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Editors Selection IGR 18-2
Anatomical Structures: Myopic disc appearance
Comment by Simon Law on:
59842 Novel screening method for glaucomatous eyes with myopic tilted discs: the crescent moon sign, Kim MJ; Kim SH; Hwang YH et al., JAMA ophthalmology, 2014; 132: 1407-1413
Myopia is the most common human eye disorder in the world, and high myopia is a risk factor for glaucoma. A myopic optic disc is notoriously difficult to be differentiated from a glaucomatous disc because of its atypical size and shape. Even with the current digital imaging technology, evaluation of myopic disc encounters many imaging artifacts.
Although the reported specificity and sensitivity seem impressive, it is important to understand that the predictive value depends on the disease prevalence
In this cross-sectional study, Kim and associates evaluated the diagnostic ability of the crescent moon (CM) sign for the detection of glaucomatous eyes with myopic tilted discs. CM sign is defined by the neural rim of the superior or inferior quadrant as not connected curvilinearly to that of the temporal quadrant. Schematically, it is defined by the tangential line, which is drawn alone the inner curvature of the neural rim margin meeting the disc margin, extending in the direction opposite to the macula. This method was found to have a high specificity and sensitivity (91% and 83%, respectively), and compared favorable to the ISNT rule. ISNT rim pattern was described by Jonas et al. in the 80's to describe the normal neural rim profile in eyes with discernable rim and has since been applied clinically.
Although the authors rightfully included patients with glaucoma confirmed by RNFL and visual field defect and disease progression, certain selection bias may be unavoidable. The sensitivity and specificity will undoubtedly be high if the diagnostic method used for case inclusion and the one to be studied are similar. Indeed, in myopic tilted discs, the nasal and temporal rims are often difficult to be evaluated due to crowding of the vessels in the nasal rim and temporal sloping of the rim, leaving only the superior and inferior rims for evaluation. In addition, the performance of this test in those eyes with earlier stages of glaucoma is unknown.
Although the reported specificity and sensitivity seem impressive, it is important to understand that the predictive value depends on the disease prevalence. For example, in a busy clinical practice where one thousand patients with myopic tilted discs were evaluated for glaucoma annually and assuming the prevalence of glaucoma is 5% in patients with myopic tilted disc, 50 of the one thousand patients have glaucoma, and 950 patients do not have glaucoma. A test of 91% sensitivity and 83% specificity will diagnose glaucoma in 45 glaucoma patients (true positives) and 161 patients without glaucoma (false positives). That is, in 100 myopic tilted disc patients with CM sign, about 22 have glaucoma and the rest are normal. Of course, the exact prevalence of glaucoma in this group of patients is unknown.
Even with these limitations, the authors should be commended for proposing such a very practical method that can be applied qualitatively or schematically to evaluate the superior and inferior rims of myopic tilted discs. This screening method underscores the importance and usefulness of stereoscopic evaluation of the optic disc.
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