The most common benign tumour of the middle ear and mastoid is the glomus tympanicum paraganglioma (O’Leary et al. 1991). Paragangliomas are neuroendocrine tumours, originating from chromaffin cells in paraganglia or chromaffin-negative glomus cells derived from the embryonic neural crest. In the middle ear, these cells are found anywhere along the course of Jacobson’s nerve (the tympanic branch of the glossopharyngeal nerve) which enters the middle ear through the tympanic canaliculus and forms the tympanic plexus, which ramifies on the surface of the cochlear promontory and medial wall of the middle ear. The term glomus jugulotympanicum refers to paragangliomas that involve both the jugular foramen and middle ear. These tumours usually arise from glomus cells derived either from Jacobson’s nerve or Arnold’s nerve (the mastoid or auricular branch of the vagus nerve). Peak incidence is in the fifth and sixth decades and there is a clear female predominance (3:1).

The most common presenting symptom is pulsatile tinnitus. If the tumour is large, it may cause conductive hearing loss. Facial nerve palsy, sensorineural hearing loss or vertigo may result if the facial nerve or inner ear becomes involved. Other cranial nerve palsies may result from extension to the jugular foramen or hypoglossal canal. Occasionally, they may be asymptomatic. On clinical examination, a vascular retrotympanic mass is seen. The symptoms and clinical findings of a glomus tympanicum tumour may be indistinguishable from glomus jugulotympanicum, aberrant internal carotid artery or a dehiscent, high-riding jugular bulb and imaging is required to differentiate these entities. On high resolution CT, a nodular soft tissue mass is typically seen in the hypotympanum near the cochlear promontory (Fig. 2). The mass may fill the middle ear cavity but generally spares the ossicles. Bone erosion is usually not a feature, even in large tumours. Tumours may fill the epitympanum, attic and antrum, resulting in fluid retention in the mastoid. On MRI, the mass is seen as a strongly enhancing lesion within the signal void of the middle ear-mastoid complex, on T1-weighted post-contrast sequences, with variable T2-weighted signal. Larger tumours may exhibit the characteristic ‘salt and pepper’ appearance on T1-weighted sequences, with the ‘pepper’ representing hypointense signal flow voids caused by the large feeding arteries and the ‘salt’ representing subacute haemorrhage within the tumour.

The next most common tumour involving the middle ear is the schwannoma. The most common schwannoma of the middle ear arises from the facial nerve, though it may arise from any nerve of the middle ear, including Jacobson’s nerve and Arnold’s nerve (Aydin et al. 2000; Wiet et al. 1985). On clinical examination, schwannomas may appear as a fleshy-white mass behind an intact tympanic membrane. On imaging, a well-circumscribed lobulated mass may be seen arising from the tympanic or mastoid segments of the facial nerve (Fig. 3) or lying separate from the facial nerve canal when the facial nerve is not involved. Post-contrast T1-weighted MRI shows enhancement of the mass.

Congenital cholesteatomas are the next most common differential for a middle ear mass. Congenital cholesteatomas typically present with a whitish mass seen behind an intact tympanic membrane, as opposed to acquired cholesteatomas, which arise from a perforation or retraction pocket in the tympanic membrane and are associated with a history of chronic otorrhoea and recurrent middle ear infection. Congenital cholesteatomas arise when there is aberrant migration of external canal ectoderm beyond the tympanic ring. As a result, the tympanic membrane is intact. Both congenital and acquired cholesteatomas form as a result of the accumulation of exfoliated keratinous material in a sac lined by squamous epithelial cells. On CT, congenital cholesteatomas appear as a nodular soft tissue mass in the hypo- or mesotympanum, with or without ossicular erosion. Prussak’s space and the scutum are typically normal. On MRI, congenital cholesteatomas demonstrate intermediate signal intensity on T2-weighted and hypointense signal intensity on T1-weighted sequences without any enhancement post contrast. Rim enhancement is occasionally seen due to surrounding granulation tissue. Diffusion weighted MRI (DWI) may be used to differentiate congenital cholesteatoma from other middle ear masses, as cholesteatomas appear hyperintense on B1000 DWI sequences. Non-echoplanar sequences are preferred, as they allow for thinner sections and are less sensitive to susceptibility artefacts at the brain-bone interface (De Foer et al. 2006; Schwartz et al. 2011).

The middle ear adenoma is a rare tumour of the middle ear, considered by some to encompass a spectrum of histological types ranging from adenoma to carcinoid, depending on the degree of glandular or neuroendocrine differentiation (Torske et al. 2002). Middle ear adenomas most commonly present with conductive hearing loss and a soft tissue mass behind an intact tympanic membrane. On CT, they appear as a soft tissue mass engulfing the ossicles, indistinguishable from the far more common congenital cholesteatoma. Larger tumours may cause ossicular erosion. On MRI, they appear as an enhancing mass not confined to the cochlear promontory and may mimic glomus tympanicum (Zan et al. 2009).

Malignant tumours of the middle ear are rare. The most common malignant middle ear tumour is SCC, which is thought to arise secondary to squamous metaplasia, mainly in the setting of chronic otitis media (Fig. 4). A high prevalence of human papilloma virus (HPV) has also been reported in middle ear SCC associated with chronic otitis media (Jin et al. 1997).

Rhabdomyosarcomas are the most common paediatric soft tissue sarcoma and most common middle ear malignancy in children, although the temporal bone is an uncommon site of involvement (Adrassy 1997). Initially, the tumour may present as chronic otitis media, unresponsive to antibiotics. Cranial nerve dysfunction is common, particularly facial nerve palsy. CT may demonstrate a soft tissue mass with extensive bony destruction. MRI is useful to delineate the extent of the tumour, the intracranial extent and to differentiate tumour from fluid, although findings are non-specific. Rhabdomyosarcomas are typically isointense to muscle on T1-weighted, iso- to slightly hyperintense to muscle on T2-weighted and diffusely enhancing on contrast enhanced T1-weighted sequences. Langerhans cell histiocytosis (LCH) is the main differential for a soft tissue mass with aggressive bony destruction in a child. LCH may indistinguishable from rhabdomyosarcoma on imaging and biopsy is often necessary. Acute coalescent otomastoiditis may also be associated with soft tissue density in the middle ear and bony destruction, however patients are usually acutely ill and respond to antibiotics.

Schwannomas are benign encapsulated tumours arising from the Schwann cells that wrap around cranial nerves. Histologically, they comprise of differentiated Schwann cells forming Antoni A (areas of compact spindle cells) and B patterns (areas of loosely arranged matrix with lipid-laden cells and cysts). Malignant schwannomas are very rare but have been reported (Balasubramaniam 1999).