Ocular Hypotony

What is Ocular Hypotony?

Ocular hypotony is a condition that can potentially be dangerous to your vision. It can occur suddenly or over a long period of time. While there’s some debate about what exact eye pressure level counts as ocular hypotony, it’s generally defined as an average eye pressure of 5 mm Hg or less. This is checked multiple times to confirm. In some cases, it has also been described as an eye pressure of less than 6.5mmHg.

Ocular hypotony can cause noticeable symptoms or it might not, depending on whether or not it causes any changes you can see or feel. It’s uncertain who will experience noticeable symptoms of hypotony. Some patients can endure low eye pressure for quite some time without losing their eyesight. Others may start showing signs of keratopathy (disease of the cornea), maculopathy (damage to the macula, the central part of the retina), or even choroidal effusion (a build-up of fluid beneath the retina). Treatment of ocular hypotony involves identifying and addressing the cause of the condition.

The key to preventing harm from ocular hypotony lies in spotting it early and managing it quickly. When this is done effectively, most patients with ocular hypotony can expect a good outcome for their vision.

What Causes Ocular Hypotony?

Ocular hypotony is a condition where the pressure in your eye (intraocular pressure) becomes too low. It can be caused by several factors: leakage of eye fluid through surgical wounds, a decrease in the production of a fluid in your eye called aqueous humor, use of certain medications, and a condition where a scar forms in the eye called proliferative vitreoretinopathy (PVR). You can also lose aqueous humor (which maintains the pressure in your eye) externally after eye surgery or trauma, or internally due to a separation in the ciliary body (part of the eye) or due to a detached retina.

It frequently occurs after surgery for glaucoma (an eye condition that damages your optic nerve). In particular, medicines used during surgery to prevent scarring such as mitomycin C (MMC) or 5 fluorouracil (5FU), can lead to ocular hypotony. Other triggers include inflammation within the eye, eye trauma, and other types of eye surgery such as vitreoretinal surgery (a procedure to treat various retinal and vitreous diseases).

To understand why ocular hypotony occurs, it helps to understand how the aqueous humor (the fluid in your eye) works. It’s produced by cells in a part of your eye called the ciliary epithelium, and it helps maintain the pressure in your eye. Most (80%) of this fluid is actively produced, while the rest is produced passively. This fluid travels from the back chamber of the eye, through the pupil to the front chamber, nourishing your eye and removing waste.

This fluid then exits the eye through a network called the trabecular meshwork, and another pathway. Factors that influence the pressure in your eye, and hence cause ocular hypotony, include:

Decrease in production of aqueous humor due to:

• Intraocular inflammation
• Inadequate blood flow (hypoperfusion)
• Medicines that reduce aqueous humor
• Reduced blood flow to the ciliary body (the part of the eye that produces aqueous humor)
• Scarring or pull to the ciliary body
• Destructive procedure on the ciliary processes

Increase in drainage of aqueous humor due to:

• Surgery for glaucoma
• Drainage devices used for glaucoma
• Surgery on the angle of the eye
• Artificially created separation in the ciliary body
• Eye injury that causes an open wound
• Drawing of fluid into spaces in the eye
• Other surgeries, such as vitreoretinal surgery, corneal surgery, or operations to fix an intraocular lens

Ocular hypotony due to medications:

Some systemic medications, including carbonic anhydrase inhibitors, antihypertensive drugs, nitrates, antivirals like Cidofovir, and hyperosmotic agents (that affect the concentration of your body fluids) can also cause ocular hypotony.

Other systemic causes:

• Pregnancy
• Dehydration
• High levels of urea in the blood (uremia)
• Diabetic coma
• Inherited conditions such as Marfan syndrome or Myotonic dystrophy

Sometimes, ocular hypotony may occur because of more than one reason, especially after glaucoma surgery. Other factors like dehydration, uremia, diabetic coma, genetic conditions, and pregnancy can also cause ocular hypotony. Additionally, certain medications taken for other health issues can also impact the pressure in your eyes.

Risk Factors and Frequency for Ocular Hypotony

Ocular hypotony, or low eye pressure, is a condition where the exact number of people affected is unknown. However, it is identified in people who have undergone eye surgery, experienced trauma to the eye, or have inflammation within the eye. In certain studies, the rate of occurrence of this condition has been reported in these groups.

• A study found a 7.7% +/- 0.9% chance of experiencing low eye pressure five years after glaucoma surgery with mitomycin C in a large group of 955 eyes.
• Another report mentioned a third of the eyes that had undergone a specific type of glaucoma surgery with mitomycin C experiencing low eye pressure.
• Low eye pressure was observed in 15.6% of juvenile patients with uveitis, an eye inflammation disorder.
• In a study of 145 eyes that had suffered blunt trauma, 6.9% experienced low eye pressure. This was significantly more common in eyes with certain types of trauma such as retinal detachment and vitreoretinopathy.
• 10% of 70 eyes that had received laser treatment for glaucoma saw the occurrence of low eye pressure.
• Lastly, the incidence of a complication known as hypotony maculopathy, usually resulting in damage to the central retina, was reported at 20% after glaucoma surgery with antimetabolite use.

Signs and Symptoms of Ocular Hypotony

Ocular hypotony is a medical term for low pressure in the eye. The symptoms a person experiences as a result of ocular hypotony largely depend on what’s causing it. It’s important for healthcare providers to know if the patient has had any past eye trauma or surgeries, or if they have any eye illnesses like uveitis or retinal detachment. These are known to result in hypotony. Additionally, some systemic diseases can cause ocular hypotony like pregnancy, diabetes, and myotonic dystrophy.

Doctors should also probe for vascular causes that may decrease blood flow to the ciliary body of the eye, such as ocular ischemic syndrome. It’s rather common that ocular hypotony is a side effect of certain medications, so it’s essential to discuss both the topical and systemic medications a patient is taking.

Patients with ocular hypotony may complain of lost vision or distorted images, which could point to significant damage to the eye. However, some patients show no symptoms at all and still maintain relatively good vision, despite having eye pressure of 5mm Hg or less. Clinical ocular hypotony could be a result of any of the aforementioned eye conditions, or could potentially cause other complications like maculopathy, keratopathy, optic disc swelling, or even changes in refractive error. In some patients with ocular hypotony, their cataracts progress rather quickly.

During a clinical examination, the patient’s visual acuity might be decreased or normal. In all cases, the intraocular pressure will be significantly low at 5mm Hg or less. There may be noticeable indications of a compromised globe integrity, such as leakage from a bleb, a perforation of the globe (eye ball), or a corneal laceration. In all cases where the aqueous humor leaks from the eye, a Seidel test should be performed using fluorescein dye. This test can identify whether the aqueous humor (fluid inside the eye) is leaking through a defect in the cornea or sclera, as shown in Figure 1.

Using a different tool, called the slit lamp biomicroscope, the doctor may notice corneal striae and corneal edema, which could potentially be related to a shallow anterior chamber, among other things. This tool can also help to identify trembling of the intraocular lens, signs of inflammation in the eye, cyclic membranes and synechiae.

In some patients with ocular hypotony, cataracts form due to decreased nourishment from the reduced circulating aqueous supply to the crystalline lens. Even though it’s more difficult, doctors might perform Gonioscopy to look for a cyclodialysis or other signs of trauma to the anterior chamber angle.

During a fundus examination, there might be signs of ciliochoroidal detachment, a condition that arises when there’s too much fluid in the vessels within the eye. In some cases, this can lead to the vessels rupturing and causing a hemorrhagic component to the serous fluid. Other possible signs of ocular hypotony a doctor could detect are tortuosity of the blood vessels, optic disc edema, choroidal foals, and macula striae, which are characteristic of hypotony maculopathy.

There may also be changes in the macula’s retinal pigment epithelial area. During the posterior segment examination, the doctor might notice further indications of ocular hypotony, such as retinal detachment, proliferative vitreoretinopathy, and signs of significant ocular ischemia.

Testing for Ocular Hypotony

Ocular hypotony, or low pressure in the eye, is typically diagnosed by examining the eye directly. In all cases of ocular hypotony, the first step is to rule out any open wounds or leaks in the eye. The Seidel test, which involves putting a dye on the surface of the eye to check for leaks, is useful here. Other diagnostic procedures can help to provide additional information about the structure of the eye, why hypotony is happening, and if it’s severe.

One of these procedures is ultrasound biomicroscopy (UBM), which uses a small ultrasound probe to create detailed images of the front part of the eye. Depending on the type of probe used, UBM can examine structures deep inside the eye, giving us important information about the condition. For example, it can show whether the chambers at the front of the eye are too shallow, whether the iris (the colored part of the eye) is bending forward, and whether there’s a tear in the ciliary body (the part of the eye that produces fluid). UBM can also reveal issues not seen during a regular eye exam, such as atrophy (tissue loss), membrane over the ciliary body, detachment of the ciliary body, and even foreign objects in the eye.

Some severe cases of ocular hypotony can lead to a build-up of fluid in the choroid, the layer of blood vessels and tissue in the back of the eye. This can be detected with a type of ultrasound imaging known as a B Scan. If the view of the back of the eye is unclear, a B Scan can also reveal whether the retina or choroid layers have detached from the back of the eye.

Various types of imaging can be used to examine the retina and choroid folds at the back of the eye. For instance, fundus fluorescein angiography (FFA) can show if there are bands of high and low brightness in the choroid folds. If the optic disc (the area where the optic nerve enters the eye) is swollen due to hypotony, this may appear as leakage on FFA. Spectral-domain Optical Coherence Tomography (OCT) can show smaller details than traditional images of the eye’s interior, providing a more accurate diagnosis of hypotony-related damage in the macula, the center of the retina.

Finally, a test called Indocyanine Green Angiography (ICG) can be used to examine the choroid folds in cases of ocular hypotony. This test may even detect folds that FFA couldn’t see. Together, these procedures give doctors a comprehensive view of the state of the eye in cases of ocular hypotony.

Treatment Options for Ocular Hypotony

The treatment for ocular hypotony, a condition that causes low pressure in the eye, can be either medical or surgical depending on the cause. Some of the treatment goals for ocular hypotony include fixing the underlying cause, maintaining the eye’s structure, and restoring any lost vision.

For milder cases, doctors may approach treatment conservatively. This might involve the patient stopping a medication that may be responsible, or surgically fixing a leaking eye injury. For other cases, the treatment may be more complex. Factors such as the state of the tissue of a lesion in the eye (a “bleb”) or repairing a large “cyclodialysis cleft” could complicate treatments.

In some situations, ocular hypotony occurs as a result of uveitis, a type of eye inflammation. With this condition, treating it promptly and aggressively is important to restore vision and prevent additional complications. Various types of medications can be used, including steroids which can provide notable benefits by reducing inflammation and increasing the production of fluids in the eye.

Alternatively, some medications can contribute to ocular hypotony. For instance, cidofovir, a medication used for treating a type of retinal infection in patients with weakened immune systems, can induce ocular hypotony and cause severe uveitis. In these cases, treating the uveitis requires local steroid treatment and potentially discontinuing cidofovir. Steroid eye drops along with other medications can be successful in treating this condition.

If the approach to treatment doesn’t work with medication alone, surgery might be necessary. A leaking bleb may require removal with surgery, allowing for the pressure in the eye to be normal again. Other surgical procedures include repair of a retinal detachment and treatment of a cyclodialysis cleft – an abnormal gap between the muscle in the eye responsible for focus and the eye wall.

Other techniques involve inserting devices to treat the eye pressure or using silicone oil in certain surgeries. Still, other techniques can increase the resistance to the fluid outflow from the eye, therefore raising the pressure inside the eye.

In extreme cases, conditions like Ocular Ischemic Syndrome (OIS) can cause ocular hypotony. Treatment of OIS involves a team of specialists aiming to enhance or restore blood flow to the eye. Surgical techniques can effectively treat the complications of ocular hypotony. For example, a vitrectomy (surgery to remove some or all of the gel inside the eye) has been shown to successfully treat specific types of eye complications such as chorioretinal folds (wrinkles in the back of the eye).

What else can Ocular Hypotony be?

When a patient presents with low eye pressure and changes in the macula (the part of the retina where sharp, central vision occurs) or folds in the part of the eye where the retina and blood vessels are located, the doctors have to consider a few different causes which include:

• Eye tumors such as choroidal melanomas and metastasis
• Eye inflammation like posterior scleritis
• New blood vessel growth in the macula as seen in age-related macular degeneration
• A mass behind the eyeball, such as an orbital tumor
• Papilledema, a condition where swollen optic nerves cause radiating chorioretinal folds

Other reasons for these eye folds could be a detached retina, materials attached to the protective white part of the eye, treatments such as scleral buckles and radioactive plaques. Also, there is a possibility of idiopathic chorioretinal folds, which simply means the underlying cause cannot be identified.

Moreover, in cases of hypotony keratopathy, a condition that results from low eye pressure, there can be folds in the Descemet membrane (a layer of the cornea, the clear front surface of the eye) and swelling of the cornea.

Certain conditions like keratoconus, post-surgical changes in the cornea and the presence of Haab striae, lines found in the eye’s cornea due to congenital glaucoma, can also cause corneal folds.

At times, a diagnosis of low eye pressure can be incorrectly given in instances where the central cornea is thinner than usual, as seen in cases of high nearsightedness. Adjusting the eye pressure appropriately for these thinner corneas may actually reveal they have a normal eye pressure.

What to expect with Ocular Hypotony

The future outlook or prognosis of ocular hypotony, a condition where the pressure within the eye is too low, largely depends on how it affects the eye and its root cause. Eyes with low pressure tend to undergo various changes that can cloud the eye’s clear structures, and this might lead to the formation of cataracts and a cloudy cornea.

There can also be changes in the way light is refracted or bent by the eye. This can lead to hyperopia (long-sightedness) with a reduction in the eye’s length, or myopia (short-sightedness) with a forward displacement of the lens. These changes can significantly affect vision.

Astigmatism, a vision condition that causes blurry vision, could result from the effect of surrounding eye muscles and tissues on the soft eyeball and its outer layer, the sclera. Changes in the back part of the eye, including detachment of the ciliary body or choroid (part of the eye that provides oxygen and nutrients) and hypotonic maculopathy (a condition causing blurry or distorted vision), can also greatly affect vision.

The cause of the low eye pressure could directly impact the visual outcome. For example, trauma, inadequate blood supply to the eye, and advanced PVR (a complication from retina surgery) could all affect the prognosis. How long the low eye pressure lasts is also a factor. Chronic low eye pressure may be associated with more long-lasting changes and may fall under a condition called phthisis bulbi, characterized by a shrunken and non-functional eye.

However, there have been instances where successful treatments, like the revision of a surgically created reservoir to control eye pressure in glaucoma, have reversed the effects of the low eye pressure.

Possible Complications When Diagnosed with Ocular Hypotony

Ocular hypotony, also known as low pressure in the eye, can sometimes just be a temporary issue with limited risk. However, in other cases, it can turn into a long-term problem, causing significant damage to the eye and even loss of vision. One extreme outcome could be phthisis bulbi, which is when the eye shrinks and loses function.

The problems that come with ocular hypotony often occur alongside the primary issue that caused the low eye pressure. For example, in cases of eye injury, hypotony can lead to other complications like the disorganization of the eye’s structure. If you have a condition known as uveitis, which causes inflammation in the eye, complications like adhesions in the eye, swelling of the macula (the part of the eye responsible for detailed vision), and clouding of the eye lens (cataract) could happen alongside hypotony.

Particular complications related to ocular hypotony include issues like ‘hypotony maculopathy’, where the macula gets damaged due to low pressure. Choroidal detachment (where a layer of blood vessels in the eye detaches), keratopathy (cornea disease), shifts in refraction (how light enters the eye), papilledema (swelling of the optic disc), and formation of cataracts are also possible complications of ocular hypotony.

But there’s some good news as well. These complications can often be treated effectively, and there’s a good chance of vision improving.

Common complications:

• Phthisis bulbi (shrinkage of the eye)
• Disorganization of the eye’s structure
• Adhesions in the eye
• Swelling of the macula
• Clouding of the eye lens (cataract)
• Hypotony maculopathy
• Choroidal detachment
• Keratopathy (cornea disease)
• Shifts in refraction (how light enters the eye)
• Papilledema (swelling of the optic disc)

Preventing Ocular Hypotony

Low eye pressure, or ocular hypotony, can have severe negative effects on one’s vision. It’s crucial that eye surgeons, their assistants, and the patient all put in an effort to prevent this from happening after eye surgery. This is particularly important during an operation for glaucoma, a condition that causes pressure in the eye and damages the nerve that connects the eye to the brain.

During this surgery, surgeons should be cautious when using antimetabolites, a type of medicine used to control the progression of the disease. Special care is needed to ensure these substances don’t come into contact with the edges of the conjunctiva, which is the clear tissue covering the front of the eye. This is to prevent the conjunctiva from becoming thin and causing liquid-filled blisters (or bleb leaks).

A useful method to prevent low eye pressure after surgery is to place ‘releasable sutures’. These are stitches that can be taken out after the operation. This helps control the flow of the fluid made by the eye (aqueous outflow) and subsequently adjust eye pressure. These stitches can also be helpful in avoiding leaks and low eye pressure after surgery for retinal disorders.

Advancements in eye surgery have led to a deeper understanding and better construction of wounds, thus overcoming initial problems with low eye pressure and leaks after utilizing small-gauge, through the conjunctiva (or transconjunctival) retinal surgery.

If a patient has low eye pressure due to a leak in the surgical wound, they should be aware of the risk of getting an infection at the leaking site. Topical antibiotics, applied directly to the eye, are usually prescribed. Some of these antibiotics, like topical aminoglycosides, can cause inflammation of the conjunctiva, which in turn triggers wound healing and helps close the conjunctiva’s defect. The patient should also be advised to avoid any physical contact with the affected eye to avoid causing further damage, such as eye rupture or internal bleeding in the eye. Using a protective shield over the eye while sleeping may help.