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ellucid marginal degeneration (PMD) is a rare, bilat-eral, asymmetric, ectatic disorder of the cornea most commonly associated with an arcuate band of thin-

ning in the inferior peripheral cornea approximately 1 mm from the limbus.1-4 The thinning usually occurs in the 4- to 8-o’clock position and can be severe, with as much as 80% stromal loss, resulting in corneal protrusion.4 The point of maximal protrusion in PMD occurs in the region of cornea just superior to the thinned region.1-4 This is in contrast to the more common ectatic disorder, keratoconus, in which api-cal protrusion occurs within regions of thinned central and paracentral cornea.5

Differentiating PMD from keratoconus is important for prognosis and management (Table 1). Although both kera-toconus and PMD cause reduced visual acuity, Descemet’s folds, and acute hydrops, PMD is not associated with several classic keratoconus findings, including Munson’s sign (a V-shaped deformation of the lower eyelid when gaze is di-rected downward), Rizutti’s phenomenon (sharply focused beam of light near the nasal limbus on lateral illumination), apical corneal scarring, Fleisher’s rings (hemosiderin de-position in basal epithelium), or reduced central corneal thickness.1-7 Moderate PMD can usually be differentiated from keratoconus via slit-lamp examination given the clas-sic location of crescentic thinning.8 However, early in the disease process, the signs are subtle and cases of advanced PMD can involve the entire inferior cornea and be difficult to differentiate from keratoconus. Patients with PMD may present with acute hydrops or corneal perforation as a result of progressive thinning, although this presentation is more common in keratoconus.1,2,9,10

PABSTRACT

PURPOSE: To review and evaluate current and future directions in the diagnosis and surgical management of pellucid marginal degeneration (PMD), including pen-etrating keratoplasty, full-thickness crescentic wedge resection (FTCWR), deep anterior lamellar keratoplasty (DALK), crescentic lamellar wedge resection (CLWR), crescentic lamellar keratoplasty, tuck-in lamellar kerato-plasty (TILK), toric phakic intraocular lens (PIOL) implan-tation, intrastromal corneal ring segment implantation (ICRS), corneal collagen cross-linking (CXL), and com-bined therapies. This is the first review article looking at the literature specific to PMD.

METHODS: Review of published studies.

RESULTS: Reported data for each treatment is presented. Penetrating keratoplasty is the treatment of last resort in PMD and is effective, but with considerable complica-tions. DALK provides visual outcomes similar to penetrat-ing keratoplasty without the risk of immune-mediated graft rejection, but its complexity and relative novelty limit its acceptance. FTCWR has good visual outcomes, but with significant astigmatic drift. CLWR is effective, but lacks long-term results. Crescentic lamellar keratoplasty and TILK are effective, but technically difficult and without long-term results. Toric PIOL implantation is effective, but ecta-sia progression is a concern. ICRS implantation can delay penetrating keratoplasty and improve contact lens toler-ance, but does not treat the underlying process. CXL dem-onstrates effectiveness without complications, although data are limited and long-term results are needed. Com-bining treatments such as ICRS, CXL, toric PIOL implan-tation, and refractive surgery is promising, but additional studies are needed to investigate their efficacy and safety.

CONCLUSIONS: Although little is understood about the etiology, pathophysiology, epidemiology, and genet-ics of PMD, new treatments are improving visual out-comes and reducing complications. Corneal collagen cross-linking is especially exciting because it halts dis-ease progression. Combined treatments and improved screening could eliminate the need for surgical manage-ment in most cases of PMD.

[J Refract Surg. 2014;30(7):474-485.]

From John A. Moran Eye Center, University of Utah, Salt Lake City, Utah (MM, JNE); and University of Utah School of Medicine, Salt Lake City, Utah (NLB, SMC).

Submitted: December 30, 2013; Accepted: March 11, 2014; Posted online: May 6, 2014

The authors have no financial or proprietary interest in the materials pre-sented herein.

Correspondence: Majid Moshirfar, MD, 65 Mario Capecchi Drive, Salt Lake City, UT 84132. E-mail: [email protected]

doi:10.3928/1081597X-20140429-02

Current Options in the Management of Pellucid Marginal DegenerationMajid Moshirfar, MD; Jason N. Edmonds, MD; Nicholas L. Behunin, MD; Steven M. Christiansen, MD

475Journal of Refractive Surgery • Vol. 30, No. 7, 2014

Pellucid Marginal Degeneration/Moshirfar et al

Although PMD and other ectatic disorders such as keratoconus and keratoglobus have some distinguishing phenotypic characteristics, it is unclear whether each is an individual disease entity or simply a variation in presentation of the same disease.2-4,7 Several authors propose that PMD is an inferior manifestation of kera-toconus, whereas others report combinations of PMD, keratoconus, and keratoglobus coexisting in the same cornea.1,11-15 Whatever the case, the word “pellucid” means “clear” and refers to the clarity of the diseased cornea (or the absence of corneal scarring) despite pro-trusion and ectasia.16

There is little certainty about the etiology and patho-physiology of PMD, although studies have confirmed that PMD demonstrates progression over time.1-4 Histo-pathological studies demonstrate stromal thinning, an irregular Bowman’s membrane, Bowman’s membrane breaks, increased stromal polysaccharides, epithelial edema, Descemet’s folds, and an absence of inflamma-tory cells.17 Electron microscopy further reveals irregu-larly arranged stroma and the presence of extracellu-lar, granular, electron-dense deposits.15,18

Although no formal categorization of PMD severity exists, published studies consistently use the subjec-tive terms early, moderate, and advanced to describe progressive stages of the disease.3,7,19

There have been no formal epidemiological studies of PMD, although the general consensus in the literature is that PMD is a rare disorder that is more prevalent than keratoglobus but less prevalent than keratoconus.2-4,7 The disease occurs more often in males, but afflicts all eth-nicities without geographic discrimination.1,9,10 There is no known genetic inheritance pattern in PMD, although

isolated studies describe corneal ectasia and moderate to high astigmatism in the asymptomatic family members of patients with PMD.13,14,20-22 Patients with PMD usually present between the second and fifth decades of life, whereas keratoconus commonly presents during puberty.

Penetrating keratoplasty remains the treatment of last resort in advanced PMD. However, recent ad-vances in the surgical management of PMD make it possible to postpone and possibly avoid penetrating keratoplasty altogether. The purpose of this article is to review and evaluate current and future directions for the diagnosis and management of PMD. This is the first review article looking at the literature specific to PMD.

DIAGNOSISClassically, diagnosis of PMD was made based on

slit-lamp findings of inferior corneal thinning in an arcuate or crescentic pattern and a “beer-belly” side-profile contour (Figure 1). Today, technological ad-vances allow detailed analysis of corneal topography and can assist in the diagnosis of PMD.

Topographical findings indicative of PMD include a flattened vertical meridian with abnormal inferior steepening in the corneal periphery (as much as 20 di-opters [D] in advanced cases) that extends to the inferi-or oblique meridians in a “crab-claw,” “kissing doves,” or “butterfly” appearance (Figure 2).2,4,23 However, a “crab claw” pattern on corneal topography alone is in-sufficient for diagnosis of PMD because keratoconus can create similar findings.6 More detailed analysis with slit lamp, topography, and pachymetry is sug-gested to assess corneal thickness and improve the accuracy of PMD diagnosis.24

TABLE 1Differentiation of Pellucid Marginal Degeneration, Keratoconus, and Keratoglobus

Clinical Parameter Pellucid Marginal Degeneration Keratoconus Keratoglobus

Etiology Unknown Unknown Unknown

Onset 2nd to 5th decade Puberty At birth

Location of thinning 1 to 2 mm from limbus Central, paracentral Generalized

Location of protrusion Superior to thinning Apical, same as thinning Generalized, globular

Scarring Absent Present Absent

Clinical presentation Reduced VA from astigmatism, rarely acute hydrops

Unilateral reduced VA from myopia and astigmatism, acute hydrops

Rupture with mild trauma, very rarely acute hydrops

Signs “Beer belly” side profile, slowly pro-gressive arcuate band of thinning

Conical ectasia, VS, MS, RP, FR, oil drop red reflex, “scissoring” reflex

Globular ectasia

Topography Steepening of inferior cornea, “crab claw,” “butterfly,” or “kissing dove”a

Irregular astigmatism, asymmetric bow-tie, skewed radial axis > 30°

Generalized steepening

VA = visual acuity; VS = Vogt striae; MS = Munson’s sign; RP = Rizzuti’s phenomenon; FR = Fleisher’s rings aTopographic findings alone are not sufficient for diagnosis of pellucid marginal degeneration and should be correlated with slit-lamp and pachymetric findings to improve specificity. Data from Krachmer et al.2, Romero-Jimenez et al.5, and Jinabhai et al.7



Placido disk-based topography is limited in its ability to detect PMD because it can only evaluate the anterior cornea. The Orbscan II (Bausch and Lomb, Rochester, NY) uses stereo-triangulation slit-scanning and placido rings to measure anterior corneal topog-raphy and thickness. The resulting mean keratometric

axial power map is useful in detecting PMD, but lacks specificity.6,24 Additional placido disk-based topographers that aid in PMD diagnosis include the Tomey Topographic Modeling System (Tomey Corp., Nagoya, Japan) and GALILEI Dual Scheimpflug Ana-lyzer (Zeimer, Switzerland), which combines placi-do disk-based topography with a three-dimensional Scheimpflug camera.

The Pentacam (Oculus Optikgeräte, Wetzlar, Ger-many) uses Scheimpflug photography to measure pachymetry and topography. Pentacam’s ability to im-age the cornea in cross-section allows evaluation of both the anterior and posterior curvature, as well as corneal thickness. In PMD, the corneal thickness map produced by Pentacam shows horizontal thinning of the inferior cornea near the limbus (Figure 3). Belin et al. recommends that detailed anterior and posterior elevation maps, anterior curvature topography with in-dices, and full pachymetry maps with 12 mm of corne-al coverage and thickness data be required for accurate diagnosis of PMD, especially when determining inclu-sion criteria for studies.24 Careful screening for PMD should also be performed prior to refractive surgery because cases of iatrogenic ectasia have been reported in patients with PMD following LASIK.25-28

Figure 1. Slit-lamp photograph of pellucid marginal degeneration (PMD). Unlike keratoconus, where the area of maximal protrusion is within the area of thinning, maximal protrusion in PMD is immediately superior to the area of thinning, resulting in a “beer-belly” contour as seen in this side profile of a cornea with PMD.

Figure 2. Topographical representation of pellucid marginal degeneration. Note the superior flattening with inferior steepening most pronounced between the 4- and 8-o’clock positions, and the classic “kissing dove” or “crab claw” pattern.



The newer Visante Omni system (Carl Zeiss Meditec, Dublin, CA) uses both placido ring and optical coher-ence tomography (OCT) to provide topography, thick-

ness data, and detailed images of the anterior segment. Srivannaboon et al. recently compared the Visante Omni to the Orbscan II and reported high agreement

Figure 4. Management of pellucid marginal degeneration. PK = penetrating keratoplasty; FTCWR = full-thickness crescentic wedge resection; DALK = deep anterior lamellar keratoplasty; CLWR = crescentic lamellar wedge resection; TILK = tuck-in lamellar keratoplasty; CLK = crescentic lamellar keratoplasty; EK = epikeratoplasty; LTK = lamellar thermokeratoplasty; ICRS = intrastromal corneal ring segments implantation; CXL = corneal collagen cross-linking; tpIOL = toric phakic intraocular lens implantation

Figure 3. Pentacam tomography offers more detailed measurements in pellucid marginal degeneration. Note the inferior band of steepening and thin-ning with a more normal appearance above the thinned band.



between systems with good repeatability and repro-ducibility.29 These results are promising, but addition-al studies are needed to establish the sensitivity and specificity of this technology in detecting specific pa-thologies such as PMD. The RTVue Premier (Optovue, Inc., Fremont, CA) provides a Fourier-domain OCT to examine corneal pachymetry at higher scanning speeds and improved repeatability of pachymetry maps.

MANAGEMENTThe management of PMD can be organized into

cornea-independent, structural, full-thickness, and partial-thickness interventions (Figure 4). Cornea-independent interventions involve the use of lenses, which correct visual deficits without affecting the cornea and include non-surgical interventions (spec-tacles and contact lenses) and surgical interventions (toric intraocular lens [IOL] implantation). Structural interventions involve modification of the curvature or mechanical properties of the cornea using mechani-cal devices designed to change the shape of the cornea (intrastromal corneal ring segments [ICRS]) and proce-

dures designed to strengthen the biological structure of the cornea (corneal collagen cross-linking [CXL]). Full-thickness interventions involve removal and re-placement of all layers of the cornea with total cornea replacement (as in penetrating keratoplasty) and cres-centic cornea replacement, where only the area of ab-normally thinned cornea is removed. Partial-thickness interventions involve removal of the anterior layers of corneal stroma while preserving the host corneal endothelium and can be subdivided into total cornea replacement (as in deep anterior lamellar keratoplasty [DALK]) and crescentic cornea replacement (removal of the anterior layers of only the abnormally thinned cornea). The most common surgical treatments of PMD today are penetrating keratoplasty and DALK.

Cornea-Independent InterventIonsNon-surgical Interventions. Many options exist for

the treatment of early and moderate PMD. Most patients with PMD are treated non-surgically with spectacles or contact lenses.1,9,10,30,31 Soft and hybrid contact lenses can be used in early PMD, but lose their effectiveness

TABLE 2Study Outcomes by Surgical Technique in the

Management of Pellucid Marginal Degeneration

Study (Year)Total Eyes (Total Pts) Procedure Complication (%)

Astigmatism (Δ Astigmatism) (D) Final CDVA (%)

MacLean et al. (1997) 10 (9) FTCWR Inferior pannus (30), choroidal detachment (10), corneal hydrops (10), wound leak (10), astigmatic

drift (100)

1.4 (-12.4) < 20/30 (100)

Biswas et al. (2000) 6 (6) FTCWR NR 8.9 (-2.14) < 20/40 (67)

Busin et al. (2008) 10 (10) FTCWR, CTRI Minimal astigmatic drift 4.6 (-10.5) < 20/40 (80)

Cameron (1992) 5 (4) CLWR Inferior vascularization and pannus (100), recurrence of thinning (20)

2.62 (NR)a < 20/40 (80)

Javadi et al. (2004) 15 (9) CLWR NR 4.30 (-14.7) < 20/40 (71)

Maccheron et al. (2012) 7 (6) CLWR Microperforation (14), suture infiltrates (34),

6.84 (-9.1)c < 20/40 (57)b

Rasheed & Rabinowitz (2000)

5 (5) CLK, PK Worsening with-the-rule astigmatism (80%)

4.24 (+1.02) < 20/40 (60)

Sridhar et al. (2004) 2 (2) CLK NR 10.3 (-5.35) 20/200, 20/125 (NR)

Millar et al. (2008) 2 (1) DALK None reported NR < 20/30 (100)

Al-Torbak (2013) 16 (16) DALK Perforation (12) NR 20/50

Fronterre et al. (1991) 4 (2) EK Folds beneath cap graft (100) 6.75 (approximately -17) 20/25 (100)

Kaushal et al. (2008) 12 (12)c TILK None reported 3.23 (-2.7) < 20/80 (100)

Camoriano et al. (2012) 10 (5) tpIOL Severe glare and halos (10) -0.58 (6.13) SE < 20/40 (100)

CDVA = corrected distance visual acuity; FTCWR = full-thickness crescentic wedge resection; NR = none reported; CTRI = corneal tunnel relaxing incision; CLWR = crescentic lamellar wedge resection; CLK = crescentic lamellar keratoplasty; PK = penetrating keratoplasty; DALK = deep anterior lamellar keratoplasty; EK = epikeratoplasty; TILK = tuck-in lamellar keratoplasty; tpIOL = toric phakic intraocular lens; SE = spherical equivalent; D = diopters aExcluding one eye that experienced recurrence of thinning. bFollow-up duration and thus outcomes varied among study participants. cPopulation includes 8 patients with combined keratoconus/pellucid marginal degeneration and 4 patients with keratoglobus.



as the disease progresses.31-38 In moderate PMD, the development of irregular astigmatism and progressive corneal irregularity often necessitates the use of large diameter rigid gas permeable lenses.9,10,30,31,39 Special-ized contact lenses such as reverse geometry lenses and “bi-toric” lenses have proven useful in the man-agement of moderate PMD.40-42 Customized soft contact lenses have theoretical potential in treating moderate PMD given their success in keratoconus.43 In advanced PMD, the scleral contact lens is a non-surgical treat-ment of last resort.44,45 However, tolerance is an issue with scleral contact lens because the lenses are uncom-fortable, expensive, difficult to apply, complicated to manufacture, and qualified scleral contact lens fitting experts are scarce.30,46,47

Surgical Interventions. Toric IOLs can be implanted in either the anterior or posterior cham-ber, either with or without natural lens extraction (to-ric phakic IOLs [PIOLs]), and allow for correction of refractive error and astigmatism independent of cor-nea alteration. Camoriano et al. recently reported im-proved visual acuity and astigmatism with toric PIOL implantation for PMD in 10 eyes with severe glare and

halos requiring explantation in just one eye (Table 2).48 Studies in patients with keratoconus have demonstrat-ed the safety and efficacy of toric PIOL implantation in larger patient populations.49-51 The greatest concerns with using toric PIOL in patients with PMD are rota-tional stability and ectasia progression after lens im-plantation. Because patients with PMD are often young at the time of potential lens implantation, if ectasia progression continues following implantation, it may result in the need for spectacle correction despite lens placement. Thus, patient selection is important to en-sure acceptable visual outcomes. For patients with ad-vanced PMD who experience contact lens intolerance, toric PIOL can potentially be combined with specta-cles. However, acceptable vision may be unattainable due to high irregular astigmatism and higher-order ab-errations. Finally, the reversible nature of toric PIOL implantation, although less than ideal, is an advantage to consider with this modality.

struCtural InterventIonsMechanical Devices. ICRS are semicircular poly-

methylmethacrylate rings implanted in peripheral

TABLE 3ICRS Implantation in Pellucid Marginal Degeneration

Study (Year)Total Eyes (Total Pts) Intervention Complication (%)

Cylinder (Δ Cyl) (D) K (Δ K) (D) Final CDVA (%)

Mularoni et al. (2005)

8 (8) ICRS NR -1.72 (4.59) 42.46 (1.49) < 20/40 (100)

Ertan & Bahadir (2006)

9 (6) ICRS via FS NR -0.94 (1.47) 46.90 (1.3) 20/20 (55)

Pinero et al. (2009)

21 (15) ICRS Visual deteriora-tion with eventual explantation (19)

-3.21 (2.15) 43.19 (1.76) 20/30 mean, range: 20/70 to

-20/15 (NR)

Kubaloglu et al. (2010)

16 (10) Single ICRS White deposits around segments

(12.5)

-2.38 (2.01) 46.08 (3.62) < 20/40 (72.7)

ICRS = intracorneal ring segment; cyl = cylinder; K = keratometry; CDVA = corrected distance visual acuity; NR = none reported; FS = femtosecond laser; D = diopters

TABLE 4Case Reports of Collagen Cross-Linking and Combined Techniques

in Pellucid Marginal Degeneration

Study (Year)Total Eyes (Total Pts) Intervention Complication (%)

Cylinder (Δ Cylinder) (D)

Postop K (Preop K) (D) Final CDVA

Spadea (2010) 1 (1) CXL NR (1.40) 47.60/54.00; (47.40/55.30)

20/63

Kymionis et al. (2009)

2 (1) PRK, CXL NR Right: -2.50 (5.50), Left: -5.50 (3.00)

“Improved” Right: 20/25, Left: 20/32

Kymionis et al. (2010)

1 (1) ICRS, PRK, CXL NR -4.00 (3.00) 44.92/39.01; (46.73/38.67)

20/25

D = diopters; postop = postoperative; K = keratometry; preop = preoperative; CDVA = corrected distance visual acuity; CXL = collagen cross-linking; NR = none reported; PRK = photorefractive keratectomy; ICRS = intracorneal ring segment



corneal stroma outside the visual axis. These ring seg-ments create an arc-shortening effect in the stroma, which flattens the cornea. The procedure for ICRS im-plantation involves creation of a radial stromal tun-nel either manually or via femtosecond laser followed by insertion of the ICRS. Originally designed for re-fractive correction, ICRS have been used successfully in keratoconus and PMD for delaying penetrating keratoplasty and improving contact lens tolerance. Studies show that ICRS placement is a safe, revers-ible, and relatively conservative approach to surgical management in early and moderate PMD (Table 3).19 However, ICRS placement has several limitations, including regression over time, unpredictable visual outcomes, and the rare occurrence of corneal melt.52-56 In addition, placement of ICRS does not address the underlying cause of thinning and instability, so dis-ease progression continues.19,57-59

Kubaloglu et al.60 and Ertan and Bahadir61 have re-ported good outcomes with placement of single ICRS in corneas with PMD. Because the degree of corneal flattening depends on the thickness and diameter of ring segments, a single, wider segment may produce more flattening with less induced astigmatism.61 Sharma et al. demonstrated that single-segment ICRS placement provided significantly better outcomes than double-segment ICRS in peripheral keratoconus.62 Although the location of corneal thinning makes en-suring adequate peripheral thickness perhaps more critical in patients with PMD, the similarities between keratoconus and PMD give reason to believe that these results could be mirrored in patients with PMD. Future studies should compare single- and double-segment ICRS implantation in patients with PMD.

Interventions Changing Biological Structure. CXL is the creation of covalent bonds between collagen mol-ecules, which results in increased strength of the col-lagen scaffold and greater corneal stiffness.63 The CXL procedure is minimally invasive and involves the ap-plication of riboflavin drops to the eye, either with or without epithelial debridement to maximize penetra-tion.64 The treated eye is then exposed to ultraviolet-A light at energy levels ranging from 3 to 30 mW/cm2 for time intervals of 3 to 30 minutes.65

Only case reports demonstrate successful CXL in PMD (Table 4). The efficacy, safety, and complica-tions of CXL have been more widely reported in kera-toconus, with studies consistently showing improved astigmatism and stabilization of corneal degeneration since 1999.66-68 However, CXL produces only minimal improvement in topography and visual acuity.69,70 Safety is an important issue for CXL. Although CXL has been widely studied around the world with good

Figure 5. Selected surgical techniques in pellucid marginal degeneration. (A) Full-thickness crescentic wedge resection involves excision of the abnormally thinned cornea and re-approximation and closure of the residual normal-thickness corneal edges. (B) Crescentic lamellar wedge resection involves excision of the area of abnormally thinned corneal stroma, leaving the pos-terior lamella intact. The residual normal-thickness stromal edges are then re-approximated and closed. (C) Crescentic lamellar keratoplasty involves excising the area of abnormally thinned stroma. A trimmed donor tissue stromal graft (gray) is fitted to the excised area (arrows). The donor tissue stromal graft (gray) is then sutured to the residual stromal edges. (D) Tuck-in lamellar keratoplasty involves removal of the corneal stroma and creation of a 180° inferior stromal pocket. A peripheral, partial-thickness flange of pos-terior stromal tissue is created on the donor lenticule (gray) and is integrated into the 180° inferior stromal pocket of the host cornea (white) where it is sutured to the residual stromal edges.



safety and efficacy, approval in the United States is still pending because long-term results have not yet been established.63,70-72 Snibson has cautioned that mea-sures should be undertaken to ensure limbal stem cells be protected from ultraviolet-A exposure.66 Finally, the safety of CXL in PMD may be uniquely challenging due to the perilimbal location of ectatic thinning.

Full-thICkness InterventIonsTotal Cornea Replacement. Penetrating keratoplasty

is the treatment of last resort in PMD and is considered when contact lenses can no longer be tolerated or fail to provide adequate correction of visual acuity.9 This procedure involves replacement of the diseased cor-nea with donor corneal tissue. Although penetrating keratoplasty has a record of good results in PMD, these patients are generally poor candidates for penetrating keratoplasty because the location of corneal thinning requires large grafts (8.5 to 9.5 mm) to be positioned close to the limbus, increasing the risk of neovascu-larization, suture complications, secondary glaucoma due to angle structure damage, and graft rejection.1,3,73 In addition, penetrating keratoplasty results in irregu-lar astigmatism that can be difficult to correct given the discrepancy between donor and recipient corneal thicknesses.11 Long-term graft survival is also an im-portant issue, especially in young patients. Penetrating keratoplasty is also difficult to justify conceptually in PMD because the central cornea and endothelium is essentially unaffected by the disease. Because of these considerable concerns, additional surgical treatments have been developed with the goal of postponing or preventing penetrating keratoplasty.

Crescentic Cornea Replacement. Full-thickness crescentic wedge resection is a modification of penetrat-ing keratoplasty that involves removal of only the ab-normally thinned cornea followed by re-approximation and suturing of the normal-thickness residual corneal edges (Figure 5).74,75 This procedure has the advantages of not requiring donor tissue (thus avoiding the risk of graft rejection inherent in penetrating keratoplasty), preserving the normal central cornea, a stronger wound, and shorter visual rehabilitation times.76 Studies have demonstrated good visual outcomes with full-thickness crescentic wedge resection in PMD, but astigmatic drift is a concern (Table 2). In a 2008 study of 10 eyes, Busin et al. combined full-thickness crescentic wedge resec-tion with corneal relaxing incisions and reported mini-mal long-term astigmatic drift.76 However, with advanc-es in surgical management of PMD, the small sample sizes of these studies and the risks associated with open globe surgery make full-thickness crescentic wedge re-section a less appealing option.25,75,76

partIal-thICkness InterventIonsTotal Cornea Replacement. DALK is a modified

approach to total cornea transplantation that spares the corneal endothelium. Studies reporting DALK outcomes in PMD are few and of limited sample size (Table 2).77,78 Studies in diseases of the anterior cor-nea indicate that DALK provides visual outcomes similar to penetrating keratoplasty without the risk of immune-mediated graft rejection.79-81 The documented benefits of DALK include a shorter time to suture re-moval, a shorter course of topical steroid treatment, reduced endothelial cell loss, and fewer postopera-tive complications.80 The surgical complexity and relative novelty of DALK are its biggest limitations. Several techniques have been described for separat-ing corneal stroma from Descemet’s membrane, all of which have significant learning curves. DALK can also cause opacification of interface layers within the cor-nea leading to decreased visual acuity. Femtosecond laser technology has been used in combination with described surgical techniques with good outcomes and could represent the future direction and possible stan-dardization of DALK technique.82 Currently, the need for more extensive study and the technical difficulty of this procedure limits its mainstream acceptance.

Crescentic Cornea Replacement. Crescentic la-mellar wedge resection is similar to full-thickness crescentic wedge resection, but only the abnormally thinned stroma is removed, followed by re-approxima-tion of the residual, normal-thickness stromal edges (Figure 5).83,84 Like full-thickness crescentic wedge re-section, crescentic lamellar wedge resection does not require donor tissue. The main advantage of crescentic lamellar wedge resection over full-thickness crescentic wedge resection is the maintenance of a closed globe.85

Several small studies have confirmed the effi-cacy of crescentic lamellar wedge resection in PMD (Table 2).83-85 Recently, Maccheron and Daya per-formed corneal wedge resection with partial or total lamellar dissection with the aim of improving corneal symmetry and optical clarity by shortening the verti-cal circumference of the cornea in eyes with combined PMD/keratoconus or keratoglobus.85 The result was a statistically significant improvement in visual acuity with reduced astigmatism, although follow-up dura-tion and outcomes varied among study participants.

Crescentic lamellar keratoplasty involves the re-moval of abnormally thinned stroma followed by placement of a donor stromal graft (Figure 5).1,86-89 The advantages of crescentic lamellar keratoplasty include maintaining a closed globe and the fact that host endo-thelial cells are maintained, reducing the risk of graft rejection.86,88



Good visual results have been reported with cres-centic lamellar keratoplasty in PMD with improved contact lens and spectacle tolerance (Table 2).1,88 The procedure has also been used to treat spontaneous cor-neal perforation in PMD.86,87 However, crescentic la-mellar keratoplasty is a technically difficult procedure and induced astigmatism and opacification have been reported.

Other lamellar surgical techniques have been devel-oped, but are less commonly used. Lamellar thermo-keratoplasty involves thermal shrinkage of the host lamellar bed prior to placement of a donor lamellar cornea.30 Few data are reported for lamellar thermo-keratoplasty in the literature. Epikeratoplasty or epik-eratophakia is an onlay lamellar keratoplasty with good results reported in the treatment of PMD (Table 2).90 The procedure involves suturing a lens made of cor-neal tissue to the anterior cornea, correcting refractive abnormalities.90 However, with the advent of newer, more sophisticated PMD treatments, epikeratoplasty is rarely performed.

Tuck-in lamellar keratoplasty is a surgical technique developed for treatment of advanced corneal ectasias including concurrent keratoconus and PMD. The pro-cedure involves creation of a peripheral, partial-thick-ness flange of posterior stromal tissue on the donor lenticule that is integrated into a 180° inferior stromal pocket in the host cornea (Figure 5).91-93 This technique is thought to provide additional tectonic support to the peripheral cornea. Kaushal et al. reported improved vi-sual acuity and reduced astigmatism with no signifi-cant complications in 8 eyes that underwent tuck-in lamellar keratoplasty for combined keratoconus and PMD (Table 2).91 Potential future directions for tuck-in lamellar keratoplasty include using femtosecond la-ser technology for flange and stromal pocket creation. More long-term outcome data are also required for tuck-in lamellar keratoplasty to become mainstream.

CombInatIon therapIesBecause CXL, toric PIOL implantation, and ICRS

each target only one aspect of the PMD disease process, combined therapies have the potential to offer compre-hensive treatment, correcting refractive error and astig-matism while halting ectatic progression (Table 4).

Kymionis et al. reported combining PRK and CXL in two eyes of one patient with PMD.94-96 Although topography-guided surface ablation is generally con-traindicated in PMD due to the risk of accelerating ec-tatic change with iatrogenic thinning of the cornea, it is thought that CXL prevents this ectatic progression before it starts. Kymionis et al. reported improved keratometry, corrected distance visual acuity, and

astigmatism with no intraoperative or short-term com-plications, although long-term follow-up is needed to ensure safety and ectasia stabilization. This study does raise the possibility that, in the future, patients at risk for iatrogenic ectasia could be pretreated with CXL pri-or to topography-guided surface ablation to avoid this dreaded complication.

Kymionis et al. also recently reported successful photorefractive keratectomy followed by same-day CXL in one patient with progression of PMD after ICRS implantation.97 By avoiding ICRS explantation, the patient was able to tolerate contact lenses postopera-tively and an acceptable visual outcome resulted. Al-though the visual results were good and there were no reported complications at 12 months, more extended follow-up is required to assess the outcome of this combination of therapies.

Additional studies are necessary to investigate the efficacy and safety of combining PMD treatments such as ICRS, CXL, toric PIOL, and topography-guided surface ablation. Although studies of combined treat-ments in keratoconus can provide clues to the impact of these studies in PMD, independent studies are nec-essary before widespread acceptance is possible.

CONCLUSIONSThe diagnosis and management of PMD has seen

considerable advancement in recent years. Although little is currently understood about the etiology, patho-physiology, epidemiology, and genetics of the disease, new treatments are improving visual outcomes and re-ducing complications. CXL is especially exciting given its ability to halt the progression of ectasia. Combined treatments and better screening could eliminate the need for surgical management of PMD in most cases. Future research aimed at discovering genetic markers for PMD and other ectactic disorders would allow early detection of affected patients and potentially lead to progression-altering treatment prior to visual distortion.

AUTHOR CONTRIBUTIONSConception and design (MM, JNE, NLB); data collection (NLB); anal-

ysis and interpretation of data (MM, JNE, NLB, SMC); writing the

manuscript (MM, JNE, NLB); critical revision of the manuscript (MM,

JNE, SMC); administrative, technical, or material support (NLB,

SMC); supervision (MM)

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