Gonioscopy

To view the angle. It's Greek. Specifically it's to view the iridocorneal angle. The primary use of gonioscopy is in glaucoma. There are a few other uses that i'll cover later. Barkan defined the different types of glaucoma with the help of gonioscopy as well as being the first to notice that the superior angle was narrower than the inferior angle. He classified glaucoma into closed and open angle. You can't view the angle with slit lamp alone. The originator of this whole thing was Maximillian Salzmann in 1915. He realised that total internal reflection obscured the anterior angle. He used a Fick type contact lens with radius smaller than that of the cornea. Then in 1919 with the development of the slit lamp a magnified view of the cornea could be obtained and with it a magnified view of the angle. In 1938 after a few less exciting developments that I can't be bothered to cover Goldmann introduced the first indirect gonioscopy mirror lens.

When I say an 'indirect' lens that means a truncated prism which has a surface that basically fits the cornea. Light from the anterior angle comes out, hits a mirror and is reflected into your own human eye via the slit lamp. The difficulty involved in viewing the angle is due to the critical angle of reflection at the cornea/air interface. The lens replaces the eye/air interface and the critical angle is eliminated by the steeply curved outer surface of the lens. Direct gonioscopy (using eg the Koeppe lens) produces an image round to the side of the lens which is much less easy to observe.

Direct Gonioscopy - Koeppe Lens

The Koeppe lens resembles a very thick high plus lens. It only has x1.5 magnification so has to be used in conjunction with another magnifying device like a hand slit lamp. The image produced is erect and virtual

Advantages - panoramic binocular view, good view of narrow angles, transillumination, can be used on bedridden patients, wide fov for teaching purposes

Disadvantages - time consuming, requires large working area as px really needs to be supine. You would have to walk all the way around the patient in order to see the full 360 degree view the lens provides. You might even need an assistant. It needs separate mag and illum, mag is low and it's not great for fine detailed view (eg you aren't going to see schwalbe's line)

Indirect Gonioscopy

Advantages - focal illumination allows the location of schwalbe's line. If you can't see schwalbe's line then you have a very narrow angle. The view is magnified and is excellent for picking out fine detail. It's technically simple to use and useful for lazer treatment.

Disadvantages - stereopsis is difficult, requires coupling fluid which can be a bit messy, observation is reversed and fov is small. An anaesthetic (eg proxymetacaine) is used.

Grading System

There have been many over the years and to analyse them all now would cause me great confusion. So we're going to go with the classic one - Schaffer.

• Grade 4 = wide open = 40 degrees
• Grade 3 = open = 30 degrees
• Grade 2 = just about showing trab meshwork = possible closure = 20 degrees
• Grade 1 = 10 degrees = eventual closure
• Grade S = <10>
• Grade 0 = closed angle

The Schaffer grades correlate well with Van Herick's slit lamp grades for anterior depth. When doing gonioscopy there is a 'goniogram' which is a generic sheet on which you can mark the appearance and grades along with what's visible. This is great to reduce confusion between optom and ophthal or whoever.

Structures

From Cornea

• Schwalbe's Line
• Non Pigmented Trab Meshwork
• Pigmented Trab Meshwork
• Scleral Spur
• Ciliary Body
• Iris Root/Insertion