The Scleral Lens Education Society: Case Report 1
Management of Pellucid Marginal Corneal Degeneration with the Use of a Gas Permeable Scleral Contact Lens
Brett Larson, O.D.
2560 Associated Rd #12
Fullerton, CA 92835
(714) 393-9890
Abstract
Pellucid Marginal Corneal Degeneration (PMCD) is a progressive, non-inflammatory, idiopathic disease affecting the cornea. Currently, very little is known about the cause of this condition and there is no cure, however, much can be done to manage patients with this condition and improve their quality of life. Treatment of PMCD depends greatly on the severity of the condition or the patient’s stage of progression. Of all treatment modalities, gas permeable (GP) contact lenses provide the clearest vision for the patient. Although fitting patients with PMCD with gas permeable lenses is a challenge due to high amounts of corneal irregularity and the high probability of corneal complications, scleral lenses can provide the patient with clear vision, improved comfort, and reduced risk of future corneal complications.
Key words: Pellucid marginal corneal degeneration (PMCD), irregular astigmatism, ectasia, scleral lens.
Introduction
Pellucid marginal corneal degeneration (PMCD) is a relatively rare, bilateral, progressive, ectatic corneal disorder characterized by a noninflammatory thinning of the inferior peripheral cornea1. One of the more specific characteristics of PMCD is an area of corneal protrusion that occurs above the area of thinning rather than within this area as is seen in keratoconus. Typically, a 1 to 2 millimeter area of non-ectatic normal cornea separates the area of the corneal thinning from the limbus itself1,6,13. Topography can aid in the diagnosis of PMCD. A topographical map of the cornea will show inferior corneal steepening with a claw like pattern and against-the-rule astigmatism. Although PMCD commonly has an inferior presentation, superior PMCD has also been reported in the literature6. Patients typically present in their third to fifth decade of life with decreased visual acuity secondary to high amounts of irregular corneal astigmatism1,13. Reported complications of PMCD are hydrops and spontaneous perforation7,14.
Successful management of PMCD relies heavily on the severity of the condition. In its early stages, PMCD can be managed with the use of spectacles. Patients with high amounts of regular or irregular astigmatism cannot be managed with glasses and need to utilize gas permeable contact lenses in order to achieve acceptable visual outcomes3,8. Surgical options for the correction of PMCD include intrastromal corneal rings, and several different types of deep lamellar and penetrating keratoplasty2,13,14. All of these surgical procedures, however, have a high probability of still requiring the patient to wear gas permeable contact lenses even after surgery is completed.
A scleral contact lens is defined as a lens 18 millimeters or larger in diameter that completely vaults over the corneal surface and rests on the sclera. These lenses are a viable option in the management of PMCD because of their high probability to center over the cornea as opposed to small diameter corneal lenses11,16. Mechanical irritation of the ectatic portion of the cornea is also less likely because of the corneal vault that occurs with these lenses. Studies have shown that scleral lenses provide similar and often improved visual outcomes when compared with corneal contact lens designs11,15.
Case Report
Patient #1, a 51-year-old Hispanic male, presented to the clinic on 17 July 2008. He had been referred by an outside optometrist for the fitting of a gas permeable contact lens OD. The fitting of this lens was for the treatment of what he reported as “unilateral keratoconus OD only”. The patient reported reduced vision OD with “blurry, distorted, and double vision” occurring constantly. He had been diagnosed with keratoconus three years prior but suspected that he had suffered from the condition since his early 20’s. The patient reported that he was “not eager” to be fit with gas permeable lenses having attempted to wear them in the past without success due to discomfort. The patient showed no other significant ocular history and his medical history was negative. He denied taking medications in any form or having any allergies. He had a family history of diabetes, high blood pressure, and glaucoma. His blood pressure was 118/76. The patient was pleasant and oriented to time, place and person.
The patient’s last eye examination had been completed in June 2008. The referring doctor reported that the patient was “intolerant to gas permeable contact lenses” and “should consider surgery to successfully wear disposable soft lenses”. His record showed other wise healthy eyes with an unremarkable fundus evaluation. Subjective refraction was reported as +1.25-6.75 x 060 OD and -1.00-0.50 x 100 OS with visual acuities of 20/50- and 20/20 respectively.
The patient presented wearing soft toric disposable contact lenses with powers of -2.25-2.75 x 070OD and -0.75-0.75 x 100 with resulting acuities of 20/400 and 20/20 respectively. The habitual lenses centered well, had good rotational stability and moved well on blink. He reported wearing +1.75 OTC readers over his contact lenses when reading with adequate vision. The patient’s uncorrected distance visual acuity was counting fingers at distance and near OD and 20/40 at distance, 20/25 at near OS. Best corrected visual acuity showed 20/50 OD and 20/20 OS with a manifest refraction of +1.25-6.75 x 060 OD and -1.00-0.50 x 100 OS. Pinhole acuity was not documented. Pupils were equally round and reactive to light with no afferent pupillary defect OU. Confrontation visual fields were full to finger count OD/OS. Extraocular muscles were unrestricted in all gazes. Binocularity measures were not performed at this visit secondary to decreased visual acuity OD. Intraocular pressures were 16mmHg OD and OS at 10:20 a.m. with Goldman applantation tonometry. Slit lamp examination revealed the bulbar and palpebral conjuntiva to be white and quiet OU, clean eyelid margins and lashes OU, brown irises OU, clear lenses OU, and open anterior chambers that were deep and quiet OU. Evaluation of corneal health with the slit lamp showed inferior corneal thinning and associated Fuch’s Striae OD, and clear corneal tissue OS. An undilated examination of the poster pole with a 90 diopter fundus lens revealed cup-to-disc ratios of 0.3 round OU. Retinal arteries and veins had a normal appearance with a ratio of 2/3 respectively. Macular areas were flat and avascular with a positive foveal reflex OU. The fundus background was homogenous with no significant retinal findings.
Corneal topography was taken OD/OS. OD topography showed an irregular corneal surface with a corneal toricity of approximately 14 diopters. Simulated keratometry showed values of 37.17 x 51.53 diopters with corneal steepness increasing in the inferior 1/3rd of the cornea. The topographical pattern OD was indicative of pellucid corneal degeneration. OS simulated keratometry values were recorded as 41.00 x 41.26 diopters. The topographical pattern showed a nearly spherical corneal surface with no signs of an irregular surface. The patient was therefore diagnosed with unilateral pellucid marginal corneal degeneration (PMCD) OD.
The differential diagnoses in this case include:
- Pellucid Marginal Corneal Degeneration (PMCD)
· On topography, PMCD has a “claw-like” appearance with the inferior portion of the cornea steepening drastically as compared to the superior cornea.
· This type of patient will typically show a large amount of against-rule-corneal toricity and against-the-rule astigmatism on refraction.
- Keratoconus
· On topography, keratoconus has the appearance of localized steepening which is commonly inferior and temporal (although the localized area can be located anywhere on the cornea).
· Ocular signs will include Fleischer’s ring, Fuch’s striae, and Munson’s sign in advanced cases.
- High Amount of Regular Corneal Astigmatism
· On topography, regular astigmatism will show a somewhat symmetrical bow-tie-pattern rather than a localized area of steepening (as in keratoconus) or a claw like appearance (as in PMCD).
· Patients with high amounts of regular corneal astigmatism will not show signs of corneal thinning such as Fuch’s striae for example.
· Regular astigmatism theoretically should be correctable to 20/20 visual acuity with glasses if there are no other confounding factors present (such as amblyopia).
Several small diameter trial lenses designed for fitting corneas with keratoconus were placed on the right eye in order to determine the most appropriate contact lens. The goal of the fitting process was to determine the particular base curve, diameter, and peripheral curve system that would provide adequate lens centration and movement while still maintaining acceptable ocular health by lightly touching, or completely vaulting the area of corneal ectasia. After several trial contact lenses were placed on the eye, the following order was placed OD:
Material: Fluroperm 60
Base Curve: 52.75(6.39)
Power: -13.75 DS
Diameter: 8.8
Optic zone: 6.3
Secondary curve: 8.8
Tertiary curve: 12.50
Although this particular lens tended to decenter inferiorly, it was determined that this lens was a good “starting point” to begin the fitting process. The patient was counseled on the risks of PMD and was scheduled to return in 1 week for a dispensing of the ordered gas permeable lens.
Follow up #1
The patient returned on 23 July 2009 to have his gas permeable contact lens dispensed. He reported no changes to his vision or ocular symptoms. His visual acuities through his habitual soft toric disposable lenses were consistent with the previous examination (20/400 OD and 20/20 OS). The previously ordered lens with the following parameters was placed on the right eye and fit and vision were assessed.
Material: Fluroperm 60
Base Curve: 52.75(6.39)
Power: -13.75 DS
Diameter: 8.8
Optic zone: 6.3
Secondary curve: 8.8
Tertiary curve: 12.50
With the gas permeable lens in place, the patient’s vision improved to 20/20- OD. Fit assessment showed that the lens was decentered inferiorly, had minimal movement on blink, showed moderate apical touch inferiorly over the ectatic portion of the cornea, had small bubbles superiorly within the optic zone, and displayed minimal peripheral clearance. The patient reported poor comfort with the lens in place. It was determined that the lens would not be dispensed at this visit, and instead would be re-ordered with a steeper base curve to reduce the amount of corneal touch, a smaller optic zone to eliminate the bubbles within optic zone, and a steeper tertiary curve to reduce the amount of edge lift. The re-ordered lens would have the following parameters.
Material: Fluroperm 60
Base Curve: 53.50(6.31)
Power: -14.00DS
Diameter: 8.8
Optic zone: 6.0
Secondary curve: 8.7
Tertiary curve: 12.0
The patient was rescheduled to return in 1 week to dispense the new gas permeable lens.
Follow up #2
The patient returned on 4 August 2008 for the dispense of his new gas permeable lens OD. He, again, reported no changes to his vision or ocular symptoms. His visual acuities through his habitual soft toric disposable lenses were consistent with previous visits (20/400 OD and 20/20 OS). The previously ordered lens with the following parameters was placed on the right eye and the fit and vision were assessed.
Material: Fluroperm 60
Base Curve: 53.50(6.31)
Power: -14.00DS
Diameter: 8.8
Optic zone: 6.0
Secondary curve: 8.7
Tertiary curve: 12.0
With the gas permeable lens in place, the patient’s vision improved to 20/20- OD. Fit assessment showed that he lens was decentered inferiorly, had minimal movement on blink, showed a feather three point apical touch inferiorly over the ectatic portion of the cornea, had no bubbles within the optic zone, and had average peripheral clearance. The patient reported poor comfort with the lens in place. The lens was dispensed to the patient and he was scheduled for a contact lens progress evaluation two weeks later. The patient was educated on the importance of full and complete blinking throughout the day in order to adequately wet the lens and achieve sufficient lens movement. He was informed that lens wear should be slowly increased over the next two weeks in order to adequately become accustomed to the lens.
Follow up #3
The patient returned on 18 August 2008 for a two week follow up on the previously dispensed lens OD. He had been wearing the lens for two hours that day and had achieved an average wear time of 8 hours per day. The patient reported inadequate comfort throughout the day. “Systane” artificial tears had been utilized as needed in an attempt to improve comfort with no relief. The patient also reported “flare and glare” in the evening, and “ghosting” of images that occurred constantly with lens use.
The patient’s visual acuities through the GP lens OD and habitual soft toric disposable lens OS were consistent with the previous visit (20/20 OD, 20/20 OS). Fit assessment of the GP lens OD showed that the lens was riding inferiorly on the cornea. A feather three point touch, no movement on blink, and minimum peripheral clearance with lens seal off were also noted. When the lens was removed, the cornea displayed a mild compression ring where the lens had been sitting with a ring of grade 2 SPK at the lens border. Mild staining was also reported in the area of the cone apex.
The patient was advised to continue to wear the current GP for as many hours per day as possible to continue building up his wear time. The lens would be re-ordered with a larger overall diameter for better centration, a larger optic zone in order to reduce the symptoms of flare and glare, and a flatter peripheral system in order to reduce the risk of lens seal off and impression rings on the corneal surface. The re-ordered OD lens would have the following parameters.
Design: Dyna Z Intra-limbal
Material: Boston XO
Base Curve: 47.50(7.11)
Power: -8.75DS
Diameter: 10.8
Optic zone: 8.5
Peripheral Curve: 1.5 steeps flatter than base curve.
The patient was scheduled to return in 1 week to dispense the new GP contact lens OD.
Follow up #4-19
Over the following 10 months the patient’s contact lens parameters were repeatedly changed in an attempt to improve lens centration, eliminate symptoms of flare and ghosting, and improve movement on blink. Each of these lenses was a large diameter “Dyna Z Intra-limbal” lens design. The trial lenses were routinely dispensed to the patient at each visit for a week long trial. At the end of each trial period, it was determined that the lens provided inadequate corneal health and comfort. Each lens consistently provided the patient with 20/20 vision OD.