A wide range of pathology involving the external, middle, and inner ear contribute to conductive and sensorineural hearing loss. Diagnostic imaging plays a critical role in the evaluation and management of hearing loss. High resolution computed tomography (CT) of the temporal bone and magnetic resonance imaging (MRI) are the preferred imaging modalities to evaluate the ear structures for causes of hearing loss, with the specific type of hearing loss and location of defect dictating which type of imaging is preferred. In general, the external auditory canal, middle ear space, mastoid, petrous apex, and otic capsule are best visualized with CT, whereas suspicion of retrocochlear pathology warrants MRI.
Imaging of hearing loss
Diagnostic imaging plays a critical role in the evaluation and management of hearing loss. The initial evaluation consists of obtaining a history that should address pertinent otologic information. The rapidity of onset, progressive nature, and presence of unilateral or bilateral symptoms influence the diagnostic possibilities. A history of trauma, otorrhea, pain, associated vertigo, exposure to noise or ototoxic medications, previous ear disease or surgery, and a family history of hearing loss are all pertinent to achieving a diagnosis. The physical examination is critical in determining the patency of the external auditory canal, the integrity and appearance of the tympanic membrane and aeration of the middle ear space, or the presence of otorrhea and tympanic membrane perforation. Evidence of a retraction pocket or cholesteatoma should be identified. Compromise of facial nerve function can occur anywhere along its course from the cortical brain, brainstem, through the CP angle and internal auditory canal, the otic capsule, middle ear, remainder of the temporal bone, and distal to its exit at the stylomastoid foramen. The presence of hearing loss and facial paresis, synkinesis, or paralysis always warrants further investigation with imaging. The use of a tuning fork helps to identify the nature and degree of hearing loss, especially if a conductive component is present.
Unless there is an obvious source of hearing loss, such as seen with a cerumen impaction, an audiogram is always obtained. An accurate audiogram identifies the nature and degree of hearing loss and whether one or both ears are involved. Hearing can be categorized as normal, showing a conductive hearing loss, sensorineural hearing loss, or mixed loss (conductive and sensorineural).
The modalities of imaging that are most pertinent to the evaluation of hearing loss are computed tomography (CT) and magnetic resonance imaging (MRI), depending on the type, degree, and location of the hearing loss. In general, the external auditory canal, the middle ear space including the ossicles, the mastoid and petrous air cell system, and the otic capsule are best visualized with CT using bone algorithm and windowing techniques. Suspicion of retrocochlear pathology (audiometric identification of unilateral reduced word recognition scores) usually reflects involvement of the eighth cranial nerve along the pathway from the cochlea through the internal auditory canal, cerebellopontine angle, and brainstem.
Imaging of conductive hearing loss
Conductive hearing loss (CHL) is most commonly caused by pathology of the external or middle ear and on occasion the inner ear. Sometimes the cause is obvious and imaging studies are not required before intervention. Imaging can be valuable in cases in which the cause is uncertain, however.
External ear
Conductive hearing loss attributable to external ear pathology is related to blockage of the external auditory canal, which can occur because of cerumen impaction, inflammation, foreign body, neoplasm, congenital or acquired stenosis, exostoses, or osteomas. High-resolution CT of the temporal bones is the most appropriate imaging modality to evaluate external ear disorders that result in hearing loss. Cerumen appears isointense to soft tissue and does not cause associated bony erosion. Inflammatory changes attributable to otitis externa appear as thickening of the soft tissues of the external auditory canal; bony erosion is not seen unless there is associated necrotizing otitis externa. External auditory canal cholesteatoma also appears as a soft tissue mass filling a portion of the canal; this process is distinguished from cerumen by localized bony erosion adjacent to the mass ( Fig. 1 ). The erosion can be smooth or irregular, rendering it difficult to distinguish from malignant neoplasm of the external auditory canal (EAC) . Squamous cell carcinoma and basal cell carcinoma account for most of the primary malignancies of the EAC. These tumors appear as soft tissue masses within the EAC usually associated with bony erosion ( Fig. 2 ). Often the bony changes appear irregular or lytic in nature.
Stenosis of the EAC may be attributable to soft tissue plug or bony atresia. Congenital atresia is discussed in the following section on congenital hearing loss. Acquired stenosis from infection, trauma, or surgery usually appears as a soft tissue narrowing within the EAC. Physical examination reveals narrowing or complete closure of the external canal. The medial extent of the scarring process often cannot be determined by examination. CT imaging identifies the presence or absence of middle ear involvement, which critically helps in the planning of operative management ( Fig. 3 ).
Exostoses or osteomas represent bony outgrowths in the EAC. Exostoses are usually multiple and are seen in patients who have a history of frequent exposure to cold water. Exostoses are typically bilateral, multiple, nonpedunculated, and arise medially in the bony EAC. In contrast, osteoma is usually unilateral, solitary, pedunculated, and arises more laterally in the bony EAC .
Middle ear/inner ear
A wide range of pathologic conditions cause CHL in the middle or inner ear. CHL of middle ear origin may be attributable to trauma, acute or chronic otitis media, cholesteatoma, fenestral otosclerosis, and benign or malignant neoplasm. Recently, conductive hearing loss attributable to the presence of a functional third window in the inner ear in addition to the round and oval windows has been described. Dehiscence of the superior semicircular canal (SCC) can cause a conductive hearing loss similar to that seen with otosclerosis. When special audiometric tests show acoustic reflexes to be present at a lower threshold and especially if vestibular symptoms exist, recent experience verifies that imaging studies for underlying pathology in CHL should include evaluation of the bony labyrinth . Similar to CHL attributable to disease of the external ear canal, the best imaging study to assess CHL of middle or inner ear origin is the high resolution temporal bone CT. When specifically evaluating for a dehiscent SCC, direct coronal images or coronal reformations from less than 1 mm acquisition data are ideal. The Stenver plane (perpendicular to the SCC) and Pöschl plane (parallel to the SCC) may also be used for equivocal cases, but are typically not necessary for diagnosis .
Temporal bone trauma may cause conductive hearing loss because of ossicular discontinuity or hemotympanum. It is common for temporal bone fractures to be associated with ossicular chain disruption, although radiologic evidence of a fracture need not be present for this to occur. Longitudinal fractures are more often associated with CHL. The most common abnormality of the ossicular chain following temporal bone trauma is disarticulation of the incudostapedial joint with dislocation of the incus ( Fig. 4 ).
Suspicion of otosclerosis is raised when patients present with new-onset unilateral or bilateral gradually progressive hearing loss in the presence of normal physical examination of the external auditory canal, tympanic membrane, and middle ear. On rare occasion, hyperemia of the mucosa of the promontory can be seen on otoscopy (Schwartze sign). Stapes reflex testing shows absence of the acoustic reflex. Imaging is not routinely obtained. When obtained, though, imaging often demonstrates demineralization at the fissula ante fenestram indicating otosclerosis of the stapes footplate. In the presence of additional sensorineural loss (mixed hearing loss) cochlear otosclerosis is manifested by a halo of lucency surrounding the otic capsule demonstrated on noncontrast bone windowed CT imaging.
Inflammatory or infectious processes, such as acute and chronic otitis media (OM) or cholesteatoma, may cause conductive hearing loss because of the presence of fluid or soft tissue in the middle ear space. Acute otitis media appears as opacification of the middle ear and mastoid air cells on high-resolution computed tomography (HRCT) of the temporal bones. Unless there is a fulminant infection with coalescence of mastoid air cells, bony changes are not seen. In the setting of chronic otitis media and CHL, particular attention should be paid to the status of the ossicular chain and the scutum on CT. Erosion of the incudostapedial joint may be an additional cause of CHL in a patient who has chronic OM. Scutal erosion seen on coronal images, or the presence of soft tissue in Prussak space, is suspicious for cholesteatoma formation ( Fig. 5 ). It can be difficult to differentiate between cholesteatoma and fluid or inflammatory tissue secondary to OM on the CT, as all appear as a soft tissue density. The presence of bony erosion alerts the clinician to the possibility of cholesteatoma. MRI can help to distinguish between fluid, thickened mucosa, and cholesteatoma. Fluid and cholesteatoma both appear bright on T2 images; however, both primary and recurrent cholesteatomas have been found to show increased signal on diffusion weighted imaging, which can distinguish them from fluid .
Neoplasms of the middle ear include benign entities, such as paraganglioma (glomus tympanicum or jugulotympanicum), schwannoma, adenoma, hemangioma, or malignant processes, such as squamous cell carcinoma, basal cell carcinoma, rhabdomyosarcoma, lymphoma, and carcinoid tumor. Glomus jugulare tumors originate from the glomus body of the jugular bulb and may extend into the hypotympanum and mesotympanum, causing CHL. The typical appearance on HRCT is of an erosive process at the jugular foramen, with permeative bony destruction at the jugular spine ( Fig. 6 ). MRI shows intense enhancement with gadolinium and the typical “salt and pepper” imaging characteristics of paraganglioma attributable to methemoglobin and flow voids. Glomus tympanicum may also cause CHL, but appears on HRCT as a smooth soft tissue density in the mesotympanum without involvement of the jugular foramen. Other benign neoplasms of the middle ear may expand, but not erode, bone. This is the key imaging feature that distinguishes benign from malignant neoplasms.
Imaging of conductive hearing loss
Conductive hearing loss (CHL) is most commonly caused by pathology of the external or middle ear and on occasion the inner ear. Sometimes the cause is obvious and imaging studies are not required before intervention. Imaging can be valuable in cases in which the cause is uncertain, however.
External ear
Conductive hearing loss attributable to external ear pathology is related to blockage of the external auditory canal, which can occur because of cerumen impaction, inflammation, foreign body, neoplasm, congenital or acquired stenosis, exostoses, or osteomas. High-resolution CT of the temporal bones is the most appropriate imaging modality to evaluate external ear disorders that result in hearing loss. Cerumen appears isointense to soft tissue and does not cause associated bony erosion. Inflammatory changes attributable to otitis externa appear as thickening of the soft tissues of the external auditory canal; bony erosion is not seen unless there is associated necrotizing otitis externa. External auditory canal cholesteatoma also appears as a soft tissue mass filling a portion of the canal; this process is distinguished from cerumen by localized bony erosion adjacent to the mass ( Fig. 1 ). The erosion can be smooth or irregular, rendering it difficult to distinguish from malignant neoplasm of the external auditory canal (EAC) . Squamous cell carcinoma and basal cell carcinoma account for most of the primary malignancies of the EAC. These tumors appear as soft tissue masses within the EAC usually associated with bony erosion ( Fig. 2 ). Often the bony changes appear irregular or lytic in nature.
Stenosis of the EAC may be attributable to soft tissue plug or bony atresia. Congenital atresia is discussed in the following section on congenital hearing loss. Acquired stenosis from infection, trauma, or surgery usually appears as a soft tissue narrowing within the EAC. Physical examination reveals narrowing or complete closure of the external canal. The medial extent of the scarring process often cannot be determined by examination. CT imaging identifies the presence or absence of middle ear involvement, which critically helps in the planning of operative management ( Fig. 3 ).
Exostoses or osteomas represent bony outgrowths in the EAC. Exostoses are usually multiple and are seen in patients who have a history of frequent exposure to cold water. Exostoses are typically bilateral, multiple, nonpedunculated, and arise medially in the bony EAC. In contrast, osteoma is usually unilateral, solitary, pedunculated, and arises more laterally in the bony EAC .
Middle ear/inner ear
A wide range of pathologic conditions cause CHL in the middle or inner ear. CHL of middle ear origin may be attributable to trauma, acute or chronic otitis media, cholesteatoma, fenestral otosclerosis, and benign or malignant neoplasm. Recently, conductive hearing loss attributable to the presence of a functional third window in the inner ear in addition to the round and oval windows has been described. Dehiscence of the superior semicircular canal (SCC) can cause a conductive hearing loss similar to that seen with otosclerosis. When special audiometric tests show acoustic reflexes to be present at a lower threshold and especially if vestibular symptoms exist, recent experience verifies that imaging studies for underlying pathology in CHL should include evaluation of the bony labyrinth . Similar to CHL attributable to disease of the external ear canal, the best imaging study to assess CHL of middle or inner ear origin is the high resolution temporal bone CT. When specifically evaluating for a dehiscent SCC, direct coronal images or coronal reformations from less than 1 mm acquisition data are ideal. The Stenver plane (perpendicular to the SCC) and Pöschl plane (parallel to the SCC) may also be used for equivocal cases, but are typically not necessary for diagnosis .
Temporal bone trauma may cause conductive hearing loss because of ossicular discontinuity or hemotympanum. It is common for temporal bone fractures to be associated with ossicular chain disruption, although radiologic evidence of a fracture need not be present for this to occur. Longitudinal fractures are more often associated with CHL. The most common abnormality of the ossicular chain following temporal bone trauma is disarticulation of the incudostapedial joint with dislocation of the incus ( Fig. 4 ).
Suspicion of otosclerosis is raised when patients present with new-onset unilateral or bilateral gradually progressive hearing loss in the presence of normal physical examination of the external auditory canal, tympanic membrane, and middle ear. On rare occasion, hyperemia of the mucosa of the promontory can be seen on otoscopy (Schwartze sign). Stapes reflex testing shows absence of the acoustic reflex. Imaging is not routinely obtained. When obtained, though, imaging often demonstrates demineralization at the fissula ante fenestram indicating otosclerosis of the stapes footplate. In the presence of additional sensorineural loss (mixed hearing loss) cochlear otosclerosis is manifested by a halo of lucency surrounding the otic capsule demonstrated on noncontrast bone windowed CT imaging.
Inflammatory or infectious processes, such as acute and chronic otitis media (OM) or cholesteatoma, may cause conductive hearing loss because of the presence of fluid or soft tissue in the middle ear space. Acute otitis media appears as opacification of the middle ear and mastoid air cells on high-resolution computed tomography (HRCT) of the temporal bones. Unless there is a fulminant infection with coalescence of mastoid air cells, bony changes are not seen. In the setting of chronic otitis media and CHL, particular attention should be paid to the status of the ossicular chain and the scutum on CT. Erosion of the incudostapedial joint may be an additional cause of CHL in a patient who has chronic OM. Scutal erosion seen on coronal images, or the presence of soft tissue in Prussak space, is suspicious for cholesteatoma formation ( Fig. 5 ). It can be difficult to differentiate between cholesteatoma and fluid or inflammatory tissue secondary to OM on the CT, as all appear as a soft tissue density. The presence of bony erosion alerts the clinician to the possibility of cholesteatoma. MRI can help to distinguish between fluid, thickened mucosa, and cholesteatoma. Fluid and cholesteatoma both appear bright on T2 images; however, both primary and recurrent cholesteatomas have been found to show increased signal on diffusion weighted imaging, which can distinguish them from fluid .
