What do you need to know about specular microscopy?
Specular microscopy is a non-invasive photographic technique that allows visualization and analysis of the corneal endothelium. Using computer-assisted morphometry, the specular microscope analyses the size, shape and population of endothelial cells. The instrument projects light onto the cornea and captures the image reflected from the optical interface between the corneal endothelium and the aqueous humor. The reflected image is analyzed by the instrument and displayed as a specular photomicrograph. In clinical practice, specular microscopy is the most accurate way to examine the corneal endothelium.
Specular microscopy assesses the number, density and structure of corneal endothelial cells, the last component of the cornea on which the clarity of vision depends. It is a contactless analysis, which is performed in various forms of corneal disease, particularly in dystrophies, but also in lens replacement and refractive surgery.
Specular microscopy has the distinct advantage of allowing observation of the cornea without causing damage to it. In addition, the specular microscope allows the observation of reflective structures inside cells that cannot be discerned with scanning electron microscopy. The importance of determining the viability and functionality of the corneal endothelium prior to keratoplasty penetration makes specular microscopy a valuable tool as a non-invasive, non-destructive method of assessing endothelial cells.
What does specular microscopy involve?
A primary concern in evaluating corneal tissue prior to surgery or prior to its use as donor tissue in penetrating keratoplasty is the condition of the corneal endothelium, the region of the cornea that is primarily responsible for maintaining normal cornea. Thus, any morphological changes observed in the endothelium are indicative of the current status of the endothelium and its functionality, the ability of the endothelium to respond favorably to an unexpected stressor.
The specular microscope is an optical reflectance microscope that reflects a slit of light from the cornea and allows observation of this specular reflected beam.
Due to its design, the specular microscope does not allow observation of non-specular light rays, so the image that is seen is attributed solely to specular reflected light rays. A projected slit is focused on the posterior endothelial surface through a lens objective. The light that is reflected from this surface is collected by the same lens objective and is focused onto a focusing plane, generally either a film plane or the sensor of a video camera, or can be observed directly by the examiner. An image of the posterior surface is seen in the plane of focus and is viewed directly, captured on film or displayed on a video monitor screen. By focusing the specular microscope in places other than the surface of the posterior cornea, various other reflective surfaces can be observed.
In our clinics, this is performed with the Nidek CEM-530 specular microscope, a high-performance device that offers a number of advantages:
- Does not require local anesthetic or contact gel
- Low intensity lighting for patient comfort
- It is contactless, which eliminates the possibility of transmitting infections
- It is extremely fast, providing a complex analysis in about 2 min
What disorders can be diagnosed by specular microscopy?
Specular microscopy is an in vivo technique for visualizing the corneal endothelium. It is a standard method for determining cell loss or changes in cell size (polymegathism) or cell shape (pleomorphism) with ageing and after contact lens wear.
Specular reflections arise from light reflecting from material interfaces with different refractive indices. This happens when the angle of incidence is equal to the angle of reflection. Thus, the difference in refractive index between the corneal and aqueous endothelium produces a specular reflection. There are many types of specular microscopes, which can be divided into contact and contactless and clinical (horizontal) and vertical. Other types of specular microscopes are capable of viewing other layers of the cornea.
The advent of the specular microscope allowed the ophthalmologist to directly observe the corneal endothelium in patients with Fuchs’ dystrophy, posterior polymorphic dystrophy, pseudophakic bullous keratopathy and other corneal endothelial disorders. If the cornea is significantly edematous, the specular reflex may be masked – which does not allow the visualization of the corneal endothelium. Endothelial cell counts – which are reported as cells per square millimeter – are used to determine the suitability and health of the transplanted tissue. Corneal endothelial cells usually have a hexagonal shape. A deviation from the normal hexagonal endothelial mosaic is called pleomorphism. Although the size of corneal endothelial cells may vary slightly, in a healthy cornea, the cells should be of similar size. When there is great variability in cell size, this is called polymegathism.