Second Year 99/00
Clinical Optometry 3
A procedure that enables the examination of the periphery of the anterior chamber angle.
Anterior chamber angle anatomy
The anterior chamber angle is not normally visible, since light reflected by the structures posterior to the limbus is totally reflected within the 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 and Scwalbe's line.
Before the angle is assessed it is important to be familiar with the features of a normal angle. There are six ocular structures normally available for observation. The appearance of the anterior chamber angle varies according to congenital individual differences and with acquired changes due to age, injury, or disease. From anterior to posterior the gonioscopic landmarks consist of cornea, Scwalbe's line, trabeculae, scleral spur, cilary body and the last roll of the iris (Fuch's roll).
Scwalbe's line forms the termination of Descement's membrane and marks the transition from the transparent cornea to opaque scleral tissue. It also forms the anterior boundary of the trabeculae. It is a difficult structure to find and to observe.
The trabeculae tends to be pale pink to pale brown in colour and is arbitrarily divided into anterior and posterior portions, the former is usually paler. Sclemm's canal runs in the latter. They each occupy half the width of the trabecular band.
The scleral spur is a projection of the sclera beneath the trabeculae. It is whiter than any of the other structures making it the most easily recognised landmark.
The cilary body lies posterior to the scleral spur and varies in colour from pink to dark brown but is always more pigmented than the trabeculae.
Indications for gonioscopy include:
Gonioscopy is carried out using goniolenses. These effectively eliminate the cornea as a refracting surface by the use of a concave contact surface placed against the cornea. Any small difference in curvature between the lens and the fluid is minimised by interposing a fluid between the two surfaces (e.g. Total contact lens solution, artificial tears or CibaVision Viscotears).
Since the most commonly used diagnostic goniolenses are the Goldmann single-mirror and three-mirror lenses, only their use will be discussed here. The techniques required for the use of other types of lenses (e.g. Zeiss and Koeppe) are similar.
In its three-mirror form the Goldmann goniolens permits: (i) angle examination using the arc-shaped internal mirror, (ii) viewing of the mid- and far-periphery of the fundus using the additional mirrors, (iii) examination of the macular using the central lens. This has a 7.4mm concave radius, a 12-mm internal diameter and an 18-mm overall diameter.
It is good practice before commencing gonioscopy (and any other invasive procedure) to explain to the patient what is going to happen and why it is being done. This will reduce patient anxiety and improve co-operation. As with all invasive procedures corneal health should be assessed using fluroscein in conjunction with blue light.
The patient's cornea should be anaesthetised with two drops of benoxinate 0.4% topical anaesthetic (note, approximately one in one thousand, usually over 50 years of age are allergic to this, and a localised or diffuse corneal desquammation may occur. The reaction develops quickly and lasts for an hour. The corneal epithelium regenerates spontaneously and recovery can be aided with artificial tears). Alternatively proxymetacaine 0.5% can be used.
The patient should be positioned comfortably at the slit lamp with the magnification set at approx. x15 and the illuminating slit to its wide-open position. Bubble free fluid should be placed into the concavity of the Goldmann lens. With the patient looking directly ahead, the lower lip of the lens is then inserted into the lower fornix, then the upper lid is lifted over the upper lip of the lens. The D-shaped mirror should initially be located in the twelve o’clock position.
The angle can now be viewed by rotating the lens gently through 360 degrees. If a slit lamp is not available, a reasonable view can be obtained through an ophthalmoscope with a high plus lens in the viewing system. The goniolens can be removed by parting the lids so that they clear the lip of the lens. In some 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 so as to break the capillary attraction that is holding the lens in place.
There are several grading systems that depend upon the visibility of anatomical landmarks. The most widely used classification is that of Kolker and Hetherington, in which the widest angles are designated grade 4, through narrow angles to the occluded angle which is graded as zero (note, do not confuse this classification with Van Herick's). It is more practical in the clinical setting to describe in words the extent of the visibility of the structures seen.
The risk of angle closure glaucoma associated with pupil dilation can be assessed using a goniolens. The chances of this occurring are low and in some hospital eye departments closing of the angle following dilation is considered to be a provocative test and beneficial to the patient in the long term.
Optometrists and gonioscopy
The number of optometrists presently using a goniolens in private practice is very small. However with optometry as a whole becoming more clinically orientated, and with the advent of shared care schemes involving glaucoma and diabetic patients, this number is set to grow.