Imaging of sensorineural hearing loss: a pattern-based approach to diseases of the inner ear and cerebellopontine angle
Insights into Imaging volume 3, pages 139–153 (2012)
An overview is presented of the common and uncommon diseases of the inner ear and of the cochleovestibular nerve within the internal acoustic meatus and cerebellopontine angle cistern causing sensorineural deafness.
An imaging-pattern-based approach is used to help detect disease and narrow the differential diagnosis.
• The most common soft tissue mass lesions in the cerebellopontine angle are schwannoma and meningioma.
• Contrast-enhanced MRI may reveal clinically unsuspected inflammatory, auto-immune or tumoural disease.
• Hearing loss may be caused by infection, inflammation or, rarely, perineural tumour spread along the cochleovestibular nerve.
• Labyrinthitis may lead to rapidly progressive ossification of the labyrinth.
Sensorineural hearing loss points at a malfunction of the inner ear or a retrocochlear condition that affects the cochleovestibular nerve within the internal acoustic meatus and cerebellopontine angle or that involves the central auditory pathway. Imaging is performed to detect or rule out congenital, infectious, inflammatory or tumoural pathology. Determining the etiology of hearing loss is helpful to establish treatment regimens that may prevent or slow down loss of auditory function. It may also contribute to counselling of the patient and—in genetic causes—his or her family.
When reading imaging studies of these patients, an analytical pattern-based approach will help to prevent uncommon or subtle disease to be overlooked and to narrow the differential diagnosis, if applied in conjunction with medical history and clinical findings.
Imaging of the inner ear is performed with computed tomography (CT) and/or magnetic resonance imaging (MRI). These techniques render complementary information. In patients requiring anaesthesia, these exams should be planned in one session. CT forms an ideal means to evaluate the bony details of the otic capsule and labyrinth and will also allow evaluating the course of the facial nerve canal and eventual associated anomalies of middle ear structures and the external auditory canal. MRI will render supplementary information on the fine intralabyrinthine structures.
Temporal bone CT should consist of both axial and coronal images. With multidetector CT, images can be reconstructed in any desired plane, obviating the need for direct coronal scanning. Contiguous or overlapping sections from the superior most mastoid air cells to the stylomastoid foramen should be obtained with the gantry angle parallel to the infraorbital-meatal line. The display slice thickness should not exceed 1.5 mm .
The inner ear comprises the cochlea, the vestibular system and the endolymphatic duct and sac. The normal cochlea consists of two-and-a-half turns separated by interscalar septae. Within each turn the cochlea is divided into the scala tympani and scala vestibuli by the osseous spiral lamina. These scalae are connected to each other at the apical tip of the cochlea (helicotrema) and they contain perilymph. The scala media, containing the organ of Corti and filled with endolymph, cannot be visualised separately with current imaging systems. The modiolus containing the spiral ganglion can, however, be visualised in the centre of the cochlea. The cochlea is connected to the middle ear by the round window. The vestibular system is located posterior to the cochlea and contains the vestibule with the saccule and utricle and three semicircular canals which are oriented perpendicular to each other. The vestibule harbours the oval window; the stapes footplate is attached to the oval window. The endolymphatic duct is connected to the saccule and courses through a bony canal—the vestibular aqueduct—to form the endolymphatic sac in the posterior fossa (Figs. 1 and 2).
Imaging patterns of disease
Formation of the labyrinth starts at about 3.5 weeks of gestation by formation of the otic disc and is complete at about 26 weeks of gestation. The otic disc becomes invaginated and gives rise to the otic vesicle or otocyst. The otocyst divides into dorsal and ventral segments that will become vestibular and auditory parts; the endolymphatic appendage appears dorsally. The mesenchyme surrounding the otocyst becomes condensed and forms the otic capsule .
Abnormal development may cause a myriad of variations, ranging from complete absence of the labyrinth to a minor dysplasia of one semicircular canal (Fig. 3). Both ears may be affected to a different degree. Congenital malformations may be acquired, e.g. after a gestational viral infection such as by rubella or cytomegalovirus, due to a genetic cause or the result of a combination of both, as in aminoglucoside-induced deafness with underlying mitochondrial DNA mutations .
Genetic deafness may be sporadic or hereditary. The mode of inheritance may be X-linked, dominant or recessive and the mutation may be chromosomal or mitochondrial. In syndromal cases, typical features of inner ear dysplasia and/or associated anomalies may be seen on imaging (Fig. 4).
Several classification systems have been proposed to group patients with a distinct radiographic pattern [4–7]. Jackler et al.  hypothesised that dysplasias result from a developmental arrest during varying stages of inner ear organogenesis and identified the following groups: labyrinthine aplasia, cochlear aplasia, cochlear hypoplasia, common cavity malformation and incomplete partitioning. The latter may be divided into cystic cochleovestibular dysplasia (incomplete partitioning type I) and incomplete partitioning type II (true Mondini malformation) . This is a useful framework to describe and understand labyrinthine dysplasias, however more variations exist. A report should contain a precise description of the aspect of the cochlea, modiolus, cochlear aperture, vestibule and SCCs, the vestibular aqueduct and eventual associated anomalies of the middle ear or external auditory canal in order to make a proper plan for hearing rehabilitation and patient counseling.
Abnormal density or signal intensity
Abnormal density or signal intensity of the inner ear may be due to obliteration of the labyrinth by infectious or inflammatory changes, involvement by a bone dysplasia, a tumour, or may result from intracochlear haemorrhage. All of these will cause altered density or signal of the labyrinth and some will show enhancement. The imaging aspect and clinical history will be helpful in differentiating these conditions.
Labyrinthitis may be of infectious (bacterial, viral), inflammatory or auto-immune origin, and the spread of disease may occur either contiguously through the middle ear, cerebrospinal fluid and meninges or haematogenously. It may also be seen as an inflammatory reaction after a translabyrinthine fracture.
In labyrinthitis the perilymphatic space will become filled with inflammatory cells. At this stage changes are still reversible. Eventually fibroblastic proliferation may occur, followed by bone formation, leading to permanent partial or complete obliteration of the inner ear. Rapid progression and some overlap of these stages may be seen. But, not all patients will run through all stages. The disease is considered a medical emergency at the acute stage because of the possible rapid progression to irreversible obliteration of the cochlea, which might preclude the patient from cochlear implantation, considered in case of severe or complete sensorineural hearing loss [8, 9].
Labyrinthine enhancement is seen in the acute stage. This may be difficult to appreciate when the labyrinth shows high signal intensity on the native T1-weighted images due to high protein content of inflammatory tissue. Once fibroblasts or osseous tissue fills the labyrinth, signal loss will be seen on thin-sliced three-dimensional (3D) T2-weighted images. Enhancement may persist in the presence of fibro-osseous changes, reflecting ongoing inflammatory reaction. The enhancement is diffuse or patchy and will decrease on follow-up scans. On CT, only osseous changes will be seen and the extent of obliteration may be underestimated (Fig. 5).
The most common bone dysplasia affecting the temporal bone is otosclerosis. This disease is exclusive to the otic capsule and may affect the patency of the cochlea by sclerosis of the round window membrane, otospongiotic foci extending into the cochlea or secondary to complicated stapedotomy. Otosclerotic involvement of the cochlea is usually limited to the proximal part of the scala tympani .
On CT, obliteration of the cochlea will be accompanied by otospongiotic foci in the surrounding bone (Fig. 6). These may be difficult to visualise on MRI.
Intracochlear haemorrhage will cause increased signal intensity on native T1-weighted images. This is seen in the acute or subacute setting of trauma.
Purely intralabyrintine schwannomas are rare. They are thought to arise from distal branches of the cochlear or vestibular nerves and are located in the cochlea, vestibule, semicircular canals or a combination of these locations. They have been reported to originate most often in the scala tympani. Growth into the scala vestibuli may occur later . These patients present with tinnitus, vertigo and progressive hearing loss leading to a dead ear over time.
The mass causes focal signal loss on T2-weighted images, is slighty hyperintens to cerebrospinal fluid (CSF) on native T1-weighted images and shows strong enhancement on contrast-enhanced T1 images (Fig. 7). Both the clinical history and the persistent, focal, intense enhancement will help to distinguish from the more diffuse or multifocal enhancement in acute labyrinthitis.
Extension of a meatal, vestibular schwannoma into the inner ear is more frequently seen and will be discussed in the next section. If present in a patient considered for surgical removal of the tumour, this requires a transotic approach for complete removal .
Internal auditory canal/cerebellopontine angle
The imaging modality of choice to assess the internal auditory canal (IAC) and its content as well as the cerebellopontine angle (CPA) cistern is MRI. There is some controversy whether the imaging protocol should include contrast enhanced images [13, 14]. Although the yield of contrast-enhanced images will be low, it may reveal inflammatory, auto-immune or tumoural pathology of the inner ear, cochleovestibular nerve or meninges that might not be suspected clinically and impossible or difficult to see on native images. Therefore, we favour the use of contrast in the diagnostic work-up of hearing loss.
A typical imaging protocol is performed with a head coil and includes axial thin-section 3D T2-weighted images, T1-weighted images without and with contrast enhancement and coronal T1-weighted enhanced scan of the temporal bone as well as a whole-brain screen [fluid-attenuated inversion-recovery (FLAIR) or T2-weighted]. A maximum section width of 3 mm with a minimal or no inter-slice gap and a small field of view (FOV) are needed .
The CPA cistern is an anatomic CSF-filled space located between the cerebellar peduncle, pons and internal auditory canal. Several vessels and the facial and cochleovestibular nerve course through it. On its way to the internal auditory canal, the cochleovestibular nerve will divide into three branches: the cochlear nerve and the superior and inferior vestibular nerve supplying the cochlea and the vestibular system respectively. These branches lie posterior and inferior to the facial nerve (Fig. 8).
Imaging patterns of disease
The formation of the IAC depends on the development of the cochleovestibular nerve. Mesoderm envelopes the eighth nerve and transforms into cartilage and bone to form the internal acoustic meatus . Altered development of the eighth nerve will induce stenosis, atresia or duplication of the IAC [17–19] (Figs. 1d, e and 9). Abnormal development of the eighth nerve may also occur within a normal-sized IAC, either as aplasia or hypoplasia of the cochlear branch. The normal size of the cochlear nerve on MRI measures 1.8 ± 0.2 mm at the porus acousticus and 1.2 ± 0.2 mm in the mid to distal IAC; the vestibular nerves are smaller .
Enhancement along the course of the cochleovestibular nerve may be the result of viral infection (e.g. herpes simplex, herpes zoster, cytomegalovirus), bacterial infection spreading from the middle ear and mastoid or from a skull base osteomyelitis or may be seen as a manifestation of leptomeningeal disease (e.g. Lyme disease, sarcoidosis, syphillis, tuberculosis) (Fig. 10). Perineural spread of cancer presenting as isolated cochleovestibular enhancement is highly unlikely.
Infectious neuritis will cause enhancement of or around the eighth nerve and/or within the labyrinth. The edematous, inflamed nerve may present as a mass-like lesion. In inflammatory neuritis, often multiple cranial nerves are affected. Associated findings of the underlying disease, such as brainstem involvement at the facial nerve root entry zone suggesting Lyme disease or meningeal enhancement as in tuberculous meningitis, should also be looked for.
Within the IAC and CPA, soft tissue mass lesions or cystic mass lesions can be found.The soft tissue lesions are the most likely to cause vestibular symptoms or hearing loss. By far the most common soft tissue tumour in this area is the schwannoma (85%). A meningioma occurs in about 3-5%. Metastases, lipoma, vascular lesions or tumours from surrounding structures extending into the CPA are infrequent. The cystic lesions are congenital abnormalities: an epidermoid cyst results from epithelial rests and accumulation of fluid within the arachnoid membrane will lead to the formation of an arachnoid cyst.
Soft tissue mass lesions
Schwannomas arise from the inferior or superior vestibular nerve, near Scarpa’s ganglion . The imaging appearance is variable and depends on the growth pattern, size and histological features of the tumour (Fig. 11). The classic growth pattern shows growth along the eighth nerve, involving both the IAC and CPA. Less frequently the tumour is confined to the CPA or IAC. The latter may extend into the labyrinth through the macula or modiolus . With continued growth, mainly of the extracanalicular component, histology changes to a predominance of Antoni type B cells and cystic regression and necrosis will cause a cystic appearance of the tumour. Microhaemorrhages are a well-known pathological feature of schwannomas. These may be demonstrated on a T2*-weighted gradient echo sequence. There have been sporadic reports of spontaneous gross intratumoural bleeding in a vestibular schwannoma or leading to subarachnoidal haemorrhage [25–27]. These patients most often present with acute cranial neuropathies and symptoms of subarachnoid haemorrhage . The likelihood for haemorrhage may be associated with preoperative radiotherapy and vascular abnormalities .
Schwannomas are typically centred on the porus and have a sharp angle towards the dura. Such an enhancing mass in the cone-shaped IAC, bulging into the CPA, resembles the image of an ice cream on a cone. Erosion of the porus may occur. Bilateral schwannomas should trigger the diagnosis of neurofibromatosis type II.
Meningiomas arise from the dura along the petrous bone or the tentorium; hence these tumours are usually located eccentric to the porus with no or only limited involvement of the IAC. They usually have obtuse angles towards the dura, which is often thickened (dural tail) (Fig. 12). Calcifications are seen in up to 25%.
Lipomas of the IAC and CPA are rare. They present as a mass lesion with fatty signal on all sequences with engulfment of neural and vascular structures (Fig. 13).
Vascular malformations causing hearing loss are rare. Hemangiomas, arising from the vascular plexus at Scarpa’s ganglion, usually present with facial nerve palsy . If small, tumours may mimic the far more common schwannoma. There is, however, usually a descrepancy between tumour size and nerve VII and VIII deficit. Large vascular tumours have a tendency to erode surrounding bone. High-flow vascular malformations are usually accompanied with tinnitus and show fairly characteristic imaging findings (Fig. 14).
Metastases or tumours extending into the CPA
Leptomeningeal or perineural tumour spread and lesions originating in the skull base, cerebellum or fourth ventricle may mimic vestibular schwannoma (Fig. 15). In case of metastatic disease there usually is a known history of malignancy. The most frequently encoutered primary sites are lung, breast, melanoma or lymphoma. Patients present with rapidly progressive symptoms, often combined with palsy of other cranial nerves, despite the small size of the lesion. Metastatic disease will show rapid growth.
An epidermoid is a congenital lesion resulting from inclusion of ectodermal epithelial elements. However, they only become symptomatic when large enough to cause pressure-related complaints or cranial nerve palsy or headache. This is usually around the 3rd decade. The lesions may also be seen as an incidental finding in asymptomatic patients. Epidermoid cysts are well-defined lesions with a lobular or irregular nodular surface. They mimic a cystic lesion on T1- and T2-weighted images, although the cyst is often hyperintense due to CSF on T2 and may show T1 shortening. FLAIR and diffusion-weighted imaging (DWI) sequences will show incomplete attenuation and diffusion restriction respectively (Fig. 16).
The clinical presentation of arachnoid cysts resemble that of an epidermoid cyst. The distinguishing imaging feature is signal loss on FLAIR and the absence of diffusion restriction (Fig. 17). When large, arachnoid cysts may cause displacement of nerves and vessels and bone remodellation.
Other cystic lesions
Other cystic lesions, such as a neurenteric or neuroepithelial cyst, are extremely rare.
Using apropriate imaging techniques and a pattern-based approach is helpful in detecting disease and narrowing the differential diagnosis in pathology of the inner ear and CPA cistern.
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Verbist, B.M. Imaging of sensorineural hearing loss: a pattern-based approach to diseases of the inner ear and cerebellopontine angle. Insights Imaging 3, 139–153 (2012). https://doi.org/10.1007/s13244-011-0134-z