Visual
impairment
Macular degeneration
Cataract
[Glaucoma]
Diabetic retinopathy
Glaucoma
What is Glaucoma?
Glaucoma is a term to describe a group of diseases in which characteristic,
progressive loss of optic nerve cells, manifesting as changes in the optic
nerve head with matching changes in the field of vision of one or both
eyes. The optic nerve head (optic disc) is examined in stereo view by
an ophthalmologist at a slit lamp bio-microscope. The field of vision
is usually assessed using a test called automated perimetry.
In approximately two thirds of glaucoma cases, the level of pressure within the eye (termed "intraocular pressure") is higher than normal and there may be some resistance to the flow of aqueous. Elevated intraocular pressure was previously considered an essential part of the diagnosis of glaucoma. It is now known that raised intraocular pressure is not always present. This sign, however, is now regarded as the most important "risk factor" for glaucoma. Intraocular pressure is measured using a tonometer which gently touches the surface of the eye, after local anaesthetic.
Glaucoma has been termed the "sneak thief of sight" because the loss of visual field often occurs gradually over a long time and may only be recognised when it is already quite advanced. Once lost, this damaged visual field can never be recovered.
The Blue Mountains Eye Study (BMES) was the first large survey of an Australian population to accurately assess the proportion of Australians with glaucoma. The study found that 3% of the population aged 50 years or older had signs of glaucoma. The frequency rose exponentially from less than 0.5% in people aged under 60 years to over 11% of those aged 80 years or older.
Combining this information with data from the Melbourne Visual Impairment Project, a BMES report predicted that over 150,000 Australians have glaucoma. All would benefit from treatment (eyedrops to lower intraocular pressure). Of this group, only half are currently diagnosed. This suggests that there are around 75,000 Australians whose glaucoma is not yet detected, but which could be detected if they were appropriately screened.The data indicate that elevated eye pressure without optic nerve or visual field damage (called ocular hypertension) is present in a further 275,000 Australians. These people could be considered at risk of developing glaucoma. Some could be considered for treatment while others simply need to be followed.
Risk Factors
Elevated pressure within the eye (intraocular pressure) is the
most important "risk factor" for the development and progression
of glaucoma. However, although the risk of glaucoma appears to increase
continuously with higher levels of intraocular pressure, no threshold
level has been found for development of optic nerve and visual field damage.
The BMES is one of the few population-based studies to comprehensively assess the systemic factors and ocular signs associated with glaucoma in an older community. The independence of these associations was evaluated in statistical models (logistic regression), taking into consideration the effects of age and other factors.
Systemic risk factors
for glaucoma
Increasing age was strongly associated with glaucoma. The risk was
around 8% higher for each year of age (from age 50). Other systemic risk
factors found significantly associated with glaucoma were:
- family history of glaucoma in a brother, sister or parent (3-fold increased risk)
- in people with a family history, use of inhaled steroids (3-fold increased risk)
- diabetes (2-fold increased risk)
- history or signs of hypertension (high blood pressure); 80% increased risk.
Ocular signs of glaucoma
Typical changes in the appearance of the optic disc suggesting the possibility
of glaucoma include thinning of rim of the optic disc seen at the back
of the eye or enlargement of the depression usually seen in the optic
nerve called cupping.
Other eye signs found significantly associated with glaucoma in the statistical model were:
- elevated intraocular pressure over 21 mm (5-fold increased risk)
- 3 mm or more difference in intraocular pressure between the 2 eyes (3-fold increased risk)
- asymmetry (0.2 or higher) of cupping between the 2 optic discs (4-fold increased risk)
- myopia (short sightedness) of 1 dioptre or more (2-fold increased risk)
- atrophy around the optic disc, termed peripapillary atrophy (3-fold increased risk)
