Introduction
Corneal topography is used to study the shape of the corneal surface. Corneal topography is a unique photography technique for mapping the eye's cornea. The cornea is present in the front window of the eye. It provides a three-dimensional view of the cornea. A topography scan aids the physician in identifying abnormalities in the cornea's usual smooth curvature. It helps in monitoring eye disease and also helps in planning for surgery. Previously, the technology could only measure the central portion of the cornea. But with advancement in corneal refractive procedures, mapping both anterior and posterior surfaces along with peripheral parts of the cornea is possible. In addition, several new devices allow the clinician to understand the shape of the corneal shape, the power of the cornea, and the optical performance of the eye.
What Is the Structure of the Cornea?
The cornea is the transparent outer covering of the tissue of the eye. The shape of cornea is dome-shaped. The cornea contributes two-thirds to the refracting power of the eye. The cornea is made up of specialized cells. As a result, the cornea lacks blood vessels to nourish it. Instead, the cornea gets its nourishment directly from the tears on the eye's surface and the aqueous humor (a fluid present in the back chamber of the eye).
What Is the Principle Behind Corneal Topography?
Corneal topography monitors and measures changes in the shape and integrity of the eye's cornea. A corneal topographer is used to project illuminated rings on the cornea's surface. This illuminated projected ring is called a Placido disc. The rings are reflected by the eye's cornea and captured into the instrument. These reflected rings are analyzed on the device, and a computer generates a topographical map of the cornea on the screen. These images are helpful for an eye doctor to examine the health of the eyes.
What Are the Uses of Corneal Topography?
Corneal topography is used for the early diagnosis of various corneal conditions and for monitoring the progress of the disease over time. It is used in the following:
1. Corneal Ectasia: It helps diagnose corneal ectasias like keratoconus (bulging out of the cornea) and pellucid marginal degeneration (a disease in which thinning of the cornea occurs) and monitors their progress with time.
2. Corneal Diseases: It helps in monitoring the corneal changes in several conditions like Terrien's marginal degeneration (peripheral thinning of the cornea), Salzmann's nodular degeneration (formation of the white-gray nodule on the cornea), and pterygium (wedge shape growth on the cornea and conjunctiva of the eye).
3. Contact Lens Fitting: Corneal tomography helps detect the cornea's curvature. So it allows an operator to choose a contact lens of appropriate size based on the cornea's curvature. Then, based on the pattern, the shape of the lens can be altered, significantly helping reduce the chair time during lens fitting.
4. Cataract Surgery: The topographic map helps estimate the cornea's astigmatic profile (asymmetry). It helps in deciding the location of the incision during surgery. Serial topography helps to understand the effect of incision on the cornea during the postoperative period. For suture removal, postoperative topography serves as a guide.
5. Corneal Transplants: Corneal topography during corneal transplant helps in suture adjustment and removal. It also helps in evaluating the outcome of different suture techniques. In addition, it helps in comparing the astigmatic effects of other surgical procedures.
6. Keratorefractive Surgery: Corneal topography gives the cornea shape, allowing the surgeon to assess the presence of any abnormality that can affect the surgical process. It helps determine the pupil size before surgery, the optical zone's size and regularity, and the treatment zone's centration. It also helps in the follow-up of patients to check the patterns of regression of the refractive effect.
7. Contact Lens Use: Corneal topography; besides knowing the fit of contact lenses, corneal topography is also helpful in studying the outcomes of contact lens use. Patients who wear tight lenses are at risk of lens-induced warpage that can be assessed early with the help of corneal topography.
8. IOL (Intraocular Lens) Power Calculations After Corneal Refractive Surgery: Corneal refractive procedures can interfere with the accuracy of IOL power calculations. So nowadays, corneal topography provides several newer methods for IOL power calculation without interfering with the accuracy.
9. Corneal Pachymetry: It is a test to assess the thickness of the cornea. It is available only in devices that can measure the posterior corneal surface. The advantage of pachymetry with the help of corneal topography over conventional pachymetry is that these devices can measure the thickness of a large part of the cornea, not just the central cornea.
10. Corneal Visual Performance: Corneal topography can measure the corneal surface's regularity which can be compromised in tear deficiency states.
11. Other Parameters: Corneal tomography can measure values like anterior chamber depth and angle depth.
12. Corneal Retrieval: Corneal topography helps to assess donor corneas before transplant surgery.
13. Topography Guided LASIK: LASIK surgery is done to treat severe surface alterations in the cornea. Corneal topographic maps to guide the LASIK ablation gives successful results.
14. Normative Data: Corneal topography helps in generating a normative database. It helps in the classification of conditions like posterior keratoconus. Normal corneas have mirror-image symmetry, which is seen in corneal topography. It can be altered in the disease stated.
What to Expect During a Corneal Topography Scan?
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The patient is seated in a position facing a large bowl along with lighted circles inside it. The forehead and chin of the patient are at rest. Keep the head secure to get the most precise images.
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The patient is asked to stare in the bowl at a fixed target. The pictures are taken.
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The scan takes a few seconds, but the procedure can be repeated a few times.
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The procedure of corneal topography is painless and non-invasive. Nothing touches the eye or person during the scan.
Conclusion
Corneal topography is used to study the shape of the corneal surface. Corneal topography is a unique photography technique for mapping the eye's cornea. Corneal topography is used to monitor and measure changes in the shape and integrity of the eye's cornea. Additionally, corneal topography is employed in the early detection of several corneal diseases as well as the long-term tracking of disease progression. But it is important to remember that despite accuracy or increasing sophistication, a single tool cannot be used for making an accurate diagnosis. Before proceeding with corneal topography, a thorough clinical exam is essential for the cornea and the entire eye. The findings of the topographic examination should be integrated with clinical study for the complete diagnosis of the problem so appropriate treatment can be given.
