Examination of the anterior chamber is an important technique to master. It is particularly useful in the differential diagnosis of glaucoma and for finding people at risk of closed-angle glaucoma. The Van Herick technique is commonly used to estimate the depth of the anterior chamber.
Slit-lamp set up:
- 60 degrees beam angle
- Beamwidth equivalent to an optic section
- Maximum beam height
- No filter
- Medium illumination
- 10-16X magnification.
Here is a recommended procedure:
- Place a narrow slit as close to the limbus as possible and normal to the cornea.
- Narrow the beam to an optic section and instruct the patient to look straight ahead.
- Focus the beam sharply on the cornea at the very edge of the temporal limbus.
- Compare the width of the ‘shadow’ formed on the iris (representing the depth of the anterior chamber to the width of the optic section (representing the thickness of the cornea).
- The shadow is a dark interval between the light on the cornea and the light on the iris, that represents the optically empty aqueous in the anterior chamber.
- The procedure can be repeated for the nasal limbus.
The angle can be graded as follows:
- Grade 4-the ratio of aqueous to cornea is 1:1-open angle
- Grade 3-the ratio of aqueous to cornea is 1:2
- Grade 2-the ratio of aqueous to cornea is 1:4-indicates narrow-angle, which should be viewed by gonioscopy
- Grade 1-the ratio is smaller than 1:4-indicates dangerously narrow-angle, which is likely to close.
If the grades of the nasal and temporal angles are judged to be different, both readings should be recorded.
The following are some useful hints and tips:
- Errors in angle estimation may occur if the patient’s eyes are not in primary gaze.
- The most common mistake that will result in angle overestimation is if the optic section is not placed far enough peripherally at the corneoscleral junction.
- The beam should be at exactly 60 degrees to the observation system.
- Most slit lamps will have a scale for judging this angle.
- Novice practitioners often have too wide a beam; it should be just thick enough to avoid extinction.
Try to find an eye specialist in practice who can observe you conducting this procedure and give you advice on how to get good at it.