Pellucid marginal corneal degeneration in a teenager

Abstract

A teenage boy presented to the clinic complaining of poor vision in both eyes. His best-corrected visual acuity was 20/25. The slit lamp examination revealed vertically oval corneas, crescentic peripheral inferior corneal thinning and ectasia in both eyes. Corneal topography showed against-the-rule astigmatism with mean keratometry (Km) of 45.1 and 45.2 dioptres in the right and left eyes, respectively. High-resolution optical coherence tomography corroborated the clinical findings, and the patient was diagnosed with pellucid marginal corneal degeneration (PMD). Collagen cross-linking was performed in the right eye. Left eye cross-linking is awaited. Although PMD is typically seen in middle-aged (third-fourth decade) patients, it can also occur at earlier ages. Collagen cross-linking can be considered to prevent the progression of the condition.

Background

Pellucid marginal corneal degeneration (PMD) is a bilateral, non-inflammatory, peripheral corneal thinning disease characterised by a thinning peripheral crescentic band, typically in the inferior cornea.¹ The band-shaped ectatic zone, approximately 1–2 mm from the limbus, lies above the point of the maximum corneal thinning. The condition is more common in men in their second to fifth decades of life.² There are some reports that associate PMD with keratoconus and vernal keratoconjunctivitis.³ We report a case of PMD in a teenage boy with no other ocular comorbidities.

Case presentation

A boy in his early teens presented to the clinic with reports of impaired vision noted over the past 5 years. At presentation, the best-corrected visual acuity (BCVA) was 20/20 with −5 cylinder at 70° in the right eye and 20/25 with −5.75 cylinder at 140° in the left eye. The central Hirschberg corneal reflex test showed that the ocular alignment was normal. Slit lamp examination revealed slightly vertical oval corneas, peripheral corneal thinning 1–2 mm from the limbal region inferiorly extending from 5 to 7 o’clock, and ectasia (figure 1A, B) in both the eyes. The remaining anterior segment findings were unremarkable. The intraocular pressure was 10 mm and 9 mm Hg in the right and left eyes, respectively. The posterior segment was normal in both eyes. High-resolution optical coherence tomography with Optovue (RTVue XR Avanti, Optovue) showed the peripheral corneal thickness of 427 microns and 383 microns on the line scan passing through the 7 o’clock axis in the right and left eyes, respectively (figure 1C, D). Corneal topography (Oculyzer, Alcon, Fort Worth) revealed against-the-rule astigmatism. A deranged corneal thickness spatial profile and percentage thickness increase were evident in both eyes from the Belin-Ambrosio-enhanced ectasia display (figure 2A–D). In the central 8 mm of the cornea of the right eye, the mean Km and thinnest pachymetry were 45.1 dioptres (K1:43.2 and K2:47.1D) and 471 microns, respectively (figure 2A). The left eye showed a Km and thinnest pachymetry of 45.2 dioptres (K1:43.0 and K2:47.3D) and 475 microns, respectively (figure 2C) in the central 8 mm of the cornea. Corvis ST (Oculus Corvis ST, Scheimpflug Technology; Wetzlar, Germany) was used to show the corneal biomechanics of both eyes (figure 3A, B). Collagen cross-linking (CXL) was done using the accelerated Dresden protocol. The epithelium was debrided with an 8 mm ring well after applying proparacaine 0.5% eye drops for 30 s. The ring was slightly displaced inferiorly to include the area of thinning. The cornea was primed with an isotonic solution for 20 min. The site of peripheral thinning was exposed to ultraviolet (UV) light for 10 min with 9 mv/cm² energy settings. A small piece of surgical sponge was placed at the limbus to act as a physical barrier to the limbal region. A bandage contact lens was placed at the end of the procedure. The patient was started on topical moxifloxacin 0.5% for 1 week, sodium hyaluronate 0.1% four times per day for a month and loteprednol etabonate 0.5% tapering weekly. The bandage contact lens was removed after 1 week. The postoperative course was uneventful. left eye cross-linking was advised, which the patient wanted a few months later.

Figure 1.

(A–D) Slit lamp photographs showing peripheral corneal thinning in the right (A) and left eye (B). The high-resolution anterior segment optical coherence tomography illustrates the peripheral corneal thinning in both eyes (C and D).

Figure 2.

(A–D) Corneal tomography illustrating the ‘against the rule’ astigmatism with a mean keratometry of 45.1 (A), corneal thickness spatial profile (CTSP) and percentage thickness increase (PTI) in the right eye (B). The mean keratometry was 45.2 dioptres in the left eye (C), and CTSP and PTI are depicted in D.

Figure 3.

(A, B) Corneal biomechanics of the right eye showing deranged corvis biomechanical index (CBI), Belin-Ambrosio enhanced ectasia total deviation index (BAD D), and tomographic biomechanical Index (TBI) in the right eye (A). The left eye shows derangement in TBI (B).

The patients’ mother had normal eyes. She had a corneal topography with an average Km and thinnest pachymetry of 45.3 dioptres (K1 of 44.9D, K2 of 45.6D) and 566 microns in the right eye and 44.9 dioptres (K1 of 44.5, K2 of 45.4D) and 518 microns in the left eye, respectively.

Discussion

PMD is a rare form of ectasia that progresses slowly compared with keratoconus, which is a more common ectatic disease. In a recent 8 year (2012–2018) study at our centre, the prevalence of PMD and keratoconus was 0.02% and 0.62%, respectively.⁴ In the same study, the mean age of patients with PMD was 37.91±13.19 years.⁴

The aetiopathogenesis of PMD is unknown. Electron microscopy of the PMD-affected cornea revealed abnormally spaced collagen fibres with a periodicity of 100 nm to 110 nm, similar to keratoconus. Comparatively, a normal cornea shows a periodicity of 60 nm to 64 nm.⁵ Many authors have proposed that PMD could be a peripheral form of keratoconus.⁶ Moreover, keratoconus and PMD can coexist in some patients with allergic eye diseases. Although there is no evidence of genetic inheritance for PMD, Santo and Kara-Jose have described topographical evidence of corneal ectasia in asymptomatic family members of patients with classic PMD.⁷ Furthermore, a few studies have reported high astigmatism in asymptomatic family members of these patients.⁸ In a national survey of PMD in Japan, the youngest patient was 18-year old.⁹ In a study by Al-Torbak, the youngest patient with PMD who underwent keratoplasty was 19 years.¹⁰ Sridhar et al reported an 8-year-old child with concomitant PMD and keratoconus in the affected eye.³ In our recent study on the clinical and demographic distribution of PMD, from 2012 to 2020, a patient aged 9 years had concomitant PMD and keratoconus with a history of allergic eye diseases.⁴

The adolescent patient described here had PMD in both eyes without any other ocular comorbidities. There was no history of excessive rubbing of the eyes and no ocular surface features suggesting allergic eye diseases in either eye. We could only evaluate the mother of the patient, who had normal topography and no clinical features associated with ectatic diseases.

PMD is a slowly progressive disease. CXL effectively arrests the progression of corneal ectasias, including PMD. Compared with Scheimpflug imaging, the anterior segment optical coherence tomography aids in assessing ectasia and pachymetry in the peripheral location. CXL in PMD needs careful assessment and decentration of the UV-A beam to achieve the required effect at the peripheral cornea and avoid limbal tissue damage. Utmost care was taken while focusing the beam on the peripheral thinning area and ectasia.

Spadea et al demonstrated the safety and efficacy of CXL in halting the progression of PMD.¹¹ In an adult with progressive PMD, Hassan et al reported the stabilisation of PMD and improvement in BCVA after CXL.¹² The potential complications of CXL may include haze, corneal melt and infection.¹³ In our recent study, 5.25% of patients diagnosed with PMD underwent CXL without complications.⁴

In view of symmetrical corneal findings, the same duration of onset of reports in both eyes, absence of other coexisting ocular comorbidities and no family history, it is speculated that the peripheral ectasia could be inherent in this patient.

Learning points.

• Although pellucid marginal corneal degeneration (PMD) is predominantly documented in adults, it can occur in children as well.

• Anterior segment optical coherence tomography and Scheimpflug imaging are important diagnostic modalities.

• Collagen cross-linking can prevent the progression of PMD.

Ethics statements

Patient consent for publication

Consent obtained from parent(s)/guardian(s)