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J. C. Wojtowicz, I. A. Butovich, M. A. Di Pascuale, J. P. McCulley; Tear Volume: Optical Coherence Tomography vs. Fluorophotometry. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5311.
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Tear volume (TV) is important in ocular surface physiology and for ocular comfort. A large proportion of tear fluid is contained within the tear menisci (upper and lower). Tear meniscus evaluation is performed by various methods. Regardless of the method of assessment, tear meniscus height (TMH) has generally been considered to be a measure of TV. The volume of tears in the eye reflects the secretion and drainage rates of the lacrimal system, and an estimation of this parameter is important in dry eye patients evaluation. The anterior segment Optical Coherence Tomography (OCT), through a still image provides access to measure TMH. On the other hand, fluorophotometry (FP) has been widely used to measure the dynamics of the tear film, but is of limited to research purposes. The aim of this study is to evaluate the reliability of indirect measuring of tear volume using two different tests - OCT and FP.
Fourteen patients with keratoconjunctivitis sicca with or without associated meibomian gland dysfunction were evaluated for TV using FP and anterior segment OCT. FP was performed utilizing Fluorotron Master (Ocumetrics, Mountain View, CA). Each eye had a baseline fluorescein scan performed, prior to the instillation of 0.5 µl of 0.5% sodium fluorescein onto the ocular surface. Eight measurements were taken for each eye to determine the decay of tear fluorescence. The data were used to calculate TV. TV refers to the initial tear volume determined by regressing the concentration to time zero, thereby obtaining the decay constant (k) and the concentration at instillation. Both eyes were imaged with an OCT (Visante OCT; Carl Zeiss Meditec, Inc.) to obtain the height and the area of the tear menisci. TV was indirectly measured by calculating the upper and the lower tear prism volumes (TPV) and the volume of tears on the exposed ocular surface. Lid lengths and the exposed ocular surface area were measured by digital photography.
There was little difference found between the average TV of the two tests. The upper and the lower TMH and TPV were similar, although the lower lid measurements tend to record higher values. Some of the higher values of TMH and TV were observed in patients with noticeable dermatochalasis. Therefore, there could be a possible association between the measurements of TMH and TV for the patients with dermatochalasis.
OCT could be an alternative noninvasive, non-contact assessment of TMH and TV and may be used in laboratory and clinical settings for evaluating dry eye patients. Redundant and lax eyelid tissue present in elderly patients should be considered when TMH and TV are assessed.
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