|In the starting position for Smith’s method to measure the anterior depth chamber (top), a dark region is visible between the slit images on the cornea (a) and on the lens/iris (b). The slit length has to be increased until both slit images just seem to touch. In the examiner’s field of view (bottom), the slit images on the cornea (a) and on the lens/iris (b) just seem to touch. In this coincidence position, the slit length is measured.|
Tips for optometric assistants and technicians on how to improve their instrumentation skills.
As eye examinations and procedures become more complex, it is vital for optometric assistants to also become more educated and proficient. An important part of the eye examination is the dilated fundus examination. However, certain eyes are at higher risk for pathology and should not be dilated without first checking the depth of the anterior chamber. Evaluating anterior chamber depth is important because if it is too shallow there is risk that the normal circulation of fluid within the eye will be blocked and elevate the intraocular pressure (IOP).
WHAT IS THE ANTERIOR CHAMBER?
The anterior chamber is the area of the eye that is located behind the cornea and in front of the iris. It is filled with a fluid called aqueous humor, which is continuously produced by the ciliary body in the posterior chamber.
The aqueous humor is secreted by the ciliary processes of the ciliary body in the posterior chamber, flows through the pupil into the anterior chamber, and exits through a filter-like drain located where the iris inserts into the ciliary body in the peripheral angle, called the trabecular meshwork. This fluid is important in the maintenance of optimum pressure within the globe of the eye. If there is an imbalance in the amount of aqueous being produced vs. the amount leaving the eye, there will be an elevation in IOP. This abnormal pressure in the eye affects and damages the optic nerve; it is called glaucoma.
TYPES OF GLAUCOMA
Narrow angle glaucoma and angle closure glaucoma are directly related to the size of the drainage angle. Narrow angle glaucoma is caused when the drainage channels in the trabecular meshwork become blocked. This type of glaucoma can occur if the iris gets pushed against the lens of the eye and blocks the trabecular meshwork canals, thereby causing the pressure within the eye to go up very fast. The drain can also become blocked if the iris and lens get stuck together. Symptoms include severe eye pain, blurred vision, and headache, rainbow haloes around lights, nausea, and vomiting.
Chronic angle closure glaucoma occurs if the drainage angle becomes completely blocked. The only treatment for these types of glaucoma is to create another port for aqueous to leave the eye. This procedure is called a laser iridotomy. Without treatment for these conditions, blindness is the result.
Risk factors for these types of glaucoma include:
- Hyperopia. Hyperopic patients have shorter eyes, which cause crowding and narrowing of the angle structures.
- Race. People of Asian descent tend to have shallow anterior chambers, which can be more susceptible to angle closure.
- Sex. Women are more likely to develop angle closure than men.
To decrease the chance of an angle closure attack, several methods have been developed to assess the anterior chamber depth.
This is the simplest method of checking the anterior chamber depth, and is used for screening purposes when a slit lamp is not available. The penlight or transilluminator is directed from the temporal side of the eye toward the outer canthus, while the examiner observes the nasal aspect of the iris. If the entire iris is illuminated, the anterior chamber is deep and the angle is wide open. If only half of the iris is illuminated and the other half is in shadow, the iris lies forward and blocks some of the light. This results in a shallow anterior chamber with narrow angles present.
VAN HERICK’S METHOD
The most common method used for assessing anterior chamber depth is the Van Herick technique. This method of angle assessment can only be performed if the corneal limbus is clear. It cannot be performed if a pterygium or peripheral corneal scarring is present. The slit lamp biomicroscope is used to compare the width of an optic section of the cornea to the dark gap between slit lamp corneal beam and the reflected beam on the iris.
A narrow slit of light is projected onto the peripheral cornea at an angle of 60Â° as near as possible to the limbus. This results in a slit image on the surface of the cornea. If the optic section of the cornea and the iris has at least the same width as the space between the back of the cornea and front of the iris, the chamber angle is considered wide open. If the width of the anterior chamber is smaller than a quarter of the slit on the cornea, then the anterior chamber is considered narrow, and angle closure is more likely to occur. The anterior chamber drainage angle is denoted as a fraction of the corneal thickness to the anterior chamber width based on this grading scale.
- Grade 0: No distance between the posterior surface of the cornea and anterior surface of the iris. The angle is considered closed, and angle closure has occurred.
- Grade 1: The distance between the posterior surface of the cornea and anterior surface of the iris is less than one quarter the width of the slit beam on the cornea. The angle is considered very narrow, and angle closure is likely.
- Grade 2: The distance between the posterior surface of the cornea and anterior surface of the iris is one quarter the width of the slit beam on the cornea. Angle closure is possible.
- Grade 3: The distance between the posterior surface of the cornea and anterior surface of the iris is one-half the width of the slit beam on the cornea. The angle is considered open, and angle closure is unlikely.
- Grade 4: The distance between the posterior surface of the cornea and anterior surface of the iris is at least the same width of the slit beam on the cornea. The angle is considered open, and angle closure is very unlikely.
This is a less-used method of measuring the anterior chamber depth, which was proposed by Dr. Redmond Smith in 1979. It is a quantitative method of measuring the anterior chamber depth by using a slit lamp with the beam rotated so that the slit is oriented horizontally. The slit beam illumination system is angled at 60Â° from the operator’s oculars and on the patient’s right side for examination of the right eye, and on the left side for examination of the patient’s left eye.
The slit width is set for 1-2mm and the slit lamp magnification is 10-16 x. The patient is asked to look straight ahead and the corneal beam is brought into sharp focus and projected across the central cornea so that it bisects the temporal portion of the pupil and the iris beam bisects the nasal portion of the pupil. The examiner then adjusts the length of the slit beam until the two beams just touch. The reading is read for the millimeter scale on the illumination system. This number reading is then multiplied by a constant correction factor of 1.4. The resulting number will be the estimated anterior chamber depth in millimeters.
Anterior segment optical coherence tomography is a recently developed method for imaging and assessing the anterior chamber angle. This method uses a series of scans similar to an ultrasound A-scan in combination with a two-dimensional B-scan to produce a cross section of the anterior segment and visualization of the anterior chamber angle.
All these methods are adequate for evaluation for the possibility of a shallow anterior chamber. However, each has its own limitation and is dependent of the skill level of the examiner.
Janet Hunter is president of Eye Source, LLC, and specializes in ophthalmic technician training.