Hypotony is a condition of low intraocular pressure (IOP) that can pose a risk to vision. Normal eye pressure is between 12 and 22 millimeters of mercury (mm Hg), while hypotony is defined as an IOP below 6.5 mmHg. The clinical significance depends on whether the low pressure leads to vision problems. This condition can arise from various causes and result in noticeable symptoms. Addressing hypotony involves medical and surgical strategies aimed at correcting the underlying issue and restoring normal eye pressure.
What is Hypotony? Unpacking Causes and Symptoms
Normal eye function depends on a stable intraocular pressure (IOP), maintained by the continuous production and drainage of a fluid called aqueous humor. The condition becomes clinically significant when this low pressure results in vision loss. When the pressure inside the eye drops too low, it can disrupt the structure and function of ocular tissues.
The physiological consequences of sustained low IOP can be significant. The sclera, the eye’s tough outer wall, can lose its rigidity, leading to inward folding of the choroid and retina. This can cause a condition known as hypotony maculopathy, where the macula, responsible for central vision, becomes wrinkled, leading to blurred or distorted sight. Other effects include swelling of the optic nerve head, corneal edema, and accelerated cataract formation.
A primary cause of hypotony is excessive outflow of aqueous humor, often following eye surgery. Glaucoma filtration surgeries can sometimes lead to over-filtration or a leak from the surgical site. Trauma to the eye can also create a pathway for fluid to escape. Another cause is the underproduction of aqueous humor, which can be triggered by inflammation (uveitis). Certain medications, systemic conditions like dehydration, and retinal detachment can also lead to a drop in eye pressure.
Patients experiencing hypotony may notice blurred or fluctuating vision, often accompanied by discomfort or pain in the eye. The development of hypotony maculopathy can cause a decline in central vision, and some individuals might become more farsighted. However, it is possible for a person to have a very low IOP and remain completely asymptomatic with good vision.
How Doctors Diagnose Hypotony and Its Origin
The diagnostic process for hypotony begins with measuring the intraocular pressure, a procedure known as tonometry. If the pressure is consistently low, the focus shifts to identifying the root cause of the condition. A detailed patient history, particularly regarding recent surgeries, trauma, or medications, provides initial clues.
Ophthalmologists use several specialized tools to examine the eye’s structures. A slit-lamp examination provides a magnified view of the front of the eye, allowing the doctor to check for wound leaks, signs of inflammation, or issues with a post-surgical drainage site called a bleb. To confirm a suspected leak of aqueous humor, a Seidel test is performed, which uses a fluorescein dye that appears diluted at the site of leakage.
For a more detailed look at internal structures, imaging technologies are employed. Gonioscopy is a technique that uses a special lens to inspect the anterior chamber angle, which can reveal a cyclodialysis cleft—a separation of the ciliary body from the sclera. Ultrasound Biomicroscopy (UBM) and B-scan ultrasonography use sound waves to create images of the eye’s anterior and posterior parts. These can detect ciliochoroidal detachments, which is fluid accumulation between the choroid and sclera. Optical Coherence Tomography (OCT) provides high-resolution images of the retina, which is useful for identifying hypotony maculopathy.
Medical Treatments for Low Eye Pressure
Once hypotony is diagnosed and its cause is identified, the initial approach is often medical and conservative. If the low pressure is a side effect of medication, such as eye drops used to treat glaucoma, an ophthalmologist might reduce the dosage or discontinue the use of aqueous suppressant medications to allow the eye’s pressure to normalize.
In cases where hypotony is caused by inflammation (uveitis), the treatment focuses on controlling the inflammatory response. Topical corticosteroids are frequently prescribed to reduce inflammation, which in turn can help the ciliary body resume its normal function of producing aqueous humor. This approach allows the intraocular pressure to gradually increase as the inflammation subsides.
Other non-surgical methods can be employed to manage specific causes. For small wound leaks after surgery, a doctor might apply a pressure patch or a large-diameter bandage contact lens. These devices apply gentle pressure to the eye’s surface, which can help a small leak to seal on its own. An ophthalmologist might also inject a thick, gel-like substance called viscoelastic into the eye to temporarily increase its volume and pressure.
Cycloplegic agents, such as atropine, are another class of medications used in the medical management of hypotony. These eye drops work by temporarily paralyzing the ciliary muscle, which can help to deepen the anterior chamber and stabilize the blood-aqueous barrier. This action can be effective in treating cases involving ciliary body detachment, helping the structure to reattach.
When Surgery is Necessary: Surgical Options for Hypotony
Surgical intervention becomes the next step when medical treatments fail to resolve hypotony or when there is a clear structural problem that requires direct repair. A decision to proceed with surgery is often made if the patient’s vision is significantly threatened or if the low intraocular pressure persists despite conservative measures.
For cases caused by leaks after previous surgeries, direct repair is often the most effective solution. A surgeon can resuture a leaking wound from a cataract surgery or revise an over-filtering glaucoma bleb. Bleb revision might involve placing compression sutures to reduce outflow or creating a new conjunctival flap to cover the area. These procedures are designed to close the path of excessive fluid drainage.
If diagnostic imaging reveals a cyclodialysis cleft, a separation of the ciliary body from the scleral spur, a surgical procedure is required to close it. This can be achieved through various techniques, including direct cyclopexy, where sutures are used to reattach the ciliary body to the scleral wall. Closing the cleft eliminates the abnormal pathway for aqueous humor outflow, thereby allowing intraocular pressure to normalize.
In situations involving ciliochoroidal detachment, where fluid accumulates between the choroid and sclera, a surgeon may need to drain this fluid. This procedure, known as a sclerotomy, involves creating a small incision in the sclera to allow the trapped fluid to escape. This can be combined with other procedures, such as a vitrectomy or revision of a glaucoma drainage device that is causing excessive filtration.