Imaging of the Temporal Bone: A Symptom-Based Approach

Transcription

1 Imaging of the Temporal Bone: A Symptom-Based Approach Tadesse Eshetu, MD, and Nafi Aygun, MD Some of the symptoms associated with the temporal bone diseases are nonspecific, whereas others overlap with each other. Careful history and physical examination is of paramount importance for the localization of these disorders to a certain anatomic structure. Imaging plays a critical role in diagnosis, treatment planning, and follow-up of diseases of the temporal bone. This article discusses the common diseases associated with the temporal bone in a symptom-based approach. The categories reviewed are otalgia, hearing loss, tinnitus, and vertigo. Representative imaging features of the common abnormalities in each category are discussed. When a disease is associated with more than one symptom, it is included under the predominant symptom. We also provide guidelines for the preferred imaging modalities in certain clinical scenarios. Semin Roentgenol 48: Elsevier Inc. All rights reserved. Otalgia Russell H. Morgan Department of Radiology and Radiological Sciences, Johns Hopkins Medical Institutions, Baltimore, MD. Address reprint requests to Nafi Aygun, MD, Division of Neuroradiology, Johns Hopkins University, 600 North Wolfe Street, Phipps B112B, Baltimore, MD naygun1@jhmi.edu Pain that localizes to the ear, or otalgia, can be due to either primary or secondary causes. In primary otalgia, the source is the ear itself (otogenic otalgia), and the ear examination results are usually abnormal. In secondary otalgia (referred otalgia), the source is elsewhere than the ear, and ear pain is experienced owing to complex sensory innervations of the ear. 1 In referred otalgia, the ear examination results may be normal. The overwhelming majority of the cases of otalgia are due to otitis media and otitis externa, and these do not need imaging evaluation in most cases. In the setting of infection, imaging is indicated if there is poor or no response to therapy and extension to the adjacent structures and osseous destruction are suspected. Additional evaluation with computed tomography (CT) or magnetic resonance imaging (MRI) is indicated if the suspicion for neoplasm is high. 2 Sensory innervation of the external ear is primarily via the auriculotemporal branch of the mandibular division of the trigeminal nerve (V3), and to a lesser extent, via the facial, glossopharyngeal, and vagus nerves. The middle ear sends sensory afferents primarily through the glossopharyngeal nerve as part of the tympanic plexus (over the cochlear promontory). The vestibulocochlear nerve of the inner ear plays no role in sensation. Therefore, ear pain may not be experienced until after significant progression of disease process within the inner ear 2,3 (Table 1). Otitis Media Middle ear infection is the most common cause of primary otalgia, when pain from the inflamed mucosa is mediated by the nerves described earlier in the text. On CT examination, highattenuation fluid collections can be seen within the middle ear. The fluid can be acute suppurative, serous, or mucoid. However, the ossicular chain and the surrounding osseous structures are intact without evidence of erosion or destruction. 4 In most cases of otitis media, no imaging is necessary, and there is usually a favorable response to antibiotic therapy. Major culprit organisms are Streptococcus pneumoniae (pneumococcus), Haemophilus influenzae, and less commonly, Proteus and Pseudomonas species. 5 The majority of the cases of acute otitis media resolve spontaneously, and physicians usually wait for a day or two before initiating antimicrobial therapy. 1 When acute otitis media is not responsive to therapy or when it progresses, there will be subsequent blockage of the aditus ad antrum and trapping of secretions within the mastoid air cells, resulting in acute mastoiditis. Clinical signs and symptoms of acute mastoiditis are very similar to otitis media but are of longer duration and tend to be recurrent. Complications of mastoiditis include coalescent mastoiditis, Bezold abscess, petrous apicitis, meningitis, subdural X/13/$-see front matter 2013 Elsevier Inc. All rights reserved.

2 Imaging of the temporal bone 53 Table 1 Causes of Primary Otalgia Infectious Otitis media, mastoiditis, otitis externa (necrotizing), abscess, herpes zoster oticus, Bell palsy, cellulitis Inflammatory Relapsing polychondritis, bullous myringitis, muscle spasm Environmental Trauma, frostbite, burns Eustachian tube dysfunction Otologic neoplasms Squamous cell carcinoma, adenocarcinoma, neurogenic tumors empyema, and venous sinus thrombosis. Facial nerve paralysis, hearing loss, and labyrinthitis can also be seen. Coalescent Mastoiditis Further progression of the inflammatory process within the mastoid air cells results in a localized acidosis, which leads to coalescence of the bony septations by way of osteoclastic dissolution. The air cells coalesce into larger cavities filled with pus. This process may extend into all directions within the adjacent bone. There can be a lateral spread, which may result in subperiosteal abscess and/or Bezold abscess (Fig. 1). Medial involvement can result in petrous apicitis. Less frequently, sigmoid sinus thrombosis and intracranial infection can be seen (Fig. 2). 3 Erosion of the mastoid septa is usually readily recognizable on CT. 6 However, comparison with the contralateral temporal bone would be helpful for subtle cases. Chronic Otomastoiditis Patients with chronic otomastoiditis can also present with otalgia. This is secondary to eustachian tube dysfunction and resultant decreased intratympanic pressure. Complications include middle ear effusion, granulation tissue development, tympanic membrane retraction, and ossicular fixation, which can lead to conductive hearing loss. 5 On CT, chronic otomastoiditis is seen as soft tissue attenuation within the middle ear cavity. The ossicles are usually preserved, although in advanced cases, erosion of the long process of the incus can be seen because this is the most vulnerable part of the entire ossicular chain. The causes of secondary otalgia are oral maxillofacial etiologies such as sinusitis, temporomandibular joint disorders, degenerative cervical spine disease, myofascial pain syndromes, facial paralysis, and disorders of the aerodigestive tract, including cancer. 1 Necrotizing (Malignant) Otitis Externa Necrotizing otitis externa is a rare but aggressive infection of the external auditory canal (EAC), which progresses to destruct the skull base and is potentially life threatening. The term malignant otitis externa has also been used to describe this process, although necrotizing has been gaining more favor to distinguish it from a neoplastic process. 7,8 Patients usually present with severe persistent otalgia, aural fullness, and purulent discharge from the ear. 8 Elderly diabetic patients are the most affected, but any patient group that is immunocompromised (usually secondary to HIV) is also among the vulnerable population. The major causative agent is Pseudomonas aeruginosa. The organism thrives in a hyperemic moist environment. Aspergillus fumigatus and Proteus species have also been seen in the immunocompromised population. CT is the primary imaging modality and demonstrates abnormal soft tissue in the EAC, along with osseous erosion. The infection can spread into the infratemporal fossa, nasopharynx, and parapharyngeal space, as well as the middle Figure 1 Coalescent mastoiditis with subperiosteal abscess. Axial computed tomography (CT) images using bone (A) and soft tissue (B) reconstruction algorithm show bilateral mastoiditis, with destruction of bony septa and extension of infection laterally, forming subperiosteal abscesses.

3 54 T. Eshetu and N. Aygun Figure 2 Coalescent mastoiditis with epidural abscess. Axial T2-weighted (T2W) (A) and postcontrast T1W (B) images demonstrate bilateral mastoiditis, with medial extension of infection on the left, forming an epidural abscess (thick arrow). The adjacent sigmoid sinus (thin arrow) was occluded on magnetic resonance (MR) venography examination. and inner ear structures. Septic thrombosis of the sigmoid sinus and the internal jugular vein can also be seen. Extension from the EAC into the skull base occurs through spread of the infection through small perforations in the cartilaginous portion of the floor of the EAC known as the fissures of Santorini. 9 MRI also provides additional soft tissue detail, particularly in the evaluation of intracranial and skull base extension of disease (Fig. 3). 7 Primary mode of treatment is with antibiotics. For advanced cases, aggressive surgical debridement and hyperbaric oxygen treatment may be needed. 8,10 Tinnitus Tinnitus is the auditory experience of a ringing sensation in the setting where there are no external stimuli. It is very complex and can occur as a result of a wide range of abnormalities. Different classification schemes exist for tinnitus. However, most commonly, it is divided into subjective versus objective, and pulsatile versus nonpulsatile. When pulsatile, the tinnitus is synchronous with the patient s systole (heartbeat), whereas nonpulsatile tinnitus is asynchronous with the heartbeat and discontinuous. When experienced by the patient only, it is subjective tinnitus, and when heard by others as well, it is objective tinnitus. 5 Most tinnitus cases are subjective and are caused by either radiologically identifiable or nonidentifiable lesions. Viral infections, drugs, allergy, high noise, and systemic diseases are most of the causes, which do not come to the attention of the radiologist. In contrast, vascular abnormalities and tumors have imaging correlates. 11,12 If the tinnitus is pulsatile and subjective, the imaging abnormality is a mass of vascular origin, ie, hypervascular mass, vascular malformation, or an osseous developmental abnormality. Therefore, the primary workup includes imaging using contrast-enhanced temporal bone CT. 13 If CT remains negative but high clinical suspicion persists, other modalities include carotid sonography, CT angiography, time-resolved MR angiography (MRA), and conventional angiography 14,15 (Table 2). Neoplasms The most common neoplasms to cause pulsatile tinnitus are paragangliomas or glomus tumors. Based on their location, they are named glomus tympanicum, glomus jugulare, or glomus jugulotympanicum. These are benign vascular neoplasms arising from glomus bodies or paraganglia. 13,16 Glomus Tympanicum Tumors They are found in the middle ear cavity. On otoscopic examination, these can be seen as reddish pulsatile lesions behind the tympanic membrane. 13 On temporal bone CT, a round mass arising from the cochlear promontory is commonly seen (Fig. 4). It is usually distinguished from cholesteatomas because it rather encases than erodes the adjacent ossicles. 16 Glomus Jugulare Tumors These arise from the adventitia of the jugular bulb. On CT, they have classic permeative or moth-eaten pattern of the adjacent osseous structures. They are hypervascular and show avid enhancement on postcontrast images in the early arterial phase. On MRI, they demonstrate the classic saltand-pepper pattern on T2 images. pepper represents flow voids within the vessels, and the salt component represents areas of slow flow or hemorrhage. 12 Glomus jugulotympani-

4 Imaging of the temporal bone 55 Figure 3 Necrotizing otitis externa. Coronal CT (A) shows opacified left mastoid air cells, with erosion of the cortex of the mastoid and skull base (arrows). Contrast-enhanced axial MR imaging (B) reveals extensive soft tissue inflammation centered around the mastoid (arrow) and extending into the masticator space, paraspinal muscles, skull base, and around the carotid artery (short arrow). cum is a term given to glomus jugulare tumors that have extended to the level of the cochlear promontory. Vascular Causes Dural Arteriovenous Fistulas Dural arteriovenous fistulas (DAVFs) are the most common causes of objective pulsatile tinnitus in patients with negative otoscopic examination results. They are most commonly located in the transverse, sigmoid, and cavernous sinuses, and the patients are symptomatic most of the time. 13 The DAVFs are supplied by branches of the external carotid artery, and venous drainage can be intracranial, extracranial, or both. 17 DAVFs may not be detected on CT or contrast-enhanced MRI, so the gold standard imaging modality is conventional catheter angiography. 18 Arteriovenous Malformations Arteriovenous malformations are anomalous connections between arteries and veins. These can be either congenital or acquired. 12 These can be identified on both contrast-enhanced CT and MRAs. Conventional angiogram better delineates the arterial supply and venous drainage pattern. Aberrant Internal Carotid Artery This is a vascular anomaly that is called aberrant, but it actually represents a hypertrophied inferior tympanic artery because of failure of the cervical segment of the internal carotid artery. The inferior tympanic vessel is a branch of the ascending pharyngeal branch of the external carotid artery, which courses through the middle ear and anastomoses with the petrous portion of the internal carotid artery. 15 On CT, dehiscence of the carotid canal, with abnormal course of the vessel in the middle ear, can be seen. On MRI, this can be seen as a flow void. MRA and catheter angiograms demonstrate a more lateral course of the internal carotid artery in the middle ear (Fig. 5). 19,20 Stapedial Artery A persistent stapedial artery is a remnant of fetal circulation that persists into adulthood. It is a branch of hyoid artery connecting the internal and external carotid arteries and usually regresses at birth. When present, it takes over the role of the middle meningeal artery, and therefore the foramen spinosum and middle meningeal artery do not develop. 12,16 Correct radiologic identification of vascular variants in the middle ear is essential for avoiding inadvertent biopsy and subsequent catastrophic consequence. 21 These findings require a direct communication between the radiologist and referring surgeon. Jugular Bulb Variants A jugular bulb is considered high-riding when it extends over the round window and the tympanic annulus or the basal turn of the cochlea. 12 A dehiscent high-riding jugular bulb can be seen as a bluish mass on otoscopic examination. A jugular diverticulum is an outpouching of the jugular bulb superior and medial to the jugular fossa. This can also be associated with tinnitus. Hearing Loss Hearing loss is a ubiquitous problem, which manifests with varying degrees of severity. The clinician needs to take a thorough history and conduct careful physical examination to appropriately diagnose and categorize the type of hearing loss. History should include the onset, duration, severity, and uni- or bilaterality of the symptom. Associated symptoms of otalgia, tinnitus, or vertigo should also be evaluated. Additional history Table 2 Causes of Pulsatile Tinnitus Idiopathic High-grade stenosis of internal carotid artery, fibromuscular dysplasia, dissection Arteriovenous malformations, dural arteriovenous fistulas Venous stenosis (pseudotumor cerebri) Hemangioma, otospongiosis, Paget disease Paragangliomas (glomus jugulare, tympanicum, jugulotympanicum). Aberrant carotid artery, persistent stapedial artery High-riding jugular bulb/jugular diverticulum

5 56 T. Eshetu and N. Aygun Figure 4 Glomus tympanicum. Small rounded soft tissue masses at the cochlear promontory, characteristic of bilateral glomus tumors in this patient with pulsatile tinnitus and red/purple mass behind tympanic membrane. of trauma, ototoxic drug, or excessive noise exposure should also be obtained. Hearing loss is categorized into conductive, sensorineural, or mixed (both conductive and sensorineural). Physical examination always involves otoscopic evaluation to determine patency of the EAC and the integrity of the tympanic membranes and the middle ear (ossicles). This is followed by a tuning fork test, which is instrumental in the determination of conductive versus sensorineural hearing loss (SNHL). Additional neurologic examinations for evaluation of facial nerve paralysis should also be performed. An audiogram is obtained in most of the cases, unless there is an obvious impaction of the EAC by cerumen. This provides information about the type and degree of hearing loss as well as the laterality. Standard imaging modalities for evaluation of hearing loss are high-resolution temporal bone CT and contrast-enhanced MRI. CT offers detailed osseous anatomy of the temporal bone. Erosive changes within the ossicles, the facial nerve canal, and the mastoids are identified with ease on CT; therefore, it is more suitable for the assessment of conductive hearing loss. Contrastenhanced MRI with additional high-resolution images through the temporal bone provides excellent soft tissue detail and is better suited for evaluation of SNHL. 22,23 Sensorineural Hearing Loss Acquired Causes Presbyacusis. The most common cause of SNHL is presbycusis, or age-related degeneration of the inner ear. This affects almost half of individuals aged 75 years, with varying degrees of severity. There are no imaging findings associated with this disorder, and it is always bilateral. 24 Neoplasms Vestibular Schwannomas. They are benign slow-growing neoplasms arising from the Schwann cells of the vestibulocochlear nerve. They commonly encase the vestibular portion Figure 5 Aberrant internal carotid artery. Axial contrast-enhanced CT (A) shows lack of bony covering of the internal carotid arteries bilaterally, which extend into the middle ear. MR angiography (B) of the same patient shows more lateral position of the petrous segment of the internal carotid arteries.

6 Imaging of the temporal bone 57 Figure 6 Internal auditory canal (IAC) schwannoma and meningioma. Axial (A) and coronal (B) postcontrast images show a dural-based mushroom-shaped right cerebellopontine angle/iac mass and a cylindrical left IAC mass compatible with meningioma and schwannoma, respectively, in this patient with no history of neurofibromatosis. of the eighth cranial nerve (CN VIII) within the internal auditory canal (IAC) and extend into the cerebellopontine angle (CPA) cistern. Less commonly, they arise within the vestibule and cochlea, known as labyrinthine schwannomas. These are the most common tumors of the CPA cistern (approximately 85%). 25 Asymmetric SNHL is the most common primary presenting symptom in this patient population (85%), followed by tinnitus and vertigo. 26 On MRI, they are seen as avidly enhancing lesions that have the shape of a cylinder when confined to the IAC and a light bulb when they extend into the CPA cistern. 23,26,27 Meningiomas. They are the second most common neoplasms of the CPA cistern (3%-6%). On imaging, they demonstrate enhancement and broad-based dural attachment (Fig. 6). It can sometimes be difficult to distinguish them from vestibular schwannomas if they extend into the IAC. 25 However, they commonly have mushroom cone morphology. Endolymphatic Sac Tumors. Endolymphatic sac tumors are located within the retrolabyrinthine region and are not very common. Larger lesions can extend to involve the middle ear or CPA cistern. These tumors can occur sporadically or as part of the von Hippel Lindau syndrome. These are heterogeneous lesions with cystic and solid components, calcification, and hemorrhage. Hypervascularity is seen and enlarged vessels can be identified on CT angiogram. Almost all patients with endolymphatic sac tumors present with SNHL. Other potential associated symptoms include vertigo, facial nerve palsy, and pulsatile tinnitus. 28 Other tumoral causes of SNHL are epidermoids, dermoids, arachnoid cysts, and metastases. 27 Infectious/Inflammatory Labyrinthitis. It is the infection/inflammation of the peri/ endolymphatic spaces of the inner ear. Affected patients commonly present with SNHL and vertigo. The mode of spread of the infection could be either from Figure 7 Fibrous stage of labyrinthitis ossificans. Axial high-resolution T2W image (A) shows a lack of bright fluid signal in the left cochlea (arrows). On the postcontrast T1W image (B), there is abnormal enhancement of the membranous labyrinth in the cochlea (short arrow) as well as the vestibule (arrow) and lateral semicircular canal.

7 58 T. Eshetu and N. Aygun Figure 8 Labyrinthitis ossificans. Axial CT images through the cochlea (A) and lateral semicircular canal (B) show calcium deposition and replacement of fluid attenuation (arrows) in the membranous labyrinth. direct extension of a middle ear infection, from meningitis (meningiogenic labyrinthitis), or rarely, via hematogenic spread. Meningogenic labyrinthitis is the most common cause of childhood SNHL. Etiologic agents could be viral, bacterial, or autoimmune. On imaging (postcontrast T1), there is enhancement of the labyrinthine structures the cochlea, the semicircular canals, and the vestibule. 28,29 If acute labyrinthitis fails to resolve, it can progress to a subacute to chronic form, where a fibro-osseous process develops, known as labyrinthitis ossificans. In the initial fibrous stage, there is diffuse fibroblastic proliferation within the perilymphatic spaces, which may begin as early as 2 weeks after infection. On MRI, there is replacement of the normal high T2 signal (fluid signal intensity) within the labyrinths by soft tissue signal and enhancement in most cases (Fig. 7). In the ossific stage, diffuse ossification of the labyrinths can be seen on CT (Fig. 8). On MRI, there will be a T2 hypointense replacement of the signal within the labyrinths. Advanced labyrinthitis ossificans can pose challenge to cochlear implantation. 5,27 Trauma Temporal bone fractures can result in SNHL, as they commonly involve the inner ear. The fractures are traditionally classified as either longitudinal or transverse despite the known shortcomings of this classification scheme, as most fractures are complex and do not conform to a single plane. Figure 9 Temporal bone fractures. Axial CT images of 2 different patients show a longitudinal (otic capsule sparing) fracture (short arrow in A) and dislocation of the incudomalleolar joint and a transverse (otic capsule involving) fracture (arrow in B) extending through the vestibule and lateral semicircular canal.

8 Imaging of the temporal bone 59 Table 3 Acquired Causes of Sensorineural Hearing Loss Tumors Vestibular schwannoma, meningioma, lymphoma, metastases, endolymphatic sac tumors, intralabyrinthine schwannoma. Infection/inflammation Vestibular neuritis, labyrinthitis, labyrinthitis ossificans, pachymeningitis, viral infection Trauma Transverse fracture (otic capsule fracture, perilymphatic fistula, cochlear concussion) Degenerative/idiopathic processes Osteopetrosis, Paget disease, otospongiosis Intra-axial Anterior inferior cerebellar artery (AICA) infarcts, demyelinating plaques. Others Postradiation, ototoxic drugs, superficial siderosis, autoimmune, idiopathic Transverse fractures occur in 10%-30% of the time and usually involve the otic capsule, and result in disruption of the CN VIII. Most fractures (70%-90%) are of the longitudinal type, which result in conductive hearing loss. This is primarily due to disruption of the ossicular chain (Fig. 9). 30 Other causes of acquired SNHL include intralabyrinthine hemorrhage, demyelinating disease such as multiple sclerosis, and autoimmune disease. Superficial siderosis by way of recurrent hemosiderin deposition on the CN VIII is also a well-documented cause of SNHL 31 (Table 3). Congenital SNHL Various congenital anomalies result in SNHL, and approximately 20% of these cases present with imaging abnormalities. Approximately one-half (50%) are due to genetic anomalies, and the remainder are secondary to intrauterine toxin exposure (ie, thalidomide), TORCH infections, and so forth. Universal newborn screening is performed in the United States, and congenital causes of hearing loss are diagnosed early. The role of imaging is to determine the etiology of the abnormality and identify patients who would be eligible for cochlear implantation or other types of therapy. 23 Enlarged Vestibular Aqueduct. Enlarged vestibular aqueduct or enlarged endolymphatic duct is the most common cause of congenital SNHL. The endolymphatic duct courses through the vestibular aqueduct to reach the endolymphatic duct. 32 The enlarged vestibular aqueduct can be seen on both CT and MRI (Fig. 10). Diameter mm at midsection is abnormal. An internal reference is the width of the posterior semicircular canal; if the vestibular aqueduct is larger, then it is abnormal. Labyrinthine Dysplasias. Congenital anomalies of the inner ear lie along a spectrum ranging from complete absence of the labyrinthine structures (aplasia) to mild dysplasia. Structures of the otic capsule develop between the 3rd and 10th week of gestation, and the severity of the abnormality corresponds to how early the embryonic developmental arrest occurred. The most severe form is Michel aplasia, where there is complete bony and membranous aplasia of the labyrinth. This is extremely rare and is thought to comprise only 1% of the inner ear anomalies. Cochlear aplasia occurs in the third week of gestation, and the vestibule and semicircular canals are dysplastic. The vestibule and the IAC can also be dilated. However, the EAC and the middle ear structures are normal. If there is an insult/arrest in the fourth week of gestation, common cavity deformity or Cock dysplasia occurs. In this case, there is a common cavity in place of the cochlea and the vestibule. Figure 10 Enlarged vestibular aqueduct and vestibular dysplasia. Axial CT images of 2 different patients with sensorineural hearing loss. Enlarged vestibular aqueduct (arrow in A) is seen with otherwise normal-appearing bony labyrinth. Note the normal vestibular aqueduct of the other patient (arrow in B). The vestibule appears fused with the lateral semicircular canal (short arrow in B). The cochlea is normal morphologically (not shown).

9 60 T. Eshetu and N. Aygun Figure 11 Findings of classic Mondini anomaly include a well-formed basilar turn of cochlea (A), incomplete separation of the second and apical turns of cochlea (arrows in B and C, respectively), vestibular dysplasia (long arrow in B), and vestibular aqueduct enlargement (C). Cystic vestibulocochlear anomalies occur if there is arrest in the fifth week of gestation. The cochlea is separate from the vestibule but appears cystic, without any internal architecture visible. This is also known as incomplete partition type I and comes in mild and severe forms. 27 In Mondini malformation (incomplete partition II), there is incomplete spiralization of the cochlea, with 2.5 turns. The basal turn is present, but the apical and middle turns are fused. Mondini malformation is commonly associated with a dysplastic vestibule and enlarged vestibular aqueduct (Fig. 11). Cochlear hypoplasia is a spectrum of abnormalities of cochlear formation that occur after the sixth week of gestation. In this anomaly, the cochlea is formed but is small. Varying degrees of semicircular canal maldevelopment is commonly seen in association with cochlear dysplasias. 22 Currently, most inner ear malformations, including rather severe ones such as common cavity deformity, are amenable to cochlear implantation. Thus, morphologic features of the cochlea are not necessarily considered in implant candidacy assessment, although they have significant implications on surgical approach and stimulator electrode selection. There is a higher incidence of aberrant facial nerve course in cochlear anomalies, which is also important in determining the surgical approach. Absence of the cochlear nerve (cochlear nerve deficiency) is associated with poor speech recognition after cochlear implantation and has gained better recognition recently. CT findings associated with cochlear nerve deficiency include narrow IAC and narrow/closed cochlear nerve canal at the base of the cochlea, but high-resolution MRI is necessary to evaluate the presence or absence of the cochlear nerve in cochlear implant candidates (Fig. 12; Table 4). Conductive Hearing Loss Conductive hearing loss associated with the external ear is commonly secondary to impaction by cerumen or foreign body, inflammation, neoplasm, or bony outgrowths such as exostoses. 23 Exostoses Exostoses are the most common solid tumors in the EAC and are found in its deep portion. These benign osseous masses usually occur in patients who are cold water swimmers, who undergo chronic irritation of the EAC, hence the term surfer s ear in reference to exostosis that results from prolonged exposure to seawater. CT is the best imaging modality, and exostoses are seen as sessile bony masses that are usually bilateral. They also arise medially in the bony EAC. These can result in EAC stenosis and conductive hearing loss. Osteomas are also bony masses of the EAC. In contradistinction to Figure 12 Cochlear nerve deficiency. High-resolution T2W axial (A) and sagittal (B) images through the IACs show bilateral narrow IAC with no cochlear nerve identified. The facial nerve is only seen in the superior anterior aspect of the IAC (arrow). Nonvisualization of the cochlear nerve on high-resolution images is a relative contraindication for cochlear implantation.

10 Imaging of the temporal bone 61 Table 4 Causes of Congenital Sensorineural Hearing Loss Enlarged vestibular aqueduct Cochlear and vestibular aplasia/hypoplasia eg, common cavity, Mondini malformation, Michel aplasia, etc Perilymphatic fistula Membranous labyrinthine dysplasias exostoses, they are usually unilateral, pedunculated, and more laterally positioned within the IAC. Complete absence of the EAC and markedly small canal are congenital abnormalities that arise from anomalies of the first and second branchial arches and are almost always associated with auricular atresia or dysplasia. 10,23 Cholesteatomas These can result in conductive hearing loss. They are usually encountered in the middle ear cavity but can also be rarely seen in the EAC. These represent keratinous debris from stratified squamous epithelium that grew through a perforated eardrum. Cholesteatomas are acquired most of the time. Their pathophysiology is unclear, but retraction pockets of the pars flaccida of the tympanic membrane are blamed to be the primary lesions leading to development of cholesteatomas. On CT imaging, they are characterized as nondependent soft tissue densities within the middle ear. Chronic otitis media and cholesteatoma are often seen together and may be difficult to differentiate from each other on imaging as well as clinically. Mass effect and bone erosion are important distinguishing features of cholesteatomas (Fig. 13). 16 When evalu- Figure 13 Cholesteatoma. Coronal (A) and axial (C) images of the normal right temporal bone and coronal (B) and axial (D) CT images of the left temporal bone with a nondependent soft tissue mass in the attic and associated bone erosion, which are characteristic of cholesteatoma. Cholesteatoma fills the epitympanic space, erodes the incus and malleus (arrows in C and D), and extends into the mastoid through the aditus ad antrum. Note the blunting of the scutum (arrows in A and B).

11 62 T. Eshetu and N. Aygun Figure 14 Fenestral otosclerosis. Axial CT of the temporal bone (A) demonstrates an area of demineralization anterior to the oval window at the fissula ante fenestram (arrow). A normal temporal bone CT image (B) for comparison. ating these erosive changes, particular attention should be paid to the scutum or the lateral epitympanic wall, the ossicles, the lateral semicircular canal, the tympanic segment of the facial nerve, and the roof of the middle ear cavity (tegmen tympani). In contrast, presence of calcifications around the middle ear ossicles favors the diagnosis of chronic otitis media (tympanosclerosis). On MRI imaging, cholesteatomas are hypointense on T1- weighted, hyperintense on T2-weighted, and enhanced on the postcontrast images. 2,33 Additional specificity can be achieved by diffusion-weighted imaging sequences, which help in distinguishing cholesteatomas from scar or granulation tissue and other inflammatory changes in the postoperative follow-up. 33,34 Clinically, the patients with EAC cholesteatoma usually present with chronic dull pain. Conductive hearing loss can be the presenting symptom, although less common. On imaging, high-resolution CT of the temporal bone exhibits a soft tissue density mass within the EAC and erosion of the adjacent bone. 35,36 Neoplasms Cutaneous malignancies such as basal cell or squamous cell carcinomas comprise a large proportion of EAC malignancies. Both initially involve the pinna and preauricular tissues and extend into the canal. Basal cell carcinoma is more common and involves the sun-exposed areas. Squamous cell carcinoma is more invasive and results in nodal spread. Melanoma involvement can also be seen. CT is instrumental in evaluation of bony invasion and nodal spread. 37 Otitis media, mastoiditis, and necrotizing otitis externa can also cause conductive hearing loss in addition to otalgia (refer to the Otalgia section). Longitudinal fractures can cause ossicular disarticulation and result in conductive hearing loss. 37 Otospongiosis Otospongiosis is an idiopathic process unique to the temporal bone, which results in conductive, sensorineural, or mixed hearing loss. The terms otosclerosis and otospongiosis have been used interchangeably. However, otospongiosis is gaining more favor because it accurately describes the replacement of endochondral by spongiotic bone (not sclerotic). The fenestral type of otospongiosis involves the bone anterior to the oval window, at a cleft of connective tissue known as fissula ante fenestram. On CT, it is seen as an area of demineralization in this region (Fig. 14). Fenestral otospongiosis commonly results in conductive hearing loss, secondary to encroachment on the stapes footplate by an abnormal bone. In advanced cases, it can result in mixed-type hearing loss. 38,39 Fenestral otospongiosis should be suspected in adults who present with conductive hearing loss in the absence of infection. Otospongiosis involves both ears, but hearing loss may be unilateral or asymmetric at presentation. Cochlear or retrofenestral otospongiosis is seen as an area of demineralization around the bony labyrinth on CT (Fig. 15). The patients present with SNHL, and this is hypothesized to be secondary to diffusion of proteolytic enzymes into the cochlea 40 (Table 5). Vertigo Vertigo is the illusory experience of rotational movement of surrounding or self. There are several causes of vertigo, and careful history, including the time course of the disease, is very helpful in identifying the etiology. Dizziness is a less specific symptom, which is often confused with vertigo. There are central and peripheral causes of vertigo. The peripheral components of the vestibular system include the semicircular canals, the vestibular nerve, and the utricle and the saccule. The central part includes the vestibular cortex, the vestibular nucleus, the cerebellum, the brainstem, and the spinal cord. The majority of the causes of vertigo are peripheral, and benign positional peripheral vertigo, vestibular neuritis or labyrinthitis, and Meniere disease are the most common causes. Semicircular canal dehiscence, perilymphatic fistula, herpes zoster, and labyrinthine concussion can also result in peripheral vertigo. 41 Many of the causes of nonpositional vertigo can be evaluated using MRI. Enhancement of the labyrinths can be seen in viral, autoimmune, or radiation-induced labyrinthitis. 41,42 Meniere Disease Meniere disease, also known as endolymphatic hydrops, is characterized by episodic vertigo, tinnitus, hearing loss, and

12 Imaging of the temporal bone 63 Figure 15 Cochlear otosclerosis. Axial CT of the temporal bone demonstrates extensive lucency around the bony labyrinth surrounding the cochlea bilaterally (arrow). ear fullness. 5 This is associated with enlargement of the endolymphatic space in the expense of the perilymphatic space. During acute episodes, there is expansion of the endolymph in the vestibular system, with a resultant bulging into the perilymphatic space and rupture of the membrane that separates the two. The mixture of fluid from these 2 compartments contacts the vestibular nerve receptors, which results in vertigo. Mechanical disturbance of the organ of Corti will also cause hearing loss. 5 MR diagnosis of this disease has been very challenging. Recent reports have shown that dilated endolymphatic space can be better visualized after intratympanic administration of gadolinium contrast or delayed imaging after intravenous administration of contrast. 43 Superior Semicircular Canal Dehiscence Dehiscence of the arcuate eminence of the semicircular canal can also be a cause of vertigo. Superior semicircular canal dehiscence is best evaluated with high-resolution CT of the temporal bone. On imaging, the bone covering the superior arc of the Superior semicircular canal that separates it from the middle cranial fossa is absent. Oblique reformatted images perpendicular and parallel to the canal can better demonstrate the anomaly (Fig. 16). 44 The anomaly can be associated with a clinical entity described as Tullio phenomenon. 5 This represents sound-invoked vertigo or nystagmus. 45 Central vertigo accounts for approximately 25% of the cases and arises from lesions within the brainstem, the thalamus, and the cerebral cortex. Cerebellar infarction, basilar artery occlusion, vertebral artery dissection, tumor, demyelinating disease, or Chiari malformations could be the causes Table 5 Conductive Hearing Loss Congenital causes External auditory canal atresia Ossicular anomalies Congenital cholesteatoma Acquired causes Exostoses/osteomas, external auditory canal tumors Cerumen impaction Otitis media and otitis externa Acquired cholesteatomas Tympanosclerosis Fenestral otosclerosis Trauma Figure 16 Superior semicircular canal dehiscence. Coronal-oblique reformatted CT of the temporal bone demonstrates lack of bony covering of the roof of the superior semicircular canal (arrow).

13 64 T. Eshetu and N. Aygun Table 6 Vertigo Peripheral causes Benign paroxysmal positional vertigo Meniere disease Vestibular neuritis Herpes zoster oticus Semicircular canal dehiscence Recurrent vestibulopathy Central causes Brainstem ischemia Wallenberg syndrome Cerebellar infarction and hemorrhage Multiple sclerosis Chiari malformation of vertigo. 41 Migraine has also been recognized as a cause of recurrent vertigo. It usually presents in association with other focal neurologic symptoms such as diplopia, dysarthria, weakness, and ataxia (Table 6). Conclusions Thorough understanding of the symptoms associated with temporal bone lesions aids in accurate categorization of the disease entities. Subsequent to this, imaging of the temporal bone plays a key role in assisting in the correct diagnosis. High-resolution CT of the temporal bone and contrast-enhanced MRI with high-resolution sequences through the level of the IAC remain the most frequently tools for imaging. References 1. 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