LASIK Interface Fluid

Monday, October 20, 2014

Letters to the Editor

Nonarteritic anterior ischemic optic neuropathy (NAION) and age- and refraction-matched controls.”1

Our finding that cup-to-disc (C/D) ratio increases in NAION eyes was in agreement with the report by Contreras et al.2 The reason for a more evident increase in C/D ratio in our study is most likely the longer length of observation period after the onset of NAION (3 months vs. an average of 48 months).

Contreras et al question the reason why we found a statistically significant difference in disc size between control normal eyes and NAION fellow eyes, whereas there was no difference found in their study.2 Data not included in their original manuscript,2 such as the inclusion criteria regarding refraction and the average refraction of the subjects have been presented in the letter. We believe that it is not unusual that different studies bring some different outcomes because the study designs, participants, or measurement methods are not identical. However, some plausible reasons for the difference include: (1) difference in measurement devices for disc area: as Contreras points out, Heidelberg Retina Tomograph (HRT) measurements are known to have inter-examiner variability.3 However, one experienced examiner (HS) outlined all normal and NAION subjects in this study, and although intra-examiner variability cannot be fully ignored, we did not consider the variability large enough to affect our results.

Furthermore, HRT measurements are corrected by refractive error and corneal curvature which would theoretically yield more accurate results than OCT. Leung et al4 have reported that, in subjects with refraction between _8.0D and _4.0D, disc area measured by HRT were independent of axial length, whereas measurements with OCT correlated with axial length and tended to overestimate disc area in myopic eyes4; (2) race: racial difference in optic disc morphology has been reported5 and this could cause some discordance between the studies; (3) small number of patients: there were 23 NAION patients included in the study by Contreras et al2 and 31 NAION patients in our study. Due to the fairly rare nature of the disease, both studies had to be based on relatively small numbers of subjects, which may have lead to a limitation on statistical power in both studies.

We agree with the authors that one of the main risk factors for NAION appears to be the crowded optic nerve and small C/D ratio. However, smaller disc size as a risk factor for NAION has also been reported by several investigators other than us. Mansour et al6 found a significantly smaller disc area in NAION eyes compared with their fellow eyes with fundus photographs. Because disc area shows no or minimal changes after the onset of NAION, comparison of disc area between affected NAION eyes and control normal eyes should be also valid. A study by Jonas et al7 using fundus photography and a study by Nagai-Kusuhara et al8 using HRT have reported smaller disc size in NAION affected eyes compared with control normal eyes. Taking these previous reports and our results into account, whether small disc area itself is a risk factor for NAION remains controversial and further study, preferably with a larger number of subjects, is needed.

HITOMI SAITO, MD
ATSUO TOMIDOKORO, MD
MAKOTO ARAIE, MD
Tokyo, Japan

References

1. Saito H, Tomidokoro A, Tomita G, et al. Optic disc and peripapillary morphology in unilateral nonarteritic anterior ischemic optic neuropathy and age- and refraction-matched controls. Ophthalmology 2008;115:1585–90.

2. Contreras I, Rebolleda G, Noval S, Muñoz-Negrete FJ. Optic disc evaluation by optical coherence tomography in nonarteritic anterior ischemic optic neuropathy. Invest Ophthalmol Vis Sci 2007;48:4087–92.

3. Iester M, Mikelberg FS, Courtwright P, et al. Interobserver variability of optic disk variables measured by confocal scanning laser tomography. Am J Ophthalmol 2001;132:57– 62.

4. Leung CK, Cheng AC, Chong KK, et al. Optic disc measurements in myopia with optical coherence tomography and confocal scanning laser ophthalmoscopy. Invest Ophthalmol Vis Sci 2007;48:3178–83.

5. Racette L, Boden C, Kleinhandler SL, et al. Differences in visual function and optic nerve structure between healthy eyes of blacks and whites. Arch Ophthalmol 2005;123:1547–53.

6. Mansour AM, Shoch D, Logani S. Optic disk size in ischemic optic neuropathy. Am J Ophthalmol 1988;106:587–9.

7. Jonas JB, Xu L. Optic disc morphology in eyes after nonarteritic anterior ischemic optic neuropathy. Invest Ophthalmol Vis Sci 1993;34:2260 –5.

8. Nagai-Kusuhara A, Nakamura M, Kanamori A, et al. Evaluation of optic nerve head configuration in various types of optic neuropathy with Heidelberg retina tomograph. Eye 2008;22:1154–60.

Dear Editor:
Dawson et al1 elegantly described the causes and pathogenesis of interface fluid syndrome in human eye bank corneas after LASIK. In their discussion, they distinguish diffuse lamellar keratitis from interface fluid syndrome as being associated with pain and limbal vascular injection. This is based on the description by Linebarger et al.2 In our experience, the majority of diffuse lamellar keratitis cases are painless and not associated with a red eye. It is important not to use these 2 factors to differentiate diffuse lamellar keratitis from interface fluid syndrome because it may lead to misdiagnosis.
SAMIR MELKI, MD, PHD
AMIT TODANI, MD
Boston, Massachusetts

References

1. Dawson DG, Schmack I, Holley GP, et al. Interface fluid syndrome in human eye bank corneas after LASIK: causes and pathogenesis. Ophthalmology 2007;114:1848 –59.

2. Linebarger EJ, Hardten DR, Lindstrom RL. Diffuse lamellar keratitis: identification and management. Int Ophthalmol Clin 2000;40:77– 86.
Letters to the Editor
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