Specular Microscopy in Contact Lens Practice
BY CHARLES R. EDMONDS, OD, FAAO
Specular microscopy has most commonly been used by eye banks to screen donor tissue for endothelial cell counts. Careful evaluation of the endothelium with 185x magnification (vs. 45x from a slit lamp) can also prove valuable in the contact lens practice for monitoring the corneal response to both daily and continuous wear as well as for surgical co-management.
Examining the Endothelium
Figure 1. Specular microscopy image of the corneal endothelium.
The corneal specular microscope projects light onto the cornea and uses reflected light to show a magnified image of the endothelium (Figure 1). The endothelial layer acts as a fluid barrier and metabolic pump, creating a 78-percent-by-weight hydration level that maintains the cornea's transparency and thickness between 500μm to 530μm. If this function is lost, the cornea can swell, resulting in edema and transparency loss. In addition to normal aging changes, cellular alteration such as polymegethism, pleomorphism and reduced cell density have occurred in long-time wearers of PMMA and low-Dk lens materials.
Normal endothelial cells show a regular quasi-hexagonal pattern with continuous cells of well-defined cell boundaries. The hexagonal shape is geometrically and thermodynamically the most stable to cover a surface because it minimizes total cell perimeter, thus producing minimum surface tension energy. Any type of damage to the corneal endothelium results in decreased cell density and an increase in cell size. The viable cells enlarge and spread or slide to cover the remaining posterior corneal surface.
It was once generally believed that the only mechanism that occurs when endothelial cells become damaged or lost was the expansion and change of shape of the remaining cells to fill potential gaps. We now know, as a result of recent specular microscopy studies, that endothelial cells migrate, expand, coalesce and, under certain conditions, can undergo mitosis. However, loss of endothelial function occurs when enough cells become damaged or lost.
Evaluating Lens Wearers
We use the Konan SP-400 specular microscope (Konan Medical Corporation) in our clinic for routine patient evaluation. The instrument produces a 0.2mm x 0.4mm photographic field of approximately 185x magnification of the corneal endothelial layer. We've observed polymegethism, pleomorphism, low cell density, guttata and stromal edema in long-term wearers of low-Dk/t lenses. We've documented improved endothelial appearance after refitting a patient from low-Dk/t designs to high-Dk silicone hydrogel designs.
Keep in mind that many of your baby boomer patients have worn contact lenses for decades. Careful endothelial evaluation can unveil long-term cellular changes from oxygen deprivation and demonstrate improvement following refitting into contemporary high-Dk materials.
As clinical use of specular microscopes becomes more common, it will reveal new insights into the cornea's response to contact lens wear. CLS
Dr. Edmonds is in a group contact lens specialty practice, which he founded, in Tucson, AZ. He is a diplomate of the Cornea and Contact Lens Section of the AAO and an adjunct assistant clinical professor for the Pacific University College of Optometry.