Intraocular Pressure (IOP) is the measure of the fluid pressure inside the eye, maintained by a balance of production and drainage of the aqueous humor. This internal pressure keeps the eye properly inflated and maintains its spherical shape for clear vision. While high eye pressure is often associated with glaucoma, low eye pressure, known as hypotony, also poses a risk to eye health. Hypotony is a serious condition that can lead to permanent structural damage and requires prompt medical attention.
Defining Hypotony and Normal Pressure Ranges
The eye’s internal pressure is regulated by the ciliary body, which constantly produces a clear fluid called aqueous humor, and the drainage system that removes it. A normal range for intraocular pressure is generally considered to be between 10 and 21 millimeters of mercury (mmHg). The average value for a healthy eye is approximately 15.5 mmHg.
Hypotony is typically defined as a sustained IOP below a specific threshold, often cited as 6.5 mmHg or less. This low pressure indicates an imbalance where there is either insufficient production of aqueous humor or excessive drainage. When the pressure drops too low, the eye’s delicate internal structures can begin to collapse or distort.
Common Reasons for Abnormally Low Eye Pressure
Low IOP stems from either a leak in the eye’s outer wall or a suppression of the fluid-producing mechanism. The most frequent cause is often related to eye surgery, particularly glaucoma filtration procedures like trabeculectomy. In these cases, the surgical site, or filtration bleb, may become over-filtering, causing the aqueous humor to exit the eye too quickly.
Trauma to the eye is another significant cause, including penetrating injuries or blunt force that results in a rupture of the globe’s outer layers. Even without a full rupture, trauma can create a cyclodialysis cleft, a separation between the ciliary body and the sclera, providing an abnormal route for fluid to drain. Severe intraocular inflammation, known as uveitis, can suppress the ciliary body’s function, leading to a decreased production of aqueous humor.
A retinal detachment, especially one complicated by scarring (proliferative vitreoretinopathy), can cause the ciliary body to shut down fluid production.
Serious Ocular Complications from Hypotony
The most significant consequence of prolonged hypotony is hypotony maculopathy. This occurs when the sustained low pressure causes the scleral wall to collapse inward, leading to folds in the choroid and retina. These folds distort the macula, the central part of the retina responsible for sharp, detailed vision, resulting in vision loss and distortion.
Sustained low pressure can also cause swelling of the optic nerve head, referred to as optic disc edema. This swelling is thought to result from restricted flow within the nerve fibers due to the inward bowing of the surrounding tissue. Other complications include accelerated cataract formation and corneal decompensation (clouding and swelling of the cornea). Chronic, severe hypotony can eventually lead to a shrunken and non-functional eye, a condition called phthisis bulbi.
Medical Management to Restore Eye Pressure
Treatment for hypotony is highly dependent on identifying and reversing the underlying cause of the pressure drop. Diagnosis typically involves tonometry to measure the low IOP, along with imaging techniques like ultrasound to assess internal damage and leakage. If the cause is inflammation, such as uveitis, aggressive use of topical or systemic corticosteroids is implemented to restore normal aqueous humor production.
In cases of a surgical leak or over-filtration, non-surgical approaches are often attempted first. These can include applying compression sutures, using a large bandage contact lens, or injecting autologous blood near the filtration site to promote controlled scarring and reduce the leak. If conservative measures fail, surgical intervention may be necessary to repair the leak or revise the over-filtering bleb. For severe cases where the eye’s shape is compromised, an injection of a viscous material, such as sodium hyaluronate, may be temporarily used to manually inflate the anterior chamber and restore structural integrity.