Everything there is to know about topographers
Corneal topography is a medical non-invasive imaging technique used by ophthalmologists and optometrists to illustrate the shape of the cornea, using specialized instruments; topographers, that project light rings on the eye, to create a three-dimensional color map that describes the anterior surface of the eye the same way a topographic map describes the surface of the earth.
This procedure is also known as videokeratography or photokeratoscopy. With the aid of advanced computer processing and digital photography, this technique creates a detailed analysis of the shape and curvature of the cornea, detecting even the slightest irregularities of its surface. This analysis is necessary for specific surgical interventions, such as the cross-linking surgery and the Mini Asymmetric Radial Keratotomy (M.A.R.K.).
A topographer is an essential instrument in the field of ophthalmology. It offers a valuable aid in the diagnosis and treatment of various optical conditions.
In this article, we will cover the history of corneal topographers, their indications, and some alternatives that offer the same value.
History of topographers
In 1619, the idea of visualizing the shape of the cornea appeared when Christopher Scheiner used clear glass with known diameters. He compared the reflections they produced on the eye, to determine its radius. Two centuries later, Antonio Placido used the reflection of a particular disk, the Placido disk, that contained alternating white and black rings. When reflected on the cornea, it produced dark and fair bands of light, which then was photographed and observed. This concept is the foundation of the Portuguese ophthalmologist's inventions: the hand keratoscope and the photokeratoscope. Later on, in 1896, Allvar Gullstrand added the Placido disk into his ophthalmoscope, and with the aid of a numerical algorithm, he could calculate the curvature of the cornea. Gullstrand sooner realized the potential of this technique in the ophthalmology field, stating that it can "give an examination accuracy that could not be achieved with any other approach. " However, the procedure consumed a lot of time and effort since photographs had to be compared with standardized images, and the curvature had to be manually calculated. This lasted until 1980 when computers made the process much faster and easier. A digital camera captures photographs of the projected images, which are then automatically analyzed by the computer. In 1990, the first topographer with a completely automatic system was produced in New York city by Computed Anatomy, Inc., and their use at the time was confined for research purposes.
Indications to use a topographer
The primary indication of using a topographer is the detection and management of corneal ectasia (a group of non-inflammatory disorders that affects the eye and cause peripheral or central thinning of the anterior part of the sclera). This screening is particularly essential before refractive surgery. It is also occasionally applied to get some optical measurements for contact lens fitting.
Keratoconus is a progressive disorder of the eye characterized by central or paracentral thinning of the stroma. It is associated with irregular astigmatism (defective shape of the cornea) and apical protrusion. Usually, both eyes are affected. The resulting symptoms are unilateral deterioration of vision or double vision, due to gradual onset myopia and astigmatism. With the disease's progression, there may be rapidly impaired vision in all distances, poor night vision, photophobia (sensitivity to intense light), and eye strain or itching. The classic sign of keratoconus is monocular polyopia: high contrast light sources (a candle, for example) are perceived as multiple ghost images, spread out in a chaotic pattern around a central point. When exposed to different light, the irregularities and scarring of some regions in the cornea reflect those multiple ghost images on the retina.
Keratoconus may be complicated with acute hydrops, the stretched Descemet membrane ruptures and let a sudden aqueous influx. It is manifested by decreased vision, pain, and photophobia. The breach heals in 6–10 weeks with the clearing of edema. Sometimes a variable amount of scarring develops.
The use of a Corneal topographer allows for highly sensitive detection of this condition, even before the development of symptoms, the device is also necessary for the monitoring of disease.
Treatment in advanced cases consists of corrective surgeries. Screening by a topographer is necessary before the cross-linking procedure, which includes the application of riboflavin (vitamin B2) and UVA light to stimulate collagen strands recovery. Thus, it gives some mechanical strength to the eye surface.
Corneal dystrophy is a hereditary, rare disorder that causes the opacification of the eye lens. It is characterized by progressive anomalous deposition of substances on one of the four corneal layers: Epithelial and subepithelial, Bowman, stromal and endothelial dystrophy. All of these layers are generally transparent in a healthy sclera. Abnormal deposition of proteins, fats, or cholesterol crystals alters its transparency, causing irregular opacities and corneal clouding, and thus, it affects the visual acuity. There are only hereditary risk factors for developing dystrophy, no other risk factors like injury, diets, or an association with a chronic disease have been identified. Both eyes are affected equally, and the pathological process may begin in one of the layers and then spreads to the rest of the tissue.
The disease is classified into four groups according to the location of the defect, and clinical manifestations vary widely among the different entities. However, all of them are outlined by the sudden opacification and clouding of both eyes, especially when a positive family history of the same condition is present. The topography is one of the screening methods for this condition, and the topographer can reveal the location, thickness, and shape of the opacities. The topographer is also used for monitoring purposes.
Corneal ulceration or abscess
Ulcerative keratitis is characterized by painful open sores in the superficial epithelial layer that extends into the stroma. Deeper ulcers may expand into the endothelium and result in perforation; these sores expose the eye to various infections and may result in severe scarring. Ulcers are common ophthalmic disorders, and they are mainly caused by physical or chemical trauma (abrasion of the cornea ), dry and exposed eyes, contact lens, and infections.
The examination makes the diagnosis of corneal scraping under a microscope in cases where the infection is suspected. Or, by direct observation of the eye under a split lamp using special stains (Rose-Bengal dye or fluorescein stain), however, these substances can be irritating. And in such cases, the topographer is used for a detailed description of the abscess or scarring in the cornea.
Ulcerative keratitis can also be associated with an autoimmune disorder, particularly in:
- Mooren ulcer: a rare autoimmune disease in which there is progressive stromal ulceration, starting from the periphery and then proceeding to the center. It is associated with stromal thinning that leaves the cornea vulnerable to perforations, after minor trauma.
- Rheumatoid arthritis and Wegener granulomatosis: these systemic autoimmune disorders have ocular manifestations. They can induce an unexplained infiltration, thinning, and ulceration via the deposition of immune complexes in the capillaries and the activation of inflammatory cells. The condition may be complicated with perforation or contact lens cornea.
The topographer can detect structural irregularities even before symptoms of the systemic disease are manifested.
Keratoplasty (Corneal transplantation or grafting) is a surgical procedure that aims to replace the damaged tissue with donated healthy tissue from a recently dead person, with no previous ocular disorders. The procedure is prevalent and has a wide range of indications:
- To improve visual acuity: conditions like bullous keratopathy, keratoconus, and dystrophy are often associated with loss of strength and deformities.
- Reconstructive surgery: in cases of perforations, thinning, or ulcerations that will not heal spontaneously.
- Therapeutic: viral or bacterial infections that do not respond to treatment.
Patients have to undergo corneal topographer screening prior to refractive surgery and after the surgery to evaluate the outcome.
Alternative instruments for corneal topographer
Holographic interferometry is a non-invasive technology that evaluates the subject structure's elasticity and surface deformities using laser light to light the subject (the cornea) from different directions. The reflected light is then recorded to produce a very detailed holograph. This technique measures irregularities of inaccessible regions of the sclera, by creating a precise 3-dimensional image of its surfaces.
This technique has a similar mechanism to the previous one. But instead of using pulsed-laser light to illuminate the subject surface, superimposed electromagnetic waves are produced to create a phenomenon called the interference, which is then used to extract information about cornea's consistency. These instruments are not as available as the corneal topographer; thus, their use is minimal compared to topographers, which are widely applied in the optometry field.
Brands and market leaders
Topcon Medical Systems
This brand offers easy-to-use devices, mainly the CA-800 Analyzer, to evaluate the corneal surface.
The OCULUS Keratograph 5M is an advanced topographer for corneal topography and keratometry. An exclusive feature is the meibomian glands examination.
This American brand offers a Multi-Diagnostic device that can perform wavefront barometry, scheimpflug tomography, and topography.
The topographer is an advanced instrument that combines the use of a digital camera and computer analysis to create a very detailed description of the corneal anterior and posterior surfaces. The topographer use in the field of optometry is essential for diagnostic and monitoring of the commonly Performed Corneal transplantation surgery.
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