Crosslinking for pellucid marginal degeneration (PMD)
Pellucid marginal degeneration (PMD) is a rare, degenerative disease of the cornea that leads to thinning and deformation of the inferior cornea.
It shares some similarities with keratoconus, but differs in several important aspects.
Features of PMD
- Thinning of the cornea: In PMD, the thinning of the cornea typically occurs in the lower (inferior) area, often close to the corneal border.
- Change in shape: This thinning leads to a flattening of the upper cornea and a steepening of the lower cornea, resulting in a characteristic “crow’s wing” shape when looking at the corneal topography.
- Visual impairment: The irregular shape of the cornea causes significant distortion and vision loss, similar to keratoconus, but the pattern of distortion is different.
Differences between PMD and keratoconus
- Localization of the dilution:
- PMD: Thinning occurs mainly in the lower part of the cornea, near the edge.
- Keratoconus: The thinning is more central or paracentral and often forms a cone-shaped protrusion.
- Form of deformation:
- PMD: Leads to a flattening of the upper cornea and a steepening of the lower cornea.
- Keratoconus: Leads to a generalized cone-shaped protrusion of the cornea.
- Onset and progression:
- PMD: Often develops later in life and progresses more slowly.
- Keratoconus: Typically begins in adolescence or young adulthood and may progress more rapidly.
- Diagnosis:
- PMD: Diagnosed by corneal topography and characteristic patterns of corneal deformation showing inferior peripheral thinning.
- Keratoconus: Also diagnosed by corneal topography, but shows a central or paracentral cone-shaped protrusion.
Treatment of PMD
Crosslinking (CXL) is a proven method for the treatment of keratoconus and can also be used in PMD to stabilize the cornea and slow the progression of the disease.
Preparation and implementation of crosslinking with PMD
- Preliminary examination:
A thorough examination of the cornea using corneal topography and pachymetry (measurement of corneal thickness) is necessary to determine the exact extent and localization of the thinning.
- Anesthesia:
Local anesthetic eye drops are used to numb the eye during the procedure.
- Epithelial management:
Epi-off crosslinking: The epithelium (the top layer of the cornea) is removed to allow better penetration of the riboflavin.Epi-on crosslinking: The epithelium remains intact.
However, this method is used less frequently for PMD, as the penetration of the riboflavin may be less effective.
- Riboflavin application:
Riboflavin (vitamin B2) is dripped onto the cornea and must be allowed to act for a sufficiently long time so that it can penetrate deep into the cornea.
This usually takes 20-30 minutes.
- UV irradiation:
The cornea is irradiated with UV-A light.
This irradiation activates the riboflavin, which leads to a hardening of the collagen fibers in the cornea.
The irradiation lasts about 30 minutes.
- Conclusion:
After radiotherapy, a therapeutic contact lens is used to protect the eye and promote healing.
Antibiotic and anti-inflammatory eye drops are also prescribed.
Special considerations in the treatment of PMD
- Localization of thinning: As PMD usually affects the inferior cornea, it is important to ensure that the riboflavin penetrates this area evenly and that the UV irradiation is strong enough to treat the entire affected region.
- Corneal thickness: In the case of very thin corneas, the thickness must be carefully monitored, as a cornea that is too thin increases the risk of complications during UV irradiation.
In such cases, special techniques can be used to temporarily thicken the cornea. - Long-term monitoring: After crosslinking, regular check-ups are necessary to monitor the stability of the cornea and to detect possible complications at an early stage.
Alternative treatment methods for PMD
- Special contact lenses:
Scleral lenses: These large contact lenses bridge the irregular cornea and provide an even, smooth surface.Hybrid lenses: Combine a firm, gas permeable central area with a soft outer zone for better comfort.RGP (Rigid Gas Permeable) lenses: Hard, gas permeable lenses that can provide better vision correction.
- Intrastromal corneal rings (intacs):
Small, semi-circular rings that are implanted into the cornea to change its shape and stabilize it.
- Corneal transplantation:
Lamellar keratoplasty: A partial corneal transplant in which only the affected layers of the cornea are replaced.Penetrating keratoplasty: A full corneal transplant in which the entire cornea is replaced.
- Phakic intraocular lenses (PIOLs):
Lens implants that are inserted into the eye in addition to the natural lens to improve visual acuity.
- Therapeutic eye injections:
Injections of medication to reduce inflammation and improve corneal stability.
- Corneal topography-guided photorefractive keratectomy (PRK):
A laser treatment to correct corneal deformation and improve the quality of vision.
These alternative methods can be used depending on the severity of the PMD and individual patient needs.