4-1. What happens at the consultation?

The purpose of the consultation is to determine if you are a good candidate for LASIK. It consists of medical and ophthalmic evaluation to see if LASIK will be safe and beneficial for you.

4-2. What procedures are carried out at the consultation?

A discussion about your eyes takes place with the doctor and the laser center’s staff. You will be asked questions about your use of glasses and/or contact lenses, your medical history, your family history, and your expectations about what LASIK can accomplish. An examination of the eyes takes place, and certain diagnostic tests are performed.

4-3. What is a refraction?

A refraction determines your current, up-to-date, glasses prescription. It is usually performed at the consultation. Looking at letters through a device, known as a “phoropter,” each eye is tested separately to determine which lens, or combination of lenses, produces the sharpest vision.

4-4. What is a “slit lamp exam?”

A slit lamp is a “biomicroscope.” It allows the doctor to examine the eyes through some magnification, usually about 2-1/2 times the true size. Many features, and abnormalities, of the eye can be detected with the slit lamp microscope.

4-6. Is a glaucoma test performed at the consultation?

A glaucoma test is performed. This test measures the pressure within the eyes, the “intraocular pressure.” If the pressure is elevated, some of the delicate structures within the eye could be damaged. Glaucoma runs in families, so if a parent, or a sibling, has it, you are at risk. Glaucoma does not prevent you from having LASIK, but it is an important screening test.

4-7. What other tests are performed?

Corneal topography is an important diagnostic test, which evaluates the anatomy of the cornea. It looks like a topographic map of land. It maps the steep, flat, and irregular areas of the cornea, and assigns a color scale to the different elevations. The cornea has very subtle topographic features, but they can be detected, and magnified by topography.

4-8. How is the steepness of the cornea displayed on a topographic map?

Usually, cool colors, like blues and greens, represent flat areas of the cornea. Warm colors, like reds, browns, and yellows, represent steep areas. In a normal cornea, the center is slightly steep, and the periphery is slightly flat. Any irregularities detected by topography might indicate an abnormal cornea.

4-9. What does astigmatism look like on corneal topography?

Astigmatism is displayed as a bow tie pattern on topography. The loops of the bow tie are oriented along the steepest axis of the cornea (Figure 10).

Figure 10

4-10. What does the topography look like after laser treatment?

In a nearsighted eye, the central cornea shows flattening compared with preoperative topography.

4-11. How is corneal thickness measured?

Corneal thickness is measured with a pachymeter, an ultrasonic device that can measure corneal thickness to the micron. (A micron is 1/1000 of a millimeter, or 1/25000 of an inch.) A cone-shaped probe is placed on the cornea, and the machine records the corneal thickness beneath the probe.

4-12. What is the normal thickness of the cornea?

The normal thickness of the center of the cornea is about 550 microns (a little more than half a millimeter, or about 2/100 of an inch). The peripheral cornea is somewhat thicker. Corneal thickness can vary, and it is not unusual to find normal corneas with central measurements as thin as 490 microns, or as thick as 620 microns.

4-13. What is the ideal corneal thickness for LASIK?

There is no ideal thickness for LASIK, but, in general, thicker corneas permit treatment of larger amounts of nearsightedness and farsightedness. Corneas that are too thin may only allow small amounts of nearsightedness and farsightedness to be corrected by LASIK.

4-14. What other measurements are made?

Corneal thickness is measured with a device, known as a “pachymeter.” Most of these measure the thickness of the cornea with ultrasound. It is important to determine the corneal thickness since the laser removes some corneal tissue and makes the cornea thinner. If the cornea is made too thin, vision may fluctuate and be of poor quality.

4-15. Are fees and financing discussed?

Fees and financing are important subjects, and they should be discussed at the consultation.

4-16. Do I need to sign a consent form for LASIK?

You will usually be given a consent form at the consultation, but it does not need to be signed right away. The consent form explains the LASIK procedure in simple terms, and discusses complications and alternative treatments. You can read over the consent form at your leisure, and make a list of questions to be asked before having LASIK.

4-17. How can I calculate the amount of correction my cornea can tolerate?

If you know your corneal thickness, and your glasses or contact lens prescription, you can do the calculation. Let’s say you have an average cornea, 550 microns in thickness. If your contact lens or glasses prescription is -5.00, the laser will remove approximately 60 microns of tissue. This leaves your cornea 490 microns thick after LASIK.

4-18. How thin can my cornea become without destabilizing it?

The rule of thumb is to leave the “posterior bed” at least 250 microns in thickness. Remember, with LASIK, laser treatment does not begin at the surface of the cornea. Rather, it begins beneath the flap, in the corneal bed. Most corneal flaps are 160 to 180 microns. Therefore, corneal thinning, in effect, begins a third of the way through the cornea.

4-19. How much correction can be done in a normal cornea?

A good generalization about the upper limit of correction for LASIK is -10.00 diopters of nearsightedness. Ten diopters of treatment means that 120 microns of tissue would be removed. If the cornea begins with a thickness of 550 microns, a 160-micron flap is made, and 60 microns of tissue is removed by the laser, a corneal bed 270 microns in thickness will remain. This is considered an adequate posterior bed, because it is thicker than the 250 micron “requirement.”

4-20. What if my correction is higher than -10.00 diopters, say -11.00 or -12.00 diopters?

It may still be possible to perform LASIK for corrections higher than -10.00 diopters. If the cornea is thick enough, more treatment can be performed while maintaining a corneal bed of at least 250 microns. For example, if the corneal thickness is 574 microns, instead of 550 microns, an additional 2 diopters of nearsightedness can be treated (24 12 = 2). In other words, a cornea which is 574 microns in thickness allows treatment of nearsightedness up to at least 12 diopters. Another possibility is to make a thinner corneal flap. Some instruments allow the surgeon to create flaps of any thickness. If a flap of 136 microns is made, instead of 160 microns, an additional 2 diopters of treatment can be achieved while maintaining the same thickness of the posterior corneal bed.

4-21. If I am farsighted, and my correction is above +5.00 diopters, can LASIK still be performed?

Theoretically, LASIK can correct farsightedness above +5.00 diopters. But, most lasers are not approved for these higher, farsighted corrections, and it is believed that +5.00 diopters is roughly the upper limit for steepening a cornea. Other alternatives exist for correcting higher levels of farsightedness, such as intraocular lenses.

4-22. Can PRK be used to correct higher levels of nearsightedness?

Yes. PRK works well, and most lasers are approved for corrections up to -12.00 diopters. Because treatment begins at the surface, it is easier to maintain a 250-micron corneal bed with PRK than with LASIK. For example, a -12.00 diopter treatment in a 550-micron cornea, would leave a posterior bed of 406 microns (550 - 144 = 406). Thus, another solution to treating large amounts of nearsightedness in average or thin corneas is to have PRK instead of LASIK.

4-23. Can PRK be used to treat farsightedness above +5.00?

It is theoretically possible to treat farsightedness above +5.00 diopters with PRK, but optical considerations would suggest that the quality of vision may not be good enough. Again, intraocular lenses might be a better choice.

4-24. Is the measurement of eye pressure affected by LASIK?

Yes. After LASIK, eye pressure readings are usually a few millimeters lower than before LASIK. This seems to be because LASIK reduces the thickness of the cornea, and thin corneas have lower readings than thicker corneas. Glaucoma patients typically have their eye pressure checked every three or four months. It is important for the eye doctor to know if a patient has had LASIK and to take into account the effect of LASIK on the pressure of the eye.

4-25 What is Contrast Acuity Assessment, or CAA?

The CAA is an extensive vision test developed by the United Kingdom Civil Aviation Authority. The test assesses visual performance across a range of tasks with varying contrast and lighting conditions that reflect the situation in the real world, rather than in the doctor’s office. The CAA has been used to compare wavefront-guided LASIK with conventional LASIK. Twenty people were tested with the CAA at Moorefield’s Eye Hospital in London, and very little difference was found between eyes corrected by wavefront and eyes corrected by conventional LASIK. This does not mean that wavefront LASIK was better or worse than conventional LASIK. It simply means, that in this rather small study, no great difference was found between wavefront and conventional LASIK.

4-26. What diseases affect pupil size?

Diseases which affect pupil size include neurologic conditions, such as stroke, brain tumors, and head injuries. Eye conditions, like uveitis (inflammation of the iris), and Adie’s pupil (a sluggish pupil of unknown cause), can produce pupils of unequal size.

4-27. Are the pupils dilated?

Eyedrops are instilled to dilate, or enlarge, the pupils. This allows the doctor to see the back of the eye, and to perform a “cycloplegic” refraction. A cycloplegic refraction temporarily removes the influence of certain eye muscles that could alter the measurements of the eye.

4-28. Is the size of the pupil measured?

The size of the pupil is measured with a pupillometer, a device which measures the diameter of the pupil in dim light. The doctor will want the diameter of the laser treatment to be larger than the diameter of the pupil in dim light.