RE: Tear Function and Ocular Surface Alterations After Accelerated Corneal Collagen Crosslinking in Progressive Keratoconus
Matalia et al.2 stated that the lateral diffusion of ultraviolet-A (UVA) irradiation is less than 20 mm during corneal collagen crosslinking (CXL). Because of the possibility of accidental irradiation and the lateral diffusion of UVA light on limbal epithelial stem cells, there are some applications such as usage of polymethyl methacrylate rings and Vidaurri rings to protect the limbal region during the CXL procedure.3 Ultraviolet-A exposure has been further implicated in ocular surface squamous neoplasia,4 and in our study metaplastic changes were observed in the epithelial cells on the superficial layer of the exposed and unexposed conjunctiva adjacent to the corneal limbus. There was no alteration in tear function, and also drug toxicity was ruled out; so, we suggest that the most likely cause of impression cytology changes was UVA toxicity.
Another issue raised by the authors of the letter is the efficiency of the accelerated CXL versus conventional CXL. This issue is still a matter of debate; however, recently, Cheng et al.5 showed that the conventional method still seems to have a better clinical outcome, with greater corneal flattening and reduction in mean keratometry. The demarcation line was also shown to be deeper in conventional CXL.5 Consequently, the accelerated technique still needs further improvement before acceptance within the clinic.
As you pointed out, the wrong publication has been referred to the statement that tear osmolarity is the most sensitive and specific diagnostic method for diagnosis and classification of dry eye disease. The relevant reference is the study by Lemp et al.6 This study demonstrated that a cut-off threshold of more than 308 mOsms/L measured by TearLab was found to be the most sensitive objective parameter in differentiating normal from mild to moderate subjects.6 Also, in a recent report, compared with a panel of the most commonly used objective tests for dry eye disease,7 tear osmolarity was found to represent the best single test for the diagnosis of dry eye and is suitable for use in the clinical setting.8,9
An increase in tear osmolarity is a hallmark of dry eye disease and is believed to be the central mechanism in the pathogenesis of ocular surface damage in the disease, as noted in the Dry Eye Workshop Report,10 and tear osmolarity has been reported to be the single best marker for dry eye disease.10 A new tear osmometer TearLab enables the clinician to collect and measure osmolarity in a 50-nL sample with minimal disturbance of the tear film and is shown to be substantially equivalent to laboratory osmometers.11
As you stated, tear osmolarity is a debated value, and there are many articles suggesting contradictory results. In the peer-reviewed literature, Potvin et al.12 evaluated the evidence regarding the use of tear osmolarity as a physiological marker to diagnose, grade severity, and track therapeutic response in dry eye disease. This literature broadly supports the use of tear film osmolarity as an objective numerical measure for diagnosing, grading severity, and managing treatment of dry eye disease.