- Pictorial Review
- Open Access
Imaging of sensorineural hearing loss: a pattern-based approach to diseases of the inner ear and cerebellopontine angle
© The Author(s) 2011
- Received: 31 May 2011
- Accepted: 11 October 2011
- Published: 9 December 2011
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.
- Computed tomography
- Magnetic resonance imaging
- Inner ear
- Cerebellopontine angle
- Hearing loss
- Pictorial review
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 .
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 .
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].
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 .
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.
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 .
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 .
Imaging patterns of disease
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 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.
Metastases or tumours extending into the CPA
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.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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