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OCT can evaluate anterior chamber reaction in nontransparent cornea

An advantage is the objective quantification and documentation of the anterior chamber reaction.

Issue: May 10, 2015

ByDhivya Ashok Kumar, MD, FRCS, FICO, FAICO

ByAmar Agarwal, MS, FRCS, FRCOphth

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The Tyndall effect, the phenomenon in which light is scattered by very small particles in its path, forms the basis behind the visualization of anterior chamber cells by the slit lamp. There are clinical situations in which the aqueous may not be clearly visible due to corneal edema or loss of corneal transparency. Under such circumstances, the visibility of cells becomes poorer, which necessitates an objective method of examination.

In 2008, the objective method of diagnosing anterior chamber cells in anterior uveitis with time domain anterior segment OCT was reported. However, the limitation with time domain OCT has been its low resolution and slow acquisition time. The more recent spectral domain or Fourier domain OCT systems enable examination of both the anterior and posterior segments in a single setup. In this column, we will present a new OCT-based grading of anterior chamber cells, especially in nontransparent corneas.

Posterior segment OCT for anterior segment imaging

The SD-OCT machine (Cirrus HD OCT, model 4000, Carl Zeiss Meditec) utilized was a posterior segment OCT with anterior segment module (Figure 1). It had the 840 nm wavelength super luminescent laser diode as the optical source. The axial resolution was 5 µm, and transverse resolution was 25 µm in tissue. It had a scanning speed of up to 27,000 A-scans per second. Other features included 2-mm scanning depth in tissue and optical power at the cornea of less than 725 µW. An anterior segment raster pattern in the horizontal axis of 0° to 180° was used for analysis. The raster scan had five lines with a length of 3 mm and a distance of 250 µm in between the lines. The line scanning quasi-confocal ophthalmoscope illumination of the retina mode was turned off, and the internal fixation target was centered.

OCT grading of anterior chamber

The OCT ordinal scale was defined as grade 0.5+ (one cell), grade 1+ (two or three cells), grade 2+ (four to seven cells), grade 3+ (eight to 14 cells) and grade 4+ (15 cells or more). Anterior chamber cells were seen as hyperreflective particles in the anterior chamber.

Fifteen eyes of 15 patients who had corneal edema in which anterior chamber visualization was not possible were examined by OCT. OCT detected cells in all 15 eyes with corneal edema in which slit lamp grading was not possible (Figure 2). Ten of 15 eyes (66.6%) had herpetic keratouveitis, and five eyes (33.3%) had postoperative uveitis. The mean central corneal thickness in those eyes in which slit lamp grading was not possible was 691.8 ± 60.2 µm (range: 614 µm to 802 µm). Out of 15 eyes, four were grade 3+, three were grade 2+, six were grade 1+, and two were grade 0.5+.

Figure 1. OCT showing grade 4+ reaction in an eye with corneal edema. Note the central corneal thickness is 684 µm.

Images: Agarwal A

Figure 2. Clinical photograph showing dense corneal edema (left). OCT of eye showing grade 4+ reaction (right).

Figure 3. Comparison of the anterior chamber as seen by OCT in a normal cornea (left) and an edematous cornea (right).

Advantages of OCT over other methods

In 1988, Sawa and colleagues introduced an objective method to count anterior chamber cells through laser flare photometry. Ladas and colleagues reported that counting of cells by laser photometry was effective and linear in controlled laboratory situations but appeared to be less accurate than flare measurements in vivo. Moreover, its reliability in an edematous cornea has not been proved so far. Laser flare photometry has been known to be affected by the aqueous humor protein concentration, the mydriatic agent used, and the presence of red blood cells.

In this column, we have shown that OCT cell counting is possible in eyes with clinical corneal edema and that it can be useful in follow-up of patients for prognosis analysis (Figure 3). To objectively quantify anterior chamber cells, one can use OCT instead of a subjective system of grading, especially in nontransparent corneas. Moreover, documentation and telemedicine are also possible in digital data. Therefore, the major advantage of OCT as compared with slit lamp grading is the objective quantification and documentation of the anterior chamber reaction at each visit. This will facilitate in follow-up management and retrospective analysis.