Pellucid marginal degeneration: Treatment options and our experience
Pellucid marginal degeneration is a corneal ectatic disorder primarily involving the peripheral inferior quadrant from the 4 o’clock to 8 o’clock position.
The inferior crescent-shaped thinning of the cornea (Figure 1) is marked by normal corneal thickness just above the ectatic area, characteristically differentiating it from keratoconus. The crescent band of corneal thinning is around 1 mm to 2 mm from the limbus and noninflammatory in nature, with the disorder typically manifesting between the second and fifth decade of life with increased prevalence in men. The inferior bulge of the cornea often gives it an appearance of a “beer belly” configuration on slit lamp examination (Moshirfar and colleagues).
Source: Priya Narang, MS, and Amar Agarwal, MS, FRCS, FRCOphth
Patients are usually asymptomatic in the early stages of the condition; eventually, they complain of decreased visual acuity and manifestation of against-the-rule astigmatism, with the condition often misdiagnosed as keratoconus. Corneal topography is an effective tool to assess the curvature of the cornea and demonstrates a “crab claw” appearance due to flattening of the vertical meridian with simultaneous steepening of the inferior periphery. Topography analysis should be performed meticulously because early pellucid marginal degeneration (PMD) may be misdiagnosed as a case having astigmatic refractive error, and to make matters worse, these cases are poor candidates for corneal refractive surgery. Pachymetry evaluation would add value to the investigative tests along with tomography.
The majority of patients with PMD are managed using spectacles and contact lenses, although in many cases, it is difficult to fit in contact lenses due to irregular curvature. Surgical procedures are usually deferred in the initial stages and considered only when spectacles and contact lenses fail to provide adequate visual output. Penetrating keratoplasty is often considered for cases with advanced PMD; however, the procedure has its own limitations. Due to the eccentric location of the corneal thinning in PMD, the graft chosen is usually larger in size or eccentrically placed, predisposing it to higher risk for graft rejection and development of corneal vascularization. Some surgeons have contemplated using intracorneal ring implants; however, they run the risk of abrupt corneal perforation due to associated corneal thinning.
In our series of cases, we have performed pinhole pupilloplasty (PPP) with gratifying results (Figure 2). There are various aspects as to how PPP works best for PMD cases. First, the corneal thinning in PMD usually manifests significantly after the fourth or fifth decade of life, and surgical consideration is needed for most of the cases. PPP works on the principle of small aperture optics and helps to channelize rays through the central cornea, which is usually clear in PMD, thereby nullifying the effect of irregular astigmatism. Second, lens removal with IOL implantation is not a major concern in these patients; PPP can eventually be performed, and the center of the pupil can be centered around the Purkinje 1 (P1) reflex, which serves as a marker for centration. Third, in cases with marked corneal thinning that may require PK in the future, the procedure can be performed with PPP still being intact and fully functional. The added advantage in these patients would be that PPP would nullify the astigmatism associated with the PK graft and suturing.
The technical difficulty that is usually encountered while performing PPP is the decentration of the P1 reflex due to associated irregular astigmatism. This might lead to formation of a slightly off-centered pinhole pupil. Clinically, this does not have much effect as the cones in the pinhole area usually adapt themselves and a gradual shift is seen to align with the pupil.
PPP works effectively in PMD as well as cases with irregular corneal astigmatism and eyes with higher-order aberrations.
- References:
- Moshirfar M, et al. J Refract Surg. 2014;doi:10.3928/1081597X-20140429-02.
- Narang P, et al. J Cataract Refract Surg. 2019;doi:10.1016/j.jcrs.2018.12.007.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at aehl19c@gmail.com; website: www.dragarwal.com.
- Priya Narang, MS, of Narang Eye Care & Laser Centre, Ahmedabad, India, can be reached at narangpriya19@gmail.com.
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