Just like a football needs air inside to maintain its (spherical) shape, our eyes also need something (in this case, a fluid called aqueous humor or simply aqueous) to maintain its shape and perform its visual function. Aqueous is constantly formed within the eye and drained out of the eye at the same time. There is a fine balance between these two processes. Any reduction in the formation of aqueous results into a soft eye whereas any obstruction in the outflow of aqueous leads to high intraocular pressure (IOP) which is an important risk factor for glaucoma.
There is lot of confusion especially among the glaucoma patients about what the normal IOP range is and what we mean by ‘high pressure’. This confusion stems from the fact that intraocular pressure in the past was included in the definition of glaucoma but now in the light of new information and research, has been excluded from the definition of glaucoma and is considered to be a major risk factor for glaucoma.
Currently normal range of intraocular pressure is considered to be between 10 and 20 mmHg (millimeter of Mercury) by some while 10-21 mmHg by others but in reality the level of intraocular pressure is not really associated with absence or presence of glaucoma.
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It would be natural to assume that if the intraocular pressure is normal, there will be no glaucoma and if the IOP is higher than the normal range, the eye will have glaucoma. But that is not the case. Consider the clinical scenarios as shown in the picture above.
1. In a condition called ‘Ocular Hypertension‘, the intraocular pressure is higher than normal (for example 25 mmHg) but there is no damage to the optic nerve and there is no loss of visual fields. These two features are important criteria for the diagnosis of glaucoma.
2. In another condition called ‘Normal Tension Glaucoma‘, the IOP is within normal range but there is damage to optic nerve and loss of visual field.
This simply means that for every individual, the level of intraocular pressure which can cause glaucomatous damage is different. For example an IOP of 18 mmHg may be normal for one individual but may cause glaucoma in another one. Similarly an IOP of 25 mmHg may cause glaucoma in some people while others may be completely normal with the same pressure.
Since there is no specific level of high IOP which would cause glaucomatous damage and there is no specific level of low IOP which could be considered safe, the IOPs should be individualized and should be taken into account when it comes to the treatment and prognosis of glaucoma.