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# Drainage angle

The drainage angle is the corner where the iris meets the cornea; it drains the eye's fluid, and when it clogs or closes, eye pressure rises (glaucoma).

Official name: Iridocorneal angle

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## What it is

The corner where the iris meets the cornea, with a sponge-like filter and a ring canal.

## What it does

Drains the aqueous humour; when it clogs or closes, eye pressure rises (glaucoma).

## Key points

- On gonioscopy, from the iris outwards: iris root, ciliary body band, scleral spur, trabecular meshwork (the pigmented band) and Schwalbe line
- Most aqueous leaves by the conventional route (meshwork, Schlemm canal, aqueous and episcleral veins); a smaller share seeps through the ciliary muscle (uveoscleral route)
- Acute angle closure: red painful eye, hazy cornea, mid-dilated poorly reacting pupil, rock-hard globe, headache and vomiting; a sight-threatening emergency
- In open-angle glaucoma the angle looks normal but drains poorly; the disease is silent until peripheral vision is lost

## Key facts

**Where**
Junction of the cornea and iris (limbus)

**Function**
Trabecular meshwork -> Schlemm's canal -> aqueous veins; blockage raises eye pressure (glaucoma)

## Relations

**Course**
Aqueous crosses the uveal, corneoscleral and juxtacanalicular layers of the trabecular meshwork into Schlemm canal; collector channels and aqueous veins then carry it to the episcleral veins

**In front**
Schwalbe line, where Descemet membrane ends at the edge of the [cornea](https://durgical.com/en/anatomy/cornea/)

**Behind**
[Iris](https://durgical.com/en/anatomy/iris/) root and the [ciliary body](https://durgical.com/en/anatomy/ciliary-body/) band

**Superficial**
[Sclera](https://durgical.com/en/anatomy/sclera/) at the limbus, inside which Schlemm canal runs as a ring

**Deep**
[Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/), whose aqueous enters the meshwork

## How to find it

It cannot be seen straight on, because its light is totally reflected inside the cornea; a gonioscopy [lens](https://durgical.com/en/anatomy/lens/) with mirrors placed on the eye shows it at the slit lamp. On the outside it lies under the limbus, the grey-blue junction of cornea and sclera.

## Examination

- Gonioscopy: a visible scleral spur or meshwork means an open angle; only Schwalbe line, or no structures, means a narrow or closed angle; pressing on the cornea (indentation) tells reversible from scarred closure
- Van Herick estimate at the slit lamp: a peripheral chamber depth under a quarter of corneal thickness suggests a narrow angle
- Tonometry: raised pressure supports poor outflow, although glaucoma can occur at normal pressures

## On imaging

Anterior segment OCT (no contact) and ultrasound biomicroscopy measure the angle width and show causes of closure such as plateau iris; they complement gonioscopy rather than replace it.

## Common problems

- Open-angle glaucoma: the meshwork drains poorly, pressure creeps up and side vision fades with no pain.
- Angle-closure glaucoma: the iris blocks the angle; sudden eye pain, redness, halos, headache and vomiting: an emergency.

## How it is diagnosed

Eye-pressure measurement (tonometry), viewing the angle with a special lens (gonioscopy), [optic disc](https://durgical.com/en/anatomy/optic-disc/) exam and visual fields.

## Treatment

- Daily eye drops to lower the pressure.
- Laser to open the drainage (trabeculoplasty) or make a hole in the iris (iridotomy).
- Surgery to make a new drainage path.

## Clinical notes

- Laser peripheral iridotomy makes a small hole in the iris so aqueous bypasses the pupil and the iris falls back; the other eye is usually treated as well
- Laser trabeculoplasty, trabeculectomy (a filtering bleb under the conjunctiva) and tube shunts all aim to improve outflow
- Secondary glaucoma: pigment, exfoliation material, blood, inflammatory cells or lens protein can clog the meshwork; new vessels in diabetes or vein occlusion can seal the angle (neovascular glaucoma)
- Angle recession after blunt trauma, often with a hyphema, can cause glaucoma years later
- Drugs that dilate the pupil (anticholinergic or sympathomimetic) can set off angle closure in an eye with a narrow angle

## Anatomical variants

- Iris processes: fine strands from the iris root to the scleral spur or meshwork, common on gonioscopy and normal unless broad and numerous
- Posterior embryotoxon: a thickened Schwalbe line set forward, seen as a white ring just inside the limbus; present in a minority of normal eyes, and part of Axenfeld–Rieger and Alagille syndromes

## Development

Forms from neural crest mesenchyme; incomplete maturation of the meshwork causes primary congenital glaucoma (large hazy cornea, tearing, light sensitivity), and anterior segment dysgenesis (Axenfeld–Rieger) carries a glaucoma risk.

## Related conditions

- [Open-angle glaucoma](https://durgical.com/en/conditions/open-angle-glaucoma/)
- [Acute angle-closure glaucoma](https://durgical.com/en/conditions/acute-angle-closure-glaucoma/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which structures are seen on gonioscopy of the drainage angle, from the iris outwards?

Iris root, ciliary body band, scleral spur, trabecular meshwork (the pigmented band) and Schwalbe line.

## What is the conventional outflow route of aqueous humour from the drainage angle?

Trabecular meshwork, Schlemm canal, then aqueous and episcleral veins; a smaller share leaves through the ciliary muscle (uveoscleral route).

## What are the features of acute angle closure?

Red painful eye, hazy cornea, mid-dilated poorly reacting pupil, rock-hard globe, headache and vomiting: a sight-threatening emergency.

## How does open-angle glaucoma differ from angle closure at the drainage angle?

The angle looks normal but drains poorly; the disease is silent until peripheral vision is lost.

## What can cause glaucoma years after blunt trauma with a hyphema?

Angle recession: damage to the drainage angle at the time of injury.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
angulus iridocornealis TA2 6793

**Official Spanish (DeCS)**
Ángulo iridocorneal DeCS D014129

**English (TA2)**
Iridocorneal angle

**Also called**
trabecular meshwork · Schlemm's canal · angulus iridis

## Related

- [Iris](https://durgical.com/en/anatomy/iris/)
- [Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=14522); [Open Angle Glaucoma](https://www.ncbi.nlm.nih.gov/books/NBK441887/); [Acute Angle-Closure Glaucoma](https://www.ncbi.nlm.nih.gov/books/NBK430857/); [Physiology, Aqueous Humor Circulation](https://www.ncbi.nlm.nih.gov/books/NBK553209/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/iridocorneal-angle/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/iridocorneal-angle/