Corneal topography, also known as photokeratoscopy or videokeratography, is a

non-invasive A medical procedure is defined as ''non-invasive'' when no break in the skin is created and there is no contact with the mucosa, or skin break, or internal body cavity beyond a natural or artificial body orifice. For example, deep palpation and p ...

medical imaging technique for mapping the anterior curvature of the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...

, the outer structure of the eye. Since the cornea is normally responsible for some 70% of the eye's

refractive power In optics, optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the ...

, its topography is of critical importance in determining the quality of

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

and corneal health. The three-dimensional map is therefore a valuable aid to the examining ophthalmologist or optometrist and can assist in the diagnosis and treatment of a number of conditions; in planning

cataract surgery Cataract surgery, also called lens replacement surgery, is the removal of the natural lens of the eye (also called "crystalline lens") that has developed an opacification, which is referred to as a cataract, and its replacement with an intra ...

and

intraocular lens Intraocular lens (IOL) is a lens (optics), lens implanted in the human eye, eye as part of a treatment for cataracts or myopia. If the natural lens is left in the eye, the IOL is known as Phakic intraocular lens, phakic, otherwise it is a pseudop ...

implantation; in planning refractive surgery such as

LASIK LASIK or Lasik (''laser-assisted in situ keratomileusis''), commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and an actual cure for astigmatism, sinc ...

, and evaluating its results; or in assessing the fit of contact lenses. A development of keratoscopy, corneal topography extends the measurement range from the four points a few millimeters apart that is offered by keratometry to a grid of thousands of points covering the entire cornea. The procedure is carried out in seconds and is painless.

Operation

The patient is seated facing the device, which is raised to eye level. One design consists of a bowl containing an illuminated pattern, such as a series of concentric rings. Another type uses a mechanically rotated arm bearing a light source. In either type, light is focused on the anterior surface of the patient's cornea and reflected back to a

digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...

at the device. The topology of the cornea is revealed by the shape taken by the reflected pattern. A computer provides the necessary analysis, typically determining the position and height of several thousand points across the cornea. The topographical map can be represented in a number of graphical formats, such as a ''sagittal map'', which color-codes the steepness of curvature according to its dioptric value.

Development

The corneal topograph owes its heritage to the Portuguese ophthalmologist Antonio Placido, who, in 1880, viewed a painted disk (''Placido's disk'') of alternating black and white rings reflected in the cornea. The rings showed as

contour line A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional grap ...

s projected on the corneal tear film. The French ophthalmologist Louis Émile Javal incorporated the rings in his ophthalmometer and mounted an eyepiece which magnified the image of the eye. He proposed that the image should be photographed or diagrammatically represented to allow analysis of the image. In 1896, Allvar Gullstrand incorporated the disk in his

ophthalmoscope Ophthalmoscopy, also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part ...

, examining photographs of the cornea via a microscope and was able to manually calculate the curvature by means of a numerical

algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...

. Gullstrand recognized the potential of the technique and commented that despite its laboriousness it could "give a resultant accuracy that previously could not be obtained in any other way". The flat field of Placido's disk reduced the accuracy close to the corneal periphery and in the 1950s the Wesley-Jessen company made use of a curved bowl to reduce the field defects. The curvature of the cornea could be determined from comparison of photographs of the rings against standardized images. In the 1980s, photographs of the projected images became hand-digitized and then analysed by computer. Automation of the process soon followed with the image captured by a digital camera and passed directly to a computer. In the 1990s, systems became commercially available from a number of suppliers. The first completely automatic system was the Corneal Modeling System (CMS-1) developed by Computed Anatomy, Inc. in New York City, under the direction of Martin Gersten and a group of surgeons at the New York Eye and Ear Infirmary. The price of the early instruments was initially very high ($75,000), largely confining their use to research establishments. However, prices have fallen substantially over time, bringing corneal topographs into the budget of smaller

clinic A clinic (or outpatient clinic or ambulatory care clinic) is a health facility that is primarily focused on the care of outpatients. Clinics can be privately operated or publicly managed and funded. They typically cover the primary care needs ...

s and increasing the number of patients that can be examined.

Use

Computerized corneal topography can be employed for diagnostics. It is, in fact, one of the exams the patients have to undergo prior to the Cross-linking and the Mini Asymmetric Radial Keratotomy (M.A.R.K.). For example, the ''KISA% index'' (keratometry, I-S, skew percentage, astigmatism) is used to arrive at a diagnosis of keratoconus, to screen the suspect keratoconic patients and analyse the degree of corneal steepness changes in healthy relatives. Nevertheless, topography in itself is a measurement of the first reflective surface of the eye (tearfilm) and is not giving any additional information beside the shape of this layer expressed in curvature. Keratoconus in itself is a pattern of the entire cornea, therefore every measurement just focusing on one layer, might not be enough for a state of the art diagnosis. Especially early cases of keratoconus might be missed by a plain topographic measurement, which is critical if refractive surgery is being considered. The measurement is also sensitive to unstable tearfilms. Also, the alignment of the measurement can be difficult, especially with eyes that have Keratoconus, a significant astigmatism, or sometimes after refractive surgery. Corneal topography instruments generate a measurement called simulated keratometry (SimK), which approximates the classic measurement of the widely used keratometer. Another novel use of corneal topographic data is called CorT, which has been shown to quantify refractive

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at n ...

more accurately than SimK and other approaches. CorT utilizes data from all Placido rings across the cornea compared with SimK, which is based on only one ring. While corneal topography relies on reflected light from the front (anterior) of the cornea, a technique called corneal tomography also provides a measure of the back (posterior) shape of the cornea.Fung, MW, et al (2015)
"Corneal Topography and Imaging"
Medscape website. Updated 17 March 2016. Accessed 6 May 2017.Guttman Krader, C (2015)
"Astigmatism measurement based on total corneal power"
''EuroTimes'' website. Posted 3 November 2015. Accessed 6 May 2017. A measure called CorT total includes this posterior corneal data and more accurately reflects refraction compared with regular CorT, SimK, and other techniques.

Operation

Development

Use

References

Further reading