A narrow angle in the eye refers to a reduced space between the iris and the cornea. This anatomical configuration restricts the eye’s natural drainage area. If this angle closes completely, it can lead to a sudden and severe rise in internal eye pressure, a medical emergency known as acute angle-closure glaucoma.
Understanding the Anterior Chamber Angle
The anterior chamber angle is the junction where the iris meets the cornea, forming a space that houses the eye’s drainage system. The eye continuously produces a fluid called aqueous humor, which flows from behind the iris, through the pupil, and into this anterior chamber. This fluid is responsible for maintaining the eye’s shape and internal pressure.
The aqueous humor then drains out of the eye through a spongy tissue network within the angle called the trabecular meshwork. An angle is considered narrow when the peripheral iris is situated too close to the meshwork. This narrow structure increases the risk that the iris will physically press against and block the meshwork, preventing fluid outflow and creating the potential for angle-closure glaucoma.
The Primary Diagnostic Method (Gonioscopy)
The definitive way doctors check for a narrow angle is through a direct visualization technique called gonioscopy. This procedure is the gold standard because it allows the eye doctor to look directly at the drainage structure inside the eye. Without this specialized test, the angle structures are obscured due to the way light reflects off the cornea.
To overcome the total internal reflection of light, a special mirrored contact lens, known as a goniolens, is placed directly on the eye after a numbing drop is applied. The lens’s mirrors redirect the light, allowing the examiner to see the angle structures using a slit lamp microscope.
The doctor systematically identifies four main structures within the angle. The extent to which these structures are visible helps the doctor determine the angle’s width, which is often graded using systems like the Shaffer or Spaeth classifications. The Shaffer system uses a scale of 0 to 4, where a lower number indicates a narrower angle and a higher risk of closure.
Angle Structures
- Schwalbe’s line
- The trabecular meshwork
- The scleral spur
- The ciliary body band
A dynamic variation of this test, called indentation or compression gonioscopy, is performed by applying light pressure to the cornea with certain lenses. This maneuver temporarily pushes the aqueous humor toward the angle, which can mechanically open an angle that is only closed by contact between the iris and the cornea. This helps to differentiate a temporary closure, which can be reversed, from a synechial closure, where permanent scar tissue has formed.
Non-Contact Screening and Imaging Techniques
While gonioscopy is the definitive diagnostic tool, several non-contact methods are used for initial screening or to provide detailed anatomical mapping. One common screening method is the Van Herick technique, which uses a narrow beam of light from the slit lamp to estimate the depth of the peripheral anterior chamber without touching the eye. The doctor compares the depth of the space between the cornea and the iris to the thickness of the adjacent cornea.
This comparison is graded as a ratio, allowing for a quick estimation of the angle’s width and the likelihood of closure risk. The Van Herick test is a rapid technique used to identify patients who need a more comprehensive gonioscopy examination, particularly those with a very shallow anterior chamber.
For more detailed, objective documentation, doctors use advanced imaging technologies like Anterior Segment Optical Coherence Tomography (AS-OCT). This non-contact method uses light waves to generate high-resolution, cross-sectional images of the anterior chamber angle structures. AS-OCT provides precise measurements of the angle’s opening distance and is useful for monitoring subtle changes over time.
Another imaging technique is Ultrasound Biomicroscopy (UBM), which uses high-frequency sound waves to create detailed images of the eye’s front portion. UBM has the distinct advantage of being able to visualize structures located behind the iris, such as the ciliary body, which is not possible with light-based AS-OCT. Although UBM is a contact procedure, it is invaluable for diagnosing specific causes of angle narrowing, such as plateau iris configuration.
Interpreting Results and Next Steps
The results from gonioscopy and imaging determine the appropriate next step for the patient. A finding of a narrow angle means the patient is at an elevated risk for developing angle-closure glaucoma, but it does not mean they currently have the disease. If a narrow angle is confirmed, the primary goal is prevention to avoid the permanent vision damage associated with an acute angle-closure attack.
The most common preventative treatment is a procedure called Laser Peripheral Iridotomy (LPI). In this quick, in-office procedure, a laser creates a tiny opening in the peripheral iris. This opening acts as a bypass, allowing the aqueous humor to flow freely from behind the iris to the front of the eye. By equalizing the pressure, the iris usually moves backward, physically widening the drainage angle and reducing the risk of the iris blocking the trabecular meshwork.