The Right Angle

Most of you reading this probably view gonioscopy as the gold standard method for determining if an eye has an occludable angle. I would argue that anterior segment imaging through ultrasound biomicroscopy (UBM) and/or optical coherence tomography (OCT) is better – and has several clear advantages. Here are my reasons why.

First, anterior segment imaging is objective: you can make quantitative assessments through measuring the angle, anterior chamber depth, corneal thickness, and so on. Second, several publications have shown that imaging is better than gonioscopy in terms of reproducibility and agreement: intra-observer repeatability is higher with imaging (1) and there is a high agreement between gonioscopy and UBM when both are performed in a darkened room (2). Third, anterior segment imaging can be a great patient education tool – patients can see their angle closure and response to treatment.

Although I think UBM and OCT are both great, it’s difficult to say which is best; they each have their own benefits. As OCT is a non-contact method, it can be performed in post-operative eyes as soon as a day after surgery – obviously UBM isn’t recommended for this. OCT also has a higher axial resolution than UBM – 5 µm versus 25 µm. On the other hand, you can’t always see the scleral spur with OCT, but it can be located consistently with UBM. Penetration is also better with UBM, meaning that it can help diagnose cases of plateau iris. Whilst you can visualize the angle and the flat iris with OCT, you may not be able to see the ciliary body processes, but with UBM, you can see the process very clearly, and you can also assess whether there is space in the sulcus. So to diagnose plateau iris, you need to use UBM – you would sometimes struggle to accurately diagnose these cases with gonioscopy alone even with indentation.

To me, using anterior segment imaging instead of gonioscopy is a no-brainer. It’s more precise, it offers greater consistency with angle assessment and it represents the true angle. I also find that the cross-sectional view is more robust when variations in the iris profile are present. We know that the agreement between gonioscopy and anterior imaging is high (2), meaning that sensitivity and specificity of the two are similar. So why not choose the method with higher reproducibility and precision?