Prakash Gaurav, Srivastava Dhruv, Choudhuri Sounak, Thirumalai Sandeep Mark, Bacero Ruthchel
Department of Cornea and Refractive Surgery, NMC Eye Care, New Medical Center Specialty Hospital, Abu Dhabi, United Arab Emirates.
Acta Ophthalmol. 2016 Mar;94(2):e118-29. doi: 10.1111/aos.12899. Epub 2015 Nov 2.
To evaluate the differences in central and non-central keratoconus (based on cone location), and their effect on the objective screening thresholds for keratoconus.
This comparative case series was performed at tertiary care cornea and refractive surgery service. Three groups were made: KC apex within central 2 mm (central keratoconus, n = 50), apex outside central 2mm (non-central keratoconus, n = 50) and normal controls (n = 100, with 50 cases each with apex within and outside central 2 mm). All cases underwent clinical evaluation and corneal topography (CSO, Sirius, Italy). Apex keratometry (ApexK), simulated keratometry at 3 mm (SimK), central corneal thickness (CCT) and minimum corneal thickness (MCT), anterior corneal higher-order aberrations root mean square (HOARMS), and Zernike's coefficients up to fourth order at different zones were measured.
In spite of the keratoconic groups having comparable ApexK (p > 0.05), central keratoconus had higher SimK and thinner CCT and MCT (p < 0.001). HOARMS was significantly more for central keratoconus at 3 mm zones. These findings had moderate to large effect size (Cohen's d). Receiver operating curve analysis was carried out to compare central keratoconus and non-central keratoconus with control group. ApexK and HOARMS had best discriminative parameters. Using single parametric suspicion cut-offs of 'either SimK steep >47.2 D or CCT < 491.6 μ' had a good sensitivity (0.98) for central keratoconus, but not for non-central keratoconus (0.80). Changing this cut-off to 'either SimK steep K ≥ 45.8 D or CCT ≤ 503 μ' gave a sensitivity and specificity of 0.95 and 0.87 for non-central keratoconus and 0.99 and 0.87 for central keratoconus.
Non-central keratoconus has lesser effect on SimK, pachymetry and smaller-aperture HOARMS. Using 'SimK steep >47.2 D or CCT < 491.6 μ' may miss timely referral for topography in many of these cases. Using more stringent criteria of SimK steep K ≥ 45.8 D or CCT ≤ 503 μ to get a corneal topography done to rule out keratoconus is recommended, especially in cohorts with higher risk.
评估中央圆锥角膜和非中央圆锥角膜（基于圆锥位置）的差异，以及它们对圆锥角膜客观筛查阈值的影响。
本比较病例系列研究在三级医疗角膜和屈光手术科室进行。分为三组：圆锥顶点位于中央2mm以内（中央圆锥角膜，n = 50），顶点位于中央2mm以外（非中央圆锥角膜，n = 50）和正常对照组（n = 100，中央2mm以内和以外各50例）。所有病例均接受临床评估和角膜地形图检查（意大利CSO公司的Sirius）。测量顶点角膜曲率（ApexK）、3mm处模拟角膜曲率（SimK）、中央角膜厚度（CCT）和最小角膜厚度（MCT）、前角膜高阶像差均方根（HOARMS）以及不同区域直至四阶的泽尼克系数。
尽管圆锥角膜组的ApexK相当（p > 0.05），但中央圆锥角膜的SimK更高，CCT和MCT更薄（p < 0.001）。在3mm区域，中央圆锥角膜的HOARMS明显更高。这些发现具有中等至较大的效应量（科恩d值）。进行受试者操作特征曲线分析以比较中央圆锥角膜和非中央圆锥角膜与对照组。ApexK和HOARMS具有最佳判别参数。使用“SimK陡峭>47.2 D或CCT < 491.6μm”的单一参数可疑截断值对中央圆锥角膜具有良好的敏感性（0.98），但对非中央圆锥角膜则不然（0.80）。将此截断值改为“SimK陡峭K≥45.8 D或CCT≤503μm”，对非中央圆锥角膜的敏感性和特异性分别为0.95和0.87，对中央圆锥角膜分别为0.99和0.87。
非中央圆锥角膜对SimK、角膜厚度测量和小光圈HOARMS的影响较小。使用“SimK陡峭>47.2 D或CCT < 491.6μm”可能会使许多此类病例错过及时进行地形图检查的时机。建议使用更严格的标准SimK陡峭K≥45.8 D或CCT≤503μm来进行角膜地形图检查以排除圆锥角膜，尤其是在高危人群中。
