The Zhongshan Angle-Closure Prevention (ZAP) Trial found that the risk of primary angle closure (PAC) in likely suspects is less than 1% per year even among untreated eyes. Therefore, current challenges of clinical practice include identifying which patients with evidence of early angle closure will develop more severe disease and could benefit from laser or surgical treatment. By convention, clinical assessments of parameters related to angle closure using AS-OCT and gonioscopy are conducted in the dark, when angles tend to be the narrowest. Despite significant associations, ocular biometrics measured under dark conditions alone have appeared incompletely predictive of clinical outcomes. A recent retrospective cohort study, published in American Journal of Ophthalmology, has demonstrated biometric data obtained under other lighting conditions could provide additional information about angle closure progression risk. Notably, that static biometric parameters measured in the light and dark were predictive of six-year progression from PAC suspect to PAC.

These findings raise questions about the clinical convention of solely assessing angles in the dark as well as the ideal lighting conditions to risk-stratify patients who are PAC suspects for more severe disease. Photo: Michael Cymbor, OD. Click image to enlarge.

The researchers used longitudinal data from the ZAP Trial to assess and compare static and dynamic ocular biometric risk factors for angle closure progression. The team analyzed 861 eyes of 861 participants. In total, 11 biometric parameters describing the anterior segment were measured in each AS-OCT image obtained at the initial visit. Dynamic change parameters were calculated by subtracting light measurements from dark measurements.

On univariable analysis, both light and dark measurements of the trabecular iris space area bounded by the angle opening distance 500µm anterior to the scleral spur (TISA500) were associated with progression, whereas dynamic change parameters were not.

In the primary multivariable model, older age (hazard ratio; HR=1.09 per year), higher IOP (HR = 1.13 per 1mm Hg), and smaller TISA500 in the light (HR=1.28 per 0.01mm²) were significantly associated with greater risk of progression. Dark TISA500 measurements had similar significance (HR=1.28) when replacing light TISA500. Risk of progression was more predictive among eyes in the lowest quartile of light TISA500 measurements (HR=4.56) compared with dark measurements (HR=2.89).

“Angles tend to be narrowest in the dark on average; therefore, it is logical from an anatomical perspective to evaluate the angle in the dark using AS-OCT or gonioscopy,” the authors wrote in their paper. “However, from a clinical perspective, it is important to recognize that the angle likely assumes this configuration for only a few brief moments throughout the day due to the miotic effects of external lighting and dark adaptation.”

The team proposed that, due to the angle-widening effect of pupillary constriction, imaging PAC suspect eyes in the light produced a more even distribution of angle width measurements than in the dark.

While the ZAP Trial showed that the majority of PAC suspect eyes do not progress, at least within a six-year time period, the researchers of this study emphasized that “identifying and treating a small subset of eyes at higher risk of progression may help reduce future vision loss, especially in regions with lower access to eye care and cataract surgery.”