The optic nerve (cranial nerve II) is a continuation of the axons of the ganglion cells in the retina) and transmits visual information from the retina to the brain.
The optic nerve is made up of about 1 million small individual wire-like nerve fibers that come from the retina. The optic nerve begins at the optic disk (also known as the optic nerve head), a structure that is 1.5 mm (0.06 inch) in diameter and is located at the back of the eye. The optic disk forms from the convergence of ganglion cell output fibres (called axons). The fibers bend about 90 degrees as they leave the retina and form the optic nerve. Since there are no photo-receptors (cones and rods) in the optic nerve head - where the optic nerve leaves the eye - this area cannot respond to light stimulation. As a results it is known as the 'blind spot' and everybody has one a blind spot in each eye. The reason we normally do not notice our blind spots is because, when both eyes are open, the blind spot of one eye corresponds to retina that is seeing properly in the other eye and the visual fields overlap.
Normally, there is a small crater-like depression seen at the front of the optic nerve head. This depression is known as the cup. Its diameter is smaller than the diameter of the optic nerve. Looking at the nerve with a magnification device, the nerve head looks like a cup on a saucer (or disc). The normal cup to disc ratio (the diameter of the cup divided by the diameter of the whole nerve head or disc) is about 1/3 or 0.3. Glaucoma can cause the cup to enlarge.
In glaucoma the optic nerve is damaged because the intraocular pressure (IOP) is higher than the retinal ganglion cells can tolerate. An internal pressure more than that which the eye can tolerate can deform the lamina cribrosa, the small cartilaginous section of the sclera at the back of the eye through which the optic nerve passes.This eventually results in the death of the ganglion cells and their axons. However, although glaucoma typically is associated with elevated IOP, the amount of pressure which will cause glaucoma varies from eye to eye and person to person. Many people actually have IOP's in the normal range ('low tension' glaucoma), possibly indicating that their lamina cribrosa are too weak to withstand even normal amounts of pressure. Conversely, many people with IOP's which would be considered high have no evidence of glaucomatous damage or visual field test loss.
Glaucomatous changes in the optic disk (optic nerve head) usually can be detected over time. If the optic cup within the optic disk increases in size over a period of months or years, if notching is observed anywhere around the nerve head rim and / or asymmetry is observed between the optic cups of the two eyes, then that person may be considered to be a 'glaucoma suspect'. The optic nerve damage corresponds to visual field decrease.
The Visual field test is used to help decide if an unusual looking optic nerve is glaucomatous or not (or whether the known glaucomatous nerve is getting worse). Determining that requires multiple visual field test exams. But while rapid progression is usually detectable with only a few exams, smaller rates of change require many visual field tests and usually a longer period of time to confirm. Visual field abnormalities occur with many optic nerve diseases. The glaucomatous pattern of visual field loss is typically on the nasal side of the field and usually is more dense at the top or bottom of the field .
In advanced Glaucoma, the visual field in the peripheral retina is decreased or lost, leaving vision in the central retina (macular area) intact. This result in 'tunnel vision' .