Ophthalmologists are trained to evaluate the severity of anterior segment inflammation by examining the aqueous humor by slit-lamp biomicroscopy. Various systems have been reported to quantify the cell number and estimate the amount of protein in the AC.
5,8 The slit-lamp cell grading system utilizes ordinal levels to represent a hierarchy of increasing cell numbers. The grading scale is not linear and is only semiquantitative. The ability to quantify inflammation relies heavily on the experience of the examiner. A method to quantify the amount of cells more precisely and objectively is needed to evaluate and manage many conditions characterized by intraocular inflammation. Laser flare-cell photometry is an automated technique to quantify cells and protein levels (“flare”) in the aqueous humor.
9 It was first reported by Sawa et al.
3 in 1988 and was commercialized later (KOWA Company, Ltd., Tokyo, Japan). It has been used in a variety of research and clinical situations to assess anterior segment inflammation.
9 However, laser flare-cell photometry requires a specialized device dedicated for AC cell and flare measurements. It is not routinely used at uveitis centers worldwide.
10
In contrast, OCT is a versatile noncontact imaging technique that can provide micrometer-scale cross-sectional and three-dimensional imaging of biological tissue in real time. OCT has already become a standard diagnostic tool in ophthalmology. It enables more sensitive diagnosis of diseases in both the anterior and posterior segments of the eye, monitors disease progression, and evaluates response to treatments. Our group was the first to evaluate AC inflammation by using an anterior segment OCT instrument (Lowder C, et al.
IOVS 2004;45:ARVO E-abstract 3372). In 2009, Agarwal et al.
1 published the first journal paper on this topic. They used a similar anterior segment OCT system (Visante; Carl Zeiss Meditec) to evaluate the AC inflammation in eyes with either clear or edema-opacified corneas.
1 Using both computer algorithm and manual identification of AC cells in OCT images, they quantified inflammation and cells, even in corneas with reduced clarity. Thus, AC cell grading is one of the new clinical applications that benefits from the versatility of OCT.
In this study, we presented an objective method to quantify cells present in aqueous humor in both uveitis and normal subjects. The in vitro calibration experiment demonstrated that the particle concentration measured automatically by OCT was highly correlated to the latex microsphere concentration. The efficiency of OCT particle counting was 0.72. The in vivo clinical study showed that the OCT AC cell counts correlated well with the slit-lamp cell grades in both nongranulomatous and granulomatous eyes.
The average OCT cell count per grade in nongranulomatous eyes was almost doubled that in granulomatous eyes (3.7 cells/grade vs. 2.0 cells/grade). One explanation could be that the visibility of the AC cells under the slit-lamp microscope may depend on both the cell size and the cell type. Different types of cells, such as inflammatory cells, macrophages, and pigment granules, may be presented in aqueous humor with intraocular inflammation. The average size of the lymphocytes, neutrophils, and monocytes ranges from 10 to 20 μm. Macrophages are usually larger and pigment cells are usually smaller than that range. Lymphocytes and plasma cells predominate in nongranulomatous inflammation. Macrophages comprise a major portion of granulomatous inflammatory infiltrates.
11 Cells presented in nongranulomatous cases may be less visible under a slit-lamp microscope. In contrast, OCT provides a more objective cell count because it identifies faint cells as well as obvious ones.
Cells are rarely present in the AC of normal individuals (Lowder C, et al.
IOVS 2004;45:ARVO E-abstract 3372). Accordingly, the current clinical standard, established by the Standardization of Uveitis Nomenclature (SUN) Working Group grading scheme for AC cells, has a higher cutoff criteria of <1 cell in the field for Grade 0,
8 instead of 0 cell used in the Hogan system.
5 The inferior AC OCT cell counts were abnormally high in five quiescent uveitis cases (categorized as QU-H), with a slit-lamp cell grade of zero. Moreover, the distribution index of inferior to total AC cells in QU-H eyes was significantly higher than that of normal eyes (0.89 vs. 0.59). However, the central and superior OCT cell counts in QU-H eyes were not significantly different from those of normal eyes. This discrepancy could be due to the fact that OCT examined more inferior regions (2- and 4-mm-diameter circular scans centered at the pupil) than the slit-lamp biomicroscopy (central 1-mm
2 fields). The cells distributed inferiorly were likely to have been missed by the slit-lamp examination but captured in OCT scans.
We hypothesize that the uneven distribution of cells in the AC is caused by the thermally driven aqueous humor circulation inside the AC. The aqueous humor is cooler near the cornea and warmer near the iris due to heat conduction. The cooler aqueous will flow downward and the warmer aqueous will flow upward to form a circulation inside the AC (
Fig. 6). The aqueous circulation may agitate small and light cells to be evenly distributed in the AC. However, this gentle aqueous current may not be strong enough to circulate the large and relatively heavy cells; therefore, the large and heavier cells can become trapped in the inferior part of the AC. They are likely to be missed by the slit-lamp biomicroscopy, which examines only the central part of the AC.
One limitation of this study was that we used a time-domain OCT system with a relatively low scan speed (2000 Hz) and relatively low resolutions (17 μm in tissue). The size of the AC cells was not investigated in this study. More recently, Fourier-domain OCT systems scanning 10–100 times faster than time-domain OCT instruments and providing much higher axial resolution (2–5 μm) became available.
12 The composition of the AC cells is often unknown without an invasive diagnostic aqueous tap. In future studies, we will use a Fourier-domain OCT system with high axial resolution (3–5 μm) to further investigate the cell sizes. Benefitting from the faster scan speed and higher resolution of the newer generation OCT, a more comprehensive scan pattern can be designed to sample the AC with a better resolution and shorter scan time. It may provide important information to decipher AC inflammatory cell subtypes that can aid in differential diagnosis of the anterior uveitis. Another limitation of the study was that the presence of inferior AC cells in quiescent cases was not confirmed by slit-lamp examinations. Nevertheless, the central OCT cell count of the quiescent cases agreed well with clinical grading (central cell count equivalent in QU-H, QU-L, and normal cases,
P > 0.05,
Table 3). It suggested that the inferior hyperreflective particles identified by OCT in quiescent cases were likely to be cells. Moreover, the superior OCT cell counts in quiescent cases were equivalent to those of normal eyes (
P > 0.05,
Table 3). It supported our speculation that the QU-H inferior cells were possibly heavier cells such as macrophages. In future studies, we plan to have the uveitis specialist to examine the superior and inferior regions of the AC in addition to grade the AC cells centrally.
In summary, OCT provided objective and quantitative information on AC inflammatory cells. The OCT cell counts correlated well with the slit-lamp microscope cell grades. OCT, which detected particles in the inferior region of the AC missed by clinical slit-lamp examination, might be a valuable tool in the management of anterior uveitis.