Corneal topography, also called photokeratoscopy or videokeratography, is an ophthalmic medical imaging technique. It allows us to identify any visual defect in order to propose to the patient the appropriate medical or surgical treatment. All the curves of the cornea are analyzed: but how is the cornea made? Why should this examination be performed? How is it carried out? Discover all the answers to these questions, and more, in this article by the COP9 team!
Corneal topography: understanding the cornea
Together with the crystalline lens, the cornea constitutes the optical system of the eye. This part of the eyeball is responsible for allowing light to converge at the back of the eye, on the retina.
The cornea is located at the front of the eyeball. It is a fibrous and transparent membrane. It is the most powerful part of the eye, with an average of 42 diopters.
A membrane that is both transparent and fibrous, the cornea is divided into three parts, which are called cell layers:
- the epithelium, on the corneal surface ;
- the stroma is the intermediate part ;
- the endothelium is the anterior segment.
Searching for corneal deformity, or any pathology of the cornea such as keratitis, requires this imaging performed by your ophthalmologist in their surgery: the corneal topography.
What is corneal topography ?
The corneal topographer is a device that allows you to analyze the three corneal surfaces that we presented in the previous section.
This ophthalmological equipment projects the Placido disc onto the cornea. It is a disc with concentric circles. Your ophthalmologist or orthoptist will then photograph the image reflected on your cornea. Based on the results obtained, any deformations of the cornea are analyzed and diagnosed.
Corneal topography is one of the oldest ophthalmological analyses: it was invented by the ophthalmologist Antonio Placido in 1880! Nowadays, it is performed with a machine in the form of a hemispherical dome located in front. And this examination continues to evolve: the corneal topographers are equipped with a rotating camera of the Scheimpflug type allowing to collect images in cross-section of the cornea thanks to a system of luminous slit scanning.
Why perform corneal topography?
Corneal topography can be requested by your ophthalmologist and performed by themselves or one of the members of their orthoptist team.
Here is an exhaustive list of the pathologies and reasons for which you may be asked to perform this ophthalmological examination:
- monitor the evolution of corneal astigmatism, differentiate between benign congenital astigmatism and progressive corneal disease (such as marginal pellucid degeneration) ;
- diagnose diabetic retinopathy and pigmentary retinopathy;
- monitor the evolution of astigmatism caused by a corneal pathology (corneal abscess, pterygium, trauma, etc.) ;
- monitor a corneal graft (called keratoplasty) ;
- monitor the evolution of a wound healing ;
- a preoperative check-up for the purpose of refractive surgery ;
- a post-operative evaluation after an operation, such as myopia, hyperopia and astigmatism by laser treatment.
Corneal topography is a compulsory examination before any refractive surgery is performed. It is through the cornea that the laser beam passes (whether the equipment is SMILE, LASIK or PRK). It is therefore absolutely necessary that the cornea is analyzed and evaluated: its curvature, its thickness, its regularity…
Any contraindication revealed by the examination can cause serious complications, with permanent alteration of the cornea. Therefore, you should not go against the ophthalmologist surgeon’s result!
How is corneal topography performed?
Corneal topography can be done at any age, child, adolescent, adult or elderly. It is performed in the ophthalmology surgery, by a member of the medical team. The examination is quick, painless and without contact with the eye:
- you are seated in front of the corneal topographer, a hemispherical dome ;
- your head is placed on a chinrest, adjusted to your height ;
- with both eyes, you stare at a luminous point representing concentric light rings ;
- wait a few seconds…and the examination is over.
The result is then analyzed by the ophthalmologist who will inform you of their conclusions.
For contact lens wearers, a precaution should be taken to ensure an optimal diagnosis: remove them a few hours before the exam, ideally the day before, so as not to influence the shape of your cornea.
Apart from this simple point, no eye drops, or other preparations are required for corneal topography.
What are the 2 techniques of Corneal topography?
There are two techniques of corneal topography:
- specular corneal topography (conventional) ;
- elevation topography.
For each technique, a color code is used to analyze the corneal topography of elevation: the high points are in warm colors (red, orange, etc.) and the low points are in cold colors, such as blue.
The specular corneal topography will calculate the curvature of the cornea according to the deformation of the test patterns. The software is created from a neural network. This allows for an automatic diagnosis. The curvature maps are calculated from the digitized image of the corneal reflection of the Placido disk.
Elevation topography is a more elaborate technique. It allows the study of the cornea in relation to a reference sphere. The variations of the curvature are studied thanks to an optical scan. Light slits scan the cornea, reconstructing it in three dimensions. Your specialist will have access to the corneal volume, to the anterior surface (the corneal cap) as well as the posterior chamber.
The COP9 team would like to conclude this article with the necessity to monitor your visual acuity through the presented examination. Any deformation, complication (such as following a corneal graft or implant), ophthalmic pathologies can be detected in this way. Very useful in the context of a future or past surgical intervention, corneal topography is a painless, quick and accessible examination that will allow you to take care of your eyes!