Intraocular pressure (IOP) is the fluid pressure maintained within the eye. This pressure is fundamental because it provides the necessary structural support to keep the eyeball’s shape and ensure the proper alignment of its optical components. Without adequate pressure, the eye cannot function correctly, leading to distorted vision and potential damage to the retina and optic nerve. Maintaining IOP within a specific, narrow range is paramount for long-term visual health. While high pressure is associated with conditions like glaucoma, pressure that is too low also poses a significant threat to the eye’s integrity and visual capability.
Understanding Normal Intraocular Pressure
Eye pressure is measured in millimeters of mercury (mmHg). For most healthy adults, the normal range for intraocular pressure falls between 10 mmHg and 21 mmHg. This range is maintained by the continuous circulation of a clear fluid called aqueous humor, which fills the space between the lens and the cornea.
The ciliary body, located behind the iris, constantly produces this aqueous humor. It then flows out of the eye through a drainage system, primarily the trabecular meshwork. This constant cycle of production and outflow ensures a stable IOP, which is essential for preserving the eye’s spherical shape. When the production rate equals the drainage rate, the pressure stabilizes within the healthy range.
Defining Clinically Low Eye Pressure (Hypotony)
When pressure drops below the normal range, the condition is referred to by the clinical term “hypotony.” While any pressure consistently below 10 mmHg is statistically low, the level considered clinically dangerous is generally defined as an IOP persistently measuring 5 mmHg or less. Some clinicians may use 6.5 mmHg as a threshold for concern, especially if structural changes are present.
Hypotony becomes clinically significant when low pressure begins to compromise the structure and function of the eye, leading to observable damage. The severity of resulting vision problems correlates with how far the IOP has dropped and for how long the pressure remains low. This distinction separates a statistically low reading from a pathological condition that requires intervention.
Common Causes of Hypotony
Hypotony results from an imbalance in the aqueous humor dynamics, either through inadequate production or excessive fluid drainage.
One major category of causes is a reduction in the ciliary body’s ability to produce aqueous humor. This can be triggered by severe inflammation, such as uveitis. Chronic intraocular inflammation can damage the ciliary body’s secretory tissue, leading to ciliary body atrophy and a permanent reduction in fluid generation. Certain systemic or topical medications can also suppress production as a side effect.
The second, and more common, cause involves excessive outflow or leakage of the aqueous humor from the eye. A frequent etiology is a complication following glaucoma filtering surgery, such as a trabeculectomy, where the surgically created drainage pathway (filtering bleb) over-functions, allowing too much fluid to exit the eye too quickly.
External leakage can also occur from an undetected wound leak following eye surgery or from a penetrating injury. Internally, a cyclodialysis cleft (a separation between the ciliary body and the sclera) can create an abnormal drainage route, shunting fluid directly into the suprachoroidal space and causing severe, chronic hypotony.
Effects of Hypotony on Eye Structure and Vision
Sustained low pressure causes the eye wall to lose structural rigidity, leading to serious consequences for vision. The lack of adequate pressure permits the scleral wall to collapse inward, causing the delicate internal layers of the eye to wrinkle and fold.
The most damaging results include:
- Hypotony Maculopathy: This condition is characterized by folds in the retina and choroid near the macula, the area responsible for sharp central vision. These folds distort the light-sensing tissue, leading to a decline in vision and often a shift toward farsightedness.
- Choroidal Detachment: Fluid accumulates between the choroid and the sclera, pushing the choroid and retina inward and further reducing aqueous humor production.
- Corneal Edema: If the pressure remains critically low, the cornea can begin to swell, resulting in vision blurring.
- Phthisis Bulbi: In the most severe, chronic cases, the eye may shrink and become non-functional.