Fibrous Dysplasia of the Temporal Bone: Imaging Characteristics and Neurotologic Considerations

Introduction: Fibrous dysplasia (FD) is a progressive, often indolent bone disorder in which normal bone marrow is replaced by a mixture of fibrous tissue and woven bone. It can potentially involve the long bones, pelvis, and craniofacial bones, either as monostotic or polyostotic disease. FD of the skull most commonly involves the frontal and sphenoid bones. Isolated FD of the temporal bone is relatively rare.

Case Report: A 15-year-old boy presented with several weeks of progressive left-sided headaches. He was neurologically intact including facial nerve, hearing, and vestibular examinations. Imaging revealed an extradural left middle fossa lesion appearing to arise from the bones of the inner ear. Temporal bone CT demonstrated that the lesion abutted the geniculate ganglion, the labyrinthine and tympanic segments of CN VII, the superior semicircular canal, and the petrous carotid. The patient was taken to surgery, where radical resection was limited by proximity to the facial nerve, which was adjacent to the field but not compressed. The diagnosis of FD was confirmed by pathology. Postoperatively, the patient developed a delayed House-Brackmann 3 facial droop which was self-limited.

Discussion: The etiology of FD is a somatic missense mutation in the gene GNAS1, which encodes the alpha subunit of the stimulatory G protein-coupled receptor Gsα. The mutation leads to constitutive, ligand-independent activation and accumulation of cAMP, similar to the effect of continuous parathyroid hormone stimulation.

Studies of the natural history of FD have suggested that the disease becomes inactive after puberty, although as many as one third of patients have recurrence in adulthood. Postpubertal disease progression is observed more commonly in the polyostotic rather than the monostotic disease variant. The incidence of malignant transformation is reported at 0.4%, although sarcomatous transformation has been strongly associated with radiation treatment.

Radiologically, three patterns of FD have been described: pagetoid (ground-glass), the most common; sclerotic; and cystic, the least common (10% of cases) and of which the present case is an example. Retroauricular swelling and hearing disturbances are the most common presenting complaints in patients with temporal bone FD. A recent temporal bone FD case series reported eight of nine patients presenting with conductive or sensorineural hearing loss. Radical excision is generally recommended when technically and cosmetically feasible to minimize the risk of disease recurrence. Recurrence rates of 25% have been reported for subtotal resection.

Conclusions: Temporal bone fibrous dysplasia can involve the vestibular apparatus, organs of hearing, and the cranial nerve VII-VIII complex. Delayed facial nerve palsy after decompression can have a favorable prognosis. Radical resection may not be necessary for a good outcome.