2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 2: Examination Techniques for the External Eye and Cornea
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:
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).
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.
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.
Figure 2-8 Specular microscopy showing Fuchs endothelial corneal dystrophy with guttae.
(Courtesy of John E. Sutphin, MD.)
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.