Collagen cross-linking in the treatment of pellucid marginal degeneration.

Pellucid marginal degeneration (PMD) is an uncommon cause of inferior peripheral corneal thinning disorder, characterized by irregular astigmatism. We analyzed a case of bilateral PMD patient and treated one eye with corneal collagen cross-linking (CXL) therapy. Corneal topography was characteristic for PMD. Visual acuity, slitlamp examinations, tonometry, and corneal thickness were observed. Simulated keratometric and topographic index values were detected with corneal topography. Uncorrected, LogMAR visual acuity has improved from +0.8 to +0.55 during the 6 months and +0.3 during the 8 months follow-up after CXL. Pachymetry values and intraocular pressure showed no changes. Keratometric values and topografic indexes disclosed no progression of the disease. CXL may postpone or eliminate the need of corneal transplantation in cases with PMD.

Pellucid marginal degeneration (PMD) of the cornea is a progressive, rare, and uncommon noninflammatory corneal disorder characterized by the thinning in the peripheral portion of the inferior cornea with marked steepening just superior to the thinned zone.[12] It occurs in both men and women and can be differentiated from other peripheral corneal thinning disorders such as keratoconus and keratoglobus by its characteristics that this thinning occurs 1–3 mm from the limbus in the 4–8 o’clock position.[3] It is differentiated from peripheral corneal disorders associated with inflammation such as Terrien's marginal degeneration and Mooren's ulcer by the absence of vascularisation. Although histopathologically it is considered a variant of keratoconus, it differs in that the marked corneal steepening occurs more inferiorly.[1]

Corneal cross-linking was introduced by Wollensak et al. in 2003 for the treatment of progressive keratoconus and related disorders increasing the biomechanical strength of the cornea by about 300%.[4]

Here we present one case showing the features of PMD both clinically and topographically treated by collagen cross-linking (CXL). It was clinically typical bilateral PMD with a characteristic pattern of irregular astigmatism on corneal topography, with chief complaints of progressive dimness of vision caused by irregular astigmatism. The CXL treatment can stop PMD progression and produce better quality of vision.

Case Report

We analyzed a case of advanced bilateral PMD patient and treated one eye with corneal CXL. A 55-year-old male is presented, complaining of progressive dimness of vision in both eyes with decreased vision started 1 year before examinations at our clinic, but the visual loss was larger in the right eye. There was no history of excessive eye rubbing, trauma, contact lens wear, or episodic redness of the eye. There was no familiar history. Earlier, he was diagnosed with glaucoma simplex treated with timolol 0.5%. His right eye was pseudophakic, and his left eye had posterior subcapsular cataract. He had high blood pressure and type II diabetes mellitus was treated for around 9 years. The patient tried rigid gas-permeable contact lenses but could never get an acceptable fit. Visual acuity, slitlamp examination, corneal thickness map measurement (Pentacam HR, Oculus), and tonometry were carried out, and simulated keratometric (SK1, SK2) and topographic index values were detected with corneal topograph (Tomey TMS4).

Ophthalmic examination revealed his best-uncorrected visual acuity, a value of +0.8 in the right eye and +0.1 in the left eye observed with a logarithmic scale (LogMAR). Slitlamp examination of the cornea showed inferior peripheral corneal-thinning without iron lines, vascularization, or lipid deposition [Fig. 1]. The lesion was nonulcerative and inflammatory, and implied the protrusion of the cornea. Between the thinned area and the limbus, the corneal thickness was normal. However, the left eye showed a clear band of peripheral thinning about 1–2 mm wide, with anterior protrusion of the cornea just above the thinned area. A 2.0-mm zone of normal-thickness cornea was seen between the thinned area and the limbus. Scheimpflug image (Oculus Pentacam HR) of the anterior segment of the right eye shows irregular shaped central corneal region [Fig. 2a and b].

Figure 1

Slit-lamp photograph of the right eye shows inferior steepening with a clear zone between limbus and the steep band

Figure 2

(a and b) Scheimpflug image (Oculus Pentacam HR) of the anterior segment shows irregular shaped central corneal region

Both anterior chambers were 4.11 mm deep. Corneal thickness measured by ultrasonic pachymeter of the right eye were 502 μm centrally and 520 μm peripherally; in the left eye, the readings were 540 μm centrally, and 520 μm at the periphery. The intraocular pressure measured by noncontact tonometry was in normal range in both eyes.

Corneal topography was characteristic for PMD. The vertical axis images showed significant central irregular against-the-rule astigmatism, marked peripheral thinning within 2.0 mm of the limbus, and more normal corneal thickness inferior to the band of thinning. Topograph sagittal curvature map on the right eye showed vertical flatting and irregular inferior corneal astigmatism. The horizontal axis had more normal corneal contour and thickness. Keratometric readings were 53.98D/36.04D in the right eye. In both eyes, there was a bow-tie-shaped corneal astigmatism with a value of 17.96D in the right eye [Fig. 3]. This clinical picture was consistent with a diagnosis of PMD of both eyes.

Figure 3

Preoperative corneal topographic picture of the right eye shows the characteristic bow-tie appearance of corneal steepening (red zones) and midperipheral inferior corneal flattening

CXL was performed on the right eye using InPro CCL-Lix device (Norderstadt, Germany). During treatment, 8 mm of the central corneal epithelium was removed and 0.1% riboflavin solution (with 20% dextrane) was instilled on the surface of the cornea with eccentric method of Spadea[5] and was repeated every 2 minutes thereafter. In the meantime, UV-A irradiation was performed with an emission at 370 nm, with radiant energy of 3 mW/cm2, focusing distance of 60 mm, and lasted for 30 minutes. The postoperative treatment was antiobiotic eye drops for 5 days (tobramycin) and steroid eyedrops (fluorometholone) and artificial tear drops for at least 1 month. We did not use contact lens postoperatively. There was no corneal edema or endothelial damage postoperatively.

The follow-up period was 8 months. Visual acuity, simulated keratometric indexes, and corneal thickness values were detected during the postoperative period. Uncorrected visual acuity has increased from +0.8 to +0.55; best-corrected visual acuity was +0.25 with a correction of spherical equivalent of -4.0 D 8 months after CXL treatment. After CXL treatment, SK1 decreased to 50.48 D and SK2 to 33.97 D [Fig. 4]. Pre- and postoperative corneal back surface Scheimpflug pictures (Pentacam HR) do not show difference [Fig. 5].

Figure 4

Corneal topograph shows flattening of the inferior cornea 8 months after CXL treatment

Figure 5

Pre-and postoperative corneal back surface Scheimpflug pictures (Pentacam HR) do not show difference

Pachymetry map showed minimal increase in peripheral corneal thickness (from 520 μm preoperatively to 528 μm postoperatively) 8 months after treatment. No change in intraocular pressure was measured, during the follow-up period. IOP varied between 14 and 19 mmHg. Our data are summarized in Tables 1 and 2.

Table 1

Patient data before and after collage cross-linking treatment

Table 2

Patient data before and after surgery

Discussion

PMD has been postulated to the abnormality of the connective tissue, but the exact pathogenesis is still unknown. On corneal topography, marked steepening of the inferior corneal periphery can be seen, which also extends into mid-peripheral inferior corneal meridians. The mid-peripheral cornea is gradually decreased in keratometric power above the inferior oblique meridians.[36] Our patient had all these features in both eyes, and there was no stromal thickness less than 400 μm, but the left eye showed less deviation than the right eye. The usual treatments for PMD are intrastromal ring insertion, lamellar keratoplasty, crescentic lamellar keratoplasty, penetrating keratoplasty, cyanoacrylate adhesive, and a bandage contact lens. Conservative treatment is also possible.[7]

Corneal transplantation for keratectasia is a difficult procedure with a long and uncertain visual recovery. Ertan et al.[8] inserted intacs ring using a femtosecond laser to correct pellucid marginal corneal degeneration with safe and efficient results. In the report of Stojanovic et al., 12 eyes were treated with topography-guided custom ablation and CXL, which after uncorrected visual acuity increased and astigmatism and keratometric asymmetry decreased.[9] Spadea described eccentric irradiation CXL technique followed by us in case of PMD.[5] Kymionis et al.[10] treated a woman with simultaneous photorefractive keratectomy and corneal CXL with riboflavin-ultraviolet-A irradiation for the treatment of progressive pellucid marginal corneal degeneration in both eyes with good results. Histologically and clinically, CXL is a well-circumscribed and well-published therapic possibility of corneal ectasias.[1112] There are some possible complications after CXL: early pseudo-haze, bacterial infection,[13] scarring, melting, or perforation. The safety parameter of a minimal stromal thickness of 400 μm to spare the endothelium.[14]

In this case, progression had been stopped by CXL treatment, and the best-corrected visual acuity increased during the observation period. No intraoperative or early postoperative complications occurred. The CXL irradiation was safe and efficient in the treatment of certain stages of PMDs with the help of which progression can be reversed or at least stopped, and visual acuity can be improved. This method may postpone or eliminate the need of corneal transplantation in cases with PMD.