What Is Pellucid Marginal Degeneration

Introduction

Pellucid marginal degeneration (PMD) is a rare eye condition affecting the cornea, the clear lens at the front of the eye. In PMD, the cornea becomes abnormally thin and takes on an irregular shape, akin to a “beer-belly” when looked at from the side.¹ PMD is a very rare condition and often shares features with other corneal issues such as keratoconus, making the accurate diagnosis a challenge for eye specialists.²

Maps of curvature of the eye show the formation of a “butterfly-wing” or “crab-claw” pattern on the lower half of the eye in patients with PMD, where the thinning cornea creates more pronounced dips than normal.³ Individuals with PMD may encounter a gradual loss of vision and clarity, irregular astigmatism (blurriness), and, in extreme cases sudden loss of vision.⁴ This introduction provides a glimpse into the rarity, diagnostic complexities, and visual impact of PMD, opening the door to a more in-depth investigation into this unique eye condition.

Clinical features

Corneal thinning

The most common clinical feature of pellucid marginal degeneration is corneal thinning. Most common in the inferior quadrant of the cornea (the bottom section), the cornea thins about 1 to 2 millimetres in a narrow, clear band from the 4 o’clock to 8 o’clock position.² The “beer-belly” protrusion seen is the area just above the thinning, where the thickness of the cornea is normal. Whilst extremely rare, the other quadrants of the cornea (superior, nasal and temporal) can also exhibit thinning.⁵ The thinning of the cornea can occur within its connective tissue, known as the stroma, which can thin as much as 80%.⁶

Corneal thinning is a slowly progressing symptom of PMD, with extreme curvature of the cornea reflecting the late stages of the disease. The slow loss of vision accompanies this progression, and as such, the progression of the disease can be monitored by eye specialists with regular check-ups.

Astigmatism

Another common symptom of PMD is the development of astigmatism, a condition that causes blurry vision. This can be very distressing to patients and impact day-to-day life. An example of astigmatic vision is the blurring of street lamps or car lights at night, usually with large streaks of light through the centre of the source.

Astigmatism in PMD patients is caused by the thinning of the lower part of the cornea, making the lens misshapen and unable to focus light correctly. This specifically results in irregular astigmatism, which is usually not corrected through traditional glasses or soft contact lenses.⁷ Furthermore, astigmatism can lead to progressive vision loss, loss of clarity of vision, and, in some cases, further complications and sudden loss of sight.

Diagnostic tools

Corneal topography

Corneal topography is the current gold standard for diagnosing pellucid marginal degeneration. This method is quick and painless, working by the patient looking into the machine and the machine quickly scanning the patient's eye. A map of the eye will be produced, with different colours representing different levels of curves of the eye.⁸ The resulting picture can then be analysed for signs of PMD. A “kissing-doves” appearance on the topography is indicative of PMD, with two curved zones on either side of a lesser curved zone. 

This diagnostic approach helps identify PMD and sets it apart from similar conditions like certain stages of keratoconus. However, there is a crossover between the patterns caused by the two diseases. Moreover, corneal topography proves valuable in detecting early-stage corneal thinning. This aspect is crucial, especially for those considering laser eye surgery, as issues arising from thinning can be worsened by such procedures.³ The ability of corneal topography to provide a clear picture of the eye's curvature levels is an essential tool for both precise PMD diagnosis and proactive management of related vision concerns.

Slit lamp examination

Slit lamp examination is a good tool to be used in combination with corneal topography. Again, this examination method is painless but may make you sensitive to light for a short period afterwards. The slit lamp is best at diagnosing moderate cases of PMD due to the signature ring of thinning. However, in extreme cases of PMD, it is difficult to distinguish between that and keratoconus.

The information gathered from both diagnostic tools can be used to identify PMD accurately. The use of both eliminates any uncertainty each of the tests has individually.

Causes and risk factors

Pellucid marginal degeneration is an idiopathic condition, meaning that there is no known underlying cause, and it is seen to arise spontaneously. However, PMD is most common in males aged between 20 and 50 years old.⁹

Additionally, there is no known genetic link or pattern of inheritance of the disease. As such, if a family member is suffering from PMD, then it is unlikely that you or another family member will develop the disease.³

Management and treatment

Glasses and contact lenses

The first line of treatment for patients suffering from pellucid marginal degeneration is the prescription of glasses and contact lenses. This is the best option for success in the vast majority of patients, with only around 12% of PMD sufferers choosing to proceed with surgery.³

At the onset of PMD, soft contact lenses and special glasses can be used to increase the sharpness of a patient's vision; however, these become less useful when the patient develops irregular astigmatism.⁹ 

As the disease progresses, rigid gas permeable (RGP) lenses may be prescribed.¹⁰ These lenses are more stable than soft contact lenses while still providing increased vision sharpness. Additionally, these lenses can be modified to the individual patient's cornea, accounting for the change in shape caused by PMD.

In the later stages of PMD, it's harder to centre RGP lenses due to the extreme steepness of the cornea, making them more likely to dislodge with blinking. Some experts suggest using RGP scleral contact lenses in these situations.¹¹ While scleral lenses offer a reduced dislodgement risk, they have downsides, including poorer oxygen transmission, complex manufacturing, application challenges, and a shortage of experienced practitioners fitting them.

Surgery

Only a small number of PMD patients choose surgical intervention to treat the disease, however, there are several surgical options a patient can consider.

One approach is penetrating keratoplasty (PK), where the damaged portion of the cornea is replaced with a donor cornea.¹² Whilst this method has seen success there are a few drawbacks, primarily postoperative astigmatism caused by the donor cornea being a different thickness to the patient's cornea.³ Additionally, complications can arise due to cornea thinning near the limbus, the iris border and the eye's white.

Another surgical option, deep anterior lamellar keratoplasty (DALK), involves a partial corneal transplant. This is primarily used in extreme cases of PMD, where a patient suffers from sudden vision loss.¹³

Recent techniques, such as tuck-in lamellar keratoplasty (TILK) and corneal wedge excision, offer alternatives, each with its considerations, especially in managing complications associated with large grafts.³

Advanced surgical interventions include intra-stromal corneal segment rings (ICRS), small rings placed in the cornea's periphery to enhance vision, and collagen cross-linking (CXL), a procedure strengthening the cornea's structural integrity.³

Navigating these surgical options requires careful consideration and consultation with an eye care professional. Together, you can determine the most suitable approach tailored to your specific condition and preferences.

Prognosis

The prognosis of pellucid marginal degeneration varies among individuals based on the severity of the condition and the effectiveness of management and treatment strategies. In the early stages, spectacles or contact lenses may effectively address visual impairment. However, as PMD progresses, particularly in advanced cases, the potential for sight loss, blurry vision and astigmatism increases. PMD is a chronic and progressive condition, and long-term outcomes can vary, in extreme, late stages of the disease, surgical intervention may be favoured, but it can come with complications. 

Potential complications, such as rupturing one of the cornea's layers, the Descemet’s membrane, may occur, albeit rare.¹⁴ This is called corneal hydrops and can be extremely uncomfortable for the patient and cause eye swelling.³ Patients may also suffer from a sudden loss of vision and sensitivity to light. Depending on the position or severity of the rupture, this may be treated without surgery.

Regular monitoring and collaborative management with an eye care professional are crucial for optimising visual outcomes and maintaining the quality of life for individuals affected by PMD.

Summary

Pellucid Marginal Degeneration (PMD) is a rare eye condition impacting the cornea, characterised by abnormal thinning and irregular shaping, often resembling a "beer-belly." Diagnosing PMD poses challenges due to its rarity and similarities with other corneal issues like keratoconus. Clinical features include corneal thinning, particularly in the inferior quadrant, leading to a gradual loss of vision and irregular astigmatism. Diagnostic tools such as corneal topography and slit lamp examination aid in precise identification, with corneal topography crucial for early-stage thinning detection.

PMD management begins with glasses and contact lenses, evolving to rigid gas permeable (RGP) lenses as the disease progresses. Surgical interventions, while uncommon, encompass options like penetrating keratoplasty (PK), deep anterior lamellar keratoplasty (DALK), and innovative techniques such as tuck-in lamellar keratoplasty (TILK) and collagen cross-linking (CXL). Prognosis varies, with potential complications like corneal hydrops, emphasising the need for regular monitoring and collaborative management with eye care professionals for optimal outcomes and improved quality of life. The vast majority of patients do not seek surgical intervention and live a high-quality life using specialised contact lenses or glasses.