- This study aimed to determine the sensitivity and specificity of modified van Herick (VH) grading of limbal anterior chamber depth as a screening tool in community-based screening programs to identify primary angle closure (PAC)
- While most eyes with angle closure are detected with a limbal anterior chamber depth < 25%, many meeting this cutoff have open angles so screening in the community using this approach is not warranted
- VH grading had 98% sensitivity but only 26% specificity for detecting PAC suspects at a cutoff of 25%, and the positive predictive value was very low (2% to 15%, depending on the estimate of PAC prevalence)
- The high sensitivity of VH grading for PAC at a cutoff of 25% suggests the test may be useful for ruling out iridotrabecular contact in the clinic but its usefulness is limited as a population-based screening tool
Van Herick (VH) grading of limbal anterior chamber depth (LACD), a noncontact test that trained technicians can perform, has been used as a screening tool for individual patients in China and other East Asian countries where primary angle closure (PAC) glaucoma is common.
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David Friedman, MD, PhD, MPH, director of the Glaucoma Service at Mass Eye and Ear/Massachusetts General Hospital Department of Ophthalmology, and colleagues recently studied the efficacy of the VH test for screening candidates for a randomized trial in China. They found VH grading had low specificity and a low positive predictive value for identifying PAC that would limit its utility in community-based screening. Their report appears in Ophthalmology.
The researchers used VH grading to screen subjects for participation in the Zhongshan Angle Closure Prevention (ZAP) Trial. Conducted at a tertiary specialized hospital in southern China, the ZAP trial was designed to identify how many people with narrow angles eventually experience acute symptoms or develop glaucoma and whether early laser peripheral iridotomy safely reduces that risk.
3,734 subjects (69% female; mean age 59; 7,468 eyes) were evaluated for eligibility to participate in the ZAP trial. All eyes underwent LACD evaluation using modified VH grading, but only measurements of the right eye were used to calculate the sensitivity and specificity of VH in detecting PAC suspects.
Eyes with a VH grade ≤25% underwent gonioscopy to evaluate for the presence of PAC (defined as Shaffer grade <2 in ≥180°). Subjects with a VH grade >25% underwent gonioscopy only if narrow angles were suspected upon further evaluation with the Scanning Peripheral Anterior Chamber (SPAC) depth analyzer.
Gonioscopic Diagnosis of PAC
Of the 1,795 right eyes with VH grade ≤25%, 667 (37%) met the criteria for PAC on gonioscopy. Of the 1,899 right eyes with VH grade >25%, 407 (21%) underwent gonioscopy to rule out iridotrabecular contact suspected on SPAC, and 12 (2.9%) were determined to have PAC.
Cutoff of 25%
A cutoff of VH ≤25% had an unadjusted sensitivity of 98% but unadjusted specificity of only 26% for detecting PAC. False-negative rates from existing data were extrapolated to subjects with missing gonioscopy data to calculate adjusted sensitivity and specificity rates.
When the researchers assumed the false-negative rate for all subjects with VH >25% was 2.9%, a cutoff of VH ≤25% had adjusted sensitivity of 93% but adjusted specificity of only 62%.
By assuming the prevalence of PAC to be 1.5% to 11.4% based on prior Chinese population-based studies, the researchers estimated a positive predictive value for PAC of 2%-15% and a negative predictive value of 99.1%-99.9%.
Cutoff of 15%
The team also calculated the adjusted sensitivity and specificity when the cutoff was set to VH ≤15%. Using the same 2.9% false-negative rate, the results were 38% and 92%, respectively. The estimated population-based PPV was 4% to 24% and the NPV was 91.5% to 98.9%.
The area under the receiver operating characteristic curve was only 0.62 for a VH cutoff of ≤25% and 0.62 for a cutoff of ≤15%. When adjusted for missing gonioscopy data the figures improved somewhat to 0.77 and 0.65, respectively.
The high sensitivity of VH grading for PAC at a cutoff of ≤25% suggests the test may be useful for ruling out iridotrabecular contact, especially in the clinic.
However, the low specificity and PPV limit the feasibility of VH grading as a population-based screening tool, even in populations with a high prevalence of PAC. The low specificity rate would result in an unsustainable number of individuals requiring further evaluation by gonioscopy.
As a point of interest, in the ZAP study, VH grading was intended to be a cost-effective tool to identify as many potential participants with PAC as possible. However, a large number of individuals who underwent follow-up gonioscopy based on VH grade were not found to have PAC.
Even more important, ZAP participants proved to be at low risk of acute angle closure or angle-closure glaucoma, suggesting early detection of PAC may not have a large public health benefit.
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