Pellucid Marginal = Corneal Degeneration

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Pellucid Marginal Corneal=20 Degeneration

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&n= bsp; &nb= sp; &nbs= p;=20 ANIL RADHAKRISHNAN, MS

ANIL = RADHAKRISHNAN,=20 MS

Senior = Consultant, Cornea=20

Chaithanya Eye = Hospital and=20 Research Institute

Kesavadasapuram=20

Trivandrum=20 695004

Phone =96=20 91-471-2447183

Fax =96=20 91-471-2443985

E-mail =96 = anilradh@yahoo.com =

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=20 Pellucid marginal corneal degeneration [PMCD] is a = bilateral=20 peripheral ectatic corneal disorder typically affecting the inferior = cornea. The=20 cornea is clear and avascular [pellucid - means clear] with no = Fleischer=92s ring,=20 scarring, inflammation or lipid deposition ^1,5 .

=20 Though it was believed to have no racial, sexual or genetic=20 predisposition, it is quite likely that it is a variant of keratoconus = and=20 keratoglobus. In a large retrospective case series, it was found to be=20 associated with keratoconus in 10% and keratoglobus in 13% and more = common in=20 males ^6,7 .

The age of onset is second to = fifth=20 decade, usually with a high against the rule astigmatism. Rarely sudden = drop in=20 visual acuity can occur due to hydrops ^1,5 . PMCD is = characterized by=20 a band of thinning 1 to 2 mm wide, concentric to the limbus typically in = the=20 inferior cornea, between 4 and 8 =91o clock position, 1 to 2 mm inside = the limbus=20 is seen. The cornea above and below the area of thinning is of normal = thickness.=20 The uninvolved inferior cornea overhanging the area of thinning = protrudes,=20 unlike keratoconus ^1,5. Uncommonly the band of inferior = thinning=20 extends above the horizontal meridian or maybe limited to the nasal = cornea=20 ^2,5 . Rare cases of superior corneal thinning has also been = reported=20 either in isolation or along with classic PMCD ^3,4 .

Unlike = keratoconus epithelial iron ring is not seen. But descemet=92s membrane = folds may=20 occasionally be seen concentric to limbus, which disappear usually with = external=20 pressure. Acute hydrops = can occur=20 with resultant stromal oedema, scarring and vascularisation of the = cornea.

Corneal = topography in=20 classical PMCD is distinct with a typical pattern of oblique peripheral=20 steepening and superior flattening [=91butterfly=92 or =91crab claw=92 = appearance ].=20 Videokeratoscopy reveals marked flattening along vertical axis with = steepening=20 in the inferior peripheral cornea. This area of inferior steepening = extends=20 upwards along the associated horizontal semi-meridians to form a =91loop = cylinder=92=20 inferiorly. It is reported that in superior or nasal PMCD, the = topographic=20 changes are similar to classical PMCD but with superior or nasal loop = cylinder=20 respectively. If the area of inferior corneal thinning extends above = horizontal=20 meridian there is a shift in the axis of loop cylinder mirroring the = extent of=20 circumferential thinning ^3,4 .

Histopathology

The=20 epithelium, descemet=92s membrane and endothelium are normal. Bowman=92s = layer is=20 fragmented or absent. At the area of thinning, fibrous long spacing = collagen=20 [FLS] with a periodicity of 100 to 110nm is found in place of normal = collagen=20 which has a periodicity of 60 to 64 nm. FLS is also seen in keratoconus, = but not=20 in normal cornea.

Treatment=20

1] = Spectacles=20 =96 is useful in early cases. Toric spectacles can achieve good visual = acuity as=20 the astigmatism is generally regular unlike keratoconus. Appropriate = selection=20 of spectacle lens and frame can improve spectacle tolerance.

2] = Contact=20 lenses =96 CL fitting is difficult due to irregular corneal surface and = steep curvature resulting in = inferior edge=20 lift and unstable centration.

RGP lenses however can provide=20 significant improvement in visual acuity. Raizada K et al have described = a=20 nomogram for RGP lens fitting in PMCD after a retrospective review of 40 = eyes of=20 24 patients. If the = astigmatism was=20 less than 10 diopters, a base curve of 6-7mm and a diameter of 8-9mm was = seen to=20 be a good initial lens for trial and if more than 10 diopters a base = curve of 7=20 to 7.5mm and a total diameter of 10 to 10.5 mm. In their series 21 of = the 24=20 patients were comfortable with their lenses at 6 months follow-up = ^8,9=20

In=20 another study of 27 eyes of 15 patients by Kompella VB et al 95.4% of = the=20 subjects had a visual acuity of 6/12 or better with the final contact = lens.=20 After a mean follow-up of 22.91 months, 77.3% had a visual acuity of = 6/12 or=20 better with an average wearing time of 9.58 hours ^8,9 .

Gas permeable = scleral=20 contact lenses [GPSCL] can be another alternative or in patients where = RGP=20 lenses had proved unsatisfactory.

3] Surgical management =

= Surgical=20 management is probably the last resort. The variety of surgical = procedures used=20 implies the fact that optimum surgical treatment is yet to be = devised.

Wedge resection is = advocated=20 by many. The disadvantage of this procedure is the tendency for = astigmatism to=20 revert to preoperative level despite overcorrection at the time of = surgery. Most=20 studies suggest that, though long term drift occurs in most patients, it = enables=20 patients to resume contact lens wear ^10,11 .

= Crescentic=20 lamellar keratoplasty is another procedure described. After lamellar dissection of = the=20 affected area of cornea normal thickness stroma is then reappossed to = normal=20 thickness host stroma with multiple interrupted sutures ^{12, =
13}. In=20 the limited numbers studied by Cameron J et al ¹² , four out = of five=20 patients had significant improvement in visual acuity over a follow-up = period of=20 27 to 40 months. = Compressive C =96=20 shaped lamellar keratoplasty is another alternative ¹⁴ .

Penetrating keratoplasty = is much=20 less successful in PMCD [as compared to keratoconus] as the need for = larger=20 graft significantly increases the risk of rejection. If =20 the graft is placed eccentrically it tends to cause increased=20 astigmatism. Simultaneous central penetrating keratoplasty with = peripheral=20 crescentic lamellar keratoplasty has also been described with = considerable=20 success ^13,15.

Intrastromal = corneal ring is=20 emerging as a promising treatment of early pellucid marginal corneal=20 degeneration. They flatten the ectatic corneal tissue to decrease = asymmetric=20 astigmatism ^16,17 . The treatment is reversible and preserves = an=20 intact globe.

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References =

1] = Krachmer=20 JH. Pellucid marginal corneal degeneration. Arch Ophthalmol 1978;=20 96:1217-1221

2] Bower KS, Dhaliwah = DK, Barnhost=20 DA Jr, Warnicke J. Pellucid marginal degeneration with superior = thinning. Cornea=20 1997; 16: 483 =96 485.

3] Maguire LJ, Klyce = SD, Mc Donald=20 MB, Kaufman HE, Corneal topography of pellucid marginal degeneration.=20 Ophthalmology 1987; 94: 519-524.

4] Rao SK, Fogla R, = Padmanabhan P,=20 Sitalakshmi G. Corneal topography in atypical pellucid marginal = degeneration.=20 Cornea 1999; 18:265-272.

5] Krachmer JH, Feder RS, Belin MW.=20 Keratoconus and allied non-inflammatory disorders. Surv = Ophthalmol=20 1984,28:293-322.

6] Kayazawa F, = Nishimura K, Kodama=20 Y, Tsuji T, Itoi M. Keratoconus with pellucid marginal corneal = degeneration.=20 Arch Ophthalmol 1984;102:895 =96 896.

7] Santo RM, Bechara = SJ, Kara-Jose=20 N. Corneal topography in asymptomatic family members of a patient with = pellucid=20 marginal degeneration. Am J Ophthalmol 1999; 127(2):205-7.

8] Raizada K, Sridhar = MS. Nomogram=20 for spherical RGP contact lens fitting in patients with pellucid = marginal=20 corneal degeneration. Eye Contact Lens. 2003; 29(3): 168 =96 72.

9] Kompella VB, Aasuri = MK, Rao GN.=20 Management of Pellucid = Marginal=20 Corneal Degeneration with rigid gas permeable contact lenses. CLAO J = 28(3):=20 140-5.

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10] Dubroff S, Pellucid = Marginal=20 Corneal Degeneration: report on corrective surgery. J Cataract Refract = Surg.=20 1989; 15(1): 89 =96 93.

11] MacLean H, Robinson = LP,=20 Wechsler AW. Long term results of corneal wedge resection for Pellucid = Marginal=20 Corneal Degeneration. Eye 1997;11 (Pt5): 613-7.

12] Camreon JA, Results = of lamellar=20 crescentic resection for pellucid marginal corneal degeneration. Am J = Ophthalmol=20 1992 15;296-302.

13] Rasheed K, = Rabinowitz YS.=20 Surgical treatment of advanced pellucid marginal corneal degeneration.=20 Ophthalmology 2000; 107(10): 1836-40.

14] Cheng CL, Theng JT, = Tan DT.=20 Compressive C-shaped lamellar keratoplasty : a surgical alternative for = the=20 management of severe asigmatism from peripheral corneal degeneration.=20 Ophthalmology 2005(3) : 425-430.

15] Sridhar MS, Mahesh = S, Bansal=20 AK, Nutheti R, Rao GN. Pellucid marginal corneal degeneration. = Ophthalmology.=20 2004 Jun;111(6):1102-7.

16] Mularoni A, Torreggiani A, di = Biase A,=20 Laffi GL, Tassinari G. Conservative treatment of early and moderate = pellucid=20 marginal degeneration: a new refractive approach with intracorneal=20 rings.Ophthalmology. 2005 Apr;112(4):660-6.

18] Kymionis GD, Aslanides IM, Siganos = CS,=20 Pallikaris IG. = Intacs for=20 early pellucid marginal degeneration. J Cataract Refract Surg. 2004=20 Jan;30(1):230-3.