Sudanese Journal of Ophthalmology

REVIEW ARTICLE
Year
: 2020  |  Volume : 12  |  Issue : 2  |  Page : 35--42

Updates on management of pellucid marginal degeneration: Topographic patterns, differential diagnosis, and surgical options


Amr Mounir 
 Department of Ophthalmology, Sohag Faculty of Medicine, Sohag University, Sohag, Egypt

Correspondence Address:
Dr. Amr Mounir
Department of Medicine Ophthalmology, Sohag Faculty of Medicine, Sohag
Egypt

Abstract

Pellucid marginal degeneration (PMD) is a rare ectatic corneal disease involving the inferior part of the cornea. It is difficult to differentiate between keratoconus (KCN) and PMD by slit lamp, especially in the detection of early and subclinical stages of the diseases. Corneal topography is the main diagnostic tool of PMD with characteristic diagnostic patterns “crab-claw” or “butterfly.” PMD could be mistaken as KCN, keratoglobus, and other peripheral thinning conditions such as Terrien marginal degeneration and Mooren's ulcer. Spectacles, soft and rigid gas permeable contact lens are the main visual correcting method in early stage of the disease. Different surgical techniques are available for PMD management; however, none of them were found to be effective, so further studies will be needed in the future.



How to cite this article:
Mounir A. Updates on management of pellucid marginal degeneration: Topographic patterns, differential diagnosis, and surgical options.Sudanese J Ophthalmol 2020;12:35-42


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Mounir A. Updates on management of pellucid marginal degeneration: Topographic patterns, differential diagnosis, and surgical options. Sudanese J Ophthalmol [serial online] 2020 [cited 2021 Dec 3 ];12:35-42
Available from: https://www.sjopthal.net/text.asp?2020/12/2/35/311042


Full Text

 Introduction



Pellucid marginal degeneration (PMD) is an idiopathic, progressive, noninflammatory, ectatic corneal disease characterized by a peripheral crescent of inferior corneal thinning.[1] The term “pellucid” means clear and first time was used by Schlaeppi[2] to describe the corneal clarity and the absence of lipid deposition, scarring or vascularization, despite the presence of ectasia.

This ectatic disease commonly involves the inferior cornea, with an area of thinning extending from the 4-o´ clock to the 8-o´ clock positions.[3]

Patients with PMD mostly presented in their third to fourth decade of life with decreased visual acuity due to increase in against-the-rule astigmatism.[4]

According to multiple studies about the incidence of PMD, this disorder is considered as a rare condition, less common than other ectatic corneal diseases, such as keratoconus (KCN), but more common than others, such as keratoglobus or posterior KCN.[5],[6]

Several studies[7],[8] have proposed that PMD is a form of KCN of peripheral type due to the close similarities between both conditions.

 Clinical Diagnosis of Pellucid Marginal Degeneration



The clinical diagnosis of PMD depends on the suspicion of the disease and careful evaluation by various available diagnostic tools including retinoscopy, slit-lamp biomicroscopy, keratometry, keratoscopy, pachymetry, and corneal topography.[9]

Age of presentation of PMD is usually diagnosed between the second and the fifth decades,[10] in comparison to KCN, which is diagnosed mostly between the puberty and the third decade of life.[11]

This disease is usually asymptomatic; however, a progressive deterioration in uncorrected and best-corrected visual acuity can occur due to irregular astigmatism induced by the corneal ectasia in the most advanced cases.[12]

By slit-lamp biomicroscopy, a 1–2-mm margin of normal cornea was found between the thinning and limbus with peripheral corneal thinning from the 4 to 8-o'clock positions.[3]

The major limitation for differentiating between KCN and PMD by slit lamp is in the detection of early and subclinical stages of the diseases, because these are not associated with any biomicroscopic signs or these signs are unremarkable.[13]

 Topographic Patterns of Pellucid Marginal Degeneration



Corneal topographic evaluation is the main method to confirm the presence of an underlying ectatic condition. It is essential to measure the degree and location of the thinning, which is the main clinical key to distinguish the different entities of corneal ectasia.[14]

The classic topographic presentation of PMD is flattening of the vertical meridian above a cresentric band of thinning [Figure 1], with characteristic keratometric pattern of a marked “against-the-rule” astigmatism.[15]{Figure 1}

Indeed, the other typical morphological patterns which are called the “crab claw” pattern or “butterfly” pattern [Figure 2] are considered the most characteristic patterns in PMD due to the presence of the corneal protrusion below the area of thinning, with normal thickness of the central cornea.[16],[17] However, it has been found that this characteristic, claw-like pattern of peripheral steepening on corneal topographic analysis is not always associated with the sure diagnosis of PMD.[18]{Figure 2}

Another topographic pattern for PMD is that shows thinning toward the inferior part of the cornea matching what was observed by the slit-lamp biomicroscopy, a peripheral band of thinning of the inferior cornea which is the hallmark signs of the disease. This thinning is characteristic and called “bell” sign [Figure 3]. This pattern is found in advanced cases of PMD.[19]{Figure 3}

In a study of Fuchihata et al.,[7] they evaluated the characteristics of the corneal shape in patients with PMD and compared these characteristics with keratoconic eyes and eyes of normal subjects on a retrospective study, they found that in eyes with PMD, the crab claw pattern (78%) was the most common axial power map pattern followed by inferior steepening pattern (18%).

 High-order Aberrations, Corneal Biomechanical Response, and Corneal Densitometry in Pellucid Marginal Degeneration



High-order aberrations (HOAs) in cases of PMD have been reported in different studies. Oie et al.[20] evaluated the characteristics of HOAs in eyes with pellucid marginal corneal degeneration; they found that the coma aberrations were significantly lower in the PMD group (0.27 mm G 0.19 [standard deviation]) than in the KCN group (0.70 G 0.37 mm) (P < 0.05), while the spherical aberration was significantly higher in the PMD group (0.086 G 0.10 mm) than in the KCN group (0.030 G 0.13 mm) (P < 0.05).

In a study of Kamiya et al.,[21] they reported the time changes in corneal wavefront aberrations in a patient with PMD; they found that coma-like aberration displayed a gradual, apparent increase with a 1.67-fold worsening during the 11-year follow-up period while spherical-like aberration remained almost stable.

Radhakrishnan and O'Donnell[22] studied aberrometry characteristics in four patients diagnosed with corneal thinning disorders including a case of PMD; they demonstrated how aberrometry can be used to help in the differential diagnosis of patients with corneal thinning disorders.

As regards the corneal biomechanics, many studies have evaluated corneal biomechanical items in PMD using the ocular response analyzer (ORA). A study was conducted by Labiris et al.[23] who evaluated the capacity of specific biomechanical indices in PMD.

Another study was done by Lenk et al.;[24] they investigated the diagnostic capacity of corneal biomechanical response using ORA and Corvis ST devices in prospective clinical study; they found that ORA results showed that the KCN match index was significantly lower in the PMD group than in the other control group (0.031 ± 0.37 vs. 0.79 ± 0.33; P = 0.001).

In a study of Sedaghat MR et al.,[25] they evaluated and compared corneal hysteresis and corneal resistance factor in PMD, KCN, and normal eyes using the ORA; they concluded that the results of the ORA were markedly different between PMD, KCN, and normal eyes.

Further studies will be needed to evaluate the corneal biomechanics in PMD patients, especially with the emergence of new generations of ORAs.

The evaluation of corneal densitometry in patients with different corneal diseases has recently found to an important diagnostic tool.[26],[27]

Densitometric assessment provides measurable data of the corneal transparency at different depths and in different zones. In ectatic diseases, remarkable structural pathological changes occur in the corneal stroma, and both the structure and the arrangement of collagen fibrils are disrupted. Hence, the corneal densitometric values increase.[28],[29]

In a study of M Koc et al.,[30] they evaluated the topographic, densitometric properties of patients with PMD and inferior KCN in a retrospective cross-sectional study; they found that the densitometry values of PMD were significantly higher than those of the controls in all zones.

 Differential Diagnosis of Pellucid Marginal Degeneration



PMD is frequently mistaken as KCN which is the most common corneal ectatic disorder.[31] KCN is an ectatic disease which is characterized by corneal steepening with conical profile at the same position of maximal stromal thinning.[32]

Clinical typical signs of KCN include a “scissoring reflex” by retinoscopy reflex, an “oildrop” sign by ophthalmoscopic reflex, Fleischer's ring,[33] Vogt's striae,[34] and stromal thinning with conical protrusion.[35] Advanced signs of KCN include Munson's sign, Rizutti's sign, and corneal scarring at the apex.[36],[37]

Corneal topography is most valuable way to differentiate between KCN and PMD, as both of them show different topographic patterns.[5]

Moderate stages of KCN shows a classical inferior-central cone in which the steepening extends toward the inferior limbus.[30]

The differentiation between diagnoses of PMD from KCN is mandatory as the prognosis and treatment can vary between the two conditions.[38]

Keratoglobus is one of the ectatic conditions which can be mistaken as PMD. It is rare, bilateral ectasia and typically characterized by limbus-to-limbus thinning causing the cornea to assume a globular shape.[39]

Vogt's striae, Fleischer's ring, subepithelial scarring, lipid deposition, and corneal vascularization are rarely found in keratoglobus.[40],[41]

Other peripheral corneal thinning conditions misdiagnosed as PMD include Mooren's ulcer and Terrien's marginal degeneration (TMD).

Mooren's ulcer is an idiopathic disorder characterized by unilateral or bilateral painful, inflammatory thinning and ectasia of the peripheral cornea.[17] The inferior limbus is the usual site of involvement with spreading circumlimbally ulceration that extends centrally to involve the entire cornea followed by the vascularization which occurs during the healing process.[42]

TMD is an uncommon bilateral slowly progressive peripheral corneal ectasia mainly affecting the superior cornea, which may progress circumferentially.[43] It includes ectasia with a thinning band and furrowing of the peripheral cornea which is associated with lipid deposition and vascularization along the anterior edge, while vascularization spreads radially from the limbus and is located within the anterior stroma.[44]

PMD can be differentiated from Mooren's ulcer and TMD as the area of thinning in PMD is always epithelialized, without lipid deposition, absent vascularization and clear cornea.[45],[46]

[Table 1] summarizes the main clinical features of PMD differential diagnosis{Table 1}

 Treatment Varieties and Surgical Options in Pellucid Marginal Degeneration



Nonsurgical management of pellucid marginal degeneration

Spectacles are considered as an effective method for visual acuity correction in early stages of PMD with the use appropriate spherocylindrical spectacles with high refractive index lenses.[47],[48]

Soft toric contact lenses were found to be useful only before the occurrence of progression of irregular astigmatism.[49]

In a study of Mahadevan,[49] they used a novel technique for contact lens fitting of spherical rigid gas permeable contact lens (RGB) in a retrospective study by calculation of appropriate base curve of the lens by stability factor method in cases of PMD.

Multiple authors reported the effective use of hybrid contact lenses for management of corneas with PMD, including Saturn II lenses (Pilkington Barnes-Hind Inc., Sunnyvale, CA, US);[50] SoftPerm lenses (Pilkington Barnes-Hind Inc., Sunnyvale, CA, US).[51],[52] and SynergEyes lenses (SynergEyes Inc., Carlsbad CA, US).[53],[54]

Bi-toric (rigid gas permeable [RGP]) contact lenses have been found to be successful in managing patients with PMD, especially in moderate stages. Dominguez et al.,[55] Kastl and Kirby,[56] and Gruenauer-Kloevekorn et al.[57] have described the successful correction of high degrees of corneal astigmatism by the use of specialized back surface design lenses, which achieved good visual acuity.

Scleral contact lenses were used in the management of PMD; scleral lenses (PROSE, prosthetic replacement of the ocular surface ecosystem, Boston Foundation for Sightight, USA) were found to be effective in improving visual acuity in patients with PMD.[58]

Surgical management of pellucid marginal degeneration

Crosslinking treatment

As PMD is considered a rare condition, it has been evaluated much less than KCN, especially with regard to crosslinking (CXL) treatment. The main difference between PMD and KCN is the location of maximal corneal thinning and steepening. In PMD, corneal thinning is localized more peripheral than in KCN.[17] CXL treatment in PMD was needed to be more peripheral than central as in usual cases of KCN.

Case series study of 13 eyes with PMD following CXL procedures showed stability in keratometric results and good visual acuity in all eyes except one.[59] A few case reports describing CXL for PMD have been published. All of them showed improvement and stabilization of visual acuity and keratometric parameters.[60],[61],[62]

Combined CXL and laser vision correction was first reported in a study of Kymionis et al.,[63] who described a combined technique of treating PMD with a simultaneous CXL and keratorefractive procedure in the treatment of a patient with progressive PMD in both eyes. The results were favorable as regards the visual and topographic improvement.

Soon after, Kymionis et al.[64] reported a case report of a 28-year-old patient who had PRK followed by same day CXL after 1 year of intracorneal ring segment (ICRS) implantation.

Stojanovic et al.[65] have performed combined topography-guided transepithelial surface ablation with CXL in a single procedure for 6 eyes with KC and 6 eyes with PMD with favorable stable results and no topographic signs of progression during the follow-up period.

Later on, Kymionis et al.[66] presented the first study investigating the results of simultaneous combined treatment of conventional PRK with CXL in patients with PMD.

In 2019, Cagil N et al.[67] evaluated the long term visual, refractive, topographic, and aberrometric outcomes of combined transepithelial phototherapeutic keratectomy (PTK) with CXL in the treatment of PMD in a retrospective study; they found stability in visual acuity, maximum keratometry value, higher order aberration, trefoil, coma, and spherical aberration values with significant reduction in spherical equivalent (SE) and average keratometry readings throughout the 36 months of follow-up. [Table 2] summarizes the combined collagen CXL procedures in PMD.{Table 2}

Intracorneal ring segment implantation

Regarding the ICRS implantation, the first case of PMD implanted successfully with ICRS was done by Rodriguez-Prats J et al.[68]

Many authors[69],[70],[71] have reported astigmatic reductions and best-spectacle corrected visual acuity improvement after ICRS implantation in corneas with PMD.

The improvement in visual acuity caused the use of ICRS in this kind of corneas could be explained by the reduction of corneal aberrations and regularization of the corneal asymmetry.[12]

The effectiveness of complete intrastromal ring implantation (Myoring, DIOPTEX GmBH, Linz, Austria) implanted by femtosecond laser in PMD was studied by Jabbarvand et al.,[72] in a prospective case series study including thirty-three eyes with PMD; they found significant reduction cylinder with no significant change in the corneal biomechanical profile.

In a study of Kalinnikov et al.,[73] they treated a patient of PMD suffered from visual deterioration by combined intrastromal lamellar keratoplasty and ICRS (359°) implantation. This treatment aimed to improvements in visual acuity (uncorrected visual acuity). Mean keratometry, sphere and correct ametropia and corneal thinning. This technique resulted in partial cylinder reduction and reinforcement of the thinned cornea with improvement of both uncorrected distance visual acuity, corrected distance visual acuity, and a decrease of 4.50 diopters in corneal astigmatism.

Penetrating and lamellar keratoplasty

Patients with PMD are not good candidates for penetrating keratoplasty (PK) as the corneal thinning in these cases is peripheral and close to the limbus. The grafts used in these cases are eccentric grafts which positioned very close to the limbus thus increasing the liability of graft rejection, suture-induced complications, and corneal neovascularization.[3],[6]

Other surgical keratoplasty techniques have been described, including lamellar cresentric resection,[74] cresentric PK,[75] central PK,[76] corneal wedge excision,[77] and simultaneous central PK with peripheral cresentric lamellar keratoplasty.[78]

Intrastromal lamellar keratoplasty was done by addition of a donor lamellar cornea to correct corneal thinning and astigmatism. The donor tissue was cut in a cresentric shape in the size of the marked corneal pocket then inserted into the stromal pocket.[79]

Modified intrastromal lamellar sclerokeratoplasty was proposed by Guindolet et al.,[80] to correct peripheral thinning with preserving the patient's endothelial layer through a scleral tunnel after mapping using perioperative optical coherence tomography. Perioperative OCT was found to be an effective method for better detection of the diseased cornea and accurate lamellar dissection.

Deep anterior lamellar keratoplasty (DALK) has used successfully by Millar and Maloof[81] in PMD patients who suffered from a sudden vision loss due to spontaneous corneal perforation. The authors found that postoperative visual acuity remained stable even 1 year after surgery.

In a study of Al-Torbak,[82] he presented the surgical outcomes of DALK for patients of PMD in a retrospective review including 16 patients; he found that DALK surgery reduces severe corneal astigmatism with good visual and refractive outcomes. Graft-host vascularization was the main graft-related complication (2/16 eyes).

 Other Treatment Modalities of Pellucid Marginal Degeneration



de Vries et al.[83] have investigated the use of toric phakic intraocular lens implants (Verisyse/Artisan phakic) in a patient with high myopia and early-stage PMD. The preoperative refraction of the right eye was −13.0 −3.0 × 90° and in the left eye was −13.0 −1.25 × 55°. The postoperative SE was + 0.50 D after 1 year and +0.50 D after 7 years in right eye and −0.38 D after 1 year and −0.13 D after 7 years in left eye. Preoperative topographic astigmatism for the right and left eye was 2.94 and 0.81 D, respectively, and changed to 4.45 and 0.71 D after 7 years, respectively.

In a study of Balestrazzi et al.,[84] they evaluated the clinical outcomes, of the implantation of a toric intraocular lens in eyes with stable PMD, the study included eleven eyes of eight patients diagnosed as stable PMD with cataract and subjected to mini-incision 2.2 mm cataract surgery followed by the implantation of hydrophobic toric aspheric IOL (AcrySof IQ Toric IOL, Alcon, Fort Worth, TX, USA). The results were favorable with significant reduction in refractive astigmatism.

 Conclusion



PMD is a rare noninflammatory corneal ectasia. Corneal topography is mandatory for differentiating PMD from other ectatic corneal disorders. Spectacles, soft and RGP contact lens are the main visual correcting method in early cases. Many surgical techniques are available for PMD vision correction with unsatisfactory results, so further studies will be needed in the future.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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