Introduction
Corneal topography is the diagnostic test used to obtain the topographic map of the cornea .
More in detail, the execution of the corneal topography allows the study of the shape and characteristics of the cornea, thus allowing the identification of any defects or pathologies affecting this region of the eye.
Brief Review: the Cornea and its Functions
The cornea is the membrane that covers the front of the eye . It represents the first "lens" encountered by light in the path to the brain, through which it is possible to see the iris and the pupil.
The cornea represents an essential element of the ocular dioptric system, both because it allows the passage of light from the outside towards the internal structures of the eye, and because it contributes to the focusing of images on the retina. In addition to this, the cornea also provides protection and support for the eye structures.
What is that
What is Corneal Topography?
Corneal topography is a non-invasive diagnostic test that allows to study the shape and characteristics of the cornea through the generation of the so-called topographic map of the cornea .
Did you know that ...
Sometimes, in common language, the name "corneal topography" is used - albeit improperly - also to indicate the topographic map of the cornea resulting from the diagnostic examination in question.
Corneal topography is a painless exam that, unlike with other types of eye exams, does not require the use of any type of eye drops. It is not a routine exam, but is carried out only in the presence of particular disorders and pathologies. However, corneal topography is not used only for diagnostic purposes, in fact, as will be seen in the next chapter, it is also used in the surgical field and in medical contactology.
Utility
What is Corneal Topography for?
As mentioned, corneal topography is used to study the characteristics of the cornea in detail. More precisely, thanks to the corneal topography it is possible to obtain an accurate map that allows to determine with extreme precision the shape of the cornea and the curvature of the anterior corneal surface at each point .
A healthy cornea generally presents with a regular and symmetrical shape. On the contrary, in the presence of disorders, pathologies or injuries, the curvature and the shape become irregular.
On what occasions is Corneal Topography performed?
Since the corneal topography and the consequent topographic map of the cornea provide useful information about the curvature of the cornea, the execution of the diagnostic examination object of the article is useful in many situations, among which we remember:
- Diagnosis of diseases and disorders affecting the cornea, such as:
- Keratoconus (corneal topography represents the diagnostic means par excellence of this pathology);
- Corneal edema;
- Other corneal dystrophies or degeneration;
- Dry eye syndromes with corneal surface irregularity.
- Preoperative and postoperative evaluation of surgical interventions for the correction of visual defects with excimer laser ( refractive surgery ). Knowing the corneal curvature before surgery is essential to understand where and how to intervene; at the same time, knowing it later is important to assess the success of the intervention.
- Preoperative and intraoperative evaluation in case of corneal transplantation (for this purpose, corneal topographers are used during surgery).
- Controls following the cross-linking treatment in keratoconus .
- Calculation of particular intraocular lenses, including those for the correction of astigmatism (toric IOLs).
- In contactology, to identify - therefore, build - the ideal contact lenses for the eyes of each patient, be it affected by myopia, hyperopia or astigmatism.
Instrumentation
With which instrument is Corneal Topography performed?
The corneal topography examination is carried out through a specific instrument: the corneal topographer .
It is equipped with a projector of a luminous image consisting of a series of concentric circles ( Placido disk ) and of a component that detects and acquires the reflection on the cornea of the luminous circles projected by taking photographs. The images thus collected are then sent to the computer equipped with special processing software . After that, the latter will generate the topographic map of the cornea reproducing it on the instrument's monitor.
The more the corneal surface is irregular (due to pathologies, disorders, traumas, surgical interventions, etc.), the greater the reflection of the luminous circles projected by the topographer will be distorted.
Did you know that ...
Not all corneal topographers are the same. In fact, there are numerous companies that produce these instruments. Nevertheless, the effectiveness in the execution of the exam is the same, although there may be more suitable tools for the study of a particular problem than another.
Modern topographers - in addition to determining the curvature of the cornea - are even able to determine their refractive power . Some of them even, using different algorithms, are able to process more than one type of topographic map (axial, tangential, altitudinal, etc.). Each type is useful for analyzing certain characteristics, or highlighting certain corneal anomalies. The choice to analyze one type of map instead of another will be taken by the doctor depending on the reason why the corneal topography was performed.
How you do it
Performing the Corneal Topography exam
Of course, corneal topography must be performed by the ophthalmologist.
The patient is seated in a sitting position and asked to rest his chin and forehead against the special supports placed in front of the instrument.
After that, the doctor activates the surveyor asking the patient to remain still and to focus his gaze on the central point of the same. In this way, the luminous circles are projected onto the cornea of the individual and their reflection is detected and captured by the dedicated component of the instrument that takes several photos and sends it all to the processing software.
Corneal Topography and Contact Lenses
To obtain quality images it is necessary to suspend the use of contact lenses by the patient before performing corneal topography. In fact, to ensure the generation of a topographical map of the true cornea, it is necessary to avoid any type of corneal deformation that could be induced precisely by the continuous use of contact lenses.
More in detail, according to some sources the lenses must be disposed of 24 hours before the examination if they are soft and 48 hours before if they are rigid; according to other sources, however, the use of any type of contact lens should be suspended between two and three days before the corneal topography is performed. In any case, the doctor will provide the correct information in this regard.
Map reading
How to read the Topographic Map of the Cornea?
The topographic map of the cornea resulting from the examination of corneal topography consists of a chromatic map within which each color represents a degree of curvature .
The processing and transformation of the numerical values obtained by the instrument into colors takes place following sophisticated mathematical calculations on the basis of specific topographic scales.
However, without going into complicated technical details and simplifying everything, the colors shown on the topographic map of the cornea have the following meanings:
- The cold colors (blue, blue and shades) indicate the flattest areas of the corneal surface, therefore with a minor curvature ;
- The green color and its nuances, instead, indicate intermediate curvature values ;
- The warm colors (red, orange and yellow) indicate, instead, the corneal areas having a greater curvature .
The curvature values of the cornea can be expressed in the radius of curvature (millimeters), or in diopters (the most frequent option).
Clearly, the expert eye of the doctor will be able to accurately and accurately read the map in question, identifying precisely:
- The presence of any anomalies and the pathologies and disorders related to them;
- The results obtained from any treatments, surgical interventions or transplants;
- The details necessary for the construction of contact lenses or intraocular lenses most suitable for the patient.
Types of Topographic Map of the Cornea
As mentioned, the most advanced corneal topographers are able to process different types of topographic maps of the cornea that can provide different information useful in different situations. Among the various types of topographic map of the cornea that can be elaborated, we recall some of them:
- Axial : the axial map - also known as the sagittal map - measures the curvature of a point on the surface of the cornea in the axial direction with respect to the center.
- Southern : the southern map - also known as local or instantaneous map - allows the measurement of the curvature of a given point of the cornea in the southern direction with respect to the other points.
- Bypass : the tangential map allows a more sensitive measurement of small curvature changes.
- Altitudinal: compared to other types of maps, the altitudinal map provides information on the real shape of the corneal surface.
The most commonly used maps in the field of corneal topography are the axial and the altitudinal ones.
However, it is necessary to specify that, depending on the type of map taken into consideration, the reading of the colors may not be the same as above. Clearly, the doctor has all the knowledge and tools necessary to perform the correct reading of any type of topographic map of the cornea.
Limitations
Limits of Corneal Topography
Although corneal topography is considered a very useful exam, it still has some limitations. In fact, to ensure high quality images are obtained, the corneal epithelium must be intact and a good tear film must be present. On the contrary, the presence of lesions or scars could compromise the success of the corneal topography leading to the generation of a topographic map of the poor quality cornea.
