The specular microscope is used to examine the endothe-lium of the cornea, which is the layer of the cornea in contact with the aqueous humor transfer ( Fig. 42.1 ). The specular microscope permits visualization and photography of the endothelium. Analysis by computer can quantitatively and qualitatively identify the cell loss of the endothelium from contact lenses and intraocular surgery. Without a viable endothelium, the cornea would swell, lose its transparency, and become a painful debilitating organ, such as occurs in bullous keratopathy.
The specular microscope is an instrument in which light passes through a slit aperture into a system of mirrors with a direct light, moves out through an objective lens, and is attached to a “dipping cone.” This cone lens is a flat surface extension of the × 20 water immersion objective. Specular microscopes can be contact or noncontact regarding the cornea. A focusing knob adjusts the movement of the cone lens to focus the image on a cornea for different thicknesses. This process is used for an objective measurement of the corneal thickness. The light that is reflected from the endothelium and back through the objected eyepiece at × 200 magnification can be observed through an eyepiece or directed into a single-lens reflex camera. The xenon flashcube permits clear photographs despite continuous small eye movement ( Fig. 42.2 ).
Endothelium counts are expressed as cells per millimeter squared. The average central endothelium cell count rate is from 1800 to 4000 cells/mm 2 , with an average of 2800 cells/mm 2 . A significant decrease in cell density occurs with age, indicating a continuous cell loss throughout life. The endothelial cell has no capacity for cell division and reproduction so that when loss occurs there is no replacement. The endothelial cell population of the human cornea decreases from approximately 1 million cells from the first year of life to one-third of that number by the eighth decade of life.
The greatest effect of specular microscopy has been in the area of cataract extraction, particularly with regard to intraocular lens insertion. It has been shown that patients with routine cataract extraction have endothelial cell losses ranging up to 8%. With some anterior chamber intraocular lenses, cell losses can range from 24% to 62%. It has been established that such endothelial cell loss is a result of the intraocular lens touching the endothelium. Many years ago, this caused a change in our thinking to have posterior chamber lenses inserted in the capsular bag. The introduction of sodium hyaluronate (Healon) and other viscoelastics has significantly minimized endothelial trauma during surgery. The use of the specular microscope has made the practitioner aware of factors that encourage a concerted effort to minimize irreparable endothelial damage.
The clinical specular microscope has become an important clinical aid in helping the surgeon operating on the anterior segment to plan a more rational presurgical approach. In those practices in which a great deal of surgery is being performed and a specular microscope is used, it is important for the ophthalmic technician to learn how to use this instrument, to understand its significance, and to know how to use it for taking pictures of endothelium for the surgeon to examine (see Fig. 42.4 ).