Head and Neck Radiology

Chronic Stenosing Otitis Externa.

Recurrent otitis externa ( Fig. 61.2 ) may result in a fibrotic band of soft tissue occluding the more medial bony portion of the EAC. When considering this diagnosis, it is important for the radiologist to answer the following questions:

• Is there any erosion of the adjacent bony walls to suggest a more aggressive process such as neoplasia or necrotising otitis externa?

• What is the depth of the occluding soft tissue? Does the fibrotic tissue extend to involve the TM?

• Is there a normally pneumatised middle ear cleft deep to the occluding soft tissue?

• Always review the contralateral side as pathology at this site is commonly bilateral.

Exostoses and Osteoma of the External Auditory Canal.

EAC osteomas usually arise spontaneously, but exostoses are associated with repeated exposure to cold water and are also known as ‘surfer's ear’. The two lesions ( Fig. 61.3 ) cannot be distinguished histopathologically; however, exostoses are usually broad-based and bilateral, whereas osteomas are pedunculated, unilateral and usually lateral to the bony isthmus of the canal. They may cause sufficient narrowing of the EAC to require surgery. It is important when assessing EAC exostoses and osteomas to answer the following questions:

• What is the maximum depth and transverse diameter of the exostosis?

• What is the exact site of origin of the osteoma?

• What is the distance between the medial aspect of the exostosis and the TM and the deep aspect of the exostosis and the descending facial nerve canal?

• Are there any associated obstructed secretions in the medial EAC?

• Is the middle ear cleft normally pneumatised?

Keratosis Obturans.

Keratosis obturans ( Fig. 61.4 ) is usually a bilateral accumulation of keratin within the EAC. The computed tomography (CT) appearances are of soft tissue filling the EAC, which may be associated with expansion of the canal and remodelling but not erosion of the bony canal walls. If the bony wall of the canal is eroded, then alternative diagnoses such as neoplasia or EAC cholesteatoma should be considered. Keratosis obturans is associated with otalgia and conductive hearing loss and most frequently occurs in young men.

External Auditory Canal Cholesteatoma.

An EAC cholesteatoma ( Fig. 61.5 ) can be differentiated from keratosis obturans as the former is associated with erosion of the adjacent bony wall of the EAC. This condition is usually seen in an older population (>40 years) and it is usually indistinguishable on CT from SCC; otoscopy ± biopsy can be performed to distinguish these two conditions.

Necrotising Otitis Externa.

Also misleadingly known as malignant otitis externa, a term that was previously used due to the high mortality that was associated with this condition. The pathology is of necrosis, usually of the walls and in particular the floor of the EAC, at the bony-cartilaginous junction commonly in the elderly and crucially the immunocompromised (usually diabetic) patient who presents with severe otalgia. Pseudomonas aeruginosa is the usual causative pathogen. The infection commonly extends inferiorly via the fissures of Santorini (see the section ‘ Anatomy ’, above), causing a skull base osteomyelitis, and involves the soft tissues inferior to the skull base where the lower cranial nerves (VII to XII) may be affected ( Fig. 61.6 ). Effacement of adjacent fat planes before demineralisation and bone erosion is often the earliest indication of necrotising otitis externa (NOE) and soft-tissue windows on the HRCT should always be carefully assessed in the absence of any overt bony erosion. Anterior extension into the temporomandibular joint (TMJ) may cause destruction of the mandibular condyle and, in the absence of bony erosion, loss of the retrocondylar fat plane is a sensitive and early indication of the presence of NOE.

HRCT of temporal bone is the usual initial imaging technique. Magnetic resonance (MR) will more clearly assess any soft-tissue involvement, meningeal enhancement or oedema of the bone marrow.

It is important to answer the following questions when assessing patients with NOE:

• Is there any erosion of the bony walls of the EAC?

• Is there any inflammatory change in the soft tissues inferior to the skull base, in particular around the stylomastoid opening of the facial nerve and in the retrocondylar fat, and is there involvement/effacement of the normal parapharyngeal fat or the remainder of the fat planes inferior to the ipsilateral side of the skull base?

• Is there erosion of the skull base (look for loss of the normal dense line of the bony cortex of the clivus, petroclival region and petrous apex)?

• Is there anterior extension to involve the mandibular condyle or involvement of the contralateral side via the prevertebral and retropharyngeal soft tissues anterior to the clivus?

• Has the differential of an SCC of the EAC or a nasopharyngeal tumour with extension to the skull base been excluded—usually with clinical examination and/or biopsy?

Neoplasia of the Auricle and External Auditory Canal.

SCC and basal cell carcinoma (BCC) are rare tumours presenting in the elderly (F > M) with a painful, discharging ulcerative lesion of the EAC that most frequently extends inferiorly. CT is the usual imaging technique ( Fig. 61.7 ), but MR imaging (MRI) is recommended if there is suspected superior extension into the middle cranial fossa or medial extension into the middle ear cleft (both are rare). As in other parts of body, EAC BCC tends to be locally aggressive and rarely metastasises, whereas SCC can readily metastasise to regional lymph nodes, commonly the ipsilateral intraparotid nodes, which along with the ipsilateral upper deep cervical lymph nodes should be carefully examined. Minor salivary rests are distributed throughout the EAC and these can rarely become neoplastic. Ultimately, a biopsy is often required to distinguish the different possible neoplastic conditions from one another.

It is important to answer the following questions:

• As with all soft tissue noted within the bony EAC, is there any associated bony erosion?

• Is there extension outside of the EAC, in particular into the middle ear cleft?

• Is there any nodal involvement (in particular, review the intraparotid and upper deep cervical nodes)?

Congenital Atresia/Stenosis of the External Auditory Canal.

The EAC may be stenosed or completely atretic (absent). The atresia may be membranous, bony or a mixture of the two. The atresia is usually unilateral in non-syndromic and bilateral in syndromic patients ( Fig. 61.8 ). The rule of thumb is the more severe the EAC abnormality, the greater the auricular deformity (microtia/anotia). The inner ear structures are usually not affected unless there is severe EAC atresia with an absent auricle, small middle ear cleft and absent ossicles. If surgery is being considered it is important to assess the following on CT:

• The severity and type of atresia.

• The size and degree of pneumatisation of the middle ear cleft.

• The appearance of the ossicles, in particular:

  • Is a stapes superstructure present?

  • Is the oval window atretic?

  • The course of the tympanic segment of the intrapetrous facial nerve as the posterior genu (junction of tympanic and descending segments) is often more anteriorly and laterally positioned than normal and therefore at risk during surgery. In cases of oval window atresia, the tympanic portion of the facial nerve canal is usually relatively low-lying and tends to lie at the level of the atretic oval window, which would put the nerve at risk if surgery is considered. An alternative to surgery in these cases is a bone conduction device such as a bone-bridge.

Cholesteatoma.

Cholesteatoma is a poor term for this lesion as it neither is a tumour nor does it contain cholesterol. It is actually effectively ectopic skin. There are two types: the common acquired (98%) and the rarer congenital (2%) cholesteatoma. In acquired cholesteatoma, as a consequence of negative middle ear pressure, a retraction pocket develops usually in the more flexible pars flaccida (superior aspect) of the TM ( Fig. 61.10A ), but occasionally in the pars tensa (inferior aspect). Desquamated skin accumulates in the retraction pocket and can enlarge, causing bony destruction most frequently of the scutum and long process of the incus, but may also extend to involve the otic capsule overlying the lateral semicircular canal (see Fig. 61.10B ), which can cause dizziness, the skull base at the tegmen tympani and the tympanic facial nerve canal (causing facial palsy).

Usually the diagnosis of cholesteatoma is apparent from the otoscopic appearances of a retraction pocket in acquired and a retrotympanic ‘pearl’ of cholesteatoma behind an intact TM in congenital cholesteatoma ( Fig. 61.11 ). The clinician needs to know how extensive the cholesteatoma is, which can be assessed by the degree of soft tissue opacification and of ossicular or bony erosion.

In particular, the following questions need to be addressed:

• Is the bony roof (tegmen tympani or mastoideum) eroded?

• Is the otic capsule overlying the lateral semi-circular canal eroded?

• Is there ossicular erosion (in particular, is the stapes eroded)?

• Does the cholesteatoma abut the tympanic facial nerve canal and, if so, is the facial canalicular wall eroded?

• Are there anatomical variants such as a low-lying tegmen or a lateralised sigmoid sinus that may affect the surgical approach?

A pars flaccida cholesteatoma is identified by a mass in the attic (epitympanium) lateral to the head of malleus and body of incus; this is associated with ossicular erosion in 70% of cases. A pars tensa cholesteatoma is identified as soft tissue in the posterior mesotympanum often extending medial to the ossicles. HRCT is used for assessing any bony involvement. Non-echo planar imaging diffusion-weighted (non-EPI DWI) MRI is increasingly being used to assess whether there is any recurrent or residual cholesteatoma in patients who have undergone canal wall preservation surgery where otoscopy provides limited visualisation. Cholesteatoma consisting of desquamated skin shows markedly restricted diffusion ( Fig. 61.12 ) permitting its differentiation from other soft tissue (granulation tissue, etc.). A non-EPI DWI sequence should be acquired as it is associated with less artefactual high signal at the soft tissue-bone interface at the skull base than echoplanar DWI.

Tympanosclerosis.

The CT appearance is of foci of calcification that are punctate or web-like in the middle ear cleft ( Fig. 61.13 ) and TM. Usually this is associated with a long history of otitis media (OM). The suspensory ligaments and muscles may also calcify. There is varying conductive hearing loss depending on the degree of ossicular fixation. If the TM only is involved (calcified), the condition is known as myringosclerosis.

Otosclerosis.

The diagnosis is usually suggested clinically. The hearing loss may be conductive, mixed or sensorineural. Otospongiosis is a term also used and describes the spongiform changes within the involved bone. In patients with otosclerosis the normal dense petrous temporal bone is replaced by foci of spongy, less dense bone. Patients usually present in the third decade. The CT appearances are commonly bilateral (85%) and there may be a family history. There are two main types: fenestral and retrofenestral.

Fenestral.

The spongiotic foci occur in the lateral wall of the otic capsule in the fissula antefenestram (anterior to the oval window; Fig. 61.14 ) where it extends posteriorly to involve the stapedial footplate, fixing the anterior crus of the stapes resulting in conductive hearing loss; this is evidenced by thickening of the stapedial footplate, which normally should not measure more than 0.2 mm in thickness. The round window and cochlear promontory also may be involved.

Retrofenestral or pericochlear.

Spongiotic foci replace the normal dense bone of the otic capsule surrounding the cochlea (see Fig. 61.14 ) and may encroach on the cochlea and, less commonly, the vestibule and semicircular canals, causing mixed or sensorineural hearing loss (SNHL). The most frequent location for a spongiotic focus is between distal half of the basal turn of the cochlea and the anterior aspect of the lateral internal auditory canal. In severe cases the cochlea is encircled, giving the appearance of a cochlea within a cochlea ( Fig. 61.15 ). CT is used for assessing otosclerosis although the foci may be visible on MRI as high signal areas on T ₂ weighted imaging that enhance post-gadolinium ( Fig. 61.16 ).

Cochlear otosclerosis is rare in the absence of fenestral disease.

Questions to answer when reviewing the CT are the following:

• Is there fenestral or pericochlear otosclerosis or both?

• Is there bilateral involvement (in 85% on CT) that may only be clinically evident on one side?

• Is the round window also involved? Round window involvement may reduce the effectiveness of surgery.

• Is the whole oval window involved at the level of the stapedial footplate and what is the depth of involvement? A markedly thickened footplate will alter the surgical technique and reduce the likelihood of a successful outcome.

• Is the facial nerve dehiscent and does it take a normal intra-petrous course as this may complicate surgery?

Ossicular disruption.

Nearly all ossicular disruption is associated with a temporal bone fracture. Incudostapedial joint (ISJ) disruption is the commonest derangement ( Fig. 61.17 ). Malleoincudal disruption results in loss of the normal ‘ice cream cone’ appearance on axial images of the head of malleus articulating with the body of the incus ( Fig. 61.18 ). Complete dislocation of the incus is the next most common finding. Much more rarely found are fractures of the individual ossicles. The incus is the ossicle most frequently disrupted as the malleus is supported by three ligaments and the tensor tympani muscle and the stapes by the annular ligament and the stapedius muscle. Fractures of the malleus can occur if a foreign body, such as a cotton bud, is introduced into the external ear and it traumatises the TM and adjacent malleus ( Fig. 61.19 ).

When assessing ossicular trauma, look first for the normal ‘ice cream cone’ appearance of the malleoincudal joint and then closely assess the alignment of the distal long and lenticular processes of the incus with the head of stapes at the ISJ and compare with the normal contralateral side.

Venous sinus thrombosis.

The sigmoid sinus has an immediate posterior relation to the mastoid air cells and mastoiditis may result in sinus thrombosis with occasional severe intracranial complications ( Fig. 61.20 ). The transverse sinus and jugular bulb may also be involved, although the latter is usually thrombosed due to retrograde extension from the neck.

Always assess the bony dural sinus plate overlying the sigmoid sinus if adjacent air cells are opacified. If there is bony erosion, MR venography or CT venography may be helpful to exclude thrombosis.

Intracranial complications.

Fortunately, intracranial complications from petromastoid infection are now rare. The commonest is extradural empyema in the posterior fossa often associated with sigmoid sinus thrombosis ( Fig. 61.20 ). Very rarely subdural empyema and intracranial abscesses may occur ( Fig. 61.21A and B ).

Vestibular schwannoma.

Vestibular schwannomas are the most common cerebello-pontine angle (CPA) tumour and the most common lesion causing asymmetrical SNHL. Most are centred within the internal auditory canal, or at the porus acusticus and occur sporadically. When bilateral they indicate the diagnosis of neurofibromatosis type 2 (NF-2) and can be associated with meningiomas and ependymomas. Only a minority (approximately 2.5%) of patients who present with tinnitus or asymmetrical hearing loss have a vestibular schwannoma. There is no relationship between the size of the tumour and the degree of hearing loss. The management approach is ‘wait and watch’ for most vestibular schwannomas as approximately 60–70% do not increase in size on follow-up (MR) imaging. Those that require intervention have two options: surgery via retrosigmoid, translabyrinthine or subtemporal middle cranial fossa approach, or gamma-knife (radiation) treatment.

MRI is the imaging technique of choice when assessing patients for a possible vestibular schwannoma, but the protocols vary. High-resolution T ₂ sequences are the most commonly used with pre- and post-gadolinium-enhanced T ₁ weighted sequences reserved for the small percentage of patients where a neuroma is identified and requires confirmation ( Fig. 61.23 ) or if other abnormalities require clarification.

Trauma.

Skull base fractures involving the petrous bone are uncommon, but are important to identify as they may be associated with cerebrospinal fluid (CSF) otorrhoea or rhinorrhoea, facial nerve palsy or ossicular disruption.

In patients undergoing CT for head trauma HRCT reconstructions of the skull vault and skull base are usually part of the routine imaging protocol. Assessing any intracranial trauma is the initial priority, but identifying skull base and, upper cervical fractures is also essential.

Classically, petrous temporal fractures have been divided broadly into transverse fractures (20%) usually secondary to occipital or frontal trauma or longitudinal fractures (80%) secondary to temporoparietal trauma. However, the fracture line frequently takes an oblique course and prognostically it is more important to accurately describe its course and the structures involved.

When reviewing images for a possible petrous fracture, remember that transverse fractures usually start at sites of weakness such as the jugular foramen ( Fig. 61.24 ) or vestibular aqueduct and longitudinal fractures frequently involve the EAC, extending to the middle ear cleft or more inferiorly the posterior genu of the intrapetrous internal carotid artery, but usually sparing the facial nerve and inner ear structures.

CSF rhinorrhoea may be secondary to a petrous temporal fracture, with CSF passing along the eustachian tube into the postnasal space. This anatomical area should be carefully reviewed for fractures as a recognised but rare cause of CSF rhinorrhoea, along with the anterior and central skull base and sinonasal regions where fractures are more common. There is a 10% annual risk of meningitis in patients with CSF rhinorrhoea.

In patients with post-traumatic facial nerve palsy it is important to identify the exact site of trauma as it will alter both the surgical approach and repair technique.

As noted in the earlier section ‘ The Middle Ear ’, the commonest ossicular disruption is of the incudostapedial joint followed by the malleoincudal joint and then incus dislocation. The malleus and stapes are rarely involved as they are more firmly held in place both by a muscle (tensor tympani and stapedius) and several ligaments.

Congenital malformations.

Congenital malformation of the cochlea and/or labyrinth may be genetic (alone or as part of a syndrome) or non-genetic.

The inner ear and middle/external ear have independent embryological developments, but still approximately 10% of patients with EAC atresia have inner ear deformities. The inner ear structures develop between the 3rd and 22nd week of intrauterine life, and inner ear malformations are usually classified according to severity from the rare Michel deformity (arrest at 3rd week and complete absence of the inner ear) to mild dysplasia of the lateral semicircular canal (22nd week).

CT and MRI are complementary investigations and, although an exhaustive discussion on this subject is outside the scope of this chapter, in cases of labyrinthine malformations, the following checklist needs to be considered:

• Cochlea: Is there a normal basal turn? Is the modiolus present ( Fig. 61.25 )? Are there a normal number of turns?

  

• Vestibule: is it enlarged? Is it separate from the cochlea?

• Semi-circular canals: Are they present or dysplastic?

• Oval and round windows: Are they normal, narrowed, atretic?

• Vestibular aqueduct: Is the vestibular aqueduct and endolymphatic sac enlarged? This is the most common imaging finding in patients with SNHL dating to infancy and is frequently missed with serious clinical consequences ( Fig. 61.26 ).

  

• Cochlear nerve: Is the nerve present and of normal size?

• Facial nerve: Does the nerve take a normal course? Is it dehiscent?

Facial palsy.

Acute facial palsy is usually secondary to Bell palsy or trauma. Bell palsy is frequently seen clinically but is uncommonly imaged. Typically the patient develops a sudden facial paralysis, which recovers fully or incompletely after 2–3 months.

Imaging is mandatory for atypical cases; that is, progressive or recurrent facial palsy ( Fig. 61.27 ). CT and MRI are complementary. MRI is preferred but CT is usually performed if pathology such as cholesteatoma or otomastoiditis is suspected on otoscopy. Both the intra- and extra-cranial course of the facial nerve must be covered or lesions such as an impalpable malignant parotid tumour will be missed. On MRI, enhancement of the facial nerve within the geniculate ganglion, tympanic and mastoid segments can be seen normally usually due to enhancement of perineural vessels. Abnormal enhancement includes intense enhancement of the labyrinthine segment. Enhancement of the facial nerve in the fundus of the internal auditory canal is always abnormal ( Fig. 61.28 ).

Glomus tumours (paragangliomas).

Paragangliomas are the most common tumours causing pulsatile tinnitus and glomus tympanicum and glomus jugulotympanicum are the most common tumours of the middle ear and the second most common tumours of the temporal bone after vestibular schwannoma. It is essential to distinguish the two types. Glomus tympanicum arise on the medial wall of the middle ear cavity on the cochlear promontory and can be removed via a mastoid approach ( Fig. 61.29 ). Glomus jugulotympanicum are tumours arising in the jugular foramen that extend into the middle ear cleft usually requiring preoperative embolisation followed by skull base surgery ( Fig. 61.30 ).

HRCT and MRI are complementary. When the tumour extends into the adjacent bone, such as in a glomus jugulotympanicum or extensive glomus tympanicum, there is a characteristic permeative bony destruction seen on HRCT. MRI demonstrates an intensely enhancing tumour that on the unenhanced T ₁ weighted sequence may demonstrate a ‘salt and pepper’ appearance with the salt representing subacute haemorrhage and the pepper representing high-velocity flow voids in large tumour vessels ( Fig. 61.31 ). Other sites of paraganglionomas in the head and neck region include glomus vagale and carotid body tumours. A number of hereditary syndromes are associated with paraganglionoma formation, including multiple endocrine neoplasia types 1 and 2, von Hippel Lindau disease and mutations in the succinate dehydrogenase (SDH) genes.

Cochlear electrode implantation.

Implantable cochlear electrodes offer a chance of hearing for some individuals, typically those whose hearing has been damaged by childhood meningitis. An array of electrodes (usually 12, 16 or 22 depending on the device) are inserted into the scala tympani of the proximal basal turn. MRI and HRCT are usually both requested preoperatively to assess for the patency of the cochlea, the size of the cochlear nerve and any malformation or anatomical variant that might alter the surgical approach or technique. Assessment of implant position can be made intraoperatively or in the early postoperative period with plain x-ray (reverse Stenver view), HRCT or CBCT. Early studies suggest CBCT can assess whether the electrodes are within the scala tympani or vestibuli ( Fig. 61.32 ). Cochlear implantation is now commonly bilateral and may be performed in patients with unilateral hearing loss and debilitating tinnitus.