Cortical dysplasia

Abstract

Fibrous dysplasia is an uncommon benign disorder of unknown etiology. It represents a disturbance of normal bone development--specifically a defect in osteoblastic differentiation and maturation that originates in the mesenchymal precursor of the bone.

Because fibrous dysplasia shows a predilection for the facial and cranial bones, where it causes deformity and dysfunction, the disease is of particular interest to the otolaryngologist. In this paper, we report a case offibrous dysplasia of the temporal bone, the first symptom of which was a mixed hearing loss. We discuss the characteristic features of this specific location of the disease, the differential diagnosis, and the treatment policy. We also address the issue of secondary sensorineural hearing loss.

Introduction

Fibrous dysplasia is an uncommon benign disorder of unknown etiology. It is associated with a defect in osteoblastic differentiation and maturation that originates in the mesenchymal precursor of the bone. In 1937, McCune and Bruch first suggested that of all the abnormalities of bone formation, this disorder should have its own place as a distinct clinical entity. [1] The following year, Lichtenstein introduced the term fibrous dysplasia. [1] There are two primary categories of the disease: monostotic fibrous dysplasia involves a single bone and represents 70% of all cases; polyostotic fibrous dysplasia involves multiple bones and makes up the remainder, [2] Also, approximately 3% of patients with fibrous dysplasia will experience McCune-Albright syndrome, in which the bony involvement is accompanied by skin lesions and endocrinopathy. [2] The male-to-female ratio is 2:1. Race predilection has been observed, with whites making up 80% of all cases, blacks 2%, and Asians 1%. [2]

Fibrous dysplasia is of particular interest to the otolaryngologist because of its predilection for the facial and cranial bones, where it causes deformity and dysfunction. In this paper, we report the case of a patient with fibrous dysplasia of the temporal bone who experienced mixed hearing loss, and we review the characteristic features of fibrous dysplasia in this region.

Case report

A 47-year-old man came to our service complaining of progressive hearing loss without any other symptoms. His personal and family medical history was unhelpful. Clinical examination showed an extensive stenosis of the left external auditory meatus. The appearance of the visible part of the left tympanic membrane was normal. The rest of the clinical examination was normal, without any visible malformations in the head and neck. Detailed hematologic and biochemical analyses were performed, and normal values were obtained.

Audiometry revealed a mixed hearing loss in the left ear; the pure-tone average was 65 dB, and the air-bone gap was 30 dB. Hearing in the right ear was normal. Results of an audiogram that had been performed 2 years earlier were normal in both ears. High-resolution computed tomography (CT) of the skull base demonstrated a bony overgrowth, with sclerosis of the left temporal bone (particularly in the squamous portion and in the anterior part of the petrous pyramid) and the left greater wing of the sphenoid bone. Moreover, the left mastoid cells were cloudy, and inflammatory tissue filled the left middle ear cavity (figures 1, 2).

The patient was operated on for left meatal stenosis under general anesthesia. A postauricular incision was made, and a generous postauricular meatal flap was created. The bony meatus was widened by removing bone from the posterior meatal wall with a cutting bur, and the meatal wall was covered with the flap. A meatoplasty was performed, which created a trap-door flap pedicled on the meatal opening. The meatus was packed with Gelfoam. Several firm, gray fragments of tissue were sent for histology, which revealed a neoplastic fibrous tissue that consisted of a number of spindle cells, with neither cytologic atypia nor an abundant collagen stroma (figure 3). No mitotic figures were identified. Narrow, curved, and misshapen newly formed bone trabeculae (woven hone) were interspersed within the fibrous tissue. Osteoblasts were not identified on the surface of the trabeculae.

The patient was discharged on the third postoperative day. Amoxicillin-clavulanate was administered until the packing was removed on the 15th postoperative day. Careful followup was performed every month for the following year.

Discussion

Fibrous dysplasia is characterized as a slow, progressive replacement of bone by an abnormal proliferative isomorphic fibrous tissue, intermixed with poorly formed and irregularly arranged trabeculae of woven bone. [3] The mode of transmission has not yet been established. If it is genetic, it is unclear whether fibrous dysplasia has an autosomal-dominant or an autosomal-recessive character. It has been suggested that monostotic fibrous dysplasia occurs secondarily to an arrest of bone maturation. [4] It has also been proposed that fibrous dysplasia might be associated with increased levels of steroid hormone receptors (receptors of estrogens or progesterone). [5]

In fibrous dysplasia, the lesion expands, which leads to a distortion and weakening of bone. Although the lesion is not encapsulated, it tends to remain enclosed within a shell of cortical bone. This shell can be thinned as a result of the pressure exerted on it. As it slowly progresses, fibrous dysplasia can cause skeletal destruction and deformity. Occasionally, certain endocrinopathies and abnormal skin pigmentation are also seen. [2]

Monostotic lesions usually involve the ribs and femurs. They grow slowly and usually appear to be stationary after puberty. [6] In contrast, polyostotic disease frequently becomes evident late in childhood. Patients experience multiple bone involvement, most often in the shoulder, pelvis, vertebral column, and craniofacial skeleton. [7] In polyostotic fibrous dysplasia, the craniofacial skeleton is affected (usually unilaterally) in 40 to 60% of all cases; in monostotic disease, craniofacial lesions occur in approximately 25% of cases. [8] Polyostotic disease commonly affects the sphenoid, frontal, maxillary, and ethmoid bones; less frequently involved are the occipital and temporal bones. [9] Polyostotic fibrous dysplasia is associated with endocrinopathies in 3 to 5% of all cases.

McCune-Albright syndrome occurs when fibrous dysplasia is accompanied by the appearance of scattered areas of melanotic skin pigmentation (cafe-au-lait spots) and sexual precocity. [10] While many endocrine associations have been identified, hyperthyroidism is the most common, encountered in 5% of patients with this syndrome. [7] A possible relationship between fibrous dysplasia and primary hyperparathyroidism has been suggested. [11] McCune-Albright syndrome occurs primarily in women. [12]

Spontaneous transformation to malignancy in fibrous dysplasia of the temporal bone has not yet been reported. [9] However, malignancies have been reported to invade other bones in 0.5% of patients [1]. Osteosarcoma is the most common malignancy, followed by chondrosarcoma, fibrosarcoma, and giant cell sarcoma. [1] The average length of time between the diagnosis of fibrous dysplasia and a malignant transformation is 13.5 years.

Symptoms. The most common symptoms of temporal bone fibrous dysplasia are visual impairment and a progressive conductive hearing loss caused by an occlusion of the eustachian tube or external auditory canal. [9] Sensorineural hearing loss attributed to this lesion, which occurs in 14 to 17% of patients, is the result of either cochlear destruction, internal auditory canal stenosis, or vestibular fistulization. [13] Our findings were not relevant to the aforementioned lesions regarding the sensorineural component of the hearing loss. The extensive stenosis of the left external auditory meatus and the inflammatory tissue filling the left middle ear cavity could explain the conductive component of the hearing loss. A thorough clinical and laboratory investigation did not indicate any evidence that the inner ear involvement was induced by the aforementioned etiologic factors (cochlear destruction, internal auditory canal stenosis, or vestibular fistulization) that could explain the development of sensorineural h earing loss. It is well established that a permanent sensorineural hearing loss can occur as a result of the spread of infection or products of inflammation (proteins, toxins, local inhibitors, bacteria) through the round window membrane. [14] The presence of inflammatory elements in the mastoid and middle ear cavity in our patient raises the possibility that the sensorineural hearing loss might have been the result of the penetration of microorganisms or toxins through the round window into the inner ear. [15,16]

Facial nerve involvement is seen in 10% of patients with fibrous dysplasia of the temporal bone; cholesteatoma (usually located in the external canal) occurs in almost 40%. [13] The lesion growth in the external auditory meatus leads to progressive stenosis, which results in the trapping of keratin debris. Full erosion of the ossicles can occur secondary to trapped cholesteatoma. [17] Other symptoms include tinnitus, dizziness, pain, trismus, and neurologic signs related to the involvement of the middle or posterior cranial fossa. [29]

Radiographic features. The radiologic features reflect the morphology of the disease and vary with the amount of fibrosis and calcification. Three patterns of fibrous dysplasia of the skull and facial bones have been described. [18] The pagetoid, or "ground-glass," pattern is the most common (56% of all cases); it appears as a mixture of dense and radiolucent areas of fibrosis. The sclerotic pattern (23% of cases) is uniformly dense. The cystic pattern (21%) is characterized by a spherical or ovoid lucidity surrounded by a dense bony shell. [18] Temporal bone lesions exhibit a sclerotic pattern in two-thirds of cases. [7] High-resolution CT helps to assess four parameters: 1) the degree of external auditory canal stenosis, 2) the involvement of middle or inner ear structures, 3) the presence of an associated cholesteatoma, and 4) the extent of a possible facial nerve involvement.

The differential diagnosis of fibro-osseous lesions of the temporal bone includes fibrous dysplasia, meningioma, aneurysmal bone cyst, unicameral cyst, ossifying or nonossifying fibroma, Paget's disease, osteochondroma, giant cell tumor, eosinophilic granuloma exostosis, osteoma, and sarcomatous neoplasms. [18] Although a plain x-ray can suggest the clinical diagnosis, high-resolution CT is usually necessary for the differential diagnosis, and histology is often required for confirmation. In our case, the diagnosis was established by high-resolution CT and confirmed by histologic examination.

When fibrous dysplasia is accompanied by significant clinical symptoms, surgery is recommended. Indications include bony encroachment of the external auditory canal (which causes sufficient narrowing to produce a conductive hearing loss), recurrent infection, and secondary canal cholesteatoma. Operative procedures should focus on three directives: the restoration of function, the prevention of complications, and the restitution of cosmesis. Radiation therapy should be avoided because of the high incidence (44%) of malignant transformation. [19]

From the Department of Otolaryngology, University of Crete School of Medicine, Heraklion, Crete, Greece.

Reprint requests: C.E. Papadakis, MD, Psaron 7 St., Heraklion, Crete 71307, Greece. Phone/fax: 30-81-326-327; e-mail: chpap@mcd.uoc.gr

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