Frontal sinus mucocele in association with polyostotic fibrous dysplasia: A case report

Abstract

Fibrous dysplasia is an uncommon developmental anomaly of bone wherein normal bone marrow is substituted and subsequently expanded by fibro-osseous tissue. Its association with mucocele formation in the paranasal sinuses is extremely rare. We present a case report of a patient with polyostotic fibrous dysplasia with frontal sinus mucocele formation, who underwent endoscopic drainage and marsupialisation via a transnasal approach.

Introduction

Frontal sinus mucoceles in association with fibrous dysplasia are an extremely rare entity. Paranasal sinus mucoceles are benign, expansile lesions lined with epithelium that develop as a result of sinus outflow tract obstruction. This obstruction may be caused by facial trauma, previous surgery, inflammation and sinonasal tumours. Such mucoceles, especially in the frontal sinus, must be considered as an important differential diagnosis when patients with craniofacial fibrous dysplasia present with proptosis, visual disturbance and periorbital swelling. This report describes a patient with underlying polyostotic fibrous dysplasia, who presented in such a manner and was subsequently treated by endoscopic drainage and marsupialisation of the mucocele via a transnasal approach.

Case report

An 18-year-old girl with no significant past medical or surgical history presented to the emergency department with a two-week history of right upper eyelid oedema, pain and tenderness. She was treated by her local general practitioner with oral antibiotics which reduced the pain and tenderness; however, the swelling continued to worsen. The patient gave a history of right-sided scalp swelling, which started five years ago around the onset of puberty. She also complained of intermittent diplopia but no other rhinological symptoms.

Nasoendoscopic examination of the patient was largely unremarkable. However, there was a large fluctuant swelling in the right supraorbital region, as well as a large bony swelling over the right side of the scalp. No abnormal cutaneous pigmentation was noted. In view of her complaints of intermittent diplopia, an ophthalmology consultation was sought, which revealed normal visual acuity, eye movements, visual fields and a healthy optic disc with no evidence of optic neuropathy.

A subsequent computed tomography (CT) scan of the paranasal sinuses and orbits showed extensive frontoparietal fibrous dysplasia with the typical ground-glass appearance involving the right orbital frontal plate, extending into the right greater wing of the sphenoid bone. There was no evidence of optic nerve compression or narrowing of the superior and inferior orbital fissures. A 37 mm cystic lesion was also seen at the superolateral aspect of the right orbit, eroding the orbital roof, as well as depressing and flattening the right globe (Figure 1). She was diagnosed as having a right frontal mucocele secondary to craniofacial polyostotic fibrous dysplasia.

Figure 1.

(a) Right frontoparietal polyostotic fibrous dysplasia. (b) Involvement of the greater wing of the sphenoid without optic nerve compression. (c) Mass effect of mucocele on right globe. (d) Right frontal mucocele.

The patient subsequently underwent endoscopic drainage and marsupialisation of the frontal sinus mucocele, assisted by image guidance. A modified Lothrop procedure was also performed to improve access to the superolateral aspect of the mucocele. Her postoperative course was uneventful, with resolution of the supraorbital swelling. Review of the patient six months after surgery revealed no evidence of mucocele recurrence.

Discussion

Fibrous dysplasia is an uncommon bony disorder with an unknown aetiology. It can be broadly classified into two forms – monostotic and polyostotic. ¹ The monostotic form is more common, tending to involve single bones in the extremities, and the bony ribs. It generally ends with the onset of puberty. The polyostotic form affects multiple bones ipsilaterally and is less common. Hallmarks of the polyostotic form include a history of recurrent spontaneous fractures, and a shepherds-crook deformity with an intracapsular femoral neck fracture. It also tends to progress even after puberty. When the polyostotic form is associated with cutaneous pigmentation and precocious puberty, it is termed McCune-Albright Syndrome. Craniofacial involvement is common in the polyostotic form, and may involve the frontal, sphenoid, ethmoid, parietal, temporal, and occipital bones, in descending order of frequency. ² Involvement of these bones results in thickening, as the diploic space of the calvarium is expanded by fibro-osseous tissue. This not only causes cosmetic deformity, but more significantly may also narrow important neural foramina, resulting in cranial nerve deficits. For example, fibrous dysplasia affecting the internal auditory canal may cause hearing loss and/or facial palsy; fibrous dysplasia affecting the optic canal may lead to visual compromise. CT scans are ideal for assessment of craniofacial fibrous dysplasia. Not only do such scans display the extent of the disease with its characteristic ground-glass appearance, but they also provide excellent bony definition of key neural foramina, and aid in surgical planning. These scans, together with a complete neurological examination, are essential in decision making with respect to timing surgical treatment.

Paranasal sinus mucoceles are a result of sinus ostia obstruction. There are various causes of obstruction, such as facial trauma, previous sinus surgery, sinonasal tumours, and in this case, fibrous dysplasia. ³ They are benign, consisting of trapped mucin in an epithelial-lined lesion. As mucin secretion continues and sinus ostia obstruction persists, the mucoceles grow, expand and exert a mass effect on the surrounding bone, resulting in bony remodelling and expansion via secretion of prostaglandins and collagenases. Occasionally, they may compress vital structures such as the orbit or optic nerve. Such mucoceles most commonly arise in the frontal sinus, followed by the ethmoid, sphenoid, and maxillary sinuses. CT scans generally show an expansile homogenous lesion, with surrounding bony resorption/remodelling. ⁴ Magnetic resonance imaging (MRI) is a good adjunct to CT scans, especially in assessing the relationship of soft-tissue structures to the mucocele. In addition, MRI may be able to differentiate paranasal sinus mucoceles from other soft-tissue neoplasms. The signal intensity of paranasal sinus mucoceles both on T1- and T2-weighted imaging is variable and dependent on the amount of water/mucin in the mucocele. ⁵ However, most tend to be hyperintense on T2-weighted imaging because of their large water content. In our patient, the radiologist was certain that the lesion seen was a mucocele, hence a subsequent MRI scan was not performed.

The incidence of frontal sinus mucoceles in patients with craniofacial fibrous dysplasia is extremely rare, with only a few cases reported in the literature. ⁶ Most of the documented cases were treated via an open craniofacial approach.^6,7 Wie et al. ⁸ reported on the first case of endoscopic transnasal drainage and marsupialisation of a craniofacial fibrous dysplasia-associated frontal sinus mucocele in 2015. To our knowledge, ours is only the second reported similar case which was treated purely endoscopically. The aetiology of these mucoceles are all similar – craniofacial involvement with subsequent obstruction of the sinus outflow tract by the disease process of fibrous dysplasia. The impact of fibrous dysplasia, and subsequent mucocele formation varies from patient to patient, as disease processes are never entirely identical. Isolated mucoceles in the sphenoid sinus are associated with headaches, and in rare cases may involve the third, fourth, fifth and sixth cranial nerves and even the pituitary gland. Mucoceles in the spheno-ethmoidal region tend to cause visual disturbances secondary to optic neuropathy, which in turn is caused by mass effect, ischaemia or inflammation of the optic nerve. Mucoceles in the fronto-ethmoidal region tend to cause mass effect on the orbit, resulting in proptosis, diplopia and periorbital swelling. Our patient as described above had a frontal mucocele with periorbital swelling. Left alone, it would have continued to enlarge, causing further mass effect on the globe with consequent ophthalmic manifestations.

Summary

In summary, craniofacial fibrous dysplasia with frontal mucocele formation is an extremely rare entity. The formation of a frontal sinus mucocele is most likely due to occlusion of the outflow tract by the bony dysplastic process. With advances in endoscopic sinus surgery, drainage and marsupialisation via a transnasal route is now a feasible option, avoiding the morbidity associated with an open craniofacial approach.