Sudanese Journal of Ophthalmology

: 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


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 2023 Feb 8 ];12:35-42
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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.


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


Conflicts of interest

There are no conflicts of interest.


1Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297-319.
2Schlaeppi V. La dystrophie marginale inférieure pellucide de la cornée [The marginal inferior pelvic cortex dystrophy]. Bibl Ophthalmol 1957;12:672-7. French.
3Sridhar MS, Mahesh S, Bansal AK, Nutheti R, Rao GN. Pellucid marginal corneal degeneration. Ophthalmology 2004;111:1102-7.
4Karabatsas CH, Cook SD. Topographic analysis in pellucid marginal corneal degeneration and keratoglobus. Eye (Lond) 1996;10 (Pt 4):451-5.
5Tummanapalli SS, Maseedupally V, Mandathara P, Rathi VM, Sangwan VS. Evaluation of corneal elevation and thickness indices in pellucid marginal degeneration and keratoconus. J Cataract Refract Surg 2013;39:56-65.
6Krachmer JH. Pellucid marginal corneal degeneration. Arch Ophthalmol 1978;96:1217-21.
7Fuchihata M, Maeda N, Toda R, Koh S, Fujikado T, Nishida K. Characteristics of corneal topographic and pachymetric patterns in patients with pellucid marginal corneal degeneration. Jpn J Ophthalmol 2014;58:131-8.
8Lee BW, Jurkunas UV, Harissi-Dagher M, Poothullil AM, Tobaigy FM, Azar DT. Ectatic disorders associated with a claw-shaped pattern on corneal topography. Am J Ophthalmol 2007;144:154-6.
9Feder RS, Kshettry P. Noninflammatory ectatic disorders. In Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea: fundamentals, diagnosis and management. 3rd. New York: Elsevier Mosby; 2005. P. 955-67.
10Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol 1984;28:293-322.
11Imbornoni LM, McGhee CNJ, Belin MW. Evolution of Keratoconus: From Diagnosis to Therapeutics. Klin Monbl Augenheilkd 2018;235:680-8.
12Piñero DP, Alio JL, Morbelli H, Uceda-Montanes A, El Kady B, Coskunseven E, et al. Refractive and corneal aberrometric changes after intracorneal ring implantation in corneas with pellucid marginal degeneration. Ophthalmology 2009;116:1656-64.
13Belin MW, Asota IM, Ambrosio R Jr., Khachikian SS. What's in a name: Keratoconus, pellucid marginal degeneration, and related thinning disorders. Am J Ophthalmol 2011;152:157-620.
14Walker RN, Khachikian SS, Belin MW. Scheimpflug photographic diagnosis of pellucid marginal degeneration. Cornea 2008;27:963-6.
15Martínez-Abad A, Piñero DP. Pellucid marginal degeneration: Detection, discrimination from other corneal ectatic disorders and progression. Cont Lens Anterior Eye 2019;42:341-9.
16Jinabhai A, Radhakrishnan H, O'Donnell C. Pellucid corneal marginal degeneration: A review. Cont Lens Anterior Eye 2011;34:56-63.
17Lohchab M, Prakash G, Arora T, Maharana P, Jhanji V, Sharma N, et al. Surgical management of peripheral corneal thinning disorders. Surv Ophthalmol 2019;64:67-78.
18Pircher N, Lammer J, Holzer S, Gschließer A, Schmidinger G. Corneal crosslinking for pellucid marginal degeneration. J Cataract Refract Surg 2019;45:1163-7.
19Sinjab MM, Youssef LN. Pellucid-like keratoconus. F1000Research 2012;1:48.
20Oie Y, Maeda N, Kosaki R, Suzaki A, Hirohara Y, Mihashi T, et al. Characteristics of ocular higher-order aberrations in patients with pellucid marginal corneal degeneration. J Cataract Refract Surg 2008;34:1928-34.
21Kamiya K, Hirohara Y, Mihashi T, Hiraoka T, Kaji Y, Oshika T. Progression of pellucid marginal degeneration and higher-order wavefront aberration of the cornea. Jpn J Ophthalmol 2003;47:523-5.
22Radhakrishnan H, O'Donnell C. Aberrometry in clinical practice: Case series. Cont Lens Anterior Eye 2008;31:207-11.
23Labiris G, Giarmoukakis A, Sideroudi H, Song X, Kozobolis V, Seitz B, et al. Diagnostic capacity of biomechanical indices from a dynamic bidirectional applanation device in pellucid marginal degeneration. J Cataract Refract Surg 2014;40:1006-12.
24Lenk J, Haustein M, Terai N, Spoerl E, Raiskup F. Characterization of ocular biomechanics in pellucid marginal degeneration. Cornea 2016;35:506-9.
25Sedaghat MR, Ostadi-Moghadam H, Jabbarvand M, Askarizadeh F, Momeni-Moghaddam H, Narooie-Noori F. Corneal hysteresis and corneal resistance factor in pellucid marginal degeneration. J Curr Ophthalmol 2017;30:42-7.
26Lopes B, Ramos I, Ambrósio R Jr. Corneal densitometry in keratoconus. Cornea 2014;33:1282-6.
27Alnawaiseh M, Zumhagen L, Wirths G, Eveslage M, Eter N, Rosentreter A. Corneal densitometry, central corneal thickness, and corneal central-to-peripheral thickness ratio in patients with fuchs endothelial dystrophy. Cornea 2016;35:358-62.
28Mathew JH, Goosey JD, Söderberg PG, Bergmanson JP. Lamellar changes in the keratoconic cornea. Acta Ophthalmol 2015;93:767-73.
29Smolek MK, Beekhuis WH. Collagen fibril orientation in the human corneal stroma and its implications in keratoconus. Invest Ophthalmol Vis Sci 1997;38:1289-90.
30Koc M, Tekin K, Inanc M, Kosekahya P, Yilmazbas P. Crab claw pattern on corneal topography: Pellucid marginal degeneration or inferior keratoconus? Eye (Lond) 2018;32:11-8.
31Panos GD, Hafezi F, Gatzioufas Z. Pellucid marginal degeneration and keratoconus; differential diagnosis by corneal topography. J Cataract Refract Surg 2013;39:968.
32Shetty R, Kaweri L, Pahuja N, Nagaraja H, Wadia K, Jayadev C, et al. Current review and a simplified “five-point management algorithm” for keratoconus. Indian J Ophthalmol 2015;63:46-53.
33Iwamoto T, DeVoe AG. Electron microscopical study of the Fleisher ring. Arch Ophthalmol 1976;94:1579-84.
34Hollingsworth JG, Bonshek RE, Efron N. Correlation of the appearance of the keratoconic cornea in vivo by confocal microscopy and in vitro by light microscopy. Cornea 2005;24:397-405.
35Erie JC, Patel SV, McLaren JW, Nau CB, Hodge DO, Bourne WM. Keratocyte density in keratoconus. A confocal microscopy study(a). Am J Ophthalmol 2002;134:689-95.
36Zadnik K, Barr JT, Gordon MO, Edrington TB. Biomicroscopic signs and disease severity in keratoconus. Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group. Cornea 1996;15:139-46.
37Zadnik K, Barr JT, Edrington TB, Nichols JJ, Wilson BS, Siegmund K, et al. Corneal scarring and vision in keratoconus: A baseline report from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Cornea 2000;19:804-12.
38Belin MW, Boyd BM, Ambrosio R Jr. Pellucid marginal degeneration vs. inferior keratoconus: Why it matters. J Cataract Refract Surg 2020;46:325-6.
39Gupta N, Ganger A. Keratoglobus: A close entity to megalophthalmos. Springerplus 2016;5:634.
40Velazquez A, Kim T. Developmental corneal anomalies of size and shape. In: Krachmer JH, Mannis MJ, Holland EJ, editors. Cornea – Fundamentals, Diagnosis and Management. 2nd ed. London: Elsevier Inc.; 2005. p. 727-38.
41Rathi VM, Murthy SI, Bagga B, Taneja M, Chaurasia S, Sangwan VS. Keratoglobus: An experience at a tertiary eye care center in India. Indian J Ophthalmol 2015;63:233-8.
42Vilaplana F, Temprano J, Riquelme JL, Nadal J, Barraquer J. Mooren's ulcer: 30 years of follow-up. Arch Soc Esp Oftalmol 2016;91:337-40.
43Ding Y, Murri MS, Birdsong OC, Ronquillo Y, Moshirfar M. Terrien marginal degeneration. Surv Ophthalmol 2019;64:162-74.
44Ceresara G, Migliavacca L, Orzalesi N, Rossetti L. In vivo confocal microscopy in Terrien marginal corneal degeneration: A case report. Cornea 2011;30:820-4.
45Chan AT, Ulate R, Goldich Y, Rootman DS, Chan CC. Terrien Marginal Degeneration: Clinical Characteristics and Outcomes. Am J Ophthalmol 2015;160:867-720.
46Yang L, Xiao J, Wang J, Zhang H. Clinical characteristics and risk factors of recurrent Mooren's Ulcer. J Ophthalmol 2017;2017:8978527.
47Tzelikis PF, Cohen EJ, Rapuano CJ, Hammersmith KM, Laibson PR. Management of pellucid marginal corneal degeneration. Cornea 2005;24:555-60.
48Biswas S, Brahma A, Tromans C, Ridgway A. Management of pellucid marginal corneal degeneration. Eye (Lond) 2000;14 (Pt 4):629-34.
49Mahadevan R, Amudhaoli A, Valarmathi A. Retrospective study of contact lens fitting in pellucid marginal degeneration. Eye Contact Lens 2008;34:207-10.
50Astin CL. The long-term use of the SoftPerm lens on pellucid marginal corneal degeneration. CLAO J 1994;20:258-60.
51Oriowo O, Briggs S. Contact lens management of pellucid marginal corneal degeneration in a Saudi Arabia teaching hospital. Int Cont Lens Clin 1994;21:147-50.
52Scheid T, Wetcher SP. SoftPerm contact lens fitting for a case of pellucid marginal degeneration. Int Cont Lens Clin 1990;17:296-9.
53Abdalla YF, Elsahn AF, Hammersmith KM, Cohen EJ. SynergEyes lenses for keratoconus. Cornea 2010;29:5-8.
54Nau AC. A comparison of synergeyes versus traditional rigid gas permeable lens designs for patients with irregular corneas. Eye Contact Lens 2008;34:198-200.
55Dominguez CE, Shah A, Weissman BA. Bitoric gas-permeable contact lens application in pellucid marginal corneal degeneration. Eye Contact Lens 2005;31:241-3.
56Kastl PR, Kirby RG. Bitoric rigid gas permeable lens fitting in highly astigmatic patients. CLAO J 1987;13:215-6.
57Gruenauer-Kloevekorn C, Fischer U, Kloevekorn-Norgall K, Duncker GI. Pellucid marginal corneal degeneration: Evaluation of the corneal surface and contact lens fitting. Br J Ophthalmol 2006;90:318-23.
58Rathi VM, Dumpati S, Mandathara PS, Taneja MM, Sangwan VS. Scleral contact lenses in the management of pellucid marginal degeneration. Cont Lens Anterior Eye 2016;39:217-20.
59Steppat MH, Raiskup-Wolf F, Sporl E, Hoyer A, Pillunat LE: Collagen Cross Linking in Patients with Pellucid Marginal Corneal Degeneration (PMD) (poster presentation). ARVO; 2008.
60Hassan Z, Nemeth G, Modis L, Szalai E, Berta A. Collagen cross-linking in the treatment of pellucid marginal degeneration. Indian J Ophthalmol 2014;62:367-70.
61Spadea L. Corneal collagen cross-linking with riboflavin and UVA irradiation in pellucid marginal degeneration. J Refract Surg 2010;26:375-7.
62Bayraktar S, Cebeci Z, Oray M, Alparslan N. Corneal collagen cross-linking in pellucid marginal degeneration: 2 patients, 4 eyes. Case Rep Ophthalmol Med 2015;2015:840687.
63Kymionis GD, Karavitaki AE, Kounis GA, Portaliou DM, Yoo SH, Pallikaris IG. Management of pellucid marginal corneal degeneration with simultaneous customized photorefractive keratectomy and collagen crosslinking. J Cataract Refract Surg 2009;35:1298-301.
64Kymionis GD, Grentzelos MA, Portaliou DM, Karavitaki AE, Krasia MS, Dranidis GK, et al. Photorefractive keratectomy followed by same-day corneal collagen crosslinking after intrastromal corneal ring segment implantation for pellucid marginal degeneration. J Cataract Refract Surg 2010;36:1783-5.
65Stojanovic A, Zhang J, Chen X, Nitter TA, Chen S, Wang Q. Topography-guided transepithelial surface ablation followed by corneal collagen cross-linking performed in a single combined procedure for the treatment of keratoconus and pellucid marginal degeneration. J Refract Surg 2010;26:145-52.
66Kymionis GD, Grentzelos MA, Plaka AD, Tsoulnaras KI, Kankariya VP, Shehadeh MM, et al. Simultaneous conventional photorefractive keratectomy and corneal collagen cross-linking for pellucid marginal corneal degeneration. J Refract Surg 2014;30:272-6.
67Cagil N, Sarac O, Yesilirmak N, Caglayan M, Uysal BS, Tanriverdi B. Transepithelial phototherapeutic keratectomy followed by corneal collagen crosslinking for the treatment of pellucid marginal degeneration: Long-term Results. Cornea 2019;38:980-5.
68Rodriguez-Prats J, Galal A, Garcia-Lledo M, de La Hoz F, Alió JL. Intracorneal rings for the correction of pellucid marginal degeneration. J Cataract Refract Surg 2003;29:1421-4.
69Akaishi L, Tzelikis PF, Raber IM. Ferrara intracorneal ring implantation and cataract surgery for the correction of pellucid marginal corneal degeneration. J Cataract Refract Surg 2004;30:2427-30.
70Barbara A, Shehadeh-Masha'our R, Zvi F, Garzozi HJ. Management of pellucid marginal degeneration with intracorneal ring segments. J Refract Surg 2005;21:296-8.
71Ertan A, Bahadir M. Intrastromal ring segment insertion using a femtosecond laser to correct pellucid marginal corneal degeneration. J Cataract Refract Surg 2006;32:1710-6.
72Jabbarvand M, Hashemian H, Khodaparast M, Bazvand F, Beheshtnejad A. Outcome of complete intrastromal ring implantation using femtosecond laser in pellucid marginal degeneration. Eye (Lond) 2015;29:783-90.
73Kalinnikov Y, Leontyeva G, Zinovyeva A, Nevrov D. Simultaneous implantation of refractive lenticule and intracorneal ring segment in the management of pellucid marginal degeneration. J Refract Surg 2019;35:606-9.
74Cameron JA. Results of lamellar crescentic resection for pellucid marginal corneal degeneration. Am J Ophthalmol 1992;113:296-302.
75Schanzlin DJ, Sarno EM, Robin JB. Crescentic lamellar keratoplasty for pellucid marginal degeneration. Am J Ophthalmol 1983;96:253-4.
76Speaker MG, Arentsen JJ, Laibson PR. Long-term survival of large diameter penetrating keratoplasties for keratoconus and pellucid marginal degeneration. Acta Ophthalmol Suppl 1989;192:17-9.
77MacLean H, Robinson LP, Wechsler AW. Long-term results of corneal wedge excision for pellucid marginal degeneration. Eye (Lond) 1997;11 (Pt 5):613-7.
78Rasheed K, Rabinowitz YS. Surgical treatment of advanced pellucid marginal degeneration. Ophthalmology 2000;107:1836-40.
79Jabbarvand M, Hashemian H, Khodaparast M, Hassanpour N, Mohebbi M. Intrastromal lamellar keratoplasty in patients with pellucid marginal degeneration. J Cataract Refract Surg 2015;41:2-8.
80Guindolet D, Petrovic A, Doan S, Cochereau I, Gabison EE. Sclerocorneal Intrastromal Lamellar Keratoplasty for Pellucid Marginal Degeneration. Cornea 2016;35:900-3.
81Millar MJ, Maloof A. Deep lamellar keratoplasty for pellucid marginal degeneration: Review of management options for corneal perforation. Cornea 2008;27:953-6.
82Al-Torbak AA. Deep anterior lamellar keratoplasty for pellucid marginal degeneration. Saudi J Ophthalmol 2013;27:11-4.
83de Vries NE, Tahzib NG, Webers CA, Hendrikse F, Nuijts RM. Use of Verisyse/Artisan phakic intraocular lens for the reduction of myopia in a patient with pellucid marginal degeneration. Cornea 2008;27:241-5.
84Balestrazzi A, Baiocchi S, Balestrazzi A, Cartocci G, Tosi GM, Martone G, et al. Mini-incision cataract surgery and toric lens implantation for the reduction of high myopic astigmatism in patients with pellucid marginal degeneration. Eye (Lond) 2015;29:637-42.