Osseous Dysplasias of the Temporal Bone: Introduction
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.
Fibrous Dysplasia
- External auditory canal stenosis
- Progressive conductive hearing loss
- Enlargement of the temporal bone
- Abnormal skin pigmentation
- Radiographic “ground glass” appearance.
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.
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.
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.
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.
The treatment for fibrous dysplasia is aimed at maintaining patency of the external auditory canal and cranial nerve conduits. For ear canal stenosis, wide meatoplasty is performed to restore an open channel and exteriorize entrapped skin. Although sarcomatous degeneration is rare for those with fibrous dysplasia, the estimated incidences are 0.4% in monostotic and polyostotic disease, and 4% in the McCune–Albright syndrome. Clinical features that suggest sarcomatous degeneration include pain, swelling, and radiographic evidence of bony destruction. The prognosis for malignant transformation is poor.
Osteopetroses
The osteopetroses are a group of inheritable metabolic bone disorders. They result in diffuse, dense sclerosis, and faulty bony remodeling. There are two forms: congenital and tarda. The congenital or lethal form is autosomal recessive, and manifests during infancy with pancytopenia secondary to obliteration of marrow spaces. Death due to hemorrhage, anemia, or overwhelming infection is common in infancy or childhood. The tarda or adult form is also known as Albers–Schönberg disease and is most commonly autosomal dominant. The adult form is benign and has a variable clinical course. Symptomatic patients present with problems that relate to bony overgrowth and foraminal stenosis. Hearing loss may be conductive or sensorineural owing to ossicular involvement or cochlear nerve impingement. The facial nerve function may be weak and spastic as a result of internal auditory canal narrowing. Other cranial nerve neuropathies may result from progressive stenosis of neural foramina.
The osteopetroses result from osteoclast dysfunction. Remodeled bone is faulty. Histologically, regions of endochondral ossification contain abnormal calcified cartilage. The osteopetrotic bone is immature, thick, dense, and brittle. This appearance gives rise to the names chalk or marble bone disease.
In osteopetrosis congenita, infants present with severe anemia and early visual problems secondary to optic nerve atrophy. Hearing loss often develops by childhood and tends to be conductive as a result of ossicular infiltration by osteopetrotic bone and exostoses. Temporal bone findings include external auditory canal stenosis, poor mastoid pneumatization, ossicular chain fixation, Eustachian tube narrowing, and stenosis of the petrous carotid and internal auditory canals. In adult-type osteopetrosis, patients suffer multiple cranial nerve palsies involving cranial nerves I, II, III, V, and VII. Facial nerve paralysis can be recurrent and results from narrowing of the internal auditory, and labyrinthine and vertical fallopian canals. Conductive or mixed hearing loss is also due to ossicular chain involvement. Sensorineural hearing loss may arise from otic capsule and internal auditory canal infiltration by osteopetrotic bone.
