What is the preoperative examination?
A rigorous preoperative examination and a correct provision of information to the patient are the first steps of an ocular surgery. Depending on the results obtained in this evaluation, the patient is informed about which procedure is more appropriate and about its advantages and disadvantages.
It is important that the patient understands the information provided by the ophthalmologist and asks any questions that may appear so that unachievable postoperative expectations are not created. It must be understood that the purpose of refractive surgery is to avoid the necessity of optical correction (glasses or contact lenses), but there is a possibility that in certain cases, some optical correction is needed for specific activities.
The ophthalmological tests conducted on every patient during the refractive surgery preoperative study are the following, making a distinction between the basic ophthalmological examination and complementary tests.
Basic ophthalmological examination
This type of examination is done to every patient as a general procedure in an ophthalmological assessment and includes a set of basic tests which evaluate the health of the patient’s eye and feasibility of performing refractive surgery. After this, and taking into account the results obtained, other more specific tests are done in order to decide the most appropriate surgical procedure for each case (LASIK, PRK, IOL, etc.).
Basic ophthalmological examination tests are:
- Visual acuity (VA) - Measurement of the visual system’s capacity to perceive and identify objects under well illumination conditions. The most frequent way to measure this is by using Snellen’s test, which consists of a chart with a series of block letters printed in each line that progressively get smaller. The more lines the patient can clearly see, the more visual acuity the patient has.
- VA without correction (VAw/c): It evaluates the eye’s visual capacity without using glasses or contact lens.
- VA with correction (VAwc): It evaluates the eye’s visual capacity once the refraction error is corrected with glasses or contact lens.
- Refraction - It measures the eye’s refraction error in diopters. This measurement will aid surgical planning. There are two ways of measuring it.
- Objective refraction: It is done using retinoscopy or refractometer which gives the exact refraction error. This may not be the refractive power the patient actually needs in order to see clearly, so it is just used as a basis for calculating the subjective refraction.
- Subjective refraction: Based on the results of objective refraction, a series of lens are placed in front of the patient’s eyes in order to evaluate the refractive power which allows the patient to see best. It is called subjective as the patient’s perception is involved in the process.
- Biomicroscopy - It allows anterior segment examination (eyelid, sclera, conjunctiva, cornea, iris, crystalline lens and anterior chamber) using a binocular microscope called slit lamp, so that any disorders which may be a contraindication to surgery can be detected (corneal lesions, dry eye syndrome, etc.). Yellow-coloured eye drops (fluorescein) may be used in order to better analyse the cornea and the tear film.
- Tonometry - It measures intraocular pressure (IOP) and if it is too high it may indicate the existence of glaucoma, which is a contraindication to refractive surgery. The most frequent ways to measure IOP are:
- Contact tonometry: It is the most exact method and it measures intraocular pressure by determining the pressure needed to flatten a certain area of the cornea. It is carried out using anaesthetic eye drops so that the patient does not find it uncomfortable.
- Non-contact or air-puff tonometry: It uses a rapid air pulse to flatten the cornea. The pressure of air needed to achieve this effect will determine intraocular pressure.
- Ophthalmoscopy - It allows posterior segment examination (retina, macula, optic disc and blood vessels). It is necessary to put eye drops to dilate the pupil and then shine a beam of light with an ophthalmoscope through the pupil in order to examine the posterior segment of the eye.
- Perimetry - Visual field examination and detection of possible defects (scotoma or area of loss of visual acuity). A reduced visual field points out the existence of ocular disorders such as glaucoma or retinopathy. It is possible to study this using various methods and the most commonly used one is computerized perimetry.
- Confrontation visual field exam: It does not require specific medical devices and it only lets us know if there is or not a defect in the patient’s visual field. The patient looks at a fixed point whilst the examiner moves an object and the patient has to signal if the object is in or out of his/her visual field.
- Goldman perimeter: It is carried out using a campimeter which emits light pulses that move across the patient’s visual field. The patient has to signal when he/she sees these pulses and when not.
- Computerized or automated perimetry: In this case the campimeter emits intermittent beams of light of different intensities in different areas of the patient’s visual field. The patient has to point out when and where does he/she see these beams of light and according with the results, a vision map is created in order to identify areas of vision loss.
Complementary tests for refractive surgery
Once basic ophthalmological examination has been carried out, a more detailed examination of the anterior segment is done in order to decide the most adequate surgical procedure.
- Corneal topography - It examines in detail the corneal surface to rule out underlying deformities which may be a contraindication to surgery, such as keratoconus at early stages which is not visible if just using slit lamp examination. In addition, this test is necessary as some refractive surgery techniques using Excimer laser (LASIK and PRK) flatten the cornea. This is not relevant if surgery involves intraocular lens as the cornea is not affected in this procedure.
- Keratometry - It determines the corneal curvature or dioptric power at various points. Excessively flattened or curved corneas will add difficulty to LASIK procedure, so in these cases, other techniques should be recommended.
- Pachymetry - It measures the corneal thickness at various points. As laser procedures reduce corneal thickness, there must be a minimum corneal tissue thickness in order to undergo these procedures. If not, LASIK technique should not be recommended.
- Pupillometry - It measures pupil dimensions under low illumination conditions in order to determine the optic area susceptible to treatment. Patients with moderate to severe myopia and significantly important pupil area, who only receive treatment in a small optic area will experience visual disruptions at night such as halos or glare. This is why, regardless of the procedure used, the optic area to be treated will be evaluated according to pupil diameter.
- Biometry - It allows to measure specific parameters of the eyeball such as the length of anterior and posterior axis. These are relevant when using intraocular lens procedure in order to determine the refractive power of the lens that is going to be used.
- Schirmer test - It evaluates if enough tears are being produced to keep the eye moist, as dry eye symptoms are a temporary side effect of laser procedures. According to the results of this test, the ophthalmologist can correctly adjust the need of lubricant eye drops in the postoperative treatment.
- Specular biomicroscopy - It allows a corneal endothelial cell count. A low corneal endothelial cell density is a contraindication to the use of anterior segment intraocular lens.
- Cycloplegic refraction - In order to accurately determine the refractive error, it is necessary to measure it again with the eye’s accommodative capacity (focusing ability) paralysed using eye drops.