Corneal Topography
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Corneal topography is also known as keratometry (computerized/digital or manual). During this painless and non-invasive examination, a colored map of the eye’s local corneal curvatures is created, enabling the ophthalmologist to diagnose and assess any abnormalities in the cornea’s structure and condition.
What Does the Examination Involve?
Corneal topography is also known as keratometry (computerized/digital or manual). During this painless and non-invasive examination, a colored map of the eye’s local corneal curvatures is created, enabling the ophthalmologist to diagnose and assess any abnormalities in the cornea’s structure and condition.
How Is the Corneal Shape Examined?
A keratoscope (Placido disk), which is a disk with black and white circles, allows observation of the shape of the circle reflections on the cornea. Manual keratoscopes are increasingly less common, as ophthalmologists and optometrists now have computerized/digital videokeratographs at their disposal. This examination uses an image of the Placido disk projected onto the corneal surface, which is then recorded by a camera, sent to a computer, and analyzed. The results of the examination include a corneal map, a cross-section of the corneal surface, and a map of the digital values of corneal curvatures.
The Gold Standard in Astigmatism Measurement
Specialists still use the Javal ophthalmometer (keratometer), a manual keratometer, to measure corneal curvatures in two main meridians. These measurements allow for precise determination of the magnitude and axes of astigmatism. Manual keratometry is the gold standard for assessing the power and axis of astigmatism in toric intraocular lens calculations.
Corneal Examination Before Lens Selection and Surgery
Corneal topography is one of the most important examinations when planning the selection of artificial implantable lenses used in the treatment of myopia, hyperopia, astigmatism and cataracts. It is a particularly important tool for qualifying patients for the appropriate type of lens. It can also diagnose keratoconus, assessing its stage and potential progression. To evaluate the patient’s eye condition, results from individual corneal topography, for example, from a year ago and the current examination, are compared. Depending on the changes that have occurred during this time, actions are taken to halt changes in the cornea. Corneal topography also allows for the diagnosis of postoperative or post-traumatic corneal deformities.
Corneal topography, as mentioned, is most often performed using a digital/computerized videokeratograph. The patient rests their chin and forehead on a support in front of a dome illuminated by red light. The dome is covered with concentric black rings that reflect on the cornea during the examination. During the examination, the patient must look steadily, without blinking, at a centrally placed light point. The reflections of the circles/rings (described above) on the cornea, their width, and distortions are recorded and analyzed by computer, then converted into corneal curvature radii. The resulting corneal map can be two- or three-dimensional.
Does a Thin Cornea Exclude Surgery?
There is a belief that this examination is decisive for a patient’s eligibility for surgery. Patients fear that a cornea that is too thin is equivalent to disqualification. However, a cornea that is too thin may, but does not necessarily, be a contraindication to the procedure. Corneal thickness is measured during an examination called pachymetry (optical non-contact or contact using an ultrasound probe).
Which Examinations Determine Surgery?
If the cornea is too thin, but other (numerous!) parameters of the eye to be laser-operated, checked by the specialist, are normal, the laser can be programmed to perform such an operation (depending on the degree of the refractive error). We distinguish between so-called Scheimpflug or OCT topographies, which also measure corneal thickness and other anatomical parameters of the anterior segment of the eye, and so-called Placido topographies, which only measure the regularity of the anterior part of the cornea (without pachymetry).
Optical Coherence Tomography (OCT) of the Eye
Corneal tomography also allows for determining the depth of the anterior chamber, the iridocorneal angle, and the size of any shape changes, scars, or inhomogeneities.
During computerized keratometry, the doctor can observe:
- The eye’s surface: This provides additional information about each corneal curvature and its characteristics. Based on this, it is assessed whether the planned correction is possible and will achieve the patient’s desired results. Anomalies of the anterior and posterior cornea are a warning sign, indicating that problems may arise in the future. Based on their analysis, the doctor decides whether to undertake laser vision correction or not. If so, which technique to use. And if not, whether phakic lens implantation or lens exchange is possible. They may also decide to completely forgo surgical intervention.
- The posterior corneal surface: This should be healthy for a decision on surgery to be made.
- Corneal thickness: This determines whether all methods are possible – i.e., surface methods, LASIK, and small incision lenticule extraction (SMILE).
- Anterior chamber depth and corneal diameter: This is important for phakic lens implantation.
More Precise Corneal Examination – Why Is It Important?
Specialists believe that a routine slit-lamp examination may cause an ophthalmologist to overlook eye diseases such as epithelial basement membrane dystrophy (EBMD). If the disease goes unnoticed, it can lead to postoperative refractive problems.
Computerized keratometry is a non-invasive and painless examination (the patient sits comfortably, resting their forehead and chin on a special support of the examination device), and the accuracy of the measurement depends on the immobilization of the patient’s eyeball – hence the need not only to stabilize their head but also for them to look centrally ahead at the light point visible in the device.
Indications for corneal topography:
- planned laser vision correction
- all procedures involving the cornea
- assessment of the cornea’s condition after these procedures, as well as after transplants and mechanical injuries
- selection of rigid contact lenses, correcting not only atypical astigmatism but also advanced myopia or hyperopia
- the need to determine the power of an artificial intraocular lens, which is implanted into the eye during cataract surgery.
The most important thing is to choose the right method for your vision defect. During the initial consultation, we will propose an appropriate laser correction technique.
