Intraocular eye pressure is related to the amount of fluid that naturally occurs in the eye. The reasons people develop low or high intraocular eye pressure and not well understood, but age, family history and past trauma to the eyes usually are considered. A low intraocular eye pressure reading that is still within normal range is a healthy condition, but a low reading beyond the normal range creates a condition called ocular hypotony.
Intraocular pressure is a measurement of the fluid pressure inside the eye. This fluid, called aqueous humour, nourishes the eye and helps it maintain its globular shape. The typical eye produces, circulates and drains about 4 cubic centimetres of fluid a day. When measured by an eye doctor with a device called a tonometer, normal eye pressure ranges from 10 and 21mm of mercury, with an average reading of 16mm.
The causes of normal variations in intraocular eye pressure are not fully understood. Low pressure (from 10 to 15mm of mercury) generally is considered beneficial because the opposite condition, high intraocular pressure (more than 21mm of mercury), is a major cause of glaucoma, which leads to blindness. High intraocular pressure results with the aqueous humour liquid does drain properly from the eye. A low normal reading is not a usually a cause for concern, although glaucoma does sometimes occur in those with low intraocular pressure--again, for medically unknown reasons--with a condition called low-tension glaucoma.
Ocular hypotony is a condition that results in a very low intraocular reading, usually 5mm of mercury or less. This condition also is related to the flow of aqueous humour liquid and can cause damage to the eyes. Hypotony is usually the result of some type of trauma to the eye, and it infrequently occurs after procedures such as glaucoma surgery.
While the causes of variation in eye pressure are not always clear, physical activity, reduced stress and reduced caffeine have been shown to lower intraocular pressure readings.
The only way to determine the level of intraoctular eye pressure is an examination by an eye doctor. A low pressure reading within normal range is a healthy result but does not necessarily mean glaucoma won't develop. Any type of trauma to the eye, which may cause very low intraocular eye pressure, should be addressed by a doctor immediately.