What is ocular hypertension?
Ocular hypertension simply means a raised pressure within the eye. It is not glaucoma, although in many cases people with glaucoma also have a raised pressure within their eyes and it does mean that someone with ocular hypertension is at increased risk of developing glaucoma, which is why it is most important for people with ocular hypertension to be monitored carefully in order that any glaucoma that does develop is detected at the earliest possible stage when treatment is most effective.
What is meant by 'raised pressure'?
Broadly speaking, the average pressure is about 16 mm Hg. The upper limit of 'normal' pressure is about 21 mm Hg. An eye is considered to have ocular hypertension if it is consistently above that level. The risk of developing glaucoma rises with rising pressure and it has been shown that the risk of developing glaucoma is about ten times greater if a person has pressures between 21 and 29 than if the pressure is below 21 mm Hg. This is why everyone with ocular hypertension should be monitored carefully for the development of glaucoma and why some people have treatment to reduce the pressure to a more 'normal' level even when they don't have glaucoma, i.e. in order to prevent the development of glaucoma.
Latest research suggests that 'normal' pressure in a Japanese population may be considerably lower than for other ethnic groups. People of Japanese origin should therefore be carefully examined in order to exclude the possibility of glaucoma. It is not yet known if this is similar for other Asiatic peoples.
What creates pressure within the eye?
Eye pressure (intraocular pressure) is controlled by a watery fluid called aqueous humour, which fills the front part of the eye. This fluid is made in the ciliary body (a ring of tissue behind the coloured part of the eye, which is called the iris). It flows through the pupil and drains away through tiny drainage channels called the trabecular meshwork. This is situated in the drainage angle between the cornea (the clear window at the front of the eye) and the iris. In a 'normal' eye there is a balance between the production and the drainage of this fluid, but in some eyes, there is an imbalance. Most cases of ocular hypertension occur because the flow of fluid out of the eye becomes restricted and the pressure in the eye rises.
Flow of aqueous humour in the eye
Outflow of aqueous humour through the drainage angle