In recent years, Fourier-Domain OCT imaging has become a valuable tool in the clinical and experimental assessment of the ocular surface, allowing for the examination of anatomical structure and measuring the thickness of various corneal layers.18 We confirm the findings of previous AS-OCT reports that corneal calcium deposits appear hyperreflective and exhibit posterior shadowing.15-17 Similar to Wirbelauer and Pham, we found that the depth of calcium as determined by AS-OCT has a wide range, close to 300 µm.17 Our distribution was skewed to more superficial lesions.
An important consideration is how well the AS-OCT calculations correspond to actual lesion depth and CCT. The AS-OCT calculations are based on refractive indices, absorptions and autocorrelations from presumed normal corneas.11 There are a number of studies looking at the consistency of AS-OCT calculations in non-calcific lesions.19-21 The true gold standard for accurate assessment of depth is comparison with corresponding histopathology specimens. Wirbelauer et al. showed no statistically significant difference between non-calcific lesion depth on AS-OCT (4Optics AG, Lübeck, Germany) and light microscopy from paired histologic specimens.20 Khurana et al. found no statistically significant difference between CCT calculated on AS-OCT (Carl Zeiss Meditec, Inc., Dublin, CA) and paired ultrasound pachymetry (Corneo-Gage Plus; Sonogage, Cleveland, OH) in eyes with non-calcific corneal opacities.21 A unique source of error in calcific lesions comes from the relatively dense posterior shadows, sometimes blocking all signals from deeper structures. The software algorithm extrapolated a CCT even in cases with dense shadowing and in calculating calcium depth we assumed it was not present within the shadow.
Nioi et al. reported cornea morphological modifications after death using portable SD-OCT on an animal model.22,23 Based on their report, we can speculate that local cornea thickness measurements or reflectivity on OCT could be altered by partial metabolic changes due to hypoxia after cornea damage. To prove this, prospective and experimental studies are needed. If these findings are revealed, the causative mechanism of changes in corneal thickness and reflectivity found in our BK study may be clarified.
We found an expected significant correlation between the depth of the calcific lesion and the duration in years of a white opacity, as determined by patient history. The regression slope was 5 µm of increased depth annually. A limitation of the study was the subject report of opacity duration. Ideally this variable would be defined by examination. An additional bias may exist due to the wide range of follow-up periods. Although there may be a certain degree of error in the reported years of duration, the correlation is still significant when the precision is rounded to decades of duration. Furthermore the correlation is supported by histologic evidence that BK begins with deposits in the epithelial basement membrane and Bowman’s layer that over time become fractured, followed by calcification of the epithelium and the anterior stroma.2
Najjar et al. found that patients with worse baseline vision had less visual improvement after mechanical debridement with EDTA.5 Similarly O’Brart et al. reported that patients with worse baseline vision had less visual improvement after PTK.6 The visual acuity in our series was primarily NLP. The predominance of severe blindness in our BK patient population may be due to local referral practice patterns. Future studies will be needed to determine whether visual acuity might correlate with outputted calcium depth, and if so, then calcium depth may be a useful prognosticator for visual improvement.
Our current treatment algorithm utilizes AS-OCT. EDTA chelation is our first line treatment, but in our experience it is less effective and efficient at removing thick plaques that extend deep into the anterior stroma. In the past we were unable to consistently identify these deep BK patients preoperatively with slit lamp biomicroscopy alone, and frequently we needed to perform a manual superficial lamellar keratectomy in addition to EDTA chelation. This operative challenge prolonged surgical time and led to unnecessary EDTA use, a chemical that is difficult to obtain in South Korea. With AS-OCT we now preoperatively stratify BK depth and only use EDTA on superficial lesions. Another potential use of AS-OCT in BK is in the prediction of post-PTK refractive outcomes. Cleary et al and Rush et al have already demonstrated the utility of preoperative AS-OCT measurements in non-calcific lesions.24,25 The current study is a first step in evaluating the potential benefit of AS-OCT, as this series is limited to patients with primarily NLP vision. As the next step, a study will be required to address the question of whether patients with better baseline vision have similar AS-OCT profiles.