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Specular Microscopy

Benetz, Beth Ann, CRA, FOPS; Lass, Jonathan H., MD

doi: 10.1097/ICO.0000000000001642
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SDC

Received February 28, 2018

Received in revised form March 06, 2018

Accepted March 06, 2018

The specular microscope was invented by David Maurice in the 1960s and further developed into a clinical and eye bank tool to evaluate the corneal endothelium by Bourne and Kaufman in the 1970s. The 1970s and 80s1–3 saw research and publications exploring the normal and pathologic structure, function,4 healing, and aging processes of the corneal endothelium5–8 including as related to penetrating keratoplasty (PK) and the transplanted cornea.9 Wiffen et al10 explored the role of specular microscopy in assessment of tissue for corneal transplantation. Their conclusion was that morphologic assessment of the donor tissue probably lessens but does not eliminate the risk of primary donor failure. The 1999 Eye Bank Association of America Medical Standards state that specular microscopy “may provide useful information in screening donor tissue to determine suitability for transplantation.”11 However, it was not until 2001 that endothelial cell density (ECD) determination became adopted as a medical standard by the Eye Bank Association of America.12 Minimal ECD requirements for suitability as a donor for corneal transplantation remain at the discretion of the local medical director. Instrument calibration on an annual basis is also a requirement of the standards.

The young, normal endothelium is observed as a single layer of finite, semipermeable hexagonal cells of similar size of the innermost layer of the cornea.4 These cells have both a barrier and pumping action, allowing the aqueous humor to pass through and nourish the cornea while pumping out excess fluid to maintain corneal clarity.4 With normal aging, the number of cells is slowly diminished and changes in the regularity of cell size and shape.7 These changes are measured and reported as ECD (in cells per square millimeter), mean cell area (in square micrometers per cell), coefficient of variation (SD of cell areas/mean cell area), and percent hexagonality (percentage of 6-sided cells).13

After age 40 years, the normal endothelial cell layer typically has an ECD of 2500 to 3000 cells/mm2 (Fig. 1). With most medical directors' established policy, the minimum ECD requirement for PK is 2000 cells/mm2, whereas surgeons commonly request a higher minimum ECD (2300–2500 cells/mm2) for endothelial keratoplasty procedures. These preferences are not necessarily based on any scientific evidence because the Specular Microscopy Ancillary Study, for example, did not show that preoperative ECD correlated with graft failure.14 In addition, although the pattern of endothelial cell loss early on is different between PK and endothelial keratoplasty,15 ultimately comparable cell loss is noted at 10 years in clear grafts for the 2 procedures.16

FIGURE 1

FIGURE 1

Trauma,17 contact lens wear,18 refractive surgeries,19–21 diabetes,22 and corneal disease13,23–25 all impact ECD and morphology. The magnified view of the corneal endothelium with the specular microscope compared with slit-lamp examination allows qualitative assessments including evaluation of endothelial disease and possible dysfunction in the form of guttae, cell dropout, and stress. Qualitative and quantitative assessment of the endothelium are important in evaluating the suitability of the donor cornea for transplantation. The healthy donor corneal endothelium demonstrates remarkable resiliency in providing for clear corneas despite significant cell losses over time.26

Specular microscopes currently commercially available for eye bank use are from 2 primary manufacturers, HAI Laboratories, Inc (Lexington, MA) and Konan Medical Inc (Irvine, CA). These instruments have integrated analysis tools to determine ECD and morphologic features. Instrument calibration and understanding of analysis tools are critical for accurate determination of ECD, coefficient of variation, and hexagonality. Confounding factors in accurate analyses include tissue temperature, tissue preparation, effects of lamellar dissection, edematous cells, folds, and focus.13 The specular images in this Atlas will demonstrate normal, pathologic, and artifact findings related to specular microscopy. Its use complementary to other technologies used in donor cornea evaluation will be demonstrated.

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