Reply to the Letter to the Editor: “Diagnostic performance and interobserver agreement of the callosal angle and Evans’ index in idiopathic normal pressure hydrocephalus: a systematic review and meta-analysis”

by Ho Young Park, Chong Hyun Suh (chonghyunsuh@amc.seoul.kr)

Diagnostic performance and interobserver agreement of the callosal angle and Evans’ index in idiopathic normal pressure hydrocephalus: a systematic review and meta-analysis

Dear Editor,

We appreciate Ryska et al. for their interest in our meta-analysis [1]. We acknowledge that the issues from their letter are important that need further discussion, although some of them have already been handled in our meta-analysis.

First, we agree that callosal angle and simplified callosal angle are different imaging biomarkers as they are measured at a different anatomical level in a different imaging plane: callosal angle measured at the posterior commissural level in an imaging plane perpendicular to bi-commissural plane vs. simplified callosal angle measured at corpus callosum midpoint in an imaging plane parallel to brain stem vertical line. Thus, a different cut-off was set for each biomarker (callosal angle: 90° vs. simplified callosal angle: 123°) [2, 3]. We acknowledge that the inclusion of simplified callosal angle may induce heterogeneous results regarding diagnostic accuracy. However, we tried to recruit as many studies as possible and performed sensitivity analysis thereafter. The sensitivity analysis excluding the study of simplified callosal angle revealed pooled sensitivity and specificity of 89% and 92%, respectively, which was comparable to the overall pooled diagnostic accuracy of callosal angle in general (callosal angle & simplified callosal angle) (pooled sensitivity: 91%, specificity: 93%). Ryska et al. presented valuable results of direct comparison between callosal vs. simplified callosal angle, showing inferior diagnostic accuracy of simplified callosal angle [4]. In contrast, the study of simplified callosal angle in our study demonstrated superior diagnostic accuracy than the studies of callosal angle [3]. The direct comparison of diagnostic accuracy of these two biomarkers needs further study.

Second, we included the diagnostic performance of callosal angle instead of simplified callosal angle from a prospective study by Ryska et al. [4]. Inclusion of both imaging markers from a single study for pooling of diagnostic performance was avoided because duplicate cohorts may confound the results. Therefore, we do not think that this is a limitation of our study.

Third, we admit the limitation of our study in that we did not include the diagnostic accuracy data of Evans’ index from the study by Ryska et al. [4]. We performed meta-analysis again after inclusion of these data, and the final results were as follows: the pooled sensitivity and specificity were 98% [95% confidence interval (CI), 55-100%] and 85% (95% CI, 80-89%), with summary AUC (area under the curve) of 0.87 (95% CI, 0.83-0.89). The pooled results after the inclusion of the additional article were not significantly changed from the original results (pooled sensitivity: 96%, specificity: 83%, summary AUC: 0.87).

Finally, as mentioned in the study by Ryska et al., the precise imaging section angulation for measuring Evans’ index was not defined in the previous literature [5]. Although several anatomical planes for measuring Evans’ index were presented in the article (bi-callosal, bi-commissural, hypophysis-fastigium, and brain stem vertical line) [4], this was not the primary interest of our study.