Gonioscopy is a technique that allows accurate evaluation of the width of the anterior chamber angle and detailed inspection of the structures within the anterior segment of the eye.
The angle is not normally visible using an ophthalmoscope or slit lamp, since light reflected by structures posterior to the limbus is totally internally reflected within the anterior chamber because of the curvature of the cornea. Also, the angle is concealed from direct observation by the projection of opaque scleral tissue over its anterior wall as far as the limbus. Gonioscopy is carried out using goniolenses of which there are several types. Those that are most commonly used are the indirect mirrored lenses.
Indirect mirrored goniolenses are plastic cone-shaped contact lenses available in various sizes and designs containing one, two, three or four mirrors and a plano-concave lens in the centre of the cone apex, which effectively eliminates the cornea as a refracting surface. Any small difference in curvature between the lens and the cornea is minimized by interposing an optical coupling solution between the two surfaces, e.g. Viscotears eye lubricant.
The single-mirror goniolens is useful for teaching purposes and for those new to this technique, as the multi-mirrors of the three- and four-mirror versions can prove confusing for the novice. The single-mirror lens is considered a good choice for examining children, and adults with small palpebral apertures.
In its three-mirror form, the indirect goniolens (often known as the Goldmann 3-mirror) permits angle examination using the smallest arc-shaped internal mirror and a good view of the mid-and far-periphery of the fundus using the additional mirrors through a dilated pupil.
The Thorpe four-mirror and the Sussman four mirror goniolenses are designed to provide a view of the angle in each mirror. This is useful in that the lens has only to be rotated slightly in order for the complete angle to be observed. The Sussman four-mirror lens has an added advantage in that its small contact surface means that an optical coupling solution is not required to create an optical interface.
The single- and three-mirror lenses only have one mirror positioned to view the angle, rotation through 360 degrees with an appropriate adjustment to the illumination is required. The slit-lamp beam should always be approximately perpendicular to the base of the arc-shaped mirror.
Some optometrists are hesitant to dilate pupils for fear of precipitating an attack of closed-angle glaucoma. Van Herick’s slit lamp technique allows an approximation of the openness of the angle, however, the use of a goniolens will provide a more accurate evaluation of angle width as well as an assessment of those structures discussed above, for anatomic anomalies and disease or the effects of trauma.
Slit-lamp set up
- Beam straight ahead
- 2 mm beam width
- Maximum beam height
- No filter
- Medium illumination
- 10-15x magnification
As with all invasive procedures, corneal health should be assessed using fluorescein in conjunction with the blue slit lamp filter.
The technique should not be carried out on a compromised cornea.
It is good practice before commencing gonioscopy to explain what is going to happen, to the patient and why it is being done since the size of the lens can prove disconcerting. The patient should be advised that the lens may feel strange but will not be uncomfortable. This will usually reduce patient anxiety and improve co-operation.
Bubble free coupling gel should be placed into the concavity of the goniolens. The cornea should be anaesthetized with one drop of topical anaesthetic (e.g. proxymetacaine 0.5%).
With the patient looking up, the lower lid can be retracted with the fore-finger, then the lower lip of the lens inserted into the lower fornix, the upper lid is then lifted over the upper lip of the lens as the patient is instructed to look forward and the lens quickly pivoted onto the cornea with the release of the upper lid. The quicker this is done the less uncomfortable it is for the patient.
When using the single – and three-mirror lenses it is advisable to locate the arc-shaped mirror superiorly for orientation purposes. In this position, the inferior portion of the angle is available for inspection and as this is usually the widest part of the anterior chamber angle it enables the novice to more easily work out the relationship of the structures located there.
The left hand should be used to hold the lens when viewing the right eye. The hand may be supported and held steady by the forehead strap or by resting the elbow on the slit lamp table. For those with short arms, a tissue box placed on the slit lamp table can provide added support
By moving the slit lamp forward to focus on the reflected mirror image and by rotating the lens between forefinger and thumb gently through 360 degrees (less for the Thorpe and Sussman four-mirror goniolenses) the anterior chamber angle can be observed.
The goniolens can be removed by parting the lids so that they clear the lip of the lens. In most cases, the lens will fall from the eye
If the lens remains attached to the cornea by capillary attraction, the lateral sclera adjacent to the rim of the lens should be pressed firmly with the tip of one finger to break the capillary attraction holding the lens in place.
Hints and tips
The size of most goniolenses can make this procedure seem initially daunting for the patient and practitioner. It is in the interests of both that the practitioner learns how to insert the lens smoothly and quickly, even if the patient becomes anxious during the insertion stage. Patient confidence in the practitioner will decrease if repeated attempts are required to insert the lens.
This often happens with a novice, as they are usually too preoccupied with the patient’s reaction as the lens is placed on the lower ready for insertion and not concentrating on the task at hand.
Once the lens is in place most beginners will mistake the optical artefact produced within the cornea for features of interest. See figure for a view of the main structures in the anterior chamber angle.
The cornea appears milky white due to internal reflection and is of little interest. A brown thick line within this opaque region may be interpreted as the trabecular meshwork or the ciliary body. The novice should in fact approach the view from the other side of the angle and locate first the iris, then the last role of the iris and then the iris root. Just above this will lie the structures of interest
Deliberate compression (dynamic gonioscopy) gives the observer a certain amount of control over the iris configuration. In an eye with a relatively narrow-angle, deeper structures can be visualised by flattening the periphery of the iris gonioscopically. It is also used to distinguish between true peripheral anterior synechiae and simple apposition of the iris to the cornea. Note the type of mirrored lens that was used e.g. 3-mirror
There are several types of notation system available that can be used to describe the openness of the anterior chamber angle. They are all cumbersome to use in practice and it is acceptable to make a note of the most posterior structure visible in each quadrant of the angle.
Draw a large X where each compartment represents a quadrant of the angle. The following abbreviations are suggested: CB ciliary body; SS scleral spur; TM trabecular meshwork; SL Schwalbe’s line.