Abstract
Abstract: :
Purpose: Alterations in the thickness of the endothelium and Descemet's membrane can be caused by endothelial disorders such as Fuch's dystrophy. However, no method of measuring these thicknesses in vivo has been reported. Interference in the reflection spectrum from the cornea has previously been used to derive the thickness of the tear film, epithelium, Bowman's layer and the complete cornea (King-Smith et al., 2000, IOVS 41, 3348). Here the endothelium and Descemet's membrane are studied by this method. Methods: 162 corneal reflection spectra (760 to 835 nm) were measured at normal incidence from 6 normal subjects (3 men, mean age 42). Measurement spot size was nominally 21 x 175 um and exposure duration was 1 s. Reflection from any corneal boundary causes oscillations in the reflection spectrum whose frequency, as a function of wave number (1/wavelength) is proportional to the depth of that boundary behind the air surface. A Fourier transform of a reflection spectrum therefore shows peaks which give the depths of different corneal boundaries. The refractive index of endothelium and Descemet's membrane was assumed to be 1.376. Results: The largest peak, corresponding to the posterior corneal surface was observed in 151 spectra. In these spectra, which were averaged for each subject, the second largest peak occurred at 12.3 to 15.5 um in front of the posterior surface and is interpreted to come from the boundary between stroma and Descemet's membrane. In 5 subjects, a smaller peaks occurred at 3.8 to 4.2 um in front of the posterior surface, interpreted to be the boundary between the endothelium and Descemet's membrane. In 3 subjects, an additional peak was observed at 9.7 to 10.2 um in front of the posterior surface, which may correspond to the boundary between anterior and posterior zones of Descemet's membrane. Peaks were also observed within the stroma, perhaps corresponding to keratocytes and interlamellar spaces. Conclusion: The average measured thickness of the endothelium, 4.0 um, and of Descemet's membrane, 10.1 um, are in good agreement with histological values. This noninvasive method has the potential to study the effects of corneal disorders on the thickness of many layers of the cornea, including the epithelium, Bowman's layer and the stroma, as well as Descemet's membrane and the endothelium.
Keywords: 371 cornea: endothelium • 432 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 370 cornea: basic science