that results from laser surgery. If a small residual prescription cannot be tolerated, laser eye surgery should be avoided.
Conditions requiring assessment
There’s a further guide to laser eye surgery and medical conditions, which you can find (here). Patients with the following conditions may be suitable for treatment, but it will depend on the individual’s health and eye history.
- Diabetes—LASIK may be suitable for diabetics who are well controlled and do not have signs of retinopathy (changes in the retina from the disease). PRK may lead to complications with healing and is not offered.
- Dry eyes—moderate dry eyes may be suitable for treatment with pre-treatment of any dryness. In patients where the dry eye is mild, it is unlikely to interfere with their suitability. All patients should expect that their eyes will be drier for 3-6 months after treatment, during which time artificial tear drops will be used frequently.
- Blepharitis—is linked to dry eyes and should be pre-treated prior to surgery. There should be no infection present. The surgeon may expect the patient to adopt lid hygiene measures and in some cases use medication for several weeks prior treatment.
- Epilepsy—the patient must be able to remain relatively still during the LASIK or PRK procedure. Patients whose epilepsy is triggered by light may not be good candidates for treatment. Patients should not have had an epileptic episode for 12 months.
- Multiple sclerosis—will depend on the nature and progression of the condition.
- Increased internal eye pressure—eye pressure needs to be under control prior to treatment and there should be no evidence of visual field loss.
- Very large pupils with high correction—patients will be at higher risk of night vision symptoms after treatment and need appropriate counselling and advice.
- Very low prescriptions—if your prescription is very low and you do not normally wear correction, treatment may not be necessarily be the right choice.
- Nystagmus—not all lasers have a tracker that can keep up with the involuntary eye movements associated with nystagmus.
Contraindications that may no longer apply
- Amiodarone—this drug was previously listed as a contraindication to have laser eye surgery. However, studies have suggested that amiodarone use may not increase risk from having laser eye surgery or change suitability for laser eye surgery. (Arch Soc Esp Oftalmol. 2016 Nov;91(11):520-525. doi: 10.1016/j.oftal.2016.05.005. Epub 2016 Jun 24.)
- 5-Hydroxytryptamine e.g. sumatriptan—patients taking this drug were thought to have a higher risk of problems from the skin coating of the cornea. Research suggests that this is not the case and treatment may still be performed. (J Refract Surg. 2005 Jan-Feb;21(1):72-6.)
Your suitability for laser eye surgery depends on many factors, but most of the time, a consultation will give you a lot of clarity on possibilities for your situation. Want to read more, we shared part 2 of this guide here.