The iris and drainage angle

• Summary
• General structure of the iris
• Anterior surface features
• Posterior surface features
• Iris blood flow
• Eye colour
• Drainage angle
• Ciliary body face
• Scleral spur
• Trabecular meshwork
• Schlemm’s canal
• Schwalbe’s line
• Gonioscopy
• References
• Author(s)

Summary

In this article, we will go over the anatomy of the iris and the drainage angle.

General structure of the iris

From anterior (front) to posterior (back), the layers of the iris are:

• Anterior border layer
• Stroma of iris
• Iris sphincter muscle
• Iris dilator muscle
• Anterior pigment myoepithelium
• Posterior pigment epithelium

The stroma consists of a pigmented fibrovascular tissue which connects to both the sphincter muscles that constrict the pupil, and the dilator muscles which pull on the iris radially to enlarge the pupil. The most posterior surface of the iris is lined by the iris pigment epithelium - a layer which is around 2 cells thick. On the contrary, the anterior surface of the iris has no epithelium.

The outer edge of the iris is known as the root, and is attached to the sclera and the anterior ciliary body. Together, the iris and the ciliary body make up the anterior uvea. Directly infront of the root of the iris is the “angle” of the eye - where the iris meets the sclera. This structure is clinically important as it is the main pathway through which aqueous humour drains out of the eye, and therefore diseases of the iris may result in effects on intraocular pressure and vision. Check out our glaucoma article!

The iris is divided into the inner pupillary zone (so called as it forms the boundary of the pupil) and outer ciliary zone (which extends all the way to the ciliary body), separated by the collarette. The collarette is also the area where the sphincter muscle and dilator muscles overlap.

Anterior surface features

• Crypts of Fuchs: these are a series of openings located on either side of the collarette, which allow the stroma and deeper tissues to be bathed in aqueous humour - these are much more visible in light eyes.
• Pupillary ruff: this is the dark brown, wrinkled rim of the normal pupil, which is the posterior pigment epithelium of the iris showing itself at the pupillary margin.
• Circular contraction folds: also known as contraction furrows, these are a series of circular folds situated midway between the collarette and the origin of the iris, due to changes in the surface of the iris as it dilates.
• Crypts at the base of the eyelids: these are additional openings that can be observed close to the outermost part of the ciliary portion of the iris.

Posterior surface features

• Radial contraction folds of Schwalbe: these are a series of fine radial folds in the pupillary portion of the iris which extend from the pupillary margin to the collarette, and the reason why the pupillary ruff has a scalloped appearance.
• Structural folds of Schwalbe: these are radial folds which extend from the border of the ciliary and pupillary zones. They are broad and widely spaced and become continuous with the "valleys" between the ciliary processes.
• Circular contraction folds: these are a fine series of ridges that run near the pupillary margin and vary in thickness of the iris pigment epithelium; others are in ciliary portion of iris.

Iris blood flow

About 5% of the total ocular blood flows through the iris. Both the iris and ciliary body are supplied by the anterior ciliary arteries, the long posterior ciliary arteries and anatosmotic connections from the anterior choroid. The anterior ciliary arteries travel with the extraocular muscles and pierce the sclera near the limbus to join the major arterial circle of the iris. The long posterior ciliary arteries (usually two) pierce the sclera near the posterior pole, then travel anteriorly between the sclera and choroid to also join the major arterial circle of the iris.

Iris blood vessels then branch off from this major arterial circle and are contained as radial coils within tube-like formations in the stroma. This arrangement allows them to remain patent when the iris is fully dilated. Iris vessels have tight junctions and no fenestrations and are hence relatively impermeable to large molecules (which is why anterior segment angiography can be done)!

Most of the venous drainage from the anterior segment is directed posteriorly into the choroid and thence into the vortex veins.

Near the collarette, the iris vasculature network forms the minor arterial circle of the iris.

Eye colour

Iris colour typically ranges from brown, hazel, green, gray, and blue. In some cases, iris colour can be due to lack of pigmentation, for example in oculocutaneous albinism, or due to increased vascularisation resulting in a red colour. Despite the wide range of iris colours possible, it is mainly melanin which contributes to iris colour.

Drainage angle

The anterior chamber angle, also known as the drainage angle, is a part of the eye located between the cornea and iris.

The drainage angle is bordered by the iris, the ciliary body face, the scleral spur, the trabecular meshwork and Schwalbe’s line.

Ciliary body face

The degree to which the ciliary body face is visible depends on the level and angle of iris insertion. In some eyes, the ciliary body face is not visible, being completely obscured by iris. The main function of the ciliary body is production of aqueous, regulation of aqueous outflow and control of accommodation. It also secretes hyaluronate into the vitreous and helps maintain the blood aqueous barrier.

Scleral spur

The scleral spur is a ring of collagen fibres which mark the posterior border of the trabecular meshwork. It projects slightly into the anterior chamber, and is seen as a pale yellow line in most eyes. The longitudinal muscle of the ciliary body attaches onto the scleral spur and pulls on it in order to open the trabecular meshwork.

Trabecular meshwork

The trabecular meshwork lies between the scleral spur and Schwalbe’s line. Approximately 90% of aqueous outflow is through this meshwork. Aqueous humor flowing through the trabecular meshwork enters Schlemm’s canal and from there flows into the scleral, episcleral, and conjunctival venous systems. For aqueous to exit the eye by this route, the intraocular pressure must be higher than the episcleral venous pressure. At pressures below episcleral venous pressure (8 to 15 mm Hg), all aqueous outflow must be via nonconventional routes.

The trabecular meshwork consists of three layers:

1. Uveal meshwork
2. Corneoscleral meshwork
3. Juxtacanalicular tissue

The uveal meshwork is the layer closest to the aqueous which extends from the ciliary body in the angle to Schwalbe’s line and covers the ciliary body face, scleral spur, and trabecular meshwork. In most eyes, this layer is not visible, however in some eyes it is dense and pigmented and gives a rough appearance to the trabecular meshwork, often obscuring portions of the scleral spur.

The middle layer, the corneoscleral layer, lies deep to the uveal meshwork. It is the central layer which extends from the scleral spur to the anterior wall of the scleral sulcus.

The deepest layer is the juxtacanalicular tissue, and is the final layer that the aqueous must course through before it enters Schlemm’s canal. It is a loose connective tissue sandwiched between trabecular endothelium and Schlemm’s endothelium. This layer, unlike the other two layers, provides resistance to aqueous outflow. The aqueous must then travel through the endothelium of Schlemm’s canal to enter the canal.

Aqueous outflow occurs primarily through the posterior portion of the trabecular meshwork, and overtime this meshwork becomes pigmented whereas the anterior meshwork usually remains nonpigmented.

The endothelial cells in the trabecular meshwork differ from corneal endothelial cells in that they are larger with less prominent cell borders. A function of endothelial cells is to digest phagocytized foreign material. After engulfing foreign material some endothelial cells undergo autolysis or migrate away from the trabecular meshwork into Schlemm’s canal (Grierson and Chisholm, 1978). With age or repeated insult the endothelial cell count decreases, as does aqueous outflow.

Schlemm’s canal

Schlemm’s canal collects aqueous and drains it into the venous system. It is usually a discrete tube 190 to 350 μm in diameter, but occasionally, the canal is a plexus rather than a single vessel. The trabecular side of Schlemm’s canal has vacuoles which allow aqueous to traverse the endothelial cells, whereas the scleral side has much smoother endothelium and is perforated by 25-35 aqueous collector channels.

As the canal is not very rigid, it collapses at high intraocular pressures which provides some resistance to aqueous outflow.

Schlemm’s canal opens up following contraction of the longitudinal muscle of the ciliary body, which pulls on the scleral spur. Cholinergic drugs have an effect on this muscle, decreasing resistance to outflow.

Schwalbe’s line

Schwalbe’s line occurs in a 50 to 150 μm transition zone (zone S) between the trabecular meshwork and the corneal endothelium. It is the anterior border of the trabecular meshwork and the posterior border of Descemet’s membrane. The Schwalbe’s line also marks a transition from the more gentle scleral curvature, to the steeper corneal curvature. This may result in the settling of pigment.

Gonioscopy

The angle of the anterior chamber of the eye cannot be seen by direct inspection, however use of a special contact lens, a light source, and a microscope can allow us to see the angle. The contact lens eliminates the corneal curve which normally impedes visualisation, and therefore the light is able to be reflected from the angle. This technique is known as gonioscopy.

References

1. Iris (Anatomy) - Wikidoc. https://www.wikidoc.org/index.php/Iris_(anatomy). Accessed 3 June 2022.
2. Forrester, John V., et al. The Eye: Basic Sciences in Practice. 4th edition, Saunders/Elsevier, 2016.
3. Kiel, Jeffrey W. Anatomy. Morgan & Claypool Life Sciences, 2010. www.ncbi.nlm.nih.gov, https://www.ncbi.nlm.nih.gov/books/NBK53329/.
4. Song, Yumi, et al. ‘A Study of the Vascular Network of the Iris Using Flat Preparation’. Korean Journal of Ophthalmology : KJO, vol. 23, no. 4, Dec. 2009, pp. 296–300. PubMed Central, https://doi.org/10.3341/kjo.2009.23.4.296.
5. ‘Anatomy of the Angle’. American Academy of Ophthalmology, 11 Aug. 2017, https://www.aao.org/disease-review/anatomy-of-angle.

Author(s)

Dr Sara Memon

Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.

Published: 26/5/22

Last updated: 23/1/25