The angle in your eye, often called the drainage angle, is a small but important space that eye care professionals routinely check to maintain your long-term eye health. This angle is formed where the colored part of your eye, the iris, meets the clear, dome-shaped front layer, the cornea. Its integrity is directly responsible for regulating the internal pressure of the eye, which is necessary for preserving the health of the optic nerve. By checking the width of this angle, doctors can assess the risk of certain conditions that can lead to permanent vision loss.
Understanding the Eye’s Drainage Angle
The function of the drainage angle is to act as the exit point for the clear, watery fluid known as aqueous humor. This fluid is continuously produced by the ciliary body, a structure located behind the iris in the posterior chamber of the eye. The aqueous humor flows through the pupil and into the anterior chamber, where it provides nourishment to structures that lack their own blood supply.
The fluid then reaches the drainage angle, where it filters out of the eye through a spongy tissue called the trabecular meshwork. This meshwork acts like a sieve, allowing the fluid to pass into Schlemm’s canal, a circular channel. From Schlemm’s canal, the aqueous humor drains into the body’s circulatory system.
The eye constantly balances the production and drainage of this fluid, with approximately 90% of the outflow occurring through the trabecular meshwork. This continuous process maintains a balanced intraocular pressure (IOP), which is necessary to keep the eye’s shape and function.
The Link Between Angle Size and Glaucoma Risk
When the drainage angle is unusually tight, it is referred to as having “narrow angles” or an “occludable angle.” This anatomical configuration physically impedes the outflow of fluid, which can cause intraocular pressure to rise.
Narrow angles become a concern because the iris can bow forward and block the trabecular meshwork completely. This condition is known as angle-closure glaucoma, which is a medical emergency that can cause irreversible vision loss within hours. While not everyone with narrow angles will develop glaucoma, the narrowness significantly increases the risk, particularly as a person ages or if they are farsighted. Early detection of an occludable angle allows for preventative action to protect the optic nerve from damage.
Clinical Methods for Measuring Eye Angles
Gonioscopy
The primary and most traditional method used by eye care professionals to check the drainage angle is Gonioscopy. This procedure uses a specialized handheld lens, which resembles a small mirrored cone, placed gently on the anesthetized surface of the eye. The mirrors inside the lens allow the examiner to see a reflected, magnified view of the angle structures directly.
Gonioscopy is the current standard for evaluating the angle because it allows the doctor to grade the width and observe specific anatomical landmarks. It is the only technique that can differentiate between the iris simply resting against the drainage area (appositional closure) and the iris permanently sticking to it (synechial closure). The examination is typically performed in a darkened room to allow the pupil to dilate slightly, which reveals the narrowest configuration of the angle.
Advanced Imaging Techniques
Advanced, non-contact imaging methods offer objective measurements of the angle anatomy and serve as complements to Gonioscopy.
Anterior Segment Optical Coherence Tomography (AS-OCT) is a non-invasive tool that uses near-infrared light to create high-resolution, cross-sectional images of the front of the eye. The patient remains seated upright, and the device quickly scans the angle, providing precise, quantitative measurements like the Angle Opening Distance (AOD).
Ultrasound Biomicroscopy (UBM) uses high-frequency sound waves to generate detailed images of the anterior segment. UBM requires an immersion technique where a small cup is placed on the eye to hold a fluid medium, as the patient is typically lying down. Unlike AS-OCT, the sound waves penetrate pigmented tissue, allowing UBM to visualize structures behind the iris, such as the ciliary body and lens, providing valuable information in complex cases.
Next Steps Following a Narrow Angle Diagnosis
If a narrow or occludable angle is confirmed, the goal of treatment is to widen the angle to ensure proper fluid outflow. The most common preventative treatment is a procedure called Laser Peripheral Iridotomy (LPI). This is an outpatient procedure that typically takes only a few minutes per eye.
During LPI, a focused laser beam is used to create a tiny, precise opening in the peripheral part of the iris. This small hole equalizes the pressure between the front and back chambers of the eye, which causes the peripheral iris to fall back slightly. The repositioning of the iris physically opens up the drainage angle, thus reducing the risk of angle-closure glaucoma. Following the procedure, anti-inflammatory drops may be prescribed, and patients require regular follow-up visits to monitor their eye pressure and the long-term effectiveness of the treatment.