The first studies on the iris microcirculation were reported in 1968 by Jensen and Lundbaek.1 They used the principles of retinal fluorescein angiography (FA) to investigate the irises of diabetic patients. Compared to biomicroscopy, this procedure gave an objective assessment of the morphology and the vascular dynamics of anterior segment vessels. However, FA is time-consuming, invasive, and can, very occasionally, give serious adverse reactions.2
During the last few years, optical coherence tomography (OCT) has been largely accepted as a noninvasive instrument for the study of retinal diseases and the recent development of OCT angiography (OCTA) allows visualization of the capillary retinal layers and the construction of microvascular flow maps.3 This procedure does not need injection of sodium fluorescein dye. OCTA has already been used to study the cornea and conjunctiva,4 but it has never been used to study the iris. This paper provides the first description of the normal iris vasculature and three-dimensional (3-D) reconstruction examined with OCTA.
Patients and Methods
This observational study was conducted at Retina 3000 Foundation, Milan, Italy, from September 1, 2015, to January 31, 2016. It satisfied all the requirements of the Declaration of Helsinki and Italian national laws for the protection of personal data; study participants gave signed informed consent. The local ethics committee ruled that no formal ethics approval was required in this particular case.
Inclusion criteria required patients to have no ocular or systemic diseases. Fourteen patients (28 eyes) were eligible for inclusion. Visual acuity and fundus were examined, tonometry was checked, and photos were taken of the anterior segment; iris OCT and OCTA were done.
The OCTA examination was done with the AngioVue system (Optovue, Fremont, CA). This instrument operates at an approximately 840 nm wavelength and at 70,000 A-scans per second to acquire OCTA volumes consisting of two repeated B-scans from 304 sequentially uniformly spaced locations. All scans were automatically processed by the internal software to reduce motion artifacts (ReVue V.2014.2.0.15).
The examination was done in AngioRetina mode, but with the anterior segment optical adaptor lens and without autofocus, manually adjusting the X,Y, and Z axes to bring the iris vessels into focus. OCTA scans were captured in both eyes, always by the same operator. The images were acquired in 3 × 3 volume cubes (Figure 1), dividing the iris into four quadrants (superior, inferior, temporal, and nasal) and in 6 × 6 cubes (Figure 2), dividing the iris into two quadrants (temporal and nasal).
Figure shows 3 × 3 volume cubes optical coherence tomography angiography (OCTA) and OCT in normal iris: inferior temporal sector in left eye.
Figure shows 6 × 6 volume cubes optical coherence tomography angiography (OCTA) and OCT in normal iris: temporal sector in left eye.
Raw data were then exported and split using an external tool provided by Optovue. The split-spectrum decorrelation algorithm outputs were then imported in ImageJ (National Institute of Health, Bethesda, MD). En face images of the iris vasculature were obtained by maximum Z projection of a stack after re-slicing along the Z axis. 3-D renders were obtained with the 3-D viewer plugin using the Spectrum lookup table (Figure 3).
Three-dimensional normal iris reconstruction of 6 × 6 volume cubes optical coherence tomography angiography image: temporal sector in left eye.
In OCTA iris scans, it is possible to observe arteries coming off the greater arterial circle, carrying blood toward the pupillary margin, running on a superficial plane (“vasa advehentes”). The veins carrying blood back toward the ciliary body run more deeply (“vasa revehentes”). Therefore, the arteriosus system is more superficially visible and the venous system more deeply visible in 3-D OCTA reconstruction (Figure 3).
It is possible to observe the minor arterial circle that surrounds the pupillary margin. Unlike the major circle, which is located on the outer edge of the iris and is covered by overlying corneal-scleral structures.
In a highly pigmented iris, OCTA only shows up in the surface vessels, as the deeper ones may be masked by pigment.
Furthermore, 6 × 6 volume cubes images show clearly the conjunctival vascular network.
Early studies with iris fluorescein angiography (FA) aimed mainly at establishing the patterns of the normal iris, giving information about either the iris vasculature or, indirectly, the retinal circulation. This may be useful when visualization of the retina is difficult because of opacities of the optic media, or to detect new vessels of the iris. It was interesting to obtain the same results using a technique without contrast dye; hence, no risk. OCTA is promising for examining the central retina, and we employed it to study the iris vascularization.
OCTA machines are actually set for retinal examination, but with some adjustments it is also possible to visualize the iris. This can be done with our instrument, employing the same programs and only making manual adjustments using the anterior segment adaptor (Figures 1 and 2).
The vascular network is less clearly visible in a highly pigmented iris. Even in these cases, however, OCTA can give useful information. If the eye presents neovascularization, this will produce some alterations to vascular markings that can be assessed, as they will normally become evident at the surface.
Unlike with FA, it is not possible to assess vascular leakage with OCTA; it is only possible to assess the presence of neovascularization or a reduction in vascular markings. However, the method does provide information on the retinal circulation in situations where this cannot be explored, and on the presence of iris neovascularization, without any dye injection.
- Jensen VA, Lundbaek K. Fluorescence angiography of the iris in recent and long-term diabetes. Diabetologia. 1968;4(3):161–163. doi:10.1007/BF01219437 [CrossRef]
- Kwiterovich KA, Maguire MG, Murphy RP, et al. Frequency of adverse systemic reactions after fluorescein angiography. Results of a prospective study. Ophthalmology. 1991;98(7):1139–1142. doi:10.1016/S0161-6420(91)32165-1 [CrossRef]
- Spaide RF, Klancnik JM Jr, Cooney MJ. Retinal vascular layers imaged by fluorescein angiography and optical coherence tomography angiography. JAMA Ophthalmol. 2015;133(1):45–50. doi:10.1001/jamaophthalmol.2014.3616 [CrossRef]
- Ang M, Sim DA, Keane PA, et al. Optical coherence tomography angiography for anterior segment vasculature imaging. Ophthalmology. 2015;122(9):1740–1747. doi:10.1016/j.ophtha.2015.05.017 [CrossRef]