The Van Herick angle grading system is a quick, non-contact method used by eye care professionals to estimate the depth of the anterior chamber angle. This screening tool utilizes a standard slit lamp microscope to assess the peripheral anterior chamber depth (PACD) relative to the thickness of the adjacent cornea. The technique provides a simple, immediate visual assessment, offering a preliminary indication of whether the angle is open or narrow. Its purpose is to identify patients at risk for angle-closure glaucoma before performing more complex diagnostic procedures.
Why Angle Grading Is Necessary
The primary reason for performing Van Herick angle grading is to screen for the risk of angle-closure glaucoma. The anterior chamber angle is the internal drainage system of the eye, located where the iris meets the cornea. This angle contains a structure called the trabecular meshwork, which is responsible for draining aqueous humor, the fluid that nourishes the eye. A healthy, open angle allows this fluid to flow out freely, maintaining a normal intraocular pressure.
If the anterior chamber angle is narrow, the iris can physically block the trabecular meshwork, leading to a sudden or rapid rise in intraocular pressure. This acute blockage of fluid outflow is known as angle-closure, which can cause severe, irreversible damage to the optic nerve. By estimating the angle depth, the Van Herick test helps practitioners determine if a patient’s anatomy places them at a higher risk for this sight-threatening condition. The test is particularly important before procedures that involve pupil dilation, as widening the pupil can push the iris forward and potentially trigger an angle closure event in a susceptible eye.
How the Van Herick Test Is Performed
The Van Herick test is performed using a slit lamp, a specialized microscope that projects a thin beam of light into the eye. The technician sets the illumination system at an angle of approximately 60 degrees relative to the observation microscope. The light beam is condensed to its narrowest possible width, creating a fine optical section.
This narrow beam is then directed onto the far peripheral cornea, near the limbus, which is the border between the cornea and the sclera. The light beam creates two visible sections: a bright reflection on the cornea, and a second, fainter reflection on the peripheral iris. The space between these two reflections represents the depth of the peripheral anterior chamber. The observer visually compares the width of this dark space to the thickness of the corneal section to determine the grade.
Decoding the Van Herick Grading Scale
The Van Herick system uses a standard grading scale from 4 down to 0, which is based on the ratio of the peripheral anterior chamber depth (AD) to the corneal thickness (CT). The progression from Grade 4 to Grade 0 signifies a continuously increasing risk of angle blockage.
- Grade 4: Represents the deepest angle (AD \(\geq\) CT). The depth is equal to or greater than the full thickness of the cornea, indicating a very low probability of angle closure.
- Grade 3: Considered open (AD \(\approx\) 1/2 CT). The depth is approximately one-half the thickness of the cornea.
- Grade 2: Classified as moderately narrow or suspicious (AD \(\approx\) 1/4 CT). The depth is about one-quarter the thickness of the cornea.
- Grade 1: Considered very narrow and high-risk (AD \(\)<[/latex] 1/4 CT). The depth is less than one-quarter the thickness of the cornea.
- Grade 0: Indicates no visible space between the corneal reflection and the iris reflection, suggesting the angle is completely closed.
Clinical Implications of the Results
The results of the Van Herick test dictate the immediate follow-up actions required to protect the patient’s vision. Grades 3 and 4 suggest a sufficiently open angle, meaning the patient is at a low risk for angle closure, and usually no immediate further testing is required. However, for any eye receiving a grade of 2, 1, or 0, the result is considered suspicious or high-risk.
Since the Van Herick method is a non-contact screening tool, it does not provide a definitive diagnosis or a complete 360-degree view of the angle structures. Patients with suspicious grades must be referred for a definitive diagnostic test, most commonly gonioscopy. Gonioscopy involves using a specialized contact lens to directly visualize the angle and its drainage structures, allowing the clinician to confirm the true status of the angle.
In some cases, imaging technologies like anterior segment Optical Coherence Tomography (OCT) may also be used to objectively measure the angle width. Early identification of a narrow angle allows for prophylactic intervention, which can significantly reduce the risk of a future acute angle-closure attack. This necessary follow-up ensures that the patient receives the appropriate management for their specific anatomical risk.