What is gonioscopy?
This is not really a ‘test’ but rather one part of the clinical examination at the slit-lamp microscope. It does require a special diagnostic lens however, which contains mirrors that allow the examiner to overcome the total internal reflection which otherwise precludes a view of the iridocorneal angle.

What is the iridocorneal angle?
It is the angle between the iris root and the cornea, that extends 360 degrees all the way around the anterior chamber of the eye. It is the site of Schlemm’s canal, which is where fluid drains out of the eye.

What problems can occur at the iridocorneal angle?
The most common problem is that it becomes too narrow, or sometimes even closed, leading to acute or chronic rise in intraocular pressure and potential visual field loss from glaucoma. This is more common in long-sighted individuals (i.e. with a ‘plus’ spectacle lens distance prescription) because the eye itself is shorter in length. Other problems with the angle, all of which can be diagnosed on gonioscopy, include:

• New, abnormal blood vessels growing in the angle (rubeosis iridis) – this can happen in diabetic retinopathy, ocular ischaemic syndrome, central retinal vein occlusion, and very rarely in central retinal artery occlusion. The new blood vessels can permanently occlude the angle and cause a type of secondary glaucoma. They are usually treated either with retinal laser therapy and/or intravitreal injections.
• Adhesions between the iris root and the angle – these can occur due to chronic touch between the iris and the cornea, or sometimes because of inflammation or a variety of very rare conditions affecting the anterior chamber.