What is Pellucid Marginal Corneal Degeneration?
Pellucid marginal corneal degeneration (PMCD) is a condition that affects both eyes and causes the outer edge of the cornea (the clear, front surface of the eye) to become thin. This thinning usually forms a band around the lower part of the cornea. The area of the cornea that is bulging out, known as the ectatic zone, is located just above the thinnest part of the cornea and is about 1-2 mm from the border of the cornea (the limbus). The name of this condition comes from a term coined in 1957 by Schalaeppi. “Pellucid,” meaning clear, represents how the cornea remains clear in spite of the bulging.
This condition is more often seen in men aged between their 20s and 50s. Prior to any eye surgery to change the way the eye focuses light (refractive surgery), it’s really important to check for any signs of PMCD and similar conditions like keratoconus that also lead to thinning and bulging of the cornea. If these conditions are not identified before surgery, they could cause unwanted complications after the operation.
What Causes Pellucid Marginal Corneal Degeneration?
The cause of PMCD (pellucid marginal corneal degeneration), an uncommon eye condition that thins and changes the shape of the cornea, is primarily unknown. PMCD shares a similar characteristic with keratoconus, another eye disorder: abnormal spacing between collagen fibers in the cornea. Normally, these fibers are spaced about 60 nm to 64 nm apart. However, in people with these eye conditions, the fibers are 100 nm to 110 nm apart.
Some studies have found a link between PMCD and conditions like obesity and sleep apnea. Why? Because these can cause a lack of oxygen, which might result in what’s called ‘anaerobic glycolysis’ and ‘stromal acidosis’. In simpler terms, this means the body breaks down glucose, or sugar, for energy without enough oxygen, turning the cornea acidic. This, in turn, might bring about the creation of damaging, inflammation-causing substances, leading to the thinning of the cornea.
Also, among obese patients, a condition known as ‘floppy eyelid syndrome’, where the eyelid is loose and can easily flip, can cause constant rubbing of the cornea, leading to thinning and an abnormal bulging outwards.
Certain eye and body conditions like atopic and vernal keratoconjunctivitis (seasonal eye allergies), retinitis pigmentosa (a genetic eye disorder which affects vision), chronic open-angle glaucoma (a common form of glaucoma that damages the optic nerve), scleroderma (a group of diseases that cause the skin and connective tissues to harden and tighten), retinal lattice degeneration (a thinning of the peripheral retina), eczema, and hyperthyroidism (overactive thyroid) have been seen in some PMCD patients. However, it’s important to note that no direct cause-and-effect relationship has been confirmed.
Risk Factors and Frequency for Pellucid Marginal Corneal Degeneration
PMCD, which is sometimes thought to be just a different form of keratoconus, may actually be more common than reported. Generally, it’s considered a rare condition. It affects both genders, but is seen more in males, and has been observed in all ethnic backgrounds with no specific geographic pattern.
- People in their 20s to 50s are usually affected, but some older individuals have reported it as well.
- While there’s no proof that PMCD is hereditary, some families seem to have a higher rate of astigmatism.
- Additionally, about 10% of patients with PMCD also have keratoconus.
Signs and Symptoms of Pellucid Marginal Corneal Degeneration
If you have PMCD, or peripheral marginal corneal degeneration, you might first notice a slow, worsening loss of vision or long-term low visual quality. However, your visual capacity, even when corrected, only really goes down in the advanced stages of this disease. When we examine your eyes, we usually see high irregular astigmatism that goes against common patterns. This is due to tissue loss in the vertical meridian of your eye and thin, weakened stroma in a crescent shape.
Your cornea’s shape also changes. It steepens and stretches at the junction of affected and unaffected tissue, which creates a high cylindrical loop. This gives your cornea a look similar to a beer belly when seen from the side. There’s also an uncommon steepening at 90 degrees, which contributes to an unusual pattern of astigmatism.
In some instances, you may experience a sudden, painful red eye with watering, a drop in vision, and sensitivity to light due to acute hydrops (fluid collection in the cornea) and/or spontaneous corneal perforation (a tear in the cornea).
When we look at your cornea, we see thinning mostly from 4 o’clock to 8 o’clock, in a crescent shape, which slowly worsens over time. The cornea is generally clear; there’s no vascularization (formation of blood vessels), Fleischer ring (a ring of iron in the cornea), lipid deposits, or changes in sensation.
Between the thinned band and the limbus (border of the cornea), there is usually a 1 to 2 mm wide area of normal, unaffected cornea. Some patients can have concentric Descemet’s folds like Vogt’s striae (stress lines in the cornea due to thinning).
PMCD usually affects both eyes, but we have seen cases where it affects only one eye. The disease has also been documented in the superior, nasal, and temporal areas of the cornea.
Testing for Pellucid Marginal Corneal Degeneration
Corneal topography, a test that maps the surface of the cornea (the clear front part of the eye), is considered the most effective method for diagnosing PMCD and other similar conditions affecting the cornea. This technology has come a long way, from early videokeratoscopy machines to advanced devices that use a technique called slit scanning and Scheimpflug imaging. These advanced tools give a more detailed view of the back of the cornea, allowing doctors to spot corneal conditions at an early stage.
In PMCD, this testing can reveal a certain pattern in the cornea’s shape, which is often described as resembling a “crab claw,” “butterfly,” or “kissing doves”.
The test also usually shows an area of heightened curvature (referred to as a ‘hot spot’) in the lower outer edge of the cornea. However, this hot spot can sometimes also be seen in advanced cases of a condition called keratoconus. This is why it’s important to interpret the results in the context of other clinical findings and measures of corneal thickness (pachymetry) to make an accurate diagnosis. In PMCD, the cornea’s thickness distribution reverses the usual pattern – it’s thinner on the edges and thicker in the center.
In some cases, other imaging techniques, like anterior segment optical coherence tomography and Scheimpflug images, can help doctors find the thinnest part of the cornea, particularly when it can’t be determined through topography alone.
Treatment Options for Pellucid Marginal Corneal Degeneration
When it comes to treating PMCD, a condition that causes the cornea of the eye to bulge and thin, the options can be broadly classified into conservative or surgical treatments. The choice of treatment is often challenging because of the advanced state of thinning and bulging in this condition. Nonetheless, for patients in the early to moderate stages, glasses or contact lenses can help improve vision. Contact lenses may be more effective in correcting high degrees of astigmatism, a condition where the eye does not focus light evenly onto the retina.
However, fitting contact lenses properly in PMCD patients is generally more difficult than in those with other eye conditions because of the large bulging area and the lens’s inability to comfortably “sit on the cone”. Using rigid gas permeable (RGP) lenses can pose some challenges in getting them centered properly. However, large diameter RGP lenses and other types, such as hybrid lenses (soft skirt), semi scleral and scleral lenses, toric lenses, and PROSE lenses can be of help in those cases.
As for surgical treatment, the options include intracorneal ring segments (ICRS), collagen cross-linking (CXL), and partial and total corneal transplantation. ICRS are segments made from a specific type of plastic that are inserted into the cornea, adding extra tissue to the outer area and thus causing it to steep and flatten the center. However, this treatment is reserved for the early and moderate stages of PMCD, and even then, its usefulness is limited due to risk factors such as potential corneal thickness requirements and complications.
CXL, a treatment that uses riboflavin and ultraviolet A (UV-A) radiation to strengthen chemical bonds in the cornea, is effective for halting the progression of keratoconus, another corneal condition. However, its role in PMCD is less clear due to the challenge of focusing the UV-A beam correctly to be effective.
Corneal transplant procedures can also be used to treat PMCD. These involve replacing part or the entire cornea with a transplant. A specific technique known as “Tuck in” lamellar keratoplasty (TILK), an innovative corneal transplant surgery that involves special reshaping of donor and host tissue, has shown very good results in PMCD patients.
Several other surgical techniques hold promise for PMCD treatment but each carries its accompanying risk factors, making the choice of treatment a very personalized and careful decision.
What else can Pellucid Marginal Corneal Degeneration be?
PMCD, a condition affecting the eye, is often mistaken for keratoconus, another similar condition. The signs that generally point to keratoconus include the eye’s central region being affected, the apex of the ‘cone’ shifting downwards and symptoms like ‘scissoring reflex’, Vogt striae, and Fleischer ring. Other conditions with overlapping symptoms include keratoglobus, where corneal thinning is observed throughout the eye.
Similarly, conditions like Terrien’s marginal corneal degeneration show signs that may be confused with PMCD. The primary distinguishing feature for PMCD is the steepening of the inferior periphery, with extension to the oblique meridians on the side.
Furrow degeneration, primarily seen in older individuals, is also sometimes mistaken for PMCD. Both these conditions are non-inflammatory and aren’t accompanied by corneal vascularization, lipid deposits, or scarring. However, the thinning seen in furrow degeneration is restricted to a certain part of the eye called the lucid interval between the limbus and arcus senilis.
Other conditions mistaken for PMCD include peripheral ulcerative keratitis and early Mooren’s ulcer. These inflammatory disorders can be differentiated from PMCD based on the associated pain, blood vessel development, and eye inflammation.
What to expect with Pellucid Marginal Corneal Degeneration
PMCD, or posterior microphthalmos, develops slowly over time. As a result, patients might not seek treatment until the condition has progressed significantly. Most people with this condition need vision rehabilitation in the early stages, and may undergo corneal transplant procedures when the condition is advanced.
While there aren’t many studies that give details about the future progression of PMCD, early to moderate cases have seen satisfying results from corneal cross-linking (CXL) and intrastromal corneal ring segments (ICRS). Corneal cross-linking is a treatment that strengthens the cornea, while intrastromal corneal ring segments are inserted into the eye to adjust its shape, both helping to improve vision.
Unfortunately, there isn’t enough data to pinpoint the exact number of patients who end up needing a corneal transplant as the disease progresses. However, total intrastromal lenticule keratoplasty (TILK), a specific type of corneal transplant procedure, has shown promising results for patients with advanced PMCD.
Possible Complications When Diagnosed with Pellucid Marginal Corneal Degeneration
Even though they are unusual, eye complications can vary from stress lines to severe conditions like corneal hydrops and spontaneous corneal perforations. In some instances, individuals may develop scarring on the back part of the stroma, a layer of the cornea. It’s also crucial to note that refractive surgery could lead to harmful outcomes if the pathology or disease isn’t recognized correctly in these patients.
Possible complications:
- Stress lines in the eye
- Corneal hydrops (swelling of the cornea)
- Spontaneous corneal perforations (unexpected holes in the cornea)
- Posterior stromal scarring (scarring on the back of a layer of the cornea)
- Potential harm from refractive surgery if the disease is not properly recognized
Recovery from Pellucid Marginal Corneal Degeneration
People who have PMCD, whether they’ve had surgery or not, will need ongoing eye care for the rest of their lives. There have been instances of stitches coming loose and organ rejections after corneal transplant surgery, so these patients need to be carefully monitored for nearly a year after the operation. Even after surgery, these people will need to wear glasses to fully restore their vision.
Preventing Pellucid Marginal Corneal Degeneration
The exact cause of PMCD, a condition that affects the cornea (the clear front surface of the eye), remains a mystery. Therefore, we can’t prevent it. However, it’s a good idea to avoid rubbing your eyes frequently, as some believe this could contribute to the condition. If you have a severe case of PMCD and your cornea has become very thin, it’s crucial to safeguard your eyes from injury because it could lead to a serious issue where the cornea ruptures. Regular visits to the eye doctor are extremely important for these patients, as it can help monitor if the condition is getting worse over time.
