Refractive errors are the most common eye disorders. For the eye to see clearly, light rays must be bent (or refracted) by the cornea and the lens, and focused on the retina. The retina receives the light rays and converts them to neural signals that are transmitted by the optic nerve to the brain, where the signals are translated into images:

Like a camera, the human eye must be properly focused to see an image clearly. If light does not bend or refract correctly and focused directly on the retina, the result is blurred vision, or a refractive error.

The four most common refractive errors are:

Most people in their 20s, 30s or early 40s, with healthy eyes, adequate cornea thickness and that haven´t had any significant changes in their lens prescription during the past year, can be considered good candidates for this procedure.

The following characteristics would make a person NOT a good candidate for lasik sugery:

1. People who required a change in their contact lens or glasses prescription in the past year. This is called refractive instability, it occurs in patients who are:
• 18 years old or younger
• Whose hormones are fluctuating
• Who are pregnant or breastfeeding
• Who are taking medications that may cause fluctuations in vision

2. People that have a disease or are on medications that may affect wound healing. Certain conditions, such as autoimmune diseases (e.g., lupus, rheumatoid arthritis), immunodeficiency states (e.g., HIV) and diabetes, and some medications (e.g., retinoic acid and steroids) may prevent proper healing after a refractive procedure.

3. People who actively participate in contact sports such as boxing, wrestling, martial arts or other activities in which blows to the face and eyes are a normal occurrence.

4. Patients with the following diseases:
• Herpes simplex or Herpes zoster (shingles) involving the eye area.
• Glaucoma, glaucoma suspect, or ocular hypertension.
• Eye diseases, such as uveitis/iritis (inflammations of the eye)
• Eye injuries or previous eye surgeries.
• Keratoconus

5. Patients which corneas are too thin.

In order to determine if the cornea has the adequate thickness and conditions for surgery, the following eye exams are required:

* Topography (measures the corneal curvature)
* Pachymetry (measures the cornea thickness)

Also, there are certain factors that need to be considered before doing these exams:

Contact Lenses
If you wear contact lenses, it is a good idea to stop wearing them before your baseline evaluation and switch to wearing your glasses full-time. Contact lenses change the shape of your cornea for up to several weeks after you have stopped using them depending on the type of contact lenses you wear. Not leaving your contact lenses out long enough for your cornea to assume its natural shape before surgery can have negative consequences. These consequences include inaccurate measurements and a poor surgical plan, resulting in poor vision after surgery. These measurements, which determine how much corneal tissue to remove, may need to be repeated at least a week after your initial evaluation and before surgery to make sure they have not changed, especially if you wear RGP or hard lenses. 

If you wear:
- soft contact lenses, you should stop wearing them for 2 weeks before your initial evaluation.
- toric soft lenses or rigid gas permeable (RGP) lenses, you should stop wearing them for at least 3 weeks before your initial evaluation.

- hard lenses, you should stop wearing them for at least 4 weeks before your initial evaluation*

Pregnancy.

Since hormones change significantly during pregnancy and breast feeding (this fluctuations alter the cornea´s shape), it is recommended to wait at least 6 months after these stages to have the evaluation and surgey performed.

Once the ophthalmologist has chosen you as a good candidate for surgery, he will proceed to inform you about the several methods available and then both of you can decide wich one is better for you.

There are several laser and non-laser methods available for correcting refractive errors:

* The laser methods are: LASIK, LASEK and PRK.

CUSTOM LASIK is a personalized laser system used to correct refractive errors (myopia, hyperopia and astigmatism) that designs each surgery based on the patient`s eye unique characteristics.

The custom lasik provides an additional precision level during surgery thanks to the wavefront technology, which measures the way light travels in the patient`s eye, and then compares it to the way it would travel in a eye with perfect vision, in order to design a specific tridimensional map that allows the surgeon to correct each refractive error in a very precise way.

The surgery combines delicate surgical techniques with an excimer laser treatment.  Anesthesia drops are applied to each eye, so the patient feels no pain during the procedure. An instrument called a microkeratome is used to create a thin, circular flap in the cornea. The surgeon folds the flap back out of the way, then removes some corneal tissue underneath using an excimer laser which is guided by the specifications calculated in the wavefront system. The excimer laser uses a cool ultraviolet light beam to precisely remove ("ablate") very tiny bits of tissue from the cornea to reshape it. When the cornea is reshaped in the right way, it works better to focus light into the eye and onto the retina, providing clearer vision than before. The flap is then laid back in place, covering the area where the corneal tissue was removed. The flap acts as a natural bandage and makes the healing process quick and painless.

Both nearsighted and farsighted people can benefit from the LASIK procedure. With nearsighted people, the goal is to flatten the too-steep cornea; with farsighted people, a steeper cornea is desired, to correct astigmatism, it smoothes an irregular cornea into a more normal shape.

Lasek is surgery is the same as Lasik (applying laser to reshape de cornea) the difference is the thickness of the flap created (only one third of the thickness used in Lasik). To help protect the wound, a contact lens is placed on the cornea to minimize risk infections and help the healing process.

 The Lasek procedure is perfect for patients that have thin corneas o certain conditions that would make them not very good candidates for Lasik.

The risk reduction depends greatly on the adequate selection of candidates and the technique chosen for the surgery. When this is done correctly, the probability of surgery-related complications is minimal (less than 0.03% risk of infection).

Thanks to state of the art technology, and the use of safe and highly reliable tools there is practically no posibility of loss of vision due to the surgery: the flap is made by a special device called microkeratome and the laser has an eye movement sensor that automatically stops if it moves beyond the established limits.

The only risks a patient faces when undergoing this surgery are:

* Not getting a perfect 20/20 vision at the end. Sometimes (depending on the original lens prescription, cornea´s thickness, etc) it is impossible to achieve a perfect vision with surgery (your doctor will explain it to you if this is your case).  In most cases, the residual graduation can be corrected with a touch-up surgery.
* Dry eyes. This can be solved with artificial tear eye-drops.

You shall arrive to surgery:

• Without perfume (since it can interfere with the air´s quality in the operating room)
• Without makeup
• With a sweater, since it can be very cold in the operating room
• Accompanied by one person only

When you get there the reception lady will ask you to sign the consent form and make the payment, she will give you your receipt and will take you to the preparation room. There a nurse will clean your eye area, apply anesthesia and antibiotic drops to your eyes and will give you a disposable hair net and  booties to place over your shoes.

Afterwards you`ll be taken to the surgery, which  is done one eye at a time. You will lay on a bed with your face under a microscope, looking at a flashing red light. The doctor will use a small tool to separate your eyelids and inmovilize your eye. Then he will use a special marker to draw a dot on the cornea where the flap will be done, will place a metalic ring over your eye and proceed to create the flap. You will experience some seconds of total darkness and feel some pressure at this point, but no pain at all.	After that you will see the light again. The doctor will ask you to stare directly at the flashing red light until the laser application has been completed. The laser has an eye-movement sensor that follows the eye and stops automaticallly in case it is moving too much. There will be a strange noise and an odd odor, this is caused by the gas used by the laser beam.

At last the nurse will place protecting goggles over your eyes (you will not take them off until the next day and then will use them only at night, during one month, to avoid any possible rubbing while you`re sleeping) and will give you some instructions.

It is very important that someone accompanies you on your way back home because, even though you will see clearly, light may bother you and you won´t be able to drive.

Thirty minutes after, the anesthesia effect will start to pass and you will feel a tingling sensation on your eyes. This feeling will last for about 2 hours, so we strongly recommend you rest and take a nap right after the surgery. At this point, the only thing you CAN NOT do is squeeze or rub your eyes, because this can make the flap move and can cause several problems.

The doctor will give you an appointment for the next day, so he can take off your googles, check your vision, make sure that the flap didn´t move and that everything is all right. He will also explain when and how you should apply the prescription drops.

By now your vision will have improved greatly, and you can do your normal routine without any trouble. You may have small bruises and experience some vague fluctuations in your vision during the first months, this is completely normal and shouldn`t alarm you.

Post- surgical care.

We recommend that from now on, you use sunglasses and lubricant eye drops. Also you will have to avoid the following activities after the surgery:

* No squeezing or rubbing your eyes (1 week)
* Avoid getting water in your eyes while taking a shower (1 week)
* No swimming (1 month)
* Avoid getting sweat on your eyes while excersizing (1 week)
* Avoid closed areas with smoke (1 week)
* No eye makeup: mascara, eye shadow, eye liner, etc (2 weeks).

Your doctor will ask you to come back after one month and one year, to do a follow up on your vision progress.

It is VERY IMPORTANT  for you to contact him immediately if you experience any of these symptoms:

* Intense and sudden pain in the eyes
* Radical changes in your vision
* Severe irritation
* Extreme sensitivity to light

In its earliest stages, keratoconus causes slight blurring and distortion of vision and increased sensitivity to glare and light. These symptoms usually first appear in the late teens and early twenties. Keratoconus may progress for 10-20 years and then slow or stabilize. Each eye may be affected differently.