Corneal Topography

Every layer of the eye has a crucial role in maintaining the proper functioning of the eye. Deformity in any layer results in the alteration of vision. The cornea has a high refractive index that reflects light entering the eye. Any defect in the corneal surface leads to distortion of vision. Corneal topography is the test to discover such defects.

What is Corneal Topography?

Corneal topography is an optical diagnostic technique that maps the curvature of the cornea. The cornea is the transparent outer layer at the front of the eye. The cornea plays a major role in maintaining the power of the eye. Any conditions resulting in abnormal corneal surfaces will impact the visual field of the person. Corneal topography is designed based on either of these principles:

Placido disc:

Placido disc is a series of concentric rings marked on a circular device. Corneal topography uses this principle of Placido disc and casts the images of Placido rings reflected by the cornea. Since the light is reflected according to the corneal pattern, any distortion in the shape or the plane of the cornea will result in distorted rings. The software inbuilt transforms the information to the color-coded three-dimensional map of the corneal surface. The hot range of color points toward the steep region in the cornea or the areas with distortions. The cooler range of color signifies a smooth and uniform surface in the cornea.

Scanning Slit techniques:

This technique creates a map through the reflection of multiple slit beams of light from the nasal to the temporal side of the eye and compares it with the control image, thus detecting distortion in the anterior or the posterior segments of the cornea.

Scheimpflug imaging:

A camera rotates to record images of corneal cross-sections illuminated by slit beams of lights at different angles. Most commonly used, the Pentacam uses this technique and has better accuracy and resolutions of the three-dimensional images.

What Conditions Are Diagnosed with Corneal Topography?

Corneal topography is used most commonly for the following purposes:

Keratoconus: Keratoconus is a condition that affects the structure of the cornea, causing abnormal vision. It is difficult to diagnose with conventional slit-lamp examination. Thus, to detect keratoconus at an early stage, corneal topography is the investigation of choice. In addition, corneal topography can be used to monitor the natural progression in patients diagnosed with keratoconus. It also aids in the decision for treatment strategies like collagen cross-linking intervention and contact lens fitting.
Pre-operative screening: It is used to screen patients undergoing eye surgery to rule out any abnormality in the surface of the cornea and the presence of keratoconus patterns.
Post-operative monitoring: During the post-operative phase, corneal topography aids in detecting dioptric changes at the cornea. It plays a vital role in diagnosing post-surgery astigmatism.
Pterygium: Pterygium is the fleshy overgrowth in the surface of the eye that is caused due to excessive exposure to ultraviolet radiations. Although it is present in the conjunctival layer, a thin membrane covering the cornea, the uneven surface is detected through the images formed by corneal topography.
Corneal scar: Corneal scar may result due to any accidental or non-accidental injuries. Corneal topography is an important tool in detecting scars of any size. It is also used to monitor the healing process.

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What Can You Expect While Undergoing Corneal Topography?

If you are experiencing abnormal vision or are about to undergo eye surgery, it is highly likely you will be examined with a corneal topographer.

1. The patient sits against the device that is usually mounted on the table. The device is connected to the computer where the examiner takes different shots of the cornea and evaluates the mathematically converted topographic images in the computer.

2. The patient is asked to put their chin on the chinrest and forehead on the forehead rest facing the device. The examiner can control the height of the stand to align the device to the target eye. A circular screen is present that radiates the Placido rings, or there may be a rotating camera depending on the device. The examiner may take a few photographic shots which are then converted into three-dimensional topographic images of the patient’s eyes.

3. You may receive the report with images and mathematical indices, which are interpreted by your ophthalmologist. They can give you further suggestions according to the report.

What Are the Pros of Corneal Topography?

The development of corneal topography has enhanced the prognosis of postoperative patients undergoing eye surgery. After the cataract surgery and post keratoplasty, corneal topography plays an important role in detecting astigmatism. Some of the pros of corneal topography over other conventional investigations to detect corneal irregularities are as follows:

Less margin of error than another testing

Hazard-free method of testing

Computer-based data that gives more accurate information

Modern corneal topography can detect the abnormality of the posterior refracting surface of the cornea which is impossible through other investigations.

What Are the Possible Setbacks of Corneal Topography?

The possible setbacks of corneal topography include:

Minor irregularities may be missed.
It cannot be used in corneas with epithelial defects and stromal ulcers.
Patients may feel uncomfortable staring at the Placido rings.
The mathematical indices interpretation requires adequate experience and training.
Any error in alignment and fixation may result in an abnormal reading of the information.
The algorithms used by different brands of topographers are non-standardized and may show inconsistent data on the comparison.

The Takeaway

Corneal topography is a computer-assisted device to diagnose the abnormality in the refractive surface of the cornea. It is of different types based on different principles. It helps to detect conditions like corneal ectasia, keratoconus being the main. It creates three-dimensional topographic images of the eye, mapping different points of the cornea. It is highly accurate, and it is preferred over other investigations to detect corneal irregularities.

It is used as a screening diagnostic as well as prognostic tool for various eye conditions. It requires highly skilled manpower to interpret the result. It contains negligible harmful effects as the impact of the investigation.

References

https://webeye.ophth.uiowa.edu/eyeforum/tutorials/Corneal-Imaging/Index.htm?fbclid=IwAR3W6F331lXTX_bggnXc9ot_C6nvtym8vyxm7-9LRL0m1sTMSuKjDohgcac

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