The eye is a pressurized structure, requiring a specific internal fluid pressure, known as intraocular pressure (IOP), to maintain its shape and function. When the intraocular pressure drops too low, the condition is medically termed hypotony. Hypotony signals a disruption in the fluid balance of the eye and, if sustained, can lead to serious and permanent damage to vision.
Defining Low Intraocular Pressure
The eye maintains its internal pressure through a continuous cycle of production and drainage of a clear fluid called aqueous humor. The normal range for intraocular pressure typically falls between 10 and 21 millimeters of mercury (mmHg). Hypotony is generally considered to be present when the IOP drops below 6.5 mmHg, although some definitions use a threshold of 5 mmHg or less, especially when discussing clinically significant changes.
The measurement of this pressure is accomplished using a device called a tonometer, with Goldmann applanation tonometry often considered the standard clinical method. This technique involves gently flattening a small area of the cornea to determine the internal fluid force. The resulting pressure reading is expressed in millimeters of mercury (mmHg). The pressure level alone is a statistical measure, but the presence of associated visual symptoms makes the condition clinically significant.
Primary Causes of Hypotony
Hypotony occurs when the production of aqueous humor decreases significantly or, more commonly, when its outflow increases excessively. The origin of this imbalance often stems from three main categories: complications following eye surgery, physical trauma, or internal inflammation.
The most frequent cause of sustained hypotony is a complication from glaucoma filtering surgery, such as a trabeculectomy. These procedures are designed to create a new drainage pathway, but sometimes this pathway, known as a filtering bleb, drains the fluid too quickly, a condition called over-filtration. A separate issue is a wound leak, where the aqueous humor escapes through an incompletely healed surgical incision.
A sudden drop in pressure can also be precipitated by blunt or perforating trauma to the eye. An injury can cause a cyclodialysis cleft, which is a tear that separates the ciliary body from the sclera, creating an abnormal, direct pathway for the aqueous humor to drain into the suprachoroidal space. This rapid loss of fluid lowers the intraocular pressure.
Inflammation inside the eye, such as iridocyclitis (inflammation of the iris and ciliary body), can suppress the function of the ciliary body. This suppression leads to a dramatic decrease in the production of aqueous humor. Systemic conditions or the use of certain medications can similarly suppress aqueous humor production, contributing to hypotony.
Serious Effects of Sustained Hypotony
When the eye remains under-pressurized for an extended period, it loses its structural integrity, leading to mechanical changes that impair vision. The most notable complication is hypotony maculopathy, which involves the swelling and folding of the retina and choroid. The low pressure causes the outer wall of the eye, the sclera, to collapse inward, wrinkling the delicate inner layers.
These chorioretinal folds often radiate outward from the center of the retina, causing central vision loss and image distortion, known as metamorphopsia. The mechanical changes can also lead to a hyperopic shift, making the patient more farsighted. Another common complication is choroidal detachment, where fluid accumulates between the choroid and the sclera (the suprachoroidal space).
Choroidal detachment, involving fluid or blood separation of the choroid layer, can further reduce aqueous humor production and worsen the hypotony. The inward collapse of the eye and the resulting displacement of internal structures can also cause the anterior chamber to become shallow. If the reduced pressure is left untreated, it may eventually lead to phthisis bulbi, a severe, shrunken, and non-functional state of the eye.
Treatment and Management Strategies
The management of hypotony is directly dependent on identifying and correcting the underlying cause of the low pressure. The overarching goal is to restore the intraocular pressure above the critical threshold to prevent permanent damage to the retinal structures.
If the hypotony is due to a surgical complication like a wound leak or over-filtration, non-surgical techniques may be attempted first. These can include pressure patching, the application of a bandage contact lens, or the injection of autologous blood to encourage scarring and closure of the leak. For a persistent leak or over-filtering bleb, surgical revision, such as resuturing the scleral flap, may be required to tighten the drainage pathway.
When inflammation is the cause, topical or systemic corticosteroids are the mainstay of treatment, as they work to suppress the inflammatory response and subsequently increase aqueous humor production. In cases of cyclodialysis clefts or ciliary body detachment, surgical intervention is often necessary to repair the structural defect. Prompt action to normalize the IOP is necessary, as delayed treatment can result in irreversible chorioretinal changes and a poor visual outcome.