Background

• Pellucid Marginal Degeneration is a rare and progressively debilitating corneal disorder that affects the outermost layer of the eye, known as the cornea. This disorder is characterized by a thinning and bulging of the cornea’s inferior portion, leading to a gradual distortion of vision. Unlike other corneal conditions such as keratoconus, PMD impacts the peripheral cornea, causing a characteristic crescent-shaped thinning region just above the corneal margin. The exact cause of PMD remains uncertain, but it is thought to involve a combination of genetic predisposition and environmental factors. 
• PMD typically manifests during early to middle adulthood and tends to affect both eyes, though their severity may differ. As the cornea becomes increasingly distorted, individuals with PMD often experience irregular astigmatism, resulting in blurred and distorted vision that cannot be adequately corrected with glasses. Instead, specialized contact lenses, such as scleral lenses, are often prescribed to provide improved visual acuity and comfort. In severe cases where vision is significantly compromised, surgical interventions like corneal cross-linking, corneal implants, or even corneal transplantation might be considered to manage the condition. While PMD can significantly impact a person’s quality of life, early diagnosis and appropriate management can help minimize its effects and preserve visual function. 

Epidemiology

• Incidence and prevalence: The exact numbers are unknown, but PMD is much less common than keratoconus.
• PMD is considered a relatively uncommon corneal condition, and its prevalence in the general population is relatively low. It tends to occur in individuals within a specific age range, typically between 20 and 50 years old, with males being more commonly affected than the females.
• Risk factors: There is no racial or geographical predisposition. It usually affects males in their 20s to 50s.

Anatomy

Pathophysiology

• The exact pathophysiology of Pellucid Marginal Degeneration (PMD) is not understood, but it is thought to involve a combination of genetic, biomechanical, and biochemical factors. PMD primarily affects the peripheral cornea, leading to thinning, steepening, and the cornea’s inferior margin protrusion. This can result in irregular astigmatism and visual distortion. 
• One hypothesis suggests that genetic factors play a role in the development of PMD. A genetic predisposition may make certain individuals more susceptible to the disorder. However, specific genes associated with PMD have not been definitively identified. 
• Biomechanical factors also likely contribute to the development of PMD. The cornea is a complex and dynamic structure with layers of collagen fibers that provide strength and shape. Abnormalities in the collagen structure and organization in the peripheral cornea could lead to weakening and thinning over time. This may result in the characteristic crescent-shaped area of thinning observed in PMD. 

Etiology

The etiology of Pellucid Marginal Degeneration (PMD) remains largely unknown, and its exact cause has not been definitively identified. 

• Genetics: There is evidence to suggest a genetic predisposition to PMD, as it often occurs in families and is more common among certain ethnic groups. However, specific genes associated with PMD have not been conclusively identified. 
• Biomechanical Factors: Abnormalities in the corneal structure and biomechanics may play a role. The cornea’s collagen fibers provide its strength and shape, and any disruptions or weaknesses in these fibers could lead to the thinning and bulging characteristic of PMD. 
• Environmental Factors: Some environmental factors, such as chronic eye rubbing, may contribute to the development or progression of PMD. Eye rubbing can create mechanical stress on the cornea, potentially exacerbating the thinning and distortion. 
• Autoimmune or Inflammatory Processes: In some cases, autoimmune or inflammatory processes may be involved in the development of PMD. These processes could contribute to the weakening and thinning of the cornea over time. 
• Hormonal Influences: Hormonal changes or imbalances may potentially influence the development of PMD, although more research is needed to understand this possible connection. 

Genetics

Prognostic Factors

The prognosis of Pellucid Marginal Degeneration (PMD) can vary widely among individuals, and several factors can influence the course and severity of the condition. While there is no definitive cure for PMD, understanding these prognostic factors can help guide treatment decisions and manage the progression of the disease.  

• Age of Onset: The age at which PMD develops can impact its progression. In some cases, PMD stabilizes or progresses more slowly as individuals age, while in others, it may progress more rapidly. 
• Severity of Thinning: The extent of corneal thinning, steepening, and protrusion is a significant prognostic factor. Individuals with more severe thinning are likely to experience more pronounced visual distortions and may have a poorer prognosis in terms of visual acuity. 
• Rate of Progression: The speed at which PMD progresses can vary. Some individuals may experience slow and gradual progression, while others may see more rapid changes in their corneal shape and visual acuity. 
• Stability: Periods of stability in corneal shape and visual acuity can occur in some individuals with PMD. These stable periods can provide opportunities for effective management and improved visual correction. 
• Management and Treatment: The success of treatment and management strategies, such as specialized contact lenses (e.g., scleral lenses) or surgical interventions (e.g., corneal cross-linking or corneal transplantation), can greatly influence the prognosis. Effective management can help stabilize or improve visual acuity and quality of life. 
• Individual Variability: Each person’s response to treatment and progression of PMD is unique. Some individuals may experience significant visual impairment and complications, while others may have a milder disease course. 
• Other Eye Conditions: Other eye conditions or complications can impact the overall prognosis. For example, individuals with PMD who have cataracts or other corneal disorders may have more complex treatment considerations. 

Clinical History

• Age Group: Pellucid Marginal Degeneration (PMD) typically presents in early to middle adulthood, usually between the ages of 30 and 50. It is rare for PMD to develop before the age of 20. 

Physical Examination

• Slit Lamp Biomicroscopy: The slit lamp is a specialized microscope that allows detailed examination of the anterior segment of the eye, inclusive of the cornea. The ophthalmologist will use the slit lamp to assess the corneal surface for signs of thinning, protrusion, and irregular astigmatism. The characteristic crescent-shaped thinning at the inferior corneal margin may be observed. 
• Corneal Topography: It is a non-invasive imaging technique that maps the curvature of the corneal surface. It helps quantify the extent of corneal irregularities and provides valuable information about corneal shape and astigmatism. Corneal topography is particularly useful for diagnosing and monitoring PMD. 
• Visual Acuity Testing: Visual acuity is assessed using an eye chart to measure how well the individual can see at different distances. Patients with PMD often have decreased visual acuity due to irregular astigmatism. 
• Refraction: Refraction determines the patient’s refractive error and the appropriate prescription for glasses/contact lenses. Individuals with PMD may require specialized contact lenses like scleral lenses to achieve optimal visual correction. 
• Pachymetry: These measures the thickness of the cornea. In PMD, the inferior corneal margin is thinning, which can be detected using pachymetry. 
• Keratometry: Keratometry measures the curvature of the cornea’s central area. In PMD, there may be a localized steepening in the inferior peripheral cornea. 
• Slit Lamp Photography: Photographs of the cornea taken with the slit lamp can aid in documenting the characteristic features of PMD and monitoring its progression over time. 
• Cycloplegic Refraction: Cycloplegic eye drops are used to temporarily relax the eye’s focusing muscles. Cycloplegic refraction can help determine the true refractive error in patients with irregular astigmatism. 
• Assessment for Corneal Hydrops (Advanced Cases): In cases of advanced PMD, the ophthalmologist will look for signs of corneal hydrops, a sudden cornea swelling due to fluid accumulation. Corneal hydrops can cause sudden and severe vision impairment. 

Age group

Associated comorbidity

• PMD is not strongly associated with specific comorbidities, but some individuals with PMD may have a history of eye rubbing, which could contribute to the condition’s development or progression.
• Activities that involve chronic eye rubbing, such as allergies or certain occupational habits, might be relevant in some cases. 

Associated activity

Acuity of presentation

The visual symptoms and acuity of the presentation can vary among individuals with PMD. Common clinical features include: 

• Irregular Astigmatism: The hallmark of PMD is irregular astigmatism, leading to distorted and blurred vision that cannot be fully corrected with glasses. Patients often complain of seeing ghosting, halos, glare, and multiple images. 
• Crescent-Shaped Thinning: PMD is characterized by thinning and protrusion of the inferior peripheral cornea. This leads to a characteristic crescent-shaped thinning area just above the corneal margin. This thinning is often observed during a clinical examination. 
• Bilateral Involvement: PMD typically affects both eyes, although the severity may vary between eyes. This bilateral involvement contributes to the distortion of depth perception and visual acuity. 
• Corneal Hydrops (Advanced Cases): In some advanced cases, the cornea may develop a sudden rupture or fluid-filled swelling, a condition known as corneal hydrops. This can cause sudden and severe vision impairment and requires urgent medical attention. 
• Decreased Visual Acuity: Irregular astigmatism often significantly compromises visual acuity. Patients may require specialized contact lenses, such as scleral lenses, for improved visual correction. 
• Unilateral vs. Bilateral Presentation: While PMD usually affects both eyes, there are rare cases where it may initially present unilaterally before involving the other eye. 

Differential Diagnoses

• Keratoconus: This is one of the most important differential diagnoses for PMD. Keratoconus also involves corneal thinning and steepening, resulting in irregular astigmatism and visual distortion. However, keratoconus typically affects the central cornea, whereas PMD primarily impacts the peripheral cornea. Corneal topography and other imaging modalities can help differentiate between the two conditions. 
• Terrien’s Marginal Degeneration: This rare degenerative corneal disorder primarily affects the superior cornea. Like PMD, it can cause thinning and peripheral distortion, but the thinning location is different. Terrien’s marginal degeneration is more common in older individuals. 
• Keratoglobus: Keratoglobus is a congenital condition characterized by generalized corneal thinning and globular protrusion. It can result in the severe visual impairment and requires specialized management. 
• Corneal Ectasia Post-LASIK: In some cases, complications from LASIK surgery can lead to corneal ectasia, which involves corneal thinning and steepening. Differentiating this from PMD is important for guiding treatment decisions. 
• Corneal Scarring or Dystrophies: Corneal scars, dystrophies, or other irregularities can cause visual disturbances similar to PMD. A thorough history and clinical examination can help differentiate these conditions. 
• Corneal Infections or Inflammatory Conditions: Conditions such as corneal infections (e.g., keratitis) or inflammatory disorders can lead to corneal thinning and irregular astigmatism, mimicking some aspects of PMD. 
• Contact Lens-Induced Corneal Changes: Improper or prolonged contact lens use can lead to corneal shape and thinning changes, causing visual distortions similar to PMD. 
• Ocular Surface Disorders: Conditions affecting the ocular surface, such as dry eye disease or other corneal irregularities, can sometimes lead to visual disturbances that must be differentiated from PMD. 

Laboratory Studies

Imaging Studies

Procedures

Histologic Findings

Staging

Treatment Paradigm

• The treatment paradigm for Pellucid Marginal Degeneration (PMD) aims to address visual impairment and improve quality of life. Management options based on the severity of the condition and individual patient factors. Corrective eyewear such as rigid gas-permeable (RGP) or scleral contact lenses are commonly prescribed in mild to moderate cases. Scleral lenses, which vault over the irregular corneal surface, can provide better visual acuity and comfort for PMD patients. Regular monitoring is essential to ensure optimal lens fit and visual correction. Surgical interventions may be considered in more advanced cases or when contact lenses are ineffective. Corneal cross-linking (CXL) can help strengthen the corneal tissue and slow disease progression. In severe cases where visual acuity is significantly compromised, or complications arise, options like corneal transplantation or other advanced surgical procedures may be explored. Early diagnosis, ongoing follow-up, and personalized treatment plans are vital to effectively manage PMD and maintain or improve visual function. 
• The treatment approach for PMD necessitates a balanced consideration of visual rehabilitation and disease stabilization. Individualized patient needs, disease severity, and response to treatment are critical factors in determining the most appropriate therapeutic path. Collaborative decision-making between the patient and healthcare provider ensures that treatment choices align with the patient’s goals and result in the best visual outcomes. 

by Stage

by Modality

Chemotherapy

Radiation Therapy

Surgical Interventions

Hormone Therapy

Immunotherapy

Hyperthermia

Photodynamic Therapy

Stem Cell Transplant

Targeted Therapy

Palliative Care

use-of-the-non-pharmacological-therapy-for-modifying-the-environment

Modifying the environment can be supportive in managing Pellucid Marginal Degeneration (PMD). However, it is important to note that environmental modifications alone are not a primary treatment for the condition. PMD primarily involves structural changes in the cornea, so interventions to improve visual acuity and comfort often focus on optical correction and surgical options.  

• Eye Protection: Wearing sunglasses with UV protection can help shield the eyes from harmful ultraviolet (UV) rays, which may contribute to corneal thinning and other eye conditions. Sunglasses can also reduce glare and improve comfort, especially in bright outdoor environments. 
• Eye Hygiene: Practicing good eye hygiene, including proper hand washing and avoiding excessive eye rubbing, can help minimize the risk of complications or exacerbations of PMD. Chronic eye rubbing could worsen the condition, so it’s important to remember this habit. 
• Avoiding Irritants: Minimizing exposure to irritants, such as allergens or environmental pollutants, can help reduce eye discomfort and potential exacerbation of symptoms. 
• Optimal Lighting: Ensuring proper lighting conditions for reading, working, or other activities can reduce eye strain and improve visual comfort. Adequate lighting can help compensate for some of the visual distortions associated with PMD. 
• Screen Use: If using digital screens for prolonged periods, following the 20-20-20 rule can help reduce eye strain and fatigue. 
• Moisturizing Eye Drops: Artificial tears or lubricating eye drops can help alleviate dryness and discomfort due to reduced blink rate or other ocular surface issues. 

Use of medical care for treating Pellucid Marginal Degeneration

several therapy options are available to manage its symptoms and improve visual function. Here are some of the medical care options: 

• Glasses and Contact Lenses: Early stages of PMD, glasses/soft contact lenses may help correct mild astigmatism and improve vision. As the condition progresses, specialized rigid gas-permeable (RGP) contact lenses or scleral lenses may be prescribed to provide better visual clarity by creating a smooth optical surface on the cornea. 
• Corneal Cross-Linking (CXL): Corneal cross-linking is a procedure used to strengthen the cornea and slow down the progression of PMD. During CXL, riboflavin eye drops are applied to the cornea, which is then exposed to ultraviolet (UV) light. This process helps create additional bonds within the corneal collagen fibers, making the cornea more stable and less prone to further thinning. 
• Intacs: Intacs are small, clear plastic inserts that are surgically placed within the cornea to flatten its shape and reduce astigmatism. This can help improve vision and stabilize the cornea’s curvature. 
• Corneal Transplant: In more advanced cases of PMD where vision cannot be adequately corrected with contact lenses or other methods, a corneal transplant (also known as keratoplasty) may be considered. During a corneal transplant, a damaged cornea is replaced with a healthy donor cornea. 
• Custom Wavefront-Guided Laser Surgery: For some individuals with PMD, custom wavefront-guided laser procedures such as LASIK or PRK may be an option to improve vision by reshaping the cornea’s surface. However, these procedures are usually not recommended if the cornea is too thin or irregular. 
• Visual Aids and Low Vision Rehabilitation: In cases where vision cannot be fully corrected, visual aids and low vision rehabilitation can help individuals make the most of their remaining vision. This may include magnifiers, telescopes, and other assistive devices.

Use of surgical care for treating Pellucid Marginal Degeneration

• Surgical interventions for Pellucid Marginal Degeneration (PMD) are typically considered when the condition is more advanced and other conservative treatments like contact lenses or corneal cross-linking have not provided sufficient improvement in vision.  
• Corneal Transplant (Keratoplasty): Corneal transplant surgery involves replacing the damaged cornea with healthy donor cornea. There are different types of corneal transplants that may be used in cases of PMD: 
• Penetrating Keratoplasty (PKP): This is the traditional full-thickness corneal transplant, where the entire central portion of the cornea is generally replaced with a donor cornea. PKP is typically reserved for cases of advanced PMD where other treatments have been ineffective. 
• Deep Anterior Lamellar Keratoplasty (DALK): In DALK, only the front layers of the cornea are replaced, leaving the back layers intact. This technique is used when the inner layers of the cornea are healthy and can provide better stability. 
• Implantable Collamer Lens (ICL): In some cases, an implantable collamer lens (ICL) may be considered. ICL surgery involves placing a synthetic lens inside the eye, just behind the iris and in front of the natural lens. This can help correct refractive errors caused by the irregular cornea in PMD. 
• Intacs Placement: Intacs are small, semi-circular plastic rings that are inserted into the cornea to reshape its curvature. This surgical procedure can help flatten the cornea and improve vision by reducing astigmatism. 
• Topography-Guided Laser Surgery: Advanced laser procedures, such as topography-guided PRK (Photorefractive Keratectomy) or LASIK (Laser-Assisted in Situ Keratomileusis), may be used to reshape the cornea’s surface and correct irregular astigmatism. These procedures use detailed corneal mapping to guide the laser treatment. 

use-of-intervention-with-a-procedure-in-treating-pellucid-marginal-degeneration

Treatment interventions for Pellucid Marginal Degeneration (PMD) can range from non-surgical approaches to surgical procedures, depending on the severity of the condition and the patient’s needs.  

• Specialized Contact Lenses: For mild to moderate PMD cases, specialized contact lenses, such as rigid gas-permeable (RGP) lenses or scleral lenses, can provide improved visual acuity by creating a smooth refractive surface over the irregular cornea. Scleral lenses, which vault over the corneal surface, are particularly effective in managing visual distortions and reducing discomfort. 
• Corneal Cross-Linking (CXL): This minimally invasive procedure aims to strengthen the corneal collagen fibers and halt disease progression. During CXL, riboflavin eye drops are applied to the cornea, followed by ultraviolet (UV) light exposure. This cross-links the collagen fibers and enhances corneal stability. 
• Intacs (Intracorneal Ring Segments): Intacs are small, semi-circular plastic segments implanted into the cornea. They help reshape the cornea and reduce irregular astigmatism, improving visual quality. 
• Corneal Transplantation: In advanced cases of PMD where other interventions are insufficient, a corneal transplant (keratoplasty) may be considered. This process involves replacing the damaged cornea with healthy donor tissue. Different types of corneal transplants, such as penetrating keratoplasty or deep anterior lamellar keratoplasty (DALK), may be used based on the extent of corneal involvement. 
• Topography-Guided Custom Ablation: This laser procedure (similar to LASIK or PRK) uses corneal topography data to guide precise corneal reshaping, reducing irregular astigmatism and improving visual outcomes. 
• Corneal Implants: Newer techniques involve implanting synthetic or biological materials to reshape and stabilize the cornea. These procedures are still under investigation and may be considered in select cases. 

use-of-phases-in-managing-pellucid-marginal-degeneration

Managing Pellucid Marginal Degeneration (PMD) typically involves a phased approach to address the condition’s various stages and optimize visual outcomes.  

• Diagnosis and Evaluation: The first phase involves a thorough clinical evaluation, including detailed corneal imaging (such as corneal topography), visual acuity testing, and a comprehensive eye examination. A proper diagnosis of PMD is essential to guide subsequent treatment decisions. 
• Non-Surgical Management: In the early stages of PMD, non-surgical approaches are often explored. This includes prescribing specialized contact lenses, such as rigid gas-permeable (RGP) or scleral lenses. These lenses provide a smoother refractive surface, improving visual acuity and reducing distortions caused by irregular astigmatism. Regular follow-up visits are essential to monitor lens fit and make adjustments as needed. 
• Corneal Cross-Linking (CXL): When PMD progresses or shows signs of worsening, corneal cross-linking (CXL) may be considered. CXL aims to strengthen the cornea’s collagen fibers and halt disease progression. During the procedure, riboflavin eye drops and ultraviolet (UV) light promote collagen cross-linking, enhancing corneal stability. CXL is most effective when performed in the early stages of PMD. 
• Advanced Interventions: In cases where non-surgical options and CXL are insufficient, more advanced interventions may be necessary. This could include procedures like Intacs (intracorneal ring segments) to reshape the cornea, topography-guided custom ablation using laser technology, or even corneal transplantation (keratoplasty) for severe cases with significant visual impairment. 
• Long-Term Monitoring: Long-term monitoring is crucial regardless of the treatment phase. Regular follow-up visits with an ophthalmologist are necessary to assess disease progression, adjust treatments, and ensure optimal visual outcomes. Monitoring helps catch any changes in the condition early and allows for timely adjustments to the management plan. 
• Patient Education and Lifestyle Management: Patient education plays a vital role in all phases of PMD management. Patients should be educated about proper eye hygiene, avoiding eye rubbing, and adhering to treatment recommendations. Lifestyle modifications, such as wearing UV-protective sunglasses and managing dry eye symptoms, can contribute to overall eye health. 

Medication

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References

• Pellucid marginal degeneration – Wikipedia 
• Pellucid Marginal Degeneration | SpringerLink 
• Pellucid Marginal Corneal Degeneration – StatPearls – NCBI Bookshelf (nih.gov)