A1 Primary Pupil Block

Aqueous humor flow resistance at the irido-lenticular contact increases the pressure in the posterior chamber, widens its area and produces an anterior iris bowing leading to angle closure.⁶ (Fig.1)
UBM and AS-OCT confirm diagnosis and document pressure balance restoration between both anterior and posterior chambers after laser peripheral iridotomy (LPI). Anterior segment imaging can also explain the reasons whereby this procedure may show unsuccessful in some angle closure situations such as incomplete LPI, goniosynechiae and plateau iris syndrome.
Iris-trabecular apposition disclosed by gonioscopy, UBM or AS-OCT do not necessarily imply an intraocular pressure increase.¹⁰ Contact may be intermittent or relatively short (circularly lower than 360º and longitudinally not reaching the Schwalbe's line) and the uveoscleral route may represent approximately 10% of drainage. Usually, mydriasis is associated with a decrease of the iris root aqueous volume. In eyes prone to angle-closure glaucoma, this phenomenon can be compromised and even be a precipitating factor.⁶ From the latter two aspects may be inferred: the relevance of assessing iris thickness and standardizing test lighting conditions when using UBM or AS-OCT for quantitative assessments.¹¹

Figure 5 Phacomorphic glaucoma. a. Shallow anterior chamber (1.28 mm), high anterior-posterior lens diameter (5.38 mm), “Mount Vesuvio” iris. b. The longer line corresponds to the anterior chamber width (ACW) -10.99 mm; the shorter line corresponding to the distance from the ACW to the lens anterior pole is the lens vault or crystalline lens rise (CLR) (1.61 mm). UBM

Figure 6 Phacomorphic glaucoma. a, b. radial sections showing a. ACA occludability b. angle closure, shallow posterior chamber, patent irido-ciliary sulcus and iris-lens diaphragm anteriorly positioned . c. (radial section) d. (horizontal isection) the presence of a patent IPL does not change  ACA structures’ topography, which remains closed. UBM 

A4 Malignant glaucoma

Some eyes from which nanophthalmos is an extreme example, are prone to angle closure due to their dimensions, to unsuccessful regulation mechanisms and vitreous resistance to fluid flow. Quigley claims that the latter determines the expansion of choroidal volume and explains why posterior lenticular pressure promotes extreme forward positioning of anterior segment structures. The mechanism afore explained leads to maximal resistance to aqueous humor flow and consequently to an intraocular pressure rise.⁹ UBM and AS-OCT document lens, iris and vitreous anteriorly positioned, absence of posterior chamber, markedly reduced anterior chamber depth until athalamia, and uveal effusion. (Figs. 7, 8 and 9) 

Figure 7 Corneal edema, highly shallow anterior chamber, anterior positioning of the iris-lens diaphragm, iris thinning, uveal effusion (red arrows). AS-OCT

Figure 8 Extreme forward positioning of the anterior segment structures. Athalamia, iris (red arrow) and lens in direct contact with the corneal epithelium. AS-OCT

A5. Other causes of angle closure

Iris or ciliary body primary or secondary cystic or solid lesions, goniosynechia, cyclitic membrane, vitreous hernia or retinal surgery tamponade material may determine partial or intermittent angle closure that can be documented by UBM and AS-OCT. (Fig. 10) 

Figure 10 a. Ciliary body cystic lesion b. Iris solid lesion c. Goniosynechia d. “Pseudo plateau”. UBM

II. The effects of light, accommodation and age on the ACA

Lighting conditions may cause relevant topography variation of anterior segment structures. Hence, we usually perform the examination under scotopic/mesopic conditions. For research purposes lightening and accommodation conditions can be standardized to obtain reliable and replicable results.¹⁴  (Fig. 11)

Anterior movement of the lens was estimated to be approximately 20µm/year with increasing age. This is associated with a progressing increase of CLR, a reduced anterior chamber depth and an ACA narrowing,¹⁵ which will be clinically more relevant the higher the anatomical predisposition to angle closure, such as in hyperopia, nanophthalmos, iris plateau, etc.

Figure 11 Effect of lighting conditions. In photopic conditons: a. axial horizontal section, b. radial section – temporal quadrant; In mesopic conditions: c. axial horizontal section, d. radial section – temporal quadrant. UBM

III. Structural aspects in Pigmentary Syndrome/Glaucoma

UBM and AS-OCT can be useful in secondary open angle glaucoma as pigmentary glaucoma and pseudoexfoliation glaucoma. The diagnostic triad of pigmentary glaucoma: Krukenberg spindle, radial transillumination defects and trabecular meshwork pigment are caused by pigment dispersion due to mechanical friction of the posterior iris surface against the zonule. Anterior segment imaging may reveal a back bowing iris touching zonular bundles, a "reverse pupillary block" and document iris flattening after LPI, confirming a pressure gradient rebalance between anterior and posterior chambers¹⁶ (Fig. 12). In pseudoexfoliation syndrome/glaucoma, UBM or AS-OCT may reveal pseudoexfoliation flecks over da lens and pupillary margin and eventually zonule fiber loss or frailty.

Figure 12. Pigmentary glaucoma associated with reverse pupil block. a. Back bowing iris b. Post-LPI flattened iris. AS-OCT. c. Reverse pupil block. Of note, are the visible structures behind the iris presented by UBM and not as such by AS-OCT

IV Biometric descriptors to screen and follow up glaucoma

Pavlin and associates devised the first biometric descriptors to use on UBM images. These increased objectivity, accuracy and reproducibility of anterior segment measurements.⁷ (Fig. 13) The concept rapidly extended to AS-OCT.^1,6 (Fig. 15) The scleral spur is the landmark from which many measurement parameters are estimated. After the manual localization of the sceral spur and by applying a semi-automated software system, the ACA can be quantitatively characterized in a fast and reproducible manner⁶. Identifying the scleral spur in angle closure cases may be difficult, especially in the upper and lower quadrants.^5,12

Research is ongoing to find one or a set of effective area or linear biometric descriptors to accurately diagnose an occludable angle and predict angle closure glaucoma.^17,18

 

Figure 13. Biometric descriptors obtained at 0.500mm (AOD500 and TISA500) distance from the scleral spur, in which AOD: Angle Opening Distance; TISA: Trabecular Iris Surface Area; C1: Anterior Chamber Depth, Lens vault: LV1, Lens Thickness: C2. UBM (axial horizontal section)

Figure 14. Biometric descriptors obtained at 0.500mm (AOD500 and TISA500) and 0.750 mm (AOD750 and TISA750) distances from the scleral spur in which AOD: angle opening distance; TISA: trabecular iris surface area. AS-OCT(axial horizontal section)

Conclusion

AS-OCT and UBM provide high-resolution cross sectional images of the anterior segment structures allowing a detailed visualization of the ACA. Both had shown to be useful in the diagnosis of glaucoma. Visualizing retroiridian structures is more challenging using AS-OCT than with UBM since the former relies on low-coherence interferometry and the pigmented posterior layer of the iris absorbs the light.

Anterior segment imaging facilitates the understanding of the underlying mechanism of the heterogenous group of angle closure glaucomas and how the effect of light, accommodation and age can influence the topography of anterior segment structures. We conclude by pointing out the relevance of quantitative anterior segment imaging.^18,19,20

Acknowledgements to

Dra. Manuela Carvalho, for her insurmountable encouragement and friendship,

All Colleagues and Friends for having entrusted their Patients, whose corresponding images illustrate this paper.