Introduction to the eye
Behind the cornea is the iris - the coloured part of the eye - with the pupil forming a hole in its centre.
The space between the cornea and the iris is filled with a clear fluid, called 'aqueous humour', which maintains the pressure in the eye (the intraocular pressure).
The pressure is determined by a balance between the fluid entering and leaving the eye.
1. Flow of aqueous humour in the eye
Aqueous humour is produced behind the iris by the ciliary body. It flows through the pupil and drains away at the angle between the cornea and iris (the drainage angle).
2. Outflow of aqueous humour through the drainage angle
Within the drainage angle, the aqueous humour passes through a porous tissue – the trabecular meshwork – into a collector channel (Schlemm’s canal), which empties into veins under the conjunctiva and thus back into the bloodstream.
Most of the aqueous humour leaves the eye through the trabecular meshwork. This is called the conventional outflow pathway.
3. Uveo-scleral outflow of aqueous humour
However, some aqueous humour also leaves the eye through the ciliary body. This is called the uveo-scleral or non-conventional outflow pathway.
1. Structure of the optic nerve head (head on)
Behind the pupil, the lens of the eye is suspended from the ciliary body by fine ligaments. The cornea and lens focus a picture of your surroundings on the retina, which is the light-sensitive layer that coats the inside of the eye. The picture of your surroundings is sent from the retina to the brain by nerve fibres, which derive from nerve cells in the retina. The optic nerve is formed by about one million of these nerve fibres collected together. The optic nerve starts at the back of the eye at the optic nerve head, which is also called the optic disc.
2. Structure of the optic nerve (side on)
The nerve fibres leave the eye through pores (holes) in the lamina cribrosa, a sieve-like structure in the optic nerve head. Blood vessels enter and leave the eye through the same structure. The nerve fibres form a rim around the edge of the optic nerve head (neuro-retinal rim), leaving a central indentation without nerve fibres called the optic cup.
What is glaucomatous cupping?
Glaucoma causes nerve fibres to die and this results in loss of vision. As nerve fibres die, the neuro-retinal rim of the optic nerve head thins and the optic cup enlarges. This is called glaucomatous cupping.
Cross-section of a healthy optic nerve head
The loss of nerve fibres is usually patchy. This gives rise to patchy loss of vision, in which some parts of the picture of your surroundings are less distinct than other parts. This loss of vision can be picked up by the visual field test.
Cross section of a glaucomatous optic nerve head