Specular microscopic photographs of the corneal epithelium were originally obtained by Laing and associates18 using a special plastic conical element that fit over the normal dipping cone objective lens. Using this conical element, saline, hydroxymethyl cellulose, or other fluids could be held between the glass surface of the contact lens and the epithelium so as to obtain epithelial images. This refractive index matching that occurred because of the fluid interface was also accomplished using the fluorite tip on the objective lens that was designed by Sherrard.11 Using this type of objective lens enabled epithelial photographs to be obtained19. When a soft contact lens having nearly the same index of refraction as the cornea (1.370) is placed on the corneal epithelium, the reflection from the epithelium can be reduced, permitting observation of the epithelial cells. If the soft lens is thick enough, the bright reflection between it and the cone of the objective lens can be avoided18,20,21. Tsubota developed an improved two-piece contact lens that enabled improved epithelial photographs to be taken20,21. One of the pieces (the tunnel lens) fits onto the objective lens dipping cone while the other (the contact lens) is placed on the patient's cornea.
With this technique, due to the slight indentation of the cornea produced by the cone lens made for some instruments, tears between the objective lens of the specular microscope and the cornea sometimes interfere with the epithelial image. Increasing the cone lens from 3.5 to 6.0 mm facilitates viewing by enabling the cornea to withstand greater pressure22 Once the contact lens is placed on the patients’s topically anesthetized cornea, the eye lids are held open by the tunnel lens, through which the objective lens of the specular microscope is applied to the contact lens. Using one of these methods, epithelial specular microscopy has been performed by a number of investigators.13,18,19,23-27 Normal Corneal Epithelium The normal corneal epithelium contains polygonal cells with specular reflected intensities (brightness) ranging from dark to light.
There are many hexagonal, pentagonal, and triangular cells but no rounded or elongated ones. While there is a continuum of brightness values, most cells are in one of three groups: dark, medium, and light. The dark cells appear almost black while the light ones are light grey. Cells of similar intensity tend to be grouped together and the brightness within any one cell is homogeneous. It should be emphasized that only the superficial layer of the epithelium can be observed with this technique. Since there are no appreciable differences of refractive index within the epithelium, the inner layer cannot be seen under normal conditions. Wound Healing
Elongation of corneal epithelial cells is typically observed in the wound healing process, such as following penetrating keratoplasty, intraepikeratophakia, excimer laser photokeratectomy , and traumatic epithelial defect. Specular microscopy reveals spindle-shaped cells, whose elongation gradually diminishes as the healing proceeds. Cell elongation is always observed in the first week following excimer laser photrefractive keratectomy, but resolves within one month. This is thought to result from an accelerated migration of peripheral cells followed by the re-establishment of the epithelium. Keratoconus and other conditions Although the corneal epithelium in ketatoconus patients shows a variety of changes, spindle-shaped cells, especially surrounding the cones’s apex, are prominent.28 Persistent elongated cells are also observed in extended wear soft contact lens wearers, aphakic patients, diabetic patients following intraepikeratophakia, and in those with persistent epithelial defects. In these patients, the epithelium is considered clinically abnormal, with increased susceptibility to trauma and infection. |
References: 18. McFarland J, Laing R, Oak S. Specular microscopy of corneal epithelium. Arch Ophthalmol. 1983;101:451. 19. Lemp M, Guimaraes R, Mahmood Me. In vivo surface morphology of the human cornea by color microscopy. Cornea. 1983;2:295. 20. Tsubota K. A contact lens for specular microscopic observation [letter]. Amer J Ophthalmol. 1988;106:627. 21. Tsubota KM Y 22. Tsubota K, Yamada ea. Specular microscopic observation of human corneal epithelial abnormalities. Ophthalmology. 1991;98:184-91. 23. Ebato B, Kasai H, Sakimoto T. Clinical specular microscopy of the corneal epithelium. Folia Ophthalmol Jpn. 1981;32:1319. 24. Lohman L, Rao G, Aquavella J. Normal human corneal epithelium in vivo micrscopic observations. Arch Ophthalmol. 1982;100:991. 25. Kasai H, Ebato B, Ueda Te. Specular microscopy of the corneal epithelium. Folia Ophthalmol Jpn. 1980;31:1704. 26. Serdarevic O, Koester C. Colour wide field specular microscopic investigation of corneal surface disorders. Trans Ophthalmologic Soc UK. 1985;104:439. 27. Marechal-Courtois I, Delcourt J. Wide-field specular microscopy of corneal epithelium. CLAO Journal. 1986;12:165. 28. Pardos G, Krachmer J. Comparison of endothelial cell density in diabetics and a control population. Amer J Ophthalmol. 1980;90:172. |
