Pellucid marginal degeneration

Signs and symptoms

Pain is not typically present in pellucid marginal degeneration, and aside from vision loss, no symptoms accompany the condition. However, in rare cases, PMD may present with sudden onset vision loss and excruciating eye pain, which occurs if the thinning of the cornea leads to perforation. While PMD usually affects both eyes, some unilateral cases have been reported.

PMD is characterized by bilateral thinning (ectasia) in the inferior and peripheral region of the cornea. The distribution of the degeneration is crescent or arcuate shaped. The cornea just above the region of thinning is of normal thickness, and may protrude anteriorly, which creates an irregular astigmatism. This is described as a "beer belly" appearance since the greatest protrusion occurs below the horizontal midline (unlike keratoconus). Normally, PMD does not present with vascularization of the cornea, scarring, or any deposits of lipid.

Pathophysiology

PMD is an idiopathic, non-inflammatory condition. The thinning of the corneas may approach 20% of normal thickness. There may be an increase in the number of mucopolysaccharides in the corneal stroma. The Bowman's layer of the cornea may be absent, irregular, or have ruptured areas.

Diagnosis

The center of the cornea shows normal thickness, with an intact central epithelium, but the inferior cornea exhibits a peripheral band of thinning, to about 1–2 mm. The portion of the cornea that is immediately adjacent to the limbus is spared, usually a strip of about 1–2 mm. along with horizontal bow ties. The inferior peripheral thinning is seen between the 4 o'clock and 8 o'clock positions.

PMD lacks apical corneal scarring, Rizutti's phenomenon, Munson's sign, and the central corneal thickness is usually normal.

The gold standard diagnostic test for PMD is corneal topography. Corneal topography may show a "crab claw-like" appearance, a finding that is seen in both keratoconus and in pellucid marginal degeneration. Thus, if corneal topography is used for diagnosis, it should be in conjunction with clinical findings of peripheral, inferior corneal thinning.

Treatment

Eye glasses

Most people can be treated non-surgically with eyeglasses, or contact lenses.

Contact lenses

The early stages of pellucid marginal degeneration may also be managed with soft contact lenses. Success has been shown with the use of rigid gas permeable contact lenses combined with over-refraction. People wearing contacts don't report increased problems with glare and contrast sensitivity, but it is not clear if this is due to the corneal disease, or the contact lenses themselves.

New studies found that the use of Scleral contact lens, a type of rigid gas permeable (RGP) lens, may be a good option for most people with PMD. Most of these lenses are in the range of 15.5mm to 18.0mm in diameter. Regardless of the lens size, it is thought that the larger the RGP lens will in most cases be more comfortable then standard rigid corneal lenses, and at times more comfortable than soft lenses, regardless of the fact that it is a rigid lens. The highlight to the scleral design and the correction of eye disorders such as pellucid marginal degeneration is that vision with these types of lenses is exceptional when fit correctly.

Intacs

The use of intacs implants has been tested as a treatment for PMD, with slight improvement in visual acuity noted after eleven months, and intacs have been used with keratoconus with success.

Corneal collagen cross-linking

Main article: Corneal collagen cross-linking

There is evidence suggesting corneal collagen cross-linking may be beneficial for patients with pellucid marginal degeneration. Research shows some promising results by combining collagen cross linking with photorefractive keratectomy, or with topography-guided transepithelial surface ablation.

Surgery

Corneal transplant surgery may be difficult due to the peripheral thinning of the cornea, even with large and off-center grafts. Therefore, surgery is usually reserved for people that do not tolerate contact lenses. Several different surgical approaches may be taken, and no one approach is currently established as the standard. Examples of surgical procedures used for PMD include: wedge resection, lamellar crescentic resection, penetrating keratoplasty, lamellar keratoplasty, epikeratoplasty and intracorneal segments. Transplantation of the entire thickness of the cornea (penetrating keratoplasty) may be performed if there is enough normal tissue present. However, if there is not enough normal tissue present, then attaching the graft is difficult. New surgical techniques are in evaluation such as Intrastromal lamellar Keratoplasties and Sclerocorneal Intrastromal Lamellar Keratoplasty (SILK), these techniques allow thickening of the inferior peripheral cornea and steepening of the vertical meridien.

Due to the thinning of the cornea, people with PMD are poor candidates for procedures such as LASIK and photorefractive keratectomy.

Prognosis

Visual function declines as a result of the irregular corneal shape, resulting in astigmatism, and causing a distortion in vision. Deterioration can become severe over time.

Epidemiology

The incidence and prevalence of PMD are unknown, and no studies have yet investigated its prevalence or incidence. However, it is generally agreed that PMD is a very rare condition. Some uncertainty regarding the incidence of PMD may be attributed to its confusion with keratoconus. PMD is not linked to race or age, although most cases present early in life, between 20 and 40 years of age. While PMD is usually considered to affect men and women equally, some studies suggest that it may affect men more frequently.

Several diseases have been observed in people with PMD. However, no causal relationships have been established between any of the associated diseases and the pathogenesis of PMD. Such diseases include: chronic open angle glaucoma, retinitis pigmentosa, retinal lattice degeneration, scleroderma, kerato-conjunctivitis, eczema, and hyperthyroidism.

References

References

1. Karim Rasheed, MD. (2005). "Pellucid Marginal Degeneration". WebMD.
2. (2011). "Pellucid corneal marginal degeneration: A review". Contact Lens and Anterior Eye.
3. (2000). "Unilateral pellucid marginal degeneration". Indian Journal of Ophthalmology.
4. (2003). "Unilateral pellucid marginal degeneration". Eye.
5. (1978). "Pellucid marginal corneal degeneration". Archives of Ophthalmology.
6. (2011). "What's in a Name: Keratoconus, Pellucid Marginal Degeneration, and Related Thinning Disorders". American Journal of Ophthalmology.
7. (2007). "Ectatic Disorders Associated with a Claw-shaped Pattern on Corneal Topography". American Journal of Ophthalmology.
8. (2005). "Management of pellucid marginal corneal degeneration". Cornea.
9. (2004). "Intacs for early pellucid marginal degeneration". Journal of Cataract & Refractive Surgery.
10. Intacs. (2005). "A new FDA approved option for keratoconus filling the gap between contact lenses and a corneal transplant". Intacs.
11. (2011). "Additional Applications of Corneal Cross Linking". The Open Ophthalmology Journal.
12. (2009). "Management of pellucid marginal corneal degeneration with simultaneous customized photorefractive keratectomy and collagen crosslinking". Journal of Cataract & Refractive Surgery.
13. (2010). "Corneal Collagen Cross-Linking with Riboflavin and UVA Irradiation in Pellucid Marginal Degeneration". Journal of Refractive Surgery.
14. Hovakimyan, Marine. (31 March 2011). "Collagen Cross-Linking: Current Status and Future Directions". Journal of Ophthalmology.
15. (June 2016). "Sclerocorneal Intrastromal Lamellar Keratoplasty for Pellucid Marginal Degeneration". Cornea.
16. (January 2015). "Intrastromal lamellar keratoplasty in people with pellucid marginal degeneration". Journal of Cataract and Refractive Surgery.
17. Ophthalmology Times. (2006). "Signs of pellucid marginal degeneration should rule out refractive surgery". Ophthalmology Times.
18. (2004). "Pellucid marginal degeneration and scleroderma". Clinical & Experimental Optometry.