Intraocular pressure (IOP) is the fluid pressure maintained inside the eye, typically measured in millimeters of mercury (mmHg). This pressure is a necessary force that keeps the eyeball inflated, ensuring the delicate structures within maintain their proper shape and function for clear vision. While the public is often aware of the dangers associated with high IOP, linked to glaucoma, pressure that is too low also represents a serious threat to eye health. When the pressure drops below a healthy threshold, the integrity of the eye is compromised, leading to a condition known as ocular hypotony. This disorder can cause significant and permanent visual impairment if not promptly addressed.
What is Ocular Hypotony?
Ocular hypotony refers to a state where the intraocular pressure drops below the range considered normal for a healthy eye. Normal IOP typically falls between 10 and 21 mmHg, a range regulated by the continuous production and drainage of a clear fluid called aqueous humor. Statistically, ocular hypotony is often defined as an IOP consistently measured at or below 6.5 mmHg. The clinical definition, however, focuses on whether the low pressure causes physical changes or visual problems. When the balance between aqueous humor production and outflow is severely disrupted, the resulting lack of internal pressure can lead to a collapse of the eye’s structural components.
Primary Causes of Low Eye Pressure
The underlying cause of ocular hypotony is always an imbalance: either the eye is producing too little aqueous humor, or it is draining or leaking too much.
The most frequent source of this imbalance involves complications arising from eye surgery, particularly glaucoma filtering procedures like trabeculectomy. These surgeries sometimes result in excessive outflow, leading to an “over-filtering bleb” where fluid escapes too quickly. A leak from the surgical wound itself, known as a bleb leak, is another common post-operative issue.
Trauma can trigger hypotony by causing a cyclodialysis cleft, a separation between the ciliary body and the sclera. This creates an abnormal pathway for the aqueous humor to exit the eye, bypassing the normal drainage system. Blunt force trauma can also cause a scleral or globe rupture, resulting in a direct leak of intraocular fluid and an immediate drop in pressure.
Severe inflammation within the eye, known as uveitis, is another significant cause. Inflammation can suppress the function of the ciliary body, leading to decreased fluid production. Certain medications used to treat glaucoma can similarly suppress aqueous production. Furthermore, a retinal detachment can alter the fluid dynamics of the eye, leading to reduced pressure.
Serious Consequences of Ocular Hypotony
Sustained low pressure compromises the eye’s structural integrity, allowing the external wall, the sclera, to collapse inward. This inward collapse is the primary mechanism that initiates vision-threatening complications.
The most serious consequence is hypotony maculopathy, which involves the development of folds in the retina and choroid. These folds typically occur in the macula, the central area of the retina responsible for sharp, detailed central vision. The folding and swelling distort the light, often causing blurred vision, visual distortion (metamorphopsia), and a shift toward farsightedness (hyperopia). If left uncorrected, these folds can lead to permanent damage and irreversible vision loss.
Low pressure also frequently causes choroidal detachment, where the choroid separates from the sclera due to fluid accumulation. Other tissues are also affected, including the optic disc, which may swell (papilledema) from restricted fluid flow within the nerve. Additionally, the cornea can develop swelling, known as corneal edema, which further reduces visual clarity.
Diagnosis and Management
Diagnosing ocular hypotony begins with measuring the intraocular pressure using a tonometer, often the Goldmann applanation method, to confirm the low reading. A comprehensive eye examination is then performed to look for clinical signs of damage, such as the characteristic retinal and choroidal folds seen in maculopathy. Specialized imaging techniques, including Optical Coherence Tomography (OCT) and B-scan ultrasonography, help visualize the folding of the retina and confirm the presence of detachments.
The management strategy for hypotony depends entirely on identifying and addressing the specific underlying cause. The goal of all treatment is to normalize the IOP as quickly as possible to prevent the severe, long-term structural changes that lead to permanent vision loss.
Medical Management
For inflammation-induced hypotony, medical treatments like topical or systemic corticosteroids are used to suppress the inflammation and restore the ciliary body’s function.
Surgical Intervention
When the cause is a surgical wound leak, conservative measures such as patching or a bandage contact lens may be used, or the leak may require surgical repair. If hypotony is caused by an over-filtering bleb, procedures such as compression sutures or injecting the patient’s own blood (autologous blood injection) can be used to reduce fluid drainage. Complex causes, like a cyclodialysis cleft or traction from a retinal detachment, often require targeted surgical interventions.