Chapter 64. Osseous Dysplasias of the Temporal Bone

Patients with osseous dysplasias of the temporal bone, notably, fibrous dysplasia, Paget's disease, osteopetroses, and osteogenesis imperfecta, present with hearing loss and external auditory canal obstruction that result in infection, lower cranial neuropathies, and temporal bone deformation. Differentiation among these entities is greatly helped by using coronal and axial high-resolution computed tomography (CT) imaging of the temporal bone and skull base. Outer, middle, and inner ear structures are detailed and foraminal stenoses are identified. Bone mineralization density appearance is the single most important imaging feature to secure a diagnosis.

Essentials of Diagnosis

• External auditory canal stenosis
• Progressive conductive hearing loss
• Enlargement of the temporal bone
• Abnormal skin pigmentation
• Radiographic “ground glass” appearance.

General Considerations

Fibrous dysplasia is perhaps the most common benign fibro-osseous disorder of the temporal bone. This poorly understood entity has three major classifications: (1) monostotic, (2) polyostotic, and (3) the McCune–Albright syndrome.

The monostotic variant is the most common variety, accounting for approximately 70% of all cases, and is seen late in childhood. The disease may enter a dormant phase in puberty. Polyostotic disease manifests as multiple bony lesions and often has long bone involvement. The active phase of the disease extends into the third and fourth decades. The McCune–Albright syndrome affects mostly females and is characterized by polyostotic fibrous dysplasia with cutaneous hyperpigmentation, and endocrinopathy, often manifested as precocious puberty. Within the skull base, the temporal bone is involved approximately 24% of the time.

Pathogenesis

The radiographic appearance of fibrous dysplasia reflects the erosion of cortical bone by fibro-osseous tissue in the medullary cavity. The cortical bone is thinned by medullary fibrous tissue that is vascular, compressible, and weak. Histologically, there are interspersed regions of predominantly soft tissue or bone. Soft areas are abundant in collagen, and occasionally contain cysts. Areas of intermediate consistency are populated by fibroblasts.

Clinical Findings

Symptoms and Signs

Common clinical manifestations of fibrous dysplasia of the temporal bone include external auditory canal stenosis and/or ossicular chain erosion, progressive hearing loss, most commonly conductive (∼80%), and increased temporal bone size presenting as painless postauricular swelling. The dysplastic process may entrap skin within the external auditory canal, resulting in cholesteatoma formation. Uncommonly, facial nerve paralysis may ensue from infected or erosive cholesteatoma.

Imaging Studies

The CT appearance of fibrous dysplasia may have several radiographic patterns: pagetoid, sclerotic, and cystic. Pagetoid (>50%) is characterized by a mixture of dense and radiolucent areas of fibrosis with bone expansion. Sclerotic (∼25%) is homogeneously dense with bone expansion. Cystic (∼20%) has either spheric or ovale lucent regions with dense boundaries.

Treatment & Prognosis

The treatment for fibrous dysplasia is aimed at maintaining patency of the external auditory canal and cranial nerve conduits. For ear canal ...