Intraocular pressure (IOP) is the fluid pressure inside the eye, necessary to maintain its spherical shape and proper function. This pressure is regulated by a continuous inflow and outflow of aqueous humor, a clear fluid that nourishes the front structures of the eye. A stable IOP ensures that delicate tissues, particularly the retina and optic nerve, are correctly supported. When the balance between fluid production and drainage is disrupted, the pressure can become too high or too low. Abnormally low pressure is known as ocular hypotony, which can lead to significant structural changes and vision problems if not addressed promptly.
The Definition of Ocular Hypotony
The health of the eye depends on its internal pressure remaining within a specific range, typically measured between 10 and 21 millimeters of mercury (mmHg). Ocular hypotony is generally defined by an intraocular pressure reading that is equal to or less than 5 mmHg, though the threshold causing damage can vary. Eye pressure is measured using tonometry, which often involves gently touching or applying a puff of air to the cornea. True hypotony involves sustained low pressure that begins to cause visible structural changes within the eye, risking vision loss.
Why Eye Pressure Drops
Low eye pressure occurs when there is either an excessive loss of aqueous humor or a severe reduction in its production by the ciliary body. The most common causes relate to previous eye surgeries. For instance, glaucoma filtering surgeries, such as trabeculectomy, sometimes result in excessive filtration, leading to an “over-filtering bleb.” Fluid can also leak externally through a non-sealing surgical wound following procedures like cataract or glaucoma surgery.
Decreased Aqueous Production
Other causes involve a decrease in aqueous humor production, often linked to inflammation or damage to the ciliary body. Severe intraocular inflammation (uveitis) can temporarily suppress the ciliary body’s ability to secrete fluid. Blunt trauma can lead to a cyclodialysis cleft, a tear that separates the ciliary body from the sclera. This separation creates an abnormal internal pathway for the aqueous humor to drain, bypassing the normal outflow system. Additionally, certain medications designed to lower eye pressure for glaucoma can suppress aqueous production too effectively, leading to hypotony.
The Damage Caused by Low Eye Pressure
Sustained low pressure causes the outer wall of the eye (the sclera) to lose structural rigidity, changing the eye’s internal anatomy. This loss of shape compresses and distorts the inner layers, leading to vision-threatening complications.
Choroidal Detachment
One common manifestation is choroidal detachment, where fluid accumulates between the choroid and the sclera. This accumulation, known as choroidal effusion, causes the choroid layer to pull away from the outer wall, sometimes forming large, dome-shaped elevations. Severe fluid accumulation or hemorrhage can push the lens and iris forward, causing further complications.
Hypotony Maculopathy
The most severe consequence is hypotony maculopathy, which occurs when low pressure causes the retina and choroid to fold in the macula, the area responsible for sharp central vision. This folding (macula striae) causes significant visual distortion and reduced visual acuity. Patients often experience a shift toward being more farsighted due to the altered axial length of the eye, along with swelling of the optic disc and engorgement of the retinal blood vessels.
Strategies for Raising Eye Pressure
Treatment for ocular hypotony depends on accurately identifying the underlying cause, as simply raising the pressure without addressing the source is ineffective.
Addressing Excessive Drainage
When the cause is excessive external drainage, such as a wound leak after surgery, interventions focus on sealing the exit point. Non-surgical methods include applying a large bandage contact lens or using a tissue adhesive to cover the leak. For a persistent leak or an over-filtering bleb, a physician might inject the patient’s own blood into the area to promote scarring and seal the site. If the hypotony is due to a structural defect like a cyclodialysis cleft, surgical repair is necessary to reattach the ciliary body and restore normal fluid dynamics.
Addressing Production Issues
For hypotony caused by inflammation, the primary approach is to treat the underlying uveitis, typically with topical or systemic steroid medications, allowing the ciliary body to resume normal aqueous humor production. If the cause is a severe reduction in aqueous production due to ciliary body damage, medication that suppresses fluid production may be temporarily stopped, or pressure-increasing drops may be used. The ultimate goal is to restore the intraocular pressure to a safe level, which often leads to the prompt resolution of structural changes like choroidal folds and maculopathy. Sustained low pressure over a long duration, however, risks irreversible scarring and permanent vision loss.