Though it has served eye doctors well for 50 years, the venerable slit-lamp technique and grading scale devised by William Van Herick to assess anterior chamber depth may be getting a fresh start. His original scheme was “known to have some drawbacks, including the inability to visualize the most peripheral anterior chamber, as it lies beneath the opacified limbal tissues, and the long slit and consequent illumination, which causes miosis and falsely widens the peripheral AC,” wrote the authors of a new report that proposes modifications. Van Herick’s method also lacked a standardized approach to illumination, slit height and exact placement of the slit at the peripheral cornea.
This team’s modified approach uses a short vertical slit lamp beam evaluation at the inferior angle, a tweak they consider an easy, accurate method to assess the peripheral anterior chamber (PAC) depth and angle. It also involves vertically straddling the inferior limbus, which allows “a comparison of the thickness of the most peripherally visible anterior chamber and the most peripheral corneal thickness before the transition to sclera,” and the iridocorneal angle can be estimated as well. A final modification—reducing the slit height to just 1.5mm on the cornea—decreased the illumination, preventing miosis and providing a better estimate of the actual angle, they wrote in the study.
Based on the study participants’ ratios of PAC depth to peripheral corneal thickness (PAC:PCT), they were divided into four groups: I (<1/4), II (1/4-1/2), III (1/2-1) and IV (>1). The researchers used a slit beam to evaluate, photograph and assess each participant’s inferior angle at the sclerolimbal junction and used software to measure the iridocorneal angle (ICA). They also measured the inferior angle at the same meridian with anterior segment optical coherence tomography (AS-OCT).
The researchers discovered that clinical assessment by short vertical beam correlated well with AS-OCT values, trabecular-iris angles (TIA) and scleral spur angles. The mean difference between ICA and TIA was 0.797°, they note. For angles graded narrow (TIA <20°), the researchers used a cut-off of peripheral PAC:PCT <1/4 and found that the area under the curve was 0.918, with a sensitivity of 85.2% and a specificity of 88.2%. They add that there was good agreement between automated imaging analysis parameters and those assessed subjectively by photographs taken during the slit beam examination by a glaucoma fellow compared with a general a ophthalmologist, where there was moderate agreement.
The study concludes that the modified grading scheme “correlated well with AS-OCT and can be used as a more reliable screening tool to identify eyes with possibly occludable angles.”
|Sihota R, Kamble N, Sharma AK, et al. ‘Van Herick Plus’: a modified grading scheme for the assessment of peripheral anterior chamber depth and angle. Br J Ophthalmol. August 1, 2018. [Epub ahead of print].|