Spark Imagewise 21

Pellucid Marginal Degeneration

Dr. Swati Mishra, Dr. Ravi Daberao, Dr. Prabhat Nangia, Dr. Sarang Lambat, Dr. Vinay Nangia 
559, Suraj Eye Institute, Nagpur.

Case Description
A male, 49 years of age, presented to us with chief complaint of blurring of vision in both eyes (BE) since 7 months. His best corrected visual acuity (BCVA) was 6/9, N6 in the right eye (RE) and 6/9, N6 in the left eye (LE) with a correction of –2.75 DS with +4.25 DC in RE and -2.50 DS with +4.00 DC in LE and a near addition of +1.75D in BE. The anterior segment examination showed inferior thinning of cornea in BE. Slit lamp examination showed crescentic stromal thinning of the inferior periphery of the cornea, extending from 4 o’clock to 8 o’clock. The intra ocular pressure was 15 mmHg in BE. Fundus examination in BE was within normal limits.

Fig. 1 Right eye diffuse illumination revealed possibility of abnormal corneal contour inferiorly (black arrow).
Fig. 2
Left eye diffuse illumination revealed possibility of abnormal corneal contour inferiorly (black arrow).
Fig.3 RE slit lamp examination showed severe thinning from 4 to 8 o’ clock in the inferior cornea (yellow arrow) just below the area of maximum ectasia (white arrow).
Fig.4 LE slit lamp examination showed severe thinning from 4 to 8 o’ clock in the inferior cornea (yellow arrow) just below the area of maximum ectasia (white arrow). The thinning appeared to be worse in left eye than right eye.
Fig 5: Axial topographical map of RE showed against-the-rule astigmatism with a typical crab claw or butterfly pattern. The steep K was 48.43 D @ 170° and the flat K was 37.73 D @ 80°. The Kmax was 49.73 D and thinnest pachymetry was 440 microns on anterior segment optical coherence tomography (ASOCT) . 
Fig 6:
 Axial topographical map of LE showed against-the-rule astigmatism with a typical crab claw or butterfly pattern. The steep K was 50.63 D @ 176° and the flat K was 37.14 D @ 86°. The K max was 51.49 D and thinnest pachymetry was 378 microns on ASOCT.
 
Fig.7 Right eye showed inferior band of gross ectasia, noted to be just above the area of maximal thinning, with posterior elevation values in the range of 50 to 190 microns.
Fig.8
Left eye showed inferior band of gross ectasia, noted to be just above the area of maximal thinning, with posterior elevation values in the range of 49 to 170 microns.
Fig 9: Right eye AS-OCT image showed localized thinning (yellow arrows) in the inferior cornea. (Thickness =440 Microns)
Fig 10: Left eye AS-OCT image showed localized thinning (yellow arrows) in the inferior cornea. (Thickness=378 Microns)

The patient was diagnosed to have pellucid marginal degeneration.

Discussion

Pellucid marginal corneal degeneration (PMCD) is a bilateral, noninflammatory, peripheral corneal thinning disease. It is characterized by a peripheral crescentic band of thinning, usually in the inferior cornea. The ectatic zone, which is 1-2 mm from the limbus, lies above the point of the maximum corneal thinning. The term pellucid marginal degeneration was coined first by Schalaeppi in 1957 as “la dystrophie marginale inferieure pellucide de la cornee”. “Pellucid,” meaning clear, signifies the clarity of the cornea despite the presence of ectasia.

The condition is more common in males in their 2nd to 5th decades of life. The etiology of PMCD is largely unknown. Similar to keratoconus, electron microscopy of the cornea in PMCD reveals abnormally spaced collagen fibers with a periodicity of 100 nm to 110 nm, as opposed to 60 nm to 64 nm found in normal corneas.

Histopathological studies report an absent or irregular Bowman’s membrane with breaks. An increase in stromal mucopolysaccharides and stromal thinning has been reported. Descemet’s membrane might show folds occasionally. Electron microscopic studies have shown the presence of irregularly arranged stromal collagen bundles along with the presence of extracellular, granular electron-dense deposits. These electron-dense areas were found to be islands of fibrous long spacing (FLS) collagen with a 100 to 110 nm periodicity in a sea of normally spaced collagen bundles (60 to 64 nm periodicity). Similar histopathological patterns have been observed in cases of advanced keratoconus with severe thinning.

Most commonly, a gradual, progressive diminution in vision or longstanding poor visual quality is the presenting feature of these patients. But best-corrected visual acuity becomes poor only in advanced stages. On evaluation, high irregular, against-the-rule astigmatism is commonly found.  The flattening in the vertical meridian is due to tissue loss and a thin stroma in a crescentic pattern. The steepening and ectasia occur at the junction of affected and unaffected tissue, leading to the typical high cylindrical loop. The high against-the-rule astigmatism stems from a paradoxical steepening perpendicular to the flattened meridian.

The gold standard investigation for PMCD and other corneal ectasias is corneal tomography. In PMCD, inferior peripheral steepening extending into the mid-peripheral, inferior oblique corneal meridians result in a characteristic “crab-claw,” “butterfly,” or “kissing doves” appearance on the curvature/keratometric map on topography. Anterior segment optical coherence tomography and Scheimpflug images might help to assess the thinnest location in such cases since topography does not provide corneal pachymetry data.

Management modalities can be classified as conservative/visual rehabilitative treatment and surgical treatment. The fitting of contact lenses in PMCD is more challenging than other ectatic diseases due to the large area of inferior protrusion and inability of the contact lens to ‘sit on the cone.’ Routine rigid gas permeable (RGP) lenses often pose decentration challenges. Large diameter RGP lenses may be particularly useful. Other contact lenses like hybrid lenses with a soft skirt, semi scleral and scleral lenses, toric lenses, and prosthetic replacement of ocular surface ecosystem (PROSE) lenses have been found to be useful. Surgical treatment comprises intracorneal ring segments (ICRS), collagen cross-linking (CXL), and partial and total corneal replacement procedures. 

In our patient the topography showed a typical crab claw pattern and AS-OCT helped in identifying the thinnest area. Vision was not improving beyond 6/9 with spectacles so, he was advised to go for visual rehabilitation with large diameter RGP lens or scleral lenses based on the fitting and comfort along with frequent follow ups. The option of crescentic lamellar corneal grafting over the area of thinning was also discussed with the patient. 

ReadWise 

  1. Sridhar MS, Mahesh S, Bansal AK, Nutheti R, Rao GN. Pellucid marginal corneal degeneration. Ophthalmology. 2004 Jun;111(6):1102-7. doi: 10.1016/j.ophtha.2003.09.035.
  2. Jinabhai A, Radhakrishnan H, O’Donnell C. Pellucid corneal marginal degeneration: A review. Cont Lens Anterior Eye. 2011 Apr;34(2):56-63. doi: 10.1016/j.clae.2010.11.007. Epub 2010 Dec 23.
  3. Koc M, Tekin K, Inanc M, Kosekahya P, Yilmazbas P. Crab claw pattern on corneal topography: pellucid marginal degeneration or inferior keratoconus? Eye (Lond). 2018 Jan;32(1):11-18. doi: 10.1038/eye.2017.198. Epub 2017 Sep 22.

Correspondence 

Dr Prabhat Nangia
DNB, FICO, FMRF, FAICO
Consultant  
Department of Cornea and Ocular Surface
Suraj eye Institute
Email – education@surajeye.org

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