Corneal Topography: An examination to map the cornea using the Pentacam

The cornea plays a crucial role in the eye. First, it protects the interior of the eyeball from mechanical irritation. It must always remain highly transparent to allow light to pass through unimpeded. Furthermore, the curvature of the cornea refracts light before the rays hit the lens of the eye.

The cornea protects the iris and the sensitive interior of the eye @ Firn /AdobeStock

Diagnosis using corneal topography is used to detect and monitor the progression of pathological changes in the cornea.

Doctors should always perform this test before surgeries that alter the eye’s total refractive power (refractive surgery).

After refractive surgery, corneal topography can be used for follow-up monitoring. It provides precise information about the anatomical structure of the cornea, which is necessary for fitting contact lenses.

Corneal topography should be used in the following cases:

• Astigmatism

Corneal topography is suitable for determining the severity of astigmatism. Astigmatism causes a refractive error in affected individuals, which in turn impairs vision.

Depending on the type of curvature, the error can have different consequences:

• A steeper curvature of the corneal surface (smaller corneal radius) results in a stronger refractive power.
• Conversely, a flatter curvature (larger corneal radius) results in a lower refractive power.

Especially in cases of severe astigmatism, it is important to distinguish it from the pronounced and progressive form (keratoconus).

Eyelid disorders, such as eyelid malpositions or eyelid tumors, can also lead to changes in corneal topography. Therefore, doctors should rely on corneal topography when treating eyelid disorders and prior to eyelid surgery.

• Corneal Injury

A corneal topography examination is also recommended in cases of corneal injuries or infections. These conditions can result in scarring, which leads to blurred vision and increased sensitivity to glare. Corneal topography allows deviations in the cornea to be measured with micrometer precision.

• Preparation for Intraocular Lens Implantation

Cataracts cause the lens to become cloudy and can severely impair vision, potentially leading to blindness. An effective treatment for this condition is the implantation of an intraocular lens.

Before implanting an intraocular lens, corneal topography is a necessary examination. It allows the power of the lens to be determined precisely. Corneal topography clarifies whether the patient is a suitable candidate for intraocular lens implantation.

• Keratoconus

Keratoconus is an excessive curvature of the cornea, which is also characterized by a conical deformation.

This congenital condition often manifests in young adulthood. Those affected experience a decline in visual acuity. There are recurring noticeable changes in the prescription values for glasses and contact lenses.

Corneal topography reveals the exact gradation of the corneal curvature. In its early stages, keratoconus can be diagnosed exclusively through this procedure.

A corneal transplant allows for early diagnosis and treatment. Initially, conservative visual aids are available for treatment; however, if the cornea becomes severely thinned, a corneal transplant may be considered.

• Vision Loss

If the cause of vision deterioration is unclear, corneal topography can aid in diagnosis. It can rule out or confirm whether the condition of the cornea is causing the visual impairment.

• Contact Lenses

Based on corneal topography, contact lenses can be precisely fitted. The individual characteristics of the cornea must also be taken into account when selecting suitable lenses. This applies even to complex corneal shapes and challenging corneal surfaces.

Corneal topography is a non-contact procedure and does not cause any additional discomfort for the patient. It takes only a few minutes.

The patient rests their head on a chin-to-forehead rest so that the ophthalmologist can take measurements of the eye using a keratograph. During the corneal topography, the patient focuses on a bright dot on the device.

In the traditional method, ring-shaped patterns are projected onto the cornea. The reflection of these rings is recorded and electronically analyzed. This reveals whether the surface is uniform or irregular. The surface is scanned with pinpoint accuracy.

The keratograph is an instrument for recording and analyzing corneal topography @ coldwaterman /AdobeStock

Placido-based and Scheimpflug-based techniques are available for corneal topography of the eye.

• Placido-based methods

The examination involves computer-based scanning of the cornea, which is performed by projecting a Placido disk onto the patient’s corneal surface. This projection consists of alternating black and light rings. The keratograph’s video camera captures an image of the reflected corneal surface.

The images are then analyzed by a computer. After the projection, the doctor analyzes the distances between the rings on the corneal surface. Asymmetrical patterns indicate pathological changes in the cornea.

Placido disk @ Anna /AdobeStock

• Scheimpflug-based methods

In this corneal topography procedure, the ophthalmologist uses a camera to take images from various viewing angles. The doctor determines the shape of the anterior and posterior surfaces of the cornea. This creates a map of corneal thickness. The overview shows the thinnest point of the cornea and the curvature along the steepest axis.

The computer-controlled system analyzes the data using various methods. To accurately capture and map the corneal surface, the system measures various parameters and values:

• Curvature

In a healthy eye, uniform curvature of the cornea ensures sharp vision. Corneal topography measures this curvature as a radius of curvature in mm or as a curvature equivalent in diopters.

• Elevation data

Elevation data consists of a three-dimensional model of the cornea. Here, too, computer-controlled systems are used for the mathematical calculations.

The elevation data are recorded as z-coordinates (x, y). These values provide precise information about the structure of the cornea.

• Representation of Refractive Power

The refractive representation enables an assessment of the corneal surface. It is based on focal lengths, which can be calculated using Snell’s law (tracing light rays). The unit for focal lengths is diopters.

• Fourier Analysis

Based on the refractive power data determined by Fourier analysis, the original size of the cornea can be calculated. The method was developed by the French physicist Jean-Baptiste Fourier (1768–1830).

The computer-controlled procedure calculates individual components from the image. 

The following data enable a precise analysis of the cornea:

• Decentration
• Irregularities
• Spherical components

These components form the basis for determining the Fourier indices. These values allow the ophthalmologist to precisely characterize the corneal surface.

• Zernike analysis

Zernike analysis is used to measure wavefront aberration.

Wavefront aberration is a subtle irregularity in the contour of the eye. The scientific model represents light optically as waves.

These wavefronts can typically be described using Zernike polynomials. The Dutch physicist Frits Zernike (1888–1966) developed a mathematical model for this purpose that determines the deviation between the actual and ideal wavefronts. 

Each individual polynomial is named after the aberration it addresses. 

Among the most important Zernike polynomials are:

• Quadrature (four-leaf aberration)
• Astigmatism
• coma

The computer-controlled corneal topography system performs Zernike analysis based on elevation data.

• Corneal indices

Corneal indices provide a meaningful summary of the data from corneal topography. These parameters allow conclusions to be drawn about the shape of the corneal surface. 

In addition, the data allow for classification.

The corneal indices are based on the following values: 

• Elevation data
• Curvature data
• Data from Fournier analysis and Zernike analysis

These indices enable the ophthalmologist to detect even minor abnormalities in the eye.

Computer-controlled corneal topography systems combine corneal measurement with the display of results. Typically, ophthalmologists can choose from different views:

• Numerical display

The computer-controlled system processes the measured data and displays it in numerical form. The doctor can then compare the values with previous findings and contrast them with standard values.

• 3-D visualization

Computer-assisted corneal topography systems provide a clear, three-dimensional representation of the cornea. This type of visualization makes it possible to visualize the curvature profile of the corneal surface. 

Using graphics, the ophthalmologist can show the patient the condition of the corneal surface. Various buttons highlight the radii of curvature.

• Color-Coded Map of Corneal Topography

In this overview, the radii of curvature are assigned different colors.

The results of the corneal topography are presented to the ophthalmologist in the following formats:

• Visual representations
• 3D images 
• List of relevant data

The options available depend on the computer-controlled system used. In most cases, the doctor can quickly review the individual values. The software often assists in interpreting the measured values.

For the following conditions, the doctor must monitor the patient’s changes and evaluate them on a case-by-case basis:

• Astigmatism

In cases of astigmatism, two focal planes are typically measured: 

• One with maximum refractive power
• One with minimum refractive power

The difference between these two principal meridians is the degree of astigmatism. Experts express this either as the difference in corneal radii or as the difference in refractive power. 

The axis is the angle between the plane of incidence and the horizontal plane. When prescribing corrective lenses, eye care professionals specify the refractive power of astigmatism in diopters and the type of correction in cylinders. They indicate the position of the astigmatism using an axis.

• Keratoconus

In cases of keratoconus, the ophthalmologist typically determines the distribution of the central radii of curvature. The patient’s cornea is characterized by a tilt. As keratoconus progresses, astigmatism generally increases.

• Preparation for Intraocular Lens Implantation

Based on the results of corneal topography, the ophthalmologist determines whether the patient meets the criteria for an intraocular lens implantation. Excessive corneal curvature or corneal dystrophy can be contraindications for such a procedure.

Artificial eye lenses @ Mohammed /AdobeStock

• Vision Deterioration

The results of corneal topography provide information on whether there is an irregularity in the cornea and, consequently, a resulting deterioration in vision. Injuries to the surface of the cornea can also be detected.

• Contact lenses

The results of corneal topography provide valuable guidance for fitting all types of contact lenses.

The patient’s individual measurements facilitate the selection of the appropriate lenses. The use of astigmatic or rigid gas-permeable contact lenses is possible thanks to the precise measurement results. Corneal topography also allows for monitoring the patient’s tolerance of contact lenses.

Corneal analysis often guides further treatment. It helps determine whether conservative measures are sufficient or whether surgery is advisable. Precise analysis is particularly indispensable before eye surgery because it improves safety and predictability. These high-resolution images also provide ophthalmologists at the eye center with detailed insights into the current condition during postoperative follow-up examinations.

This method is also extremely useful when fitting optical aids. It provides specialists with the basis for selecting contact lenses that fit as precisely as possible and ensures that the prescription remains stable over the long term. As a result, many patients can achieve optimal visual quality.