A specular microscopy is an examination that allows the observation and study of the innermost layer of the cornea, the endothelium.
It is a quick, painless and non-invasive examination. The patient positions himself with his chin on the chin cup, fixes a luminous point and sees a slight flash of light, as in a photograph. The examination requires no prior preparation on the part of the patient and is normally performed on both eyes.
The corneal endothelium is the innermost layer. It consists of a single layer of flattened hexagonal cells that covers the posterior surface of the cornea. The intact endothelium is fundamental in the maintenance and transparency of the cornea and is subject to physiological changes and traumatic or inflammatory injuries.
The main parameters analysed in a specular microscopy are cell density, i.e. the amount of cells present, the variation in size maintaining hexagonal shape (polymegatism) and shape, the cells losing their hexagonal shape (pleomorphism) and cell density.
When the examination is performed, the specular microscope projects a slit of light onto the posterior corneal surface. Most of this light is transmitted into the aqueous humour, but a small portion (about 0.02%) is reflected by the posterior portion of the cornea towards the microscope lens. This light is reflected as the image in a mirror (specular form) where the angle of reflection is equal to the angle of incidence. The specular light is transmitted to the microscope, forming an image of the corneal endothelium that can be captured and stored for analysis.
The normal endothelial pattern presents a hexagonal morphology and in the adult will have approximately 1500 cells/mm2 and 3500 cells/mm2. In children it is expected to be greater than 3500 cells/mm2 and in the elderly is less than or equal to 2000 cells/mm2 which means a cellular loss, normal with age.
