particularly for tracking endothelial cell loss after endothelial keratoplasty. Both contact and noncontact specular microscopes may include a computer for analyzing images of the corneal endothelium. The following parameters can be calculated from a specular or confocal image. (Note that these parameters have implications for the cornea’s response to surgical manipulation.)
Density. The normal endothelial cell density decreases with age. Endothelial cell density normally exceeds 3500 cells/mm2 in children and gradually declines with age to approximately 2000 cells/mm2 in older people. An average value for adults is 2400 cells/mm2 (1500–3500), with a mean cell size of 150–350 μm2. Low cell density (eg, fewer than 1000 cells/mm2) may provide for a transparent cornea, but such corneas are at greater risk for corneal decompensation with intraocular surgery.
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 normally less than 0.30. Polymegathism is increased variation in individual cell areas; it typically increases with contact lens wear. Corneas with significant polymegathism (>0.40) might not tolerate intraocular surgery. Percentage of hexagonal cells. The percentage of cells with 6 apices should ideally approach 100%. Lower percentages indicate a diminishing state of health of the endothelium. Pleomorphism is increased variability in cell shape. Corneas with high pleomorphism (more than 50% nonhexagonal) 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.)
Benetz BA, Yee R, Bidros M, Lass J. Specular microscopy. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:177–204.
Anterior Segment Fluorescein Angiography
Anterior segment fluorescein angiography has occasionally been used to study the circulatory dynamics of normal and pathologic bulbar conjunctival, episcleral, scleral, and iris blood vessels. This technique is particularly applicable to patients who might have anterior segment ischemia or focal areas of vascular nonperfusion, as in necrotizing scleritis and corneal neovascularization.
Esthesiometry
The cornea is innervated by sensory fibers of the ophthalmic branch of cranial nerve V. Esthesiometry is the measurement of corneal sensation, and its primary use is in the evaluation of neurotrophic keratopathy. The examiner should not apply topical anesthesia (or any other topical agent, preferably) to the eye if corneal sensation is to be evaluated. In most clinical circumstances, reduced corneal sensitivity can be diagnosed qualitatively without special instruments. A rolled wisp of cotton from a cotton-tipped applicator is touched lightly to corresponding quadrants of each cornea. The patient is asked to report the degree of sensation in the first eye relative to that of the fellow eye, and sensation is recorded as normal, reduced, or absent for each quadrant. This method can be used to detect most clinically relevant cases of reduced corneal sensation.
Quantitative esthesiometry is useful both in unusual cases and for research purposes. The handheld esthesiometer (Cochet-Bonnet) is a contact device that gives quantitative information about corneal sensation. This device contains a thin, flexible, retractable nylon filament. The patient’s
cornea is touched with the filament, which is extended to the full length of 6 cm. The filament is then retracted incrementally in 0.5-cm steps until it becomes rigid enough to allow the patient to feel its contact. This length is then recorded. Alternatively, the filament can be lengthened until the patient is unable to detect the contact. Esthesiometry readings may vary with user technique, but in general, a lower number or shorter filament indicates reduced corneal sensation. After the central cornea’s sensitivity is measured, a map is produced of the cornea (and sometimes of the bulbar conjunctiva) by testing the superior, temporal, inferior, and nasal quadrants sequentially.
Two noncontact esthesiometry methods have also been described, one using air, the other using air mixed with carbon dioxide. Noncontact corneal esthesiometry stimulates the corneal nerves by releasing a controlled pulse of air at a predetermined pressure (in millibars). The subject indicates verbally whether the stimulus is felt, and a stimulus threshold can be determined.
Faulkner WJ, Varley GA. Corneal diagnostic techniques. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:131–138.
Goins KM. New insights into the diagnosis and treatment of neurotrophic keratopathy. Ocul Surf. 2005;3(2):96–110.
Retinoscopy
Retinoscopy can detect irregular astigmatism by showing nonlinear or multiple reflexes that cannot be completely neutralized with a spherocylindrical lens. With a multifocal cornea, retinoscopy reveals multiple regular reflexes that move in different directions. Irregular astigmatism and multifocal cornea can occur in keratoconus, with corneal degenerations, or after keratorefractive surgery. Abnormalities found with retinoscopy can help explain why a patient with a clear cornea cannot see well. In addition, retinoscopy can disclose disrupted light reflexes caused by disturbances of the corneal surface. In cases where retinoscopic findings exceed the corresponding slit-lamp findings, retinoscopy can help gauge the relative effect of corneal surface changes on vision. See also BCSC Section 3, Clinical Optics.
Mannis MJ, Krachmer JH. Refraction of the abnormal cornea. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:125–130.