Mooren’s ulcer (MU) is an eye condition that causes damage to and degeneration of the cornea. The cornea is the outside layer of your eye that covers the front of your eye.

MU is a type of keratitis. Keratitis is inflammation of the edges of the cornea. MU is different from other types of corneal ulcers because it happens along the edge of the cornea where it meets the sclera. The sclera is the white of your eye. Because of this, it’s known as a type of peripheral ulcerative keratitis (PUK).

MU is extremely rare. There’s not much information about how common it is in the United States. Cases of MU are recorded more frequently in China, India, and Africa.

There are various classifications of Mooren’s ulcer. One common classification divides the disease into two types based on laterality (one or both eyes) and age of onset:

  • Limited (benign) type. This type of MU doesn’t cause much pain or discomfort. It usually only happens in one eye (unilaterally). Only 25 percent of cases of the benign type happen in both eyes (bilaterally). It’s more common if you’re older.
  • Atypical (malignant) type. This type is more painful and can quickly cause your cornea to break down if it’s not treated. It usually happens in both eyes. About 75 percent of cases of the malignant type happen in both eyes.

More recent classifications group Mooren’s ulcer into three types based upon their clinical presentation:

  • Unilateral Mooren's ulceration (UM). This is a painful and progressive corneal ulcer typically seen in elderly patients.
  • Bilateral aggressive Mooren's ulceration (BAM). This type occurs in young patients. The ulcer progresses circumferentially then centrally in the cornea.
  • Bilateral indolent Mooren's ulceration (BIM). This type usually occurs in middle-aged patients. It presents with progressive peripheral corneal ulceration in both eyes.

When MU first appears, the ulcer usually starts around the circumference, or edge, of the cornea. As it progresses, MU can spread to the rest of the cornea and surrounding eye tissues.

The symptoms of MU can include:

  • intense pain in the affected eye(s)
  • thinning or tearing of the corneal tissue
  • redness in the affected eye(s)
  • unusual sensitivity to light (photophobia)
  • inflammation of the uvea, the eye’s middle layer (iritis or uveitis)

If it’s not treated, complications can include:

  • sclera and eyelid Inflammation and irritation (conjunctivitis)
  • inflammation and pus in the front of your eye (hypopyon)
  • blurriness in the eye lens (cataracts)
  • punctures in the cornea (perforation)
  • optic nerve damage (glaucoma)
  • loss of vision or blindness

The exact cause of MU is unclear. Many MU symptoms are similar to those that affect the eye due to immune system disorders, such as rheumatoid arthritis. This may mean that MU is caused by an exaggerated immune response due to an autoimmune disease response to eye injury or infection.

Some research suggests that MU may be linked to hepatitis C. In several cases, people with MU were also diagnosed with long-term, or chronic, hepatitis C infections.

MU is only diagnosed when other underlying inflammatory conditions, such as rheumatoid arthritis, can be ruled out as a cause of corneal damage.

Other, more common corneal conditions, such as Terrien’s degeneration, also need to be ruled out before your doctor can give you a confident diagnosis. Unlike other corneal conditions, MU doesn’t happen along with inflammation of the whites of your eyes (scleritis), so your doctor will check for this symptom, too.

Your doctor may use several tests, such as a slit-lamp test or fluorescein staining, to diagnose MU. Your doctor will look for specific signs of the ulcer using a symptom list known as Watson’s criteria. Signs your doctor will look for include:

  • ulcer in the shape of a crescent around the edge of the cornea
  • inflammatory cells found around the edge of the ulcer (corneal infiltrates)
  • tissue damage underneath the edge of the ulcer (undermining)
  • non-inflamed whites of the eyes (no scleritus)
  • no diagnosis of other autoimmune or systemic conditions

To rule out the similar corneal condition called Terrien’s degeneration, your doctor will check if the ulcer has spread to the middle of your cornea. If it hasn’t, they can rule out Terrien’s degeneration.

Benign MU often doesn't need to be treated if it doesn’t cause pain or doesn’t have any risk of complications.

If treatment is needed, both benign and malignant MU may be treated using one or more of the following:

  • topical treatments to keep tissue from degenerating
  • antibiotics, such as moxifloxacin (Vigamox), to prevent infections
  • interferon a2b for hepatitis C infections, sometimes combined with the antiviral medication ribavirin (Rebetron)
  • resection, or surgical removal of tissues surrounding the ulcer
  • cryotherapy, which involves freezing and surgically removing ulcer tissue
  • tissue adhesion, which involves placing materials near the ulcer to stop it from spreading

Benign MU can cause discomfort but is harmless and doesn't need to be treated right away. You can often go for years without treating MU and not experience any complications.

Malignant MU can be painful and quickly cause irreversible damage to your cornea, sometimes resulting in vision loss. In many cases, this type of MU can be treated and you won’t permanently lose any vision. Prompt treatment is important to avoid long-term complications.