Dastjerdi M H, Hashemi H
Noor Eye Clinic, Tehran, Iran.
J Refract Surg. 1998 Jul-Aug;14(4):427-36. doi: 10.3928/1081-597X-19980701-09.
Detection of keratoconus has become a critical issue in patients who are potential candidates for refractive surgical procedures. In eyes with early stages of keratoconus, slit-lamp corneal changes are either absent or too subtle for detection, and keratometry may be normal. Videokeratography systems have greater sensitivity for detection of such cases, but interpretation of the topographic map is sometimes difficult, especially when pathologies with similar topographic patterns are suspected. We introduce an index using corneal topography for detection of keratoconus.
From March 1994 to December 1995, a total of 513 unoperated eyes, with no pathology other than keratoconus, from 283 consecutive patients were categorized into three groups based on slit-lamp examination and corneal topographic findings. Those with clinically obvious keratoconus were placed in the first group (53 eyes [10.3%]), those with keratoconus suspect comprised the second group (17 eyes [3.3%]), and the third group was that of normal eyes (443 eyes [86.4%]). In all videokeratographs of these eyes, the highest rate of corneal power changes (in diopters per millimeter) away from the apex (the point of maximum power) to the periphery was measured along its semimeridian. This index, called the "highest rate of steepening (HRS)", was statistically analyzed in the three groups.
The mean +/- SD of highest rate of steepening was 3.60 +/- 1.70 D/mm (range 1.59 to 7.53 D/mm) in the clinically obvious keratoconus group, 1.59 +/- 0.21 D/mm (range 1.35 to 1.98 D/mm) in the keratoconus suspect group, and 0.72 +/- 0.31 D/mm (range 0.18 to 1.63 D/mm) in normal eyes. There was a highly statistically significant difference in the means of highest rate of steepening in the three groups (p < 0.0001). A small overlap in the highest rate of steepening ranges was shown among the three groups. Considering the first and second groups as cases of keratoconus, and 1.40 D/mm as the optimum cut-off value for highest rate of steepening, a sensitivity of 95.7% (67 out of 70 cases were detected), a specificity of 96.4% (16 false-positive cases), and an accuracy of 96.3% was obtained.
The highest rate of steepening may be a useful measure for keratoconus screening with videokeratography. Validation is needed to evaluate its accuracy.
圆锥角膜的检测已成为屈光手术潜在候选患者的关键问题。在圆锥角膜早期阶段的眼睛中，裂隙灯检查发现角膜没有变化或变化过于细微难以察觉，角膜曲率计测量结果可能正常。视频角膜地形图系统对这类病例的检测具有更高的灵敏度，但地形图的解读有时会很困难，尤其是当怀疑存在具有相似地形图模式的病变时。我们引入一种利用角膜地形图检测圆锥角膜的指标。
从1994年3月至1995年12月，对283例连续患者的513只未经手术的眼睛进行研究，这些眼睛除圆锥角膜外无其他病变。根据裂隙灯检查和角膜地形图结果，将这些眼睛分为三组。临床诊断为明显圆锥角膜的患者归入第一组（53只眼[10.3%]），疑似圆锥角膜的患者归入第二组（17只眼[3.3%]），第三组为正常眼睛（443只眼[86.4%]）。在所有这些眼睛的视频角膜地形图中，沿着角膜子午线测量从角膜顶点（屈光力最大的点）到周边角膜屈光力变化的最高速率（以每毫米屈光度为单位）。这个指标，称为“最高变陡率（HRS）”，在三组中进行了统计学分析。
明显圆锥角膜组最高变陡率的平均值±标准差为3.60±1.70 D/mm（范围1.59至7.53 D/mm），疑似圆锥角膜组为1.59±0.21 D/mm（范围1.35至1.98 D/mm），正常眼睛组为0.72±0.31 D/mm（范围0.18至1.63 D/mm）。三组最高变陡率的平均值差异具有高度统计学意义（p < 0.0001）。三组之间最高变陡率范围存在小部分重叠。将第一组和第二组视为圆锥角膜病例，以1.40 D/mm作为最高变陡率的最佳截断值，灵敏度为95.7%（70例中有67例被检测到），特异性为96.4%（16例假阳性病例），准确率为96.3%。
最高变陡率可能是视频角膜地形图筛查圆锥角膜的一个有用指标。需要进行验证以评估其准确性。
