Epibulbar osseous choristoma

Abstract

Osseous choristoma constitutes the rarest category of choristoma affecting pediatric ocular regions. It predominantly composed of ectopically ossified tissue, which can involve anywhere in the eye and typically manifest as isolated anomalies. This study presents the case of an 8-year-old male patient diagnosed with epibulbar osseous choristoma. The choristoma exhibited significant adhesion to the superficial sclera, as corroborated by comprehensive diagnostic imaging, encompassing both macroscopic and pathological analyses. This case contributes to a deeper understanding of the lesion’s primary localization, pathological features, and etiological mechanisms, thereby informing differential diagnosis and surgical treatment approaches.

Introduction

Ocular choristomas are congenital proliferative lesions characterized by the ectopic growth of differentiated and mature normal tissues. These lesions are classified into simple or complex choristomas based on the number of germ layers involved. Furthermore, they can be subcategorized according to the types of tissues present, including dermoid, dermolipoma, lacrimal gland choristomas, cartilaginous choristomas, and osseous choristomas [1]. Among these subtypes, dermoids are the most prevalent, whereas osseous choristomas are exceedingly rare. Osseous choristomas constitute approximately 0.1% of all conjunctival tumors and represent 1.7-5% of all ocular choristomas, noted for their low occurrence rate [2, 3]. The appearance of osseous choristoma is nonspecific, and it is easy to be misdiagnosed without adequate preoperative examination. Consequently, definitive diagnosis hinges on histopathological analysis, which reveals mature bone tissue. Osseous choristoma is relatively stable, but potential complications may arise from tumor enlargement, mechanical irritation, recurrent inflammation, or mass effect-induced tissue compression [4]. Surgical removal of the mass is the only means of treatment [3]. However, in most case reports, patient examination data are relatively simple and fail to provide a complete overview of the pathogenesis. This study presents a detailed case of an osseous choristoma closely adhered to the superficial sclera.

Case description

An 8-year-old male patient presented with a superotemporal bulbar conjunctival mass in the right eye. The mass was incidentally noted by his parents three months before the presentation, with no reported changes in size or associated symptoms. The patient experienced no discomfort in the affected eye. The parents denied any history of ocular medical conditions. His visual acuity was 20/20 bilaterally, with normal intraocular pressures. A preoperative slit-lamp examination identified a pale-yellow nodule, approximately 5 mm in diameter, situated beneath the bulbar conjunctiva on the upper temporal side of the right eye. The nodule was hard, mildly congested, with clear borders, immobile, and non-tender (Fig. 1A-B). There was no evidence of proptosis, and the eye movements were unrestricted. The posterior segment of the right eye appeared normal (Fig. 1C). Computed tomography (CT) scan of the right eye revealed a crescent-shaped calcified nodule on the temporal side, well adhered to the sclera (Fig. 1D). In addition, to exclude hypercalcemia, sarcoidosis, and hypervitaminosis, laboratory investigations were conducted. Serum calcium (2.35 mmol/L), phosphate (1.20 mmol/L), 25-hydroxyvitamin D (65 nmol/L), and angiotensin-converting enzyme (42 U/L) were within normal ranges. The lesion was therefore considered an isolated anomaly. A diagnosis of epibulbar osseous choristoma was made based on the clinical manifestation and CT results. Surgical excision was performed to obtain histopathological confirmation and prevent complications. This decision was based on preoperative imaging demonstrating firm scleral adhesion of the lesion. During surgery, the mass was found to be tightly attached to the superficial sclera without involving the muscles (Fig. 2A). The removed mass was hard and irregularly rounded, with a small indentation on the inner surface, and overall resembled a cap, measuring approximately 9 mm × 7 mm (Fig. 2B-C). Histopathological examination revealed mature cortical bone encased in fibrous connective tissue, with proliferative subconjunctival fibrous connective tissue surrounding the mass (Fig. 3A-B). The sutures were removed one-week post-surgery. During a 6-month follow-up period, the patient maintained good ocular surface integrity with no signs of recurrence.

Fig. 1

Preoperative examination. A-B: Slit-lamp examination showed the mass located in the superotemporally of the right eye. C: B-mode Ultrasound of the right eye at 50 Hz revealed no notable abnormalities. D: CT scan images showed a crescent-shaped calcified nodule on the lateral wall of the right eyeball, clearly demarcated with no involvement of the extraocular muscles (red arrow)

Fig. 2

Intraoperative appearance of the mass. A-B: The mass exhibited characteristics similar to bone, with a cap-like appearance, approximately 9 mm × 7 mm

Fig. 3

Histopathology of the excised mass. A: The tumor contains a well-defined mature cortical bone within which acellular sequestrum is seen (arrow), encased by a thin fibrous connective capsule. B: The periphery of the tumor consisted of fragmented fibrous connective tissue

Discussion

In 1863, ophthalmic pioneer von Graefe first documented an ocular choristoma [5]. Beckman and Sugar coined the term “Osseous choristoma” in 1964, a term still in use [6]. By 2024, a total of 76 cases with epibulbar osseous choristoma have been reported, mostly as sporadic case reports. Researchers like Gayre et, al [7] and Shields et, al [8] have analyzed small cohorts of osseous choristomas, delineating several shared traits. Osseous choristoma is typically congenital and the age of onset ranged from 6 months to 38 years old, with the mean age of 11.6 years old. Females had higer predominance (66.7%) based on historical case series [9]. Approximately 76% of these cases occur in the right eye and 74% are located above the temporal side of the eyeball. It can also affect the eyelids [10], tarsal plate [11], outer canthus [12], and choroid [13]. The lesion at the eyeball is situated at the level of Tenon’s capsule or the external scleral layer, often tightly adhering to the superficial sclera or conjunctiva. It may occasionally involve the ocular muscles, predominantly the lateral rectus muscle [14]. The condition of osseous choristoma is generally stable, exhibiting no growth or only gradual growth during puberty. Most patients report no discomfort, though some seek treatment for symptoms of ocular surface irritation or for aesthetic reasons. Post-removal, osseous choristoma shows a low recurrence rate with no documented cases of metastasis or malignant transformation [15, 16]. Our case shared demographic similarities with the pediatric osseous choristoma reported by Khan et al., [17] including male patient and histological confirmation of mature bone. However, the lesion in our patient was located in the superotemporal conjunctiva, whereas theirs was at the lateral canthus, highlighting the different anatomical locations of these ectopic lesions. Recent studies highlighted the histopathological diversity of epibulbar choristomas. Angus et al. [18] described complex variants containing mixed bone and cartilage, while Singh et al. [19] reported odontogenic types with tooth-like structures. Yu et al. [20] further identified cartilaginous subtypes showing pigmented cystic changes. In contrast, our case exhibited exclusively mature lamellar bone without heterologous elements, aligning with classical osseous choristoma. This spectrum underscores the importance of detailed histopathological evaluation to guide classification.

The etiology of osseous choristomas encompasses multiple hypotheses yet lack robust evidence. Most scholars contend that ectopic ossification stems from the abnormal activation of multipotent mesenchymal cells [21]. Furthermore, some researchers describe osseous choristomas as scleral ossicles, advancing two hypotheses based on species evolution and human anatomical development: osseous choristomas may be either atavistic remnants of a scleral ossicle or results of congenital maldevelopment of the zygomatico-frontal suture [7, 8]. Kim and Henderson analyzed previously reported cases of intraocular osseous choristomas, proposing that early hormonal shifts, intraocular inflammation, ocular trauma, and disturbances in calcium and phosphorus metabolism might contribute to the formation of intraocular bone tissue [13]. Their findings potentially elucidate why osseous choristomas predominantly affect females and display variability in incidence location and randomness, yet they do not clarify why osseous choristomas are primarily located above the temporal side. Shield et al. [8] reviewed eight cases of epibulbar osseous choristoma and, upon consulting anatomical references, noted that osseous choristomas frequently develop above the temporal side near the zygomatico-frontal suture, thereby supporting the theory that osseous choristomas derive from congenital maldevelopment of this suture, implying that the bone tissue resembles human “Wormian bones” or “the pterion ossicle” during the developmental phase. Additionally, given the low prevalence of osseous choristomas and the absence of an evolutionary requirement for scleral ossicles in humans, they dismiss the notion that osseous choristomas are evolutionary remnants of scleral ossicles. Regrettably, although these theories provide some insight into the gender preference and diversity of osseous choristomas’ locations, no current research explains why osseous choristomas more frequently occur in the right eye.

Osseous choristomas may present in yellow, pink, or white hues. The differential diagnosis should encompass dermoid, dermolipoma, epithelial inclusion cysts, orbital fat prolapse, ectopic lacrimal gland, papilloma, scleral calcific plaques, and complex choroidal tumors, among others. Furthermore, systemic diseases that alter calcium and phosphate metabolism, leading to local tissue calcification and ossification such as hypercalcemia, sarcoidosis, and hypervitaminosis, must also be considered. Osseous choristomas typically occur sporadically or in isolation, but they may also coexist with various syndromes such as Coat’s disease [2], organoid nevus syndrome, and peripapillary hypopigmented fundus lesions [22].

Due to the non-specific appearance and clinical presentation of osseous choristomas, histopathological examination identifying mature bone tissue serves as the definitive diagnostic criterion [23]. If the choristoma is small, exhibits no progressive growth, and does not impact appearance or vision, intervention may be unnecessary. However, if the tumor significantly alters appearance, invades the cornea inducing astigmatism, or poses a risk of causing deprivation amblyopia, surgical removal might be warranted. Preoperative CT scans assist surgeons in evaluating the tumor’s nature and adhesion level, thereby aiding in determining the extent of lesion removal. While existing literature reports no cases of osseous choristomas with deep scleral adhesion or postoperative globe perforation, our surgical team suggested for preoperative preparation of a scleral graft to avert iatrogenic globe perforation.

This case provides new insights of epibulbar osseous choristoma. Notably, the tight scleral adhesion observed in our pediatric patient represented an underreported anatomical characteristic of osseous choristomas. To our knowledge, this is the first study to systematically exclude metabolic etiologies in pediatric epibulbar lesions, establishing a practical diagnostic framework for similar cases. The surgery preserved the fibrous capsule while employing amniotic membrane grafting achieved.