How does glaucoma develop?
Glaucoma is often, but not always, the result of increased intraocular pressure
Raised intraocular pressure exerts stress on the retinal ganglion cells and slowly kills them; their gradual loss results in glaucoma. The level of IOP at which ganglion cells die depends on the susceptibility of the optic nerve and varies from one person to another. Raised intraocular pressure is the consequence of increased resistance against the outflow of intraocular fluid at the level of the trabecular meshwork (cfr. Understanding the function of the eye). The fact that trabecular outflow resistance increases with age explains why the prevalence of glaucoma is higher in older age groups.
Not every person with raised IOP develops glaucoma
Many people will develop glaucoma if their IOP exceeds 21 mm Hg, but not everyone! The fact that your IOP is slightly above 21 mm Hg does not necessarily mean that you should be treated. The eye doctor will examine your optic nerve, the nerve fibre layer and your visual fields and if everything is normal, treatment is not yet indicated. The association of raised intraocular pressure in the bbabsence of any glaucomatous damage is called “ocular hypertension”. However, ocular hypertension is a risk factor for glaucoma and if you have this condition regular follow up, once or twice a year, is mandatory.
Not every person with a normal IOP is safe
Most people with an IOP below 21 mm Hg will not develop glaucoma, but some patients do develop glaucoma despite the fact that their IOP is within the normal range. Therefore, an normal isolated pressure reading (e.g. by an optician) does not exclude the possibility of glaucoma and may confer a false sense of security! A comprehensive examination including visual fields, nerve fibre layer analysis and optic nerve head observation sometimes shows the typical characteristics of glaucoma in patients with normal IOP. These “normal tension glaucoma” patients probably have an optic nerve that is more vulnerable. Vascular factors such as a disturbance in the perfusion of the optic nerve may play a role.