A sequential approach to the management of an advanced case of pellucid marginal degeneration

Abstract

Purpose

We report a sequential approach in the management of a case with advanced pellucid marginal degeneration (PMD). The management was tailored based on the patient's corneal topography and degree of corneal astigmatism. Full thickness corneal transplants in cases of PMD carry a prolonged rehabilitation period and the risk of graft failure.

This case consisted of lamellar wedge resection followed by wavefront guided transepithelial photorefractive keratectomy (wTPRK) combined with accelerated corneal crosslinking (aCXL) in preparation for a cataract extraction with intraocular lens.

Observation

A 68-year-old gentleman with advanced PMD underwent phacoemulsification with toric intraocular lens (T-IOL) for the right eye and a lamellar wedge resection, followed by wTPRK combined with aCXL for the contralateral eye. The left eye underwent a sequential approach to regularize the cornea, minimize the higher order aberrations (HoA) and degree of corneal astigmatism. Effective visual rehabilitation was achieved with significant visual improvement.

Conclusion and importance

Although advanced PMD may have limited options for visual rehabilitation, a sequential stepwise approach may be considered in such cases, permitting a better quality with less invasive options.

1. Introduction

Pellucid marginal degeneration (PMD) belongs to a family of bilateral corneal ectatic disorders occurring between second and fifth decade of life. Most cases present simultaneously with keratoconus or as advanced forms of its appearance. Although no known etiology is identified to date, histopathological studies concluded that Bowman's layer was either abscent or exhibited focal breaks, and the stromal layer had increased presence of mucopolysacharides leading to progressive weakness.¹ Due to rarity of the condition, no large prospective epidemiological studies are available.

Treatment options for PMD is dependent on the degree of severity (early, moderate, advanced) and are divided into conservative or surgical options. Both options aims to improve the quality of vision. Before the progression of the disease, spectacles and the wide range of contact lenses are preferences that assist in achieving that objective. Jinabhai et al. et al. in his review divided the surgical approaches into four subcategories i) Structural/Mechanical interventions using intracorneal ring segments and corneal crosslinking, ii) Partial thickness interventions, iii) Full thickness interventions, iv) Toric Intraocular Lens Implantation, v) Combination of the above.² Of the challenging approaches is the partial lamellar wedge resection. This modification establishes a normal prolate cornea shape via removal of the diseased corneal stroma and approximation of the residual stromal edges however it carries the risk of intraoperative perforation, suture infiltrates, and double anterior chamber creation. Limited number of eyes with this approach are reported in literature. The advantage with this approach, lies in the globe remaining intact, avoiding the complications associated with allograft tissue transplant and those of long-term use of corticosteroids.³ We report a case of advanced PMD in the both eyes, where the patient underwent a toric intraocular lens (T-IOL) in the right eye and a lamellar wedge resection, followed by wavefront guided transepithelial photorefractive keratectomy (wTPRK) combined with accelerated corneal crosslinking (aCXL) in the left eye. His final step of his visual rehabilitation period consisted of phacoemulsification with toric intraocular (T-IOL) implantation.

2. Case report

A 68-year-old gentleman, presented to the cornea service with poor vision in both eyes. His corrected distance visual acuity (CDVA) in the right eye was 20/80 (+3.00/-9.00x70) left eye was counting fingers (CF) not correctable with spectacles. Refraction was not possible in the left eye due to the high degree of against the rule astigmatism. On examination of both eyes, he had a large angle of sensory exotropia (45 prism diopter), a clear cornea, left eye with peripheral thinning 3 mm from limbus extending 4–7 clock hours, corneal bulging superior to that area, deep anterior chamber, and a nuclear sclerosis cataract grade 3. His tomographic/topographic maps (Pentacam, OCULUS Optikgeräte GmbH, Germany) showed a crab claw pattern consistent with advanced PMD in the right eye, keratometry (K) readings of K₁ 34.0 diopters (D), K₂ 48.9D, and an astigmatism (AST) 15.0D, thinnest corneal thickness (TCT) 478 μm, and posterior float elevation +22. Left eye had a moderate to advanced PMD, K₁ 41D, K₂ 59.4D, an AST of 18 D, TCT 393um inferiorly, and posterior float elevation +132 (Fig. 1). Documented stability of his tomography and refraction were noted.

Fig. 1.

Tomographic map of the right and left eyes, demonstrating severe inferior steepening and the classic crab claw appearance.

2.1. Right eye surgical intervention

Limited options for correcting high astigmatism exist in the market. In addition to the above investigations he had a pre-operative biometry using the (IOLMaster500, Carl Zeiss Meditec AG, Jena, Germany). It was accepted to have a residual astigmatism in his case. He underwent a right uneventful cataract extraction, phacoemulsification, with toric posterior chamber intraocular lens, (AT TORBI 709 M, Carl Zeiss Meditec AG, Jena, Germany).

2.2. Left eye surgical intervention

Two years later he presented to the clinic seeking to improve vision in his left eye. He was contact lens intolerant and was refusing to wear it for vision correction. A contact lens over refraction was performed to determine the visual potential. The recorded CDVA with a contact lens of a vault 500, a steep skirt curve and a power of −15.00 spherical D improved it to 20/40. Knowing his background with a confirmed diagnosis of PMD, a very large graft would be needed, with a prolonged rehabilitation prior to his cataract removal. The survival of the graft would be questionable as the patient is known to be noncompliant to his follow up visits. After a thorough discussion with the patient on the options available, we proceeded with lamellar wedge resection. A careful preop localization of the ectatic area was outlined preoperatively by slit lamp examination, aided by the vascular markings at the limbus in combination with correlation of the topographic findings. The procedure was performed under local peribulbar anesthesia. The following treatment plan was performed with the help of a guarded diamond blade knife,4 mm away from the limbus, 2–3mm in width, documented corneal thickness on pachymetry was consistent with the topography reading, 80 % depth was dialed into the diamond knife. The strip of tissue was excised, paracentesis of 1.2mm was created to shallow the anterior chamber, flattening the cornea allowing for a better tissue proximation and titration of the by 10-0 nylon sutures (Fig. 2A-B). Sutures were removed after 3 months with significant reduction of the astigmatism was noted, post procedure corneal astigmatism of 5.8D (Fig. 3). To provide the patient with the best possible refractive outcome, 6 months post all suture removal, his documented CDVA was 20/40 with refraction of (+5.00/-10.00 x 171) he underwent wTPRK using the excimer laser (Amaris, Schwind eye-tech- solutions GmBH & Co.KG) to the central 5.50mm optical zone, and a maximum ablation depth of 82 μm addressing the high order aberrations of trefoil, coma, and spherical aberration combined with epithelium off aCXL (fig). The aCXL was performed after additional scrapping of the of the corneal epithelium to include the 9mm zone of the cornea and was socked with Innocross-Rriboflavin isotonic solution (riboflavin 5-phosphate (0.1 %) plus 20 % dextran T 500 in 2 mL syringes; (IROCInnocross AG, Zurich, Switzerland) and accelerated UVA irradiation (370 nm, 9 mW/cm² for 4 min, a 2.16 J/cm² dose of irradiation) under sterile conditions using the UVAmachine UV-X 1000 (IROCInnocross AG, Zurich, Switzerland). Five months post procedure he reported better quality of vision with CDVA 20/40 and refraction of +3/-5.00x171. He later underwent cataract extraction, phacoemulsification, with toric intraocular lens (T-IOL) (AT TORBI 709 M, Carl Zeiss Meditec AG, Jena, Germany). Effective visual rehabilitation was achieved with significant visual improvement to CDVA of 20/30.

Fig. 2.

A. Intraoperative photograph of the left eye demonstrating the advanced degree of the PMD. B. Immediate post-operative photo of the same eye.

Fig. 3.

Tomographic map of the left eye. Three months post removal of all sutures. Demonstrating a reduction of 12.8D in Keratometry readings on axial curvature and flattening of the previously steep cornea.

3. Discussion

Managing advanced PMD is a challenging aspect in a cornea practice. The modalities are usually conservative aiming to achieve stability and visual restoration using spectacles or contact lenses. Achieving a comfortable contact lens fitting sometimes is hard to reach, or not possible at all.⁴ Invasive approaches such as full thickness cornea transplant carries a high risk of graft failure, and it requires considerable surgical experience.⁵ There is no clear guidelines to address its treatment.

This case report of an advanced PMD displayed proper counseling and customization to the options available for improvement of the vision quality. Primarily by reshaping the cornea and regularizing its surface, followed by cataract surgery in PMD patients can be planned after normalization of the surface. Monofocal/T-IOLs are the preferred choice as diffractive IOLs induce more aberrations affecting the contrast sensitivity and reducing the quality of vision. The main challenge post lamellar wedge resection is the change induced to the anterior and posterior corneal power which is not proportionate. In order to improve the T-IOL power calculation we chose the true net power and the equivalent K-reading (EKR) in the central 4mm guided by the T-IOL selection using the SRK/T formula aiming for a more negative spherical equivalent (−1.50D).^6,7 Due to the severity of the condition in the left eye it was managed in the sequential approach described earlier. Initial surgical approach to address the structural deformity associated with the disease by a lamellar wedge resection, followed by a stepwise visual rehabilitation process. This consisted of wTPRK addressing the corneal irregularities and the higher-order aberrations combined with accelerated aCXL (9 mW/cm²) for 4 min. Multiple papers looked at each approach as a separate entity, but in our case we decided to combine all of them in a successive manner based on the overall progression of the patient's vision. The decision to combine the short duration aCXL results from studies that have shown the benefits of lasik Xtra with high myopic and hyperopic corrections.^8,9 This provides a protective biomechanical stability of the cornea and halt ectatic progression by inducing intrastromal cross-links in conjunction with the enhanced functional visual correction. It was not used to treat any corneal progression of the ectatic disease. Therefore, minimizing the increased risk of stromal haze and delayed epithelial healing when combining such procedures.¹⁰

Maccheron et al.¹¹ described a series with documented improvement following corneal wedge resection with lamellar dissection of the ectatic cornea. It is important that the lips of the resulting wound be thick enough to allow for safe suturing and opposition.¹² Busin et al.¹³ also described a similar approach with the use of suture-less penetrating corneal keratotomies almost 90° away from the site of dissection. There are other papers published that demonstrated a similar approach with good outcome. The purpose of this important step is improving corneal shape, regularizing the astigmatism, maintain optical clarity without the need to introduce a donor tissue or perform a full thickness procedure.

It requires several months for visual rehabilitation, and the proximity of the sutures to the conjunctiva can invite corneal blood vessels into the area necessitating the need for them to be removed sooner than normal lamellar grafts.¹⁴ Also, visual rehabilitation with the use glasses or contact lenses can take place soon after. The leading cause of optical errors in PMD is the higher order aberrations (HoA) induced by the corneal irregularity and against the rule astigmatism. Few studies discussed that this condition in PMD.15, 16, 17 In facilities that offer excimer laser vision correction that can be used to address the higher order aberrations as in our case. aCXL demonstrated efficacy in halting the progression of PMD, even though the end visual outcome did not have significant change.^18,19

In summary, although advanced PMD may have limited options for visual rehabilitation. A sequential stepwise approach may be considered, permitting a better quality of vision and life with less invasive options.

Patient consent

Signed consent to publish case details has been obtained from the patient.

Funding

None.

Authorship

All authors attest that they meet the current ICMJE criteria for authorship.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.