2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 2: Examination Techniques for the External Eye and Cornea
Scanning
Specular Microscopy
Specular microscopy (contact and noncontact techniques) provides an objective measurement of corneal endothelial cells. The following parameters can be calculated from a specular or confocal image:
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Density. Endothelial cell density decreases with age. The endothelial cell count normally exceeds 3500 cells/mm2 in children and gradually declines with age to approximately 2000 cells/mm2 in older individuals. An average value for adults is 2400 cells/mm2 (with a range of 1500–3500 cells/mm2), with a mean cell size of 150–350 μm2. Low cell density (ie, fewer than 1000 cells/mm2) may provide for a transparent cornea, but such corneas are at greater risk for corneal decompensation with intraocular surgery. Dark, “punched out” areas might represent cornea guttae (Fig 2-8).
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Coefficient of variation. The standard deviation of the mean cell area divided by the mean cell area gives the coefficient of variation, a unitless number that is normally less than 0.30. Polymegethism is increased variation in individual cell areas; it typically increases with contact lens wear. Corneas with significant polymegethism (>0.40) might not tolerate intraocular surgery.
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Percentage of hexagonal cells. Ideally, the percentage of cells with 6 apices is close to 100%. Lower percentages indicate a diminishing state of endothelial health. Pleomorphism is increased variability in cell shape. Corneas with high pleomorphism (more than 50% nonhexagonal cells) might not tolerate intraocular surgery.
American Academy of Ophthalmology. Corneal Endothelial Photography. Ophthalmic Technology Assessment. San Francisco: American Academy of Ophthalmology; 1996. (Reviewed for currency 2003.)
Sayegh RR, Benetz BA, Lass JH. Specular microscopy. In: Mannis MJ, Holland EJ, eds. Cornea. Vol. 1. 4th ed. Philadelphia: Elsevier; 2017:160–179.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.