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 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.

Essentials of Diagnosis

• Conductive or sensorineural hearing loss
• Cranial neuropathies
• Facial dysfunction.

General Considerations

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.

Pathogenesis

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.

Clinical Findings

Symptoms and Signs

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.

Imaging Studies

Temporal bone CT findings in osteopetrosis congenita are notable for a small middle ear cavity with normal ossicles, obliteration of the mastoid antrum, and normal-appearing otic capsule.

Temporal bone CT findings in osteopetrosis tarda are remarkable for a diffusely chalky, thickened cranial vault. Stenosis of neural foramina, encroachment of pneumatic spaces, infiltration of ossicles, and involvement of the otic capsule are other findings.

Treatment & Prognosis

There is no effective medical therapy for the osteopetroses, so limited surgical intervention may be indicated to decompress cranial canals and foramina. Of note, conductive hearing loss resulting from osteopetroses may be caused by either direct bony ossicular infiltration or epitympanic fixation. Treatment of conductive hearing loss by ossiculoplasty may be technically difficult because of dense middle ear bony disease and footplate abnormalities. Nonsurgical therapy with hearing aid rehabilitation should be considered before surgical intervention. It may be necessary to perform surgery to enlarge the external auditory canal to accommodate a hearing aid. Surgical decompression of the acoustic nerve for stabilization of sensorineural hearing loss is unproven.

Facial nerve dysfunction generally presents with acute and recurrent episodes of facial palsy. Presurgical planning with a fine-resolution temporal bone CT scan can delineate stenotic sites for decompression.

Essentials of Diagnosis

• External auditory canal stenosis.
• Mixed hearing loss.
• Facial nerve dysfunction.
• Radiographic “cotton wool” appearing lesions.

General Considerations

Paget's disease, or osteitis deformans, is a disorder of excessive bone remodeling, primarily of the axial skeleton. Most cases are sporadic, but up to 15% are inherited in an autosomal pattern. The disease tends to occur after the fifth decade. The diagnosis is often made during evaluation for skeletal pain or incidentally on routine radiography.

Pathogenesis

The development of Paget's disease is believed to be due to a combination of genetic and environmental factors. Findings of inclusion bodies in osteoclasts resembling paromyxoviruses have suggested a viral etiology although recent evidence has been conflicting. Inherited forms of Paget's disease have been associated with mutations on chromosome 5 as well as chromosome 10. The histologic pattern in Paget's disease is one of alternating waves of osteoclastic and osteoblastic activity. Bone remodeling activity results in haphazard bony resorption followed by deposition of structurally weakened, demineralized cancellous bone. The early phase of the disease is dominated by bone resorption, which is seen as lytic lesions. The marrow space subsequently fills with fibrovascular tissue, which later undergoes sclerosis. Multifocal areas of lysis and sclerosis within the temporal bone and cranial base are seen.

Temporal bone findings in Paget's disease are notable for a tortuous external auditory canal, constriction of the middle ear cleft, bony changes of the ossicular chain, and demineralization of the otic capsule. Narrowing of the internal auditory canal can also cause acoustic-vestibular-facial dysfunction.

Clinical Findings

Symptoms and Signs

Patients with Paget's disease of the temporal bone present with tinnitus, vertigo, and hearing impairment. The pattern of hearing loss is mixed. The conductive component is most pronounced in the lower frequencies, whereas the sensorineural component most commonly involves the higher frequencies. Other cranial neuropathies due to foraminal stenosis are hemifacial spasm, trigeminal neuralgia, and optic atrophy.

Imaging Studies

Plain film x-rays of the skull may be diagnostic in Paget's disease. The “cotton wool” appearance (coexistence of osteolysis and sclerosis) is almost pathognomonic. The only other diagnostic consideration is the pagetoid variant of fibrous dysplasia. In approximately 10% of cases, Paget's disease may present as a sharply delineated osteolytic skull lesion, osteoporosis circumscripta cranii. The CT appearance of the temporal bone reflects varying degrees of bone remodeling activity. There are two radiographic patterns: mosaic and translucent. In the mosaic pattern, diffuse areas of radiolucency adjacent to foci of irregular sclerosis are seen. In the translucent variant, the appearance is homogeneous, “washed out,” and blurred. The otic capsule may lack its usual sharply demarcated boundary and may be accompanied by an overall diffuse demineralization of the petrous pyramid. The internal and external auditory canals and middle ear cleft may appear stenotic.

Treatment

Nonsurgical Measures

Treatment of symptomatic Paget's disease (bone pain, neuropathies, and cardiovascular stress) with calcitonin and bisphosphonates has been shown to induce biochemical and clinical improvement. Decremental levels of alkaline phosphatase and urinary hydroxyproline are seen in association with clinical improvement. Radiographic evaluation may document the arrest of bony lesions.

Surgical Measures

Surgical therapy for hearing loss and cranial neuropathy in Paget's disease should be considered only as the last resort. Surgery for conductive hearing loss in Paget's disease has not been satisfactory. Modern hearing devices are excellent alternatives to middle ear exploration and should be encouraged. Persistent symptomatic internal auditory canal stenosis with sensorineural hearing loss and facial nerve dysfunction following medical therapy may be an indication for surgical decompression.

Essentials of Diagnosis

• Fragile bones
• Blue sclera
• Conductive and sensorineural hearing loss.

General Considerations

Osteogenesis imperfecta carries the hallmark of fragile bones susceptible to easy fracture. Historically, osteogenesis imperfecta was classified as two major variants: congenita and tarda, with the congenital variant being lethal. More recently, the disease has been classified into eight different types. The majority of the cases (85–90%) are associated with autosomal dominant mutations in type I collagen and have been classified as types I–IV with type I being the mildest and most common form. Patients with osteogenesis imperfecta types V and VI present clinically similar to type IV with mild bone disease and normal appearing sclera; however, they do not have mutations in type I collagen. Types VII and VIII are inherited in an autosomal recessive manner, and tend to present in a similar manner to types II and III, which are perinatally lethal and severe phenotypes, respectively. These patients demonstrate the classic triad of blue sclera, multiple fractures in childhood, and early hearing loss.

Pathogenesis

The histopathology of osteogenesis imperfecta is marked by the deposition of osteopenic immature bony tissue that is weak and fragile. There is an increase in osteocytes in both woven and lamellar bone, and a relative reduction of matrix substance. The bone turnover rate is high. Conflicting theories have been proposed to explain the pathogenesis of this disease. Some advocate the hypothesis of osteoblast dysfunction that is responsible for immature bone deposition; others advocate the hypothesis of increased osteoclast activity. Still others implicate abnormal cell signaling due to defects of the extracellular matrix. Clinically, the regulatory defect in bone turnover results in pathologic fractures and hearing loss.

Clinical Findings

Symptoms and Signs

The clinical presentation is highly variable from a severe perinatal lethal form to extremely mild forms that are often confused with early onset osteoporosis. Osteogenesis imperfecta is a systemic disease and thus affects multiple organ systems, producing a broad array of clinical manifestations. Some of these conditions are dentinogenesis imperfecta, blue sclerae, loose joints, mitral valve prolapse, easy bruising, and growth deficiency. The hearing loss pattern may be conductive, sensorineural, or mixed. The onset of hearing loss is between the second and third decades. Hearing loss in osteogenesis imperfecta can be audiometrically indistinguishable from otosclerosis. However, osteogenesis imperfecta has an earlier onset of hearing impairment and a higher incidence of sensorineural loss compared with otosclerosis. Footplate fixation in osteogenesis imperfecta can arise either from an otospongiosis-like focus, as seen in early otosclerosis, or diffuse changes within the otic capsule.

Several operative findings during stapedectomy differentiate osteogenesis imperfecta from otosclerosis. The canal wall skin is thin and fragile, and the scutum is brittle. Crural fractures are not uncommon, and there is excessive bleeding. The sensorineural component of the hearing loss in osteogenesis imperfecta is poorly understood but may involve microfractures of the otic capsule and encroachment of the bony labyrinth by dysplastic bone. Facial nerve dysfunction is a rare complication of osteogenesis imperfecta.

Imaging Studies

Temporal bone CT findings in osteogenesis imperfecta have substantial overlap with those found in otosclerosis. Both entities may have fenestral and retrofenestral findings. In fenestral disease, CT scanning shows an excrescent mass at the level of the promontory. In retrofenestral disease, the cochlea may be demineralized, with or without sclerosis. The “double ring” sign refers to the hypodense band that spirals along the cochlea. Extensive endochondral demineralization of the otic capsule is evident in severe cochlear otosclerosis. However, diffuse resorptive changes in vast areas of the otic capsule are more often seen in osteogenesis imperfecta. The findings of extensive facial nerve canal involvement and severe proliferative otic capsule dysplasia differentiate osteogenesis imperfecta tarda from cochlear otosclerosis.

Treatment & Prognosis

The primary otologic symptom in osteogenesis imperfecta is conductive hearing loss that occurs between the second and third decades. The benefit of medical therapy with calcitonin, sodium fluoride, and vitamin D is unclear. Surgical intervention with stapedectomy to improve conductive hearing loss in osteogenesis imperfecta tarda is technically more demanding than in otosclerosis. There is a greater tendency for bleeding and difficult footplate mobilization. Despite these challenges, stapes surgery in osteogenesis imperfecta has favorable short- and long-term results. Alternately, patients may choose to improve hearing with an amplification device. In patients with severe to profound bilateral sensorineural hearing loss, cochlear implantation is a viable option, although the hypervascular spongiotic bony changes may pose additional challenges.

In the rare event of facial nerve dysfunction, CT imaging is useful to evaluate the fallopian canal. Sites obstructed by dysplastic bone can be delineated for surgical decompression.

We would like to acknowledge Karsten Munck, MD for his contribution to this chapter in the previous editions of CDT.