Eye Diseases > Cornea

Pellucid Marginal Degeneration

Evidence-based assessment and management of pellucid marginal degeneration — a non-inflammatory peripheral corneal ectasia characterised by inferior crescent-shaped thinning and irregular against-the-rule astigmatism.

Left: Inferior peripheral thinning (red) with maximal protrusion superior to the thinned zone (dashed line). Right: Characteristic "crab claw" corneal topography — inferior steepening (red/orange) with superior flattening (blue).

Pellucid marginal degeneration (PMD) is a non-inflammatory peripheral corneal ectasia of unknown aetiology. It is considered part of the corneal ectasia spectrum, potentially sharing pathogenic mechanisms with keratoconus.

Primary Causes

Idiopathic: No identifiable systemic or local cause in the majority of cases
Genetic predisposition: Familial cases reported; possible autosomal dominant inheritance pattern with variable expressivity
Mechanical trauma: Chronic eye rubbing (associated with atopic disease and vernal keratoconjunctivitis) implicated in progression
Enzymatic degradation: Increased matrix metalloproteinase (MMP) activity leading to stromal collagen breakdown in the inferior periphery
Overlap with keratoconus: PMD and keratoconus may represent different phenotypic expressions of a shared ectatic disorder; topographic overlap (PMD-keratoconus spectrum) is well-documented

Mechanism of Degeneration

Stromal thinning occurs in a crescent-shaped band in the inferior peripheral cornea, typically between the 4 and 8 o'clock positions, located 1–2 mm from the limbus
The thinned zone undergoes reduced biomechanical support, causing ectatic protrusion of the cornea immediately superior to the area of maximum thinning — a hallmark distinguishing PMD from keratoconus (where the cone is at the site of maximum thinning)
Altered keratocyte density and activity, upregulation of proteolytic enzymes (MMPs), and disruption of Bowman's layer contribute to progressive stromal loss
The resultant corneal shape produces irregular against-the-rule or oblique astigmatism — often high magnitude — due to the inferior protrusion creating an oblate inferior contour

Optical Consequences

High irregular astigmatism uncorrectable with spectacles
"Beer belly" inferior bulge on profile slit lamp view
Against-the-rule astigmatism on keratometry and topography
Characteristic "crab claw" or "kissing birds" pattern on axial topography maps

No universally accepted classification system exists for PMD. The following frameworks are used clinically:

By Severity

Grade	Thinning	Astigmatism	Vision
Mild	Minimal peripheral thinning; pachymetry >400 µm	<3 D irregular astigmatism	Correctable with spectacles
Moderate	Visible crescent thinning; pachymetry 300–400 µm	3–8 D irregular astigmatism	Requires RGP/scleral lenses
Severe	Marked thinning; pachymetry <300 µm	>8 D irregular astigmatism	Surgical candidacy; poor CL tolerance

By Arc Extent

Focal PMD: Thinning arc <90°, limited to a small inferior segment
Diffuse PMD: Thinning arc >90°, spanning a wide inferior crescent (more common)

By Ectasia Overlap

Pure PMD: Inferior peripheral thinning with protrusion superior to thinning; no central cone
PMD-keratoconus overlap: Features of both conditions on tomography; inferior thinning with additional paracentral steepening

Patient Characteristics

Age of onset typically 20–40 years
Male predominance (approximately 3:1 male-to-female ratio)
Bilateral presentation (though often asymmetric at diagnosis)
Positive family history of ectatic corneal disease

Environmental / Behavioural

Chronic eye rubbing (significant modifiable risk factor)
Atopic disease (eczema, asthma, allergic rhinitis)
Vernal keratoconjunctivitis

Systemic Associations

Connective tissue disorders (Marfan syndrome, Ehlers-Danlos syndrome)
Down syndrome
Previous contact lens overwear (mechanical corneal trauma)

Slit Lamp Findings

Inferior peripheral thinning: Crescent-shaped band of stromal thinning, 4–8 o'clock, 1–2 mm from limbus — with characteristically clear overlying epithelium (pellucid = clear/translucent)
Superior protrusion: Ectatic bulging of cornea immediately above the thinned crescent; "beer belly" profile on tangential illumination
Munson's sign: V-shaped indentation of the lower eyelid on downgaze (advanced cases)
Rizzuti's sign: Conical light reflex on the nasal limbus when a penlight is shone from the temporal side
Corneal clarity: Stroma usually clear (defining feature); no lipid deposits or active inflammation
Vogt's striae: May be present (vertical stress lines in posterior stroma) in advanced cases

Topographic / Tomographic Signs

"Crab claw" or "kissing birds" pattern: Inferior steepening with flanking areas of flattening on axial curvature map; superior relative flattening
Against-the-rule or oblique astigmatism: Often high magnitude (>3 D)
Pachymetric thinning: Thinnest point in inferior periphery (not paracentral as in keratoconus)
Posterior elevation: Posterior float elevation inferior to the best-fit sphere on Scheimpflug imaging

Blurred vision: Gradual onset, progressive; often poor spectacle best-corrected visual acuity due to irregular astigmatism
Glare and halos: Particularly around lights, especially at night
Monocular diplopia / polyopia: Multiple ghost images from the irregular corneal surface
Photosensitivity: Increased light sensitivity due to light scatter
Frequent spectacle prescription changes: Progressive increase in astigmatism magnitude and axis shift
Contact lens intolerance: Difficulty maintaining soft or conventional contact lens fit over the irregular surface

Note: Symptoms in PMD often develop insidiously. Many patients present late because early irregular astigmatism may be partially correctable with spectacles. The diagnosis is frequently first suspected on topography performed for contact lens fitting or refractive assessment.

Ocular Complications

Corneal hydrops: Acute Descemet's membrane rupture with stromal oedema — less common than in keratoconus but can occur
Corneal scarring: Post-hydrops scarring; occasionally from contact lens-related complications
Progressive visual impairment: Increasing irregular astigmatism rendering contact lens fitting increasingly complex
Contact lens intolerance: RGP lens awareness, poor centration over protrusion site

Long-term Sequelae

Corneal perforation (very rare; associated with trauma or extreme thinning)
Secondary amblyopia (if onset in childhood or adolescence)
Psychosocial impact from visual disability in working-age adults
Requirement for keratoplasty in advanced disease unresponsive to contact lens management

PMD is primarily an ocular condition without a strong or consistent systemic association. However, the following systemic conditions have been reported in association:

Associated Systemic Conditions

Connective tissue disorders: Marfan syndrome, Ehlers-Danlos syndrome — due to shared collagen fibril abnormalities
Atopic disease: Eczema, asthma, allergic rhinitis — primarily via the mechanical link of chronic eye rubbing
Down syndrome (Trisomy 21): Higher prevalence of corneal ectasia including PMD
Vernal keratoconjunctivitis: Chronic inflammation and rubbing contribute to inferior corneal stress

Systemic Workup Considerations

Systemic workup is not routinely indicated in isolated PMD
Consider referral for connective tissue disorder assessment if there are systemic features (arachnodactyly, joint hypermobility, skin hyperelasticity)
Counsel all patients on the importance of avoiding eye rubbing as a key modifiable risk factor

1. Clinical History

Age of onset, duration, and rate of vision change
History of atopy, eye rubbing, contact lens wear
Family history of ectatic corneal disease
Previous spectacle prescription history (progressive ATR astigmatism change)
Prior ocular surgeries or trauma

2. Slit Lamp Biomicroscopy

Assess inferior peripheral thinning location and extent (1–2 mm from limbus)
Evaluate corneal clarity — absence of scarring, lipid, or neovascularisation is characteristic
Check for Munson's sign (downgaze), Rizzuti's sign, Vogt's striae
Assess for acute hydrops (corneal oedema, epithelial microcysts)

3. Corneal Topography

Placido-disc topography: identifies the "crab claw" axial curvature pattern
Inferior steepening with superior relative flattening
Simulated keratometry: against-the-rule or oblique astigmatism
Key differentiating feature from keratoconus: The steepest point is located superior to the area of maximum thinning in PMD; in keratoconus the cone apex coincides with the thinnest point

4. Corneal Tomography (Scheimpflug / Pentacam)

Pachymetric map: peripheral inferior thinning (thinnest zone 1–2 mm from limbus)
Posterior elevation map: posterior float elevation inferior to best-fit sphere
Belin-Ambrosio Enhanced Ectasia Display (BAD): quantifies ectasia risk
Provides reproducible baseline for monitoring progression

5. Anterior Segment OCT

High-resolution pachymetry of the inferior thinning zone
Useful for surgical planning (DALK, ICRS) and monitoring thinning rate

Singapore Context: Scheimpflug tomography (Pentacam) is available at tertiary eye centres (SNEC, NUH). Placido-disc topography is available at specialist optometry practices and provides adequate screening for PMD. Referral for full tomographic workup is recommended when PMD is suspected on topography.

Singapore Optometry Scope Note

Optometrists in Singapore manage PMD within their scope through spectacle correction and contact lens fitting (including RGP and scleral lenses). Patients requiring surgical intervention — corneal collagen cross-linking, ICRS, DALK, or penetrating keratoplasty — must be co-managed with or referred to a registered ophthalmologist. Optometrists should not initiate or modify surgical treatment independently.

Optical Management (Optometry Scope)

Spectacle Correction

Effective only in mild disease with regular or mildly irregular astigmatism
Higher-order aberrations limit spectacle BCVA in moderate–severe disease

Rigid Gas-Permeable (RGP) Contact Lenses

First-line contact lens option for mild–moderate PMD
Vault the irregular inferior surface; tear lens fills optical irregularity
Challenges: poor centration over inferior protrusion; bearing on thinned zone
Rose K2 IC (irregular cornea) design particularly suited for PMD fitting

Scleral Contact Lenses

Preferred modality for moderate–severe PMD
Large diameter (16–18 mm) vaults entire corneal irregularity; lands on sclera
Provides stable, comfortable vision even over advanced ectasia
Requires specialist fitting assessment and fenestration management

Piggyback / Hybrid Lens Systems

RGP over soft carrier lens: reduces RGP discomfort while maintaining optical clarity
Hybrid lenses (SynergEyes): rigid centre bonded to soft skirt

Surgical Management (Ophthalmology Referral)

Corneal Collagen Cross-Linking (CXL)

Riboflavin/UVA cross-linking to halt ectasia progression
Most effective when corneal pachymetry >400 µm at treatment site
Recommended at first sign of documented topographic progression
Does not reverse existing thinning; stabilises further deterioration

Intrastromal Corneal Ring Segments (ICRS / Intacs)

Implanted in inferior stroma to flatten the ectatic zone and regularise topography
Improves contact lens tolerance post-implantation
Often combined with CXL for synergistic effect

Deep Anterior Lamellar Keratoplasty (DALK)

Preferred keratoplasty option — retains host endothelium, avoids endothelial rejection
Indicated when contact lens management fails and pachymetry is critically thin

Penetrating Keratoplasty (PK)

Full-thickness corneal transplant; reserved for end-stage disease with scarring
Higher post-operative astigmatism risk in PMD due to peripheral graft placement

Referral Criteria

Urgent Referral

Acute corneal hydrops (sudden pain, oedema, vision drop)
Suspected corneal perforation
Rapid progression with pachymetry approaching 300 µm

Routine Referral

Documented topographic progression over 2 consecutive visits
Contact lens intolerance despite optimised fitting
Candidacy assessment for CXL, ICRS, or keratoplasty

Visual Prognosis

PMD typically progresses more slowly than keratoconus. Many patients maintain functional vision with appropriate contact lens correction for years to decades. Progression tends to plateau in the fifth to sixth decade of life.

Treatment Outcomes

Scleral lenses: Excellent visual outcomes; the majority of moderate–severe PMD patients achieve functional vision
CXL: Effective at halting progression in 90–95% of cases when performed at appropriate stage; does not improve existing topography
DALK / PK: Good structural outcomes; post-keratoplasty astigmatism management may still require RGP or scleral lenses

Prognostic Factors

Favourable

Diagnosis and CXL treatment before significant thinning
Cessation of eye rubbing
Good contact lens tolerance
Adequate residual stromal thickness for surgical options
Slow progression documented on serial tomography

Unfavourable

Corneal hydrops with resultant scarring
Pachymetry <300 µm at presentation
Continued eye rubbing or atopic disease
Bilateral advanced disease
Post-PKP residual irregular astigmatism

PMD must be differentiated from other peripheral corneal thinning disorders and ectasias. Clinical signs, slit lamp findings, and topographic/tomographic patterns are key to discrimination.

Condition	Location of Thinning	Topographic Pattern	Distinguishing Features
Pellucid Marginal Degeneration (PMD)	Inferior peripheral (4–8 o'clock), 1–2 mm from limbus	"Crab claw" — inferior steep, superior flat; against-the-rule astigmatism	Protrusion superior to thinning; clear cornea; non-inflammatory
Keratoconus	Paracentral (inferior-central), usually 5–7 o'clock	Inferior-central cone with paracentral steepening; with-the-rule to oblique astigmatism	Cone apex at thinnest point; Fleischer ring; central scarring in advanced disease
Terrien's Marginal Degeneration	Superior peripheral (most common), any quadrant	Irregular peripheral flattening at site of thinning; irregular astigmatism	Lipid deposits at leading edge; superficial vascularisation; slow progression; non-inflammatory but can have pseudo-inflammatory phase
Keratoglobus	Global (360° peripheral thinning)	Generalised steepening; high myopia	Entire cornea thinned and protuberant; associated with connective tissue disorders; present from birth
Mooren's Ulcer	Peripheral, any quadrant; often superior	Irregular; secondary to ulceration	Active ulceration with stromal melt; painful; may be bilateral; associated with systemic vasculitis in some cases; inflammatory aetiology

Pearl 1 — Topography Pattern Recognition: The "crab claw" or "kissing birds" pattern on axial topography is pathognomonic of PMD. Look for the characteristic inferior steepening with flanking flat zones forming the claw shape. This pattern is distinct from keratoconus, which shows an inferior cone without the paired flanking flattening.

Pearl 2 — Distinguishing PMD from Keratoconus: The key differentiating principle: in PMD, the area of maximum steepening is located superior to the thinnest point; in keratoconus, the cone apex is at or near the thinnest point. Corneal tomography (Pentacam) clarifies this distinction when topography alone is equivocal.

Pearl 3 — Contact Lens Fitting Strategy: RGP lenses often decentre inferiorly over the protrusion in PMD. Consider a Rose K2 IC design with a larger overall diameter or early transition to scleral lenses. Scleral lenses, by vaulting the entire cornea, are the most reliable option for moderate–severe PMD and consistently achieve better visual outcomes than corneal RGPs.

Pearl 4 — Progression Monitoring: Perform Pentacam/Scheimpflug tomography at 6-monthly intervals during the active progression phase. Document: minimum pachymetry at the inferior thinning zone, Kmax, posterior elevation, and BAD-D score. Progression is defined as a ≥1.0 D increase in Kmax, ≥5% reduction in minimum thickness, or significant posterior elevation change over 12 months.

Pearl 5 — CXL Timing: CXL should be offered at the first sign of documented topographic progression, before pachymetry falls below 400 µm at the treatment site. Accelerated CXL protocols are increasingly used but standard Dresden protocol (3 mW/cm² for 30 min) remains the gold standard for efficacy. Do not delay referral awaiting further significant deterioration.

Singapore Practice Note: Many patients with PMD are first identified at optometry contact lens fitting consultations. Optometrists are often the first-line practitioners to detect the characteristic topography pattern. Timely referral to ophthalmology for tomographic confirmation and CXL assessment is a key role of the Singapore optometrist in PMD management.

Krachmer JH. Pellucid marginal corneal degeneration. Arch Ophthalmol. 1978;96(7):1217–1221.
Sridhar MS, Mahesh S, Bansal AK, Nutheti R, Rao GN. Pellucid marginal corneal degeneration. Ophthalmology. 2004;111(6):1102–1107.
Moshirfar M, Edmonds JN, Behunin NL, Christiansen SM. Current options in the management of pellucid marginal degeneration. J Refract Surg. 2014;30(7):474–485.
Tzelikis PF, Cohen EJ, Rapuano CJ, Hammersmith KM, Laibson PR. Management of pellucid marginal corneal degeneration. Cornea. 2005;24(5):555–560.
Biswas S, Brahma A, Tromans C, Ridgway A. Management of pellucid marginal corneal degeneration. Eye (Lond). 2000;14(Pt 4):629–634.
Kymionis GD, Karavitaki AE, Grentzelos MA, et al. Pellucid marginal degeneration: a literature review. Clin Ophthalmol. 2017;11:1113–1122.
Jinabhai A, Radhakrishnan H, O'Donnell C. Pellucid corneal marginal degeneration: a review. Cont Lens Anterior Eye. 2011;34(2):56–63.
Belin 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(2):157–162.
Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37–50.
Raiskup F, Theuring A, Pillunat LE, Spoerl E. Corneal collagen crosslinking with riboflavin and ultraviolet-A light in progressive keratoconus: ten-year results. J Cataract Refract Surg. 2015;41(1):41–46.
Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-A–induced collagen crosslinking for the treatment of keratoconus. Am J Ophthalmol. 2003;135(5):620–627.
Schornack MM, Patel SV. Scleral lenses in the management of pellucid marginal degeneration. Eye Contact Lens. 2010;36(5):299–302.
Woodward MA, Randleman JB. Visual rehabilitation and contact lens considerations for corneal ectasia. Curr Opin Ophthalmol. 2010;21(4):286–291.
Tan DT, Por YM. Current treatment options for corneal ectasia. Curr Opin Ophthalmol. 2007;18(4):284–289.
Ambrosio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg. 2006;32(11):1851–1859.

Clinical Illustration

Etiology