FP1722 : Pattern of Anterior and Posterior Corneal Astigmatism in Keratoconus in Indian Population

FP1722 : Pattern of Anterior and Posterior Corneal Astigmatism in Keratoconus in Indian Population
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By SiteAdmin Cornea – I June 1, 2017

Dr. Bhushan Ghodke,
G14425, Dr. Parmar Gautam Singh,

Dr. Ashok Kumar Meena, Dr. Sachin Arya

INTRODUCTION :

Keratoconus(KC) is a progressive, non-inflammatory, thinning disorder causing irregular corneal astigmatism of unknown cause.1Many keratoconus patients and suspects have steepening in an asymmetric bow tie pattern with skewed radial axes (SRAX) of astigmatism. Since the amount of anterior corneal astigmatism is far larger than that of posterior corneal astigmatism, the anterior corneal surface plays a more vital role in astigmatic correction than the posterior corneal surface. The development of new technologies, such as slit-scanning devices, Scheimpflug devices, and optical coherence tomography, has made possible the quantitative measurement of the posterior corneal curvature in irregular astigmatism patients especially keratoconus.2-4 Newer techniques, especially Scheimpflug-based tomography, have been developed to obtain a complete analysis of corneal geometry, permitting characterization of the anterior and posterior surfaces and pachymetric mapping.5,6As Keratoconus-associated changes apparently first arise on the posterior corneal surface, it has been suggested that corneal Scheimpflug tomography may readily detect topographically normal Keratoconus cases.7,8

Ho et al9 reported that neglecting the posterior corneal astigmatism may result in significant deviation in the estimation of corneal astigmatism. Such deviation results in refractive surprises when toric intraocular lens (IOL) implantation is planned, and when rigid gas permeable (RGP) lenses are worn.

Thus the purpose of this study is to retrospectively assess the magnitudes and the axis orientations of anterior corneal surface(ACA) and posterior corneal astigmatism (PCA)in keratoconic patients.

METHODS :

A retrospective study was conducted at Sadguru Netra Chikitsalaya, Chitrakoot. Medical records of 42 patients (22 males and 20 females) diagnosed with Keratoconus from September 2015 to May 2016 were retrospectively analysed. Keratoconus was diagnosed by single experienced clinician with evident findings characteristic of keratoconus (corneal topography with asymmetric bow-tie pattern with or without skewedaxes), and at least 1 keratoconus sign (stromal thinning, conical protrusion of the cornea at the apex, Fleischer ring, Vogt striae, or anterior stromal scar) on slit-lamp examination. We excluded patients with other ectatic conditions like pellucid marginal degeneration, other corneal diseases and eyes with previous ocular trauma or surgery or any sort of corneal scarring that might interfere with keratometric data acquisition. Patients with recent history of Rigid Gas Permeable (RGP) lenses and softcontact lenses were asked to stop using them for 3 and 2 weeks respectively, before Scheimpflug imaging and only such patients were included in the study. This retrospective review of records followed the tenets of the Declaration of Helsinki.

All corneal astigmatism measurements were obtained from a rotating Scheimpflug corneal tomographer (Pentacam, Oculus Optikgeräte GmbH, Wetzlar,Germany). The patient’s chin was placed on the chin rest, and the foreheadwas placed against the forehead strap. After blinking a few times, the patient was asked to open both eyes and stare at the fixation target. Proper alignment was obtained using a joystick, and then the automatic release mode started the scan using 25 single Scheimpflug images captured within 2 seconds for each eye. Only Schiempflug images with Quality specification (QS) as ‘OK’ were included in the study. Following parameters were recorded:anterior and posterior corneal astigmatism (ACA and PCA), astigmatism axis and anterior and posterior mean keratometry (K).

Anterior corneal astigmatism was classified as with-the-rule (WTR) when thesteepest meridian of the corneal surface was between 60-120 degrees and as against-the-rule (ATR) when the steepest meridian was between 0-30 or 150-180 degrees. Since the dioptric power of the posterior corneal surface is negative,posterior corneal astigmatism was classified as WTR when the steepest meridian was between 0-30 or 150-180 degrees and as ATR when the steepest meridian was between 60-120 degrees. The remaining values were classified as oblique astigmatism.

The pattern of astigmatism distribution over the anterior corneal surface and posterior corneal surface was studied. Data analysis was performed using IBM SPSS Statistics (Version 22). The normality of the data was assessed using the Kolmogorov-Smirnov test, which revealed the data of anterior and posterior cornealastigmatism was not normally distributed. The results were demonstrated by mean ± standard deviation (SD). The Chi-square test was utilized to analyse categorical variables. The Pearson correlation test was used to evaluate the correlation between different variables. To evaluate the agreement between anterior and posterior astigmatism orientations, the Cohen kappa coefficient was applied. P value <0.05 was considered statistically significant.

RESULTS:

 The study included 42 patients (22 males and 20 females) with keratoconus.The mean age of the KC patients was 21 ± 5 years.In patients with KC, the mean magnitude of ACA and PCA was 5.32 ± 3.09 diopter (D) and 0.7 ± 0.2 diopter (D) respectively. The dominant astigmatism orientation of the anterior corneal surface was WTR in patients with KC. The dominant astigmatism orientation of the posterior corneal surface was ATR in patients with KC.In KC patients, no significant differences were found between males and females regarding ACA and PCA.In the anterior corneal surface, the mean magnitude of WTR astigmatism was higher than ATR and oblique orientations in the KC patients (p<0.001).

No significant agreement was found between the axis orientations of the ACA and PCA in the normal group (ĸ=0.002 , p=0.538 ). There was a significant correlation between the magnitudes of anterior and posterior corneal astigmatism (Pearson correlation coefficient r ¼ 0.776 , P < .001).When the distribution of astigmatism was studied with respect to the progression and severity of KC, therewas a high prevalence of WTR anterior corneal astigmatism, but it was gradually decreased with the severity of keratoconus. But, the ATR astigmatism of the posterior surface had no changes with stages of keratoconus.In both of theanterior and posterior corneal astigmatism, the oblique orientation graduallydecreased with severity of KC.

DISCUSSION :

In the past, topographic analysis of the anterior corneal surface has traditionally been the principal tool for characterizing KC. But, clinically relevant posterior corneal astigmatism, as postulated by Javal10 back in 1890, cannot be assessed by conventional topography and thus the direct measurement of the posterior corneal surface was notpossible. Posterior corneal parameters were calculated based on the anteriormeasurements and through a keratometric index (1.3375 in most cases) tocompensate for the posterior corneal surface. Recently with the advancement of thenewer instruments such as Scheimpflug imaging the evaluation of the posteriorcorneal surface is achievable. It is reported that in the eyes with higher ACA andtotal corneal astigmatism, the influence of the PCA increases. Therefore, it isimportant to evaluate the PCA in the keratoconic eyes, which mostly have a highermagnitude of corneal astigmatism than normal eyes.11

In the present retrospective study, our results demonstratedthat the mean magnitudes of anterior and posterior central corneal astigmatism were approximately 5.5 D and 1D, respectively, and that approximately 75% and 90 % of eyes showed WTR astigmatism of the anterior corneal surface and ATR astigmatism of the posterior corneal surface, respectively. Also the study reveals a trend for a high prevalence of WTR anterior corneal astigmatism in eyes with keratoconus, but that it was gradually decreased with increasing severity and progression of keratoconus.Orucoglu et al.12 evaluated 656 eyes of 338 KCpatients by Pentacam and reported lower mean magnitudes of ACA and PCA in comparison to our study. The mean magnitudes of ACA and PCA were 3.05 ± 1.97 and 0.71 ± 0.44 D in their patients, respectively

In eyes with WTR astigmatism of the anterior cornealsurface, the presence of ATR astigmatism of the posterior corneal surface compensates for anterior corneal astigmatism, and thus reduces total corneal astigmatism. But in cases with ATR astigmatism of ACA, it actually increases total corneal astigmatism. Thus the present study and its findings may be helpful to consider whenever KC patients are planned for astigmatism correction with ToricIOL or RGP contact lens wear.

Higher order aberrations (HOA’s) in keratoconic eyes with RGP lenses were larger than those in normaleyes, resulting in lower visual outcome.13,14Chen and Yoon15reported that large amounts of posterior corneal HOAs contributed to the large ocular HOAs, which were not corrected by RGP lenses in keratoconic eyes. Coupled with present and previous findings, the presence of both posteriorcorneal astigmatism and posterior HOAs may explain the clinical observation that keratoconic eyes with RGP lenses have poorer visual performance than do normal eyes.

The study has several limitations. Firstly it was a retrospective study. Secondly, vector analysis of the astigmatism power was not done. Third,the magnitude and the axis orientation of corneal astigmatism was determined on the 3.0-mm ring only using the Scheimpflug system.

In the conclusion ,our study reportsthe meanmagnitudes of anterior and posterior corneal astigmatism were approximately 5.5 D and 1 D, respectively, in eyes with keratoconus, and that approximately 75% and 90% of eyes showed WTR anterior corneal astigmatism and ATR posterior corneal astigmatism, respectively .Thus this study results may be helpful for more accuratecorrection of astigmatism by means of toric IOL implantation or RGP lens wear in such patients, because the magnitude of posterior corneal astigmatism in keratoconic eyes is far larger than that in normal eyes.

Funding/Support: No funds, grants or other support were received.

Financial Disclosures: No financial disclosures.

Other Acknowledgments: There were no conflicts of interest

REFERENCES :

  1.  Elisabeth J. Cohen. Keratoconus and Corneal Noninflammatory Ectasias. In Albert and Jakobiec’s Principles and Practice of Ophthalmology, Elseiver2010 .
  2. Rao SN, Raviv T, Majmudar PA, Epstein RJ. Role of Orbscan II in screening keratoconus suspects before refractive corneal surgery. Ophthalmology 2002;109(9):1642–1646.
  3. Schlegel Z, Hoang-Xuan T, Gatinel D. Comparison of and correlation between anterior and posterior corneal elevation maps in normal eyes and keratoconus-suspect eyes. J Cataract Refract Surg 2008;34(5):789–795.
  4. de Sanctis U, Loiacono C, Richiardi L, Turco D, Mutani B, Grignolo FM. Sensitivity and specificity of posterior cornea elevation measured by Pentacam in discriminating keratoconus/ subclinical keratoconus. Ophthalmology 2008;115(9): 1534–1539.
  5. Ambrosio- Jr R, Faria-Correia F, Ramos I, et al. Enhanced screening for ectasia susceptibility among refractive candidates: the role of corneal tomography and biomechanics. CurrOphthalmol Rep 2013;1(1):28-38.
  6. Randleman JB, Akhtar J, Lynn MJ, et al. Comparison of objective and subjective refractive surgery screening parameters between regular and high resolution Scheimpflug imaging devices. J Cataract Refract Surg2015;41(2):286-294
  7. Muftuoglu O, Ayar O, Ozulken K, Ozyol E, Akinci A. Posterior corneal elevation and back difference corneal elevation in diagnosing formefrust keratoconus in the fellow eyes of unilateral keratoconus patients. J Cataract Refract Surg2013;39(9):1348-1357.
  8. Qin B, Chen S, Brass R, et al. Keratoconus diagnosis with optical coherence tomography–based pachymetric scoring system. J Cataract Refract Surg2015;39(12):1864-1871.
  9. Ho JD, Tsai CY, Liou SW. Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement. Am J Ophthalmol2009;147(5):788-795.
  10. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg2012;38(12):2080-2087.
  11. Ruiseñor-Vázquez PR, Galletti JD, Minguez N, et al. PentacaScheimpflug tomography findings in topographically normal patients and subclinical keratoconus cases. Am J Ophthalmol2014;158(1):32-40.
  12. Orucoglu F, Toker E. Comparative analysis of anterior segment parameters in normal and keratoconus eyes generated by scheimpflug tomography. J Ophthalmol2015;2015:925414
  13. Negishi K, Kumanomido T, Utsumi Y, Tsubota K. Effect of higher-order aberrations on visual function in keratoconic eyes with a rigid gas permeable contact lens. Am J Ophthalmol 2007;144(6):924–929.
  14. Marsack JD, Parker KE, Pesudovs K, Donnelly WJ 3rd, Applegate RA. Uncorrected wavefront error and visual performance during RGP wear in keratoconus. Optom Vis Sci 2007;84(6):463–470.
  15. Chen M, Yoon G. Posterior corneal aberrations and their compensation effects on anterior corneal aberrations in keratoconic eyes. Invest Ophthalmol Vis Sci 2008;49(12):5645–5652
FP1656 : Corneal Epithelial Thickness Mapping in Keratoconus Using Fourier – Domain Optical Coherence Tomography
FP1178 : Prevalence of Herpes Simplex Virus in Culture Negative Suppurative Keratitis Using PCR - a Pilot Study

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