Skull Base, Temporal Bone, Cranial Nerves

Skull Base

From nose to occipital protuberace, comprising ethmoid, sphenoid, occipital, temporal and frontal bones. Most malignant lesions are metastatic, only 2-3% are primary.

Temporal Bone Fractures

Orientation relates to petrous ridge. Any injury to the membranous labyrinth results in immediate vertigo before permanent dead ear.

  • Transverse (vertical) – Laterally through cochlea or vestibule; or medially through IAC and petrous apex. High rate of facial nerve injury. May cause sensorineural hearing loss, transection of cochlear nerve, haemotympanum.
  • Longitudinal (horozontal, much more common than transverse) – Along plane of petrous bone from EAC through middle ear towards sphenoid. Often associated with ossicular dislocation with conductive hearing loss (esp incudostapedial joint with fracture of long process incus). High rate of haemotympanum, otorrhea. Facial nerve may be injured at geniculate ganglion. Rarely involves the labyrinth. Commonly involves glenoid fossa. Common tympanic membrane perforation.
  • Complex (mixed, oblique) – Most, with vertical and horizontal components.

Cephalocoele

May be spheno-pharyngeal (through sphenoid body), spheno-orbital (through superior orbital fissure), spheno-ethmoidal (through sphenoid and ethmoids), trans-ethmoidal (through cribriform plate) or spheno-maxillary (through maxillary sinus). May include meninges, brain or CSF. Cause round, smooth bony erosion.

Tolosa-Hunt Syndrome

Idiopathic inflammation involving cavernous sinus and orbital apex with lymphocytic and plasma cell infiltration. Recurrent steady gnawing retro-orbital pain, CN deficits. MR may be normal. Enhancing soft tissue in cavernous sinus extending to orbital apex and middle cranial fossa. Narrowing of cavernous ICA, irregularity/thrombosis of superior ophthalmic vein, cavernous sinus. Dural thickening. Good response to steroids. DDx sarcoidosis, meningioma, lymphoma, metastases, neurotropic spread of tumours/infections.

Langerhans Cell Histiocytosis (LCH)

Eosinophilic granuloma has propensity to mastoids. Children and young adults. Lytic, low T1, high T2, enhances.

Fibrous Dysplasia

Increased density with ground-glass. Mastoid more commonly affected of the temporal bone. May cause conductive hearing loss.

Paget Disease

May cause sensorineural or conductive hearing loss. Early lytic and late sclerotic phases. Diffuse lysis involving bony labyrinth starting medially in petrous apex progresing laterally; peripheral to central (cf otosclerosis).

Skull Base Meningioma

Cavernous sinus meningiomas usually follow the lateral wall, may extend along the tentorium with a dove’s tail appearance. Can encase the ICA. May extend through base of skull foramina into orbit, paranasal sinuses, nasopharyngeal space, parapharyngeal space, jugular foramen, foramen ovale.

Trigeminal Neuropathy

Trigeminal neuralgia is a clinical syndrome of intense recurrent electric-shock like pain involving just V2 and/or V3 divisions. Occasionally (in type 2) pain may be more constant dull/burning pain.

  • Type 1/classic trigeminal neuralgia (most) – Intense episodic pain. Often neurovascular compression of the root entry zone.
  • Type 2/symptomatic trigeminal neuralgia – More constant pain, may have some sensory change. Other pathology involving the nerve or nucleus. Imaging will need to include the nucleus (pons down to C2), and cover length of CN5 into face.

Trigeminal neuropathy is any other symptoms involving CN5 eg pain involving V1, numbness, tingling etc. this is usually due to pathology involving the nerve (eg inflammation/tumour/infection/compression).

Neurovascular Compression (NVC)

A vessel in close proximity to a nerve may cause demyelination and ephaptic crosstalk. Most commonly involves the root entry zone of CN5, also shown to affect CN7 and CN9/10. May be contact of displacement/compression of the nerve. Usually SCA, occasionally perimesencephalic vein, AICA and vertebral a; Tx microvascular decompression. The root entry/exit zone includes the central/brainstem fibres (part of CNS) and transition zone where oligodendrocyte myelin transitions to more robust schwann cell myelin. CN7 root exit zone has an attached segment to the brainstem before detaching as the transition zone at the supraolivary fosette (best appreaciated on oblique coronal).

Trigeminal Schwannoma

Rare, from Meckel’s cave, posterior fossa or dumbbell shaped in both. Peak 30s, M=F, trigeminal neuropathy, numbness and parasthesia. Iso T1, high T2, avid enhancement, may have cystic change (more common than in meningiomas). Grow through neural foramina causing smooth enlargement (foramen ovale/rotundum, superior orbital fissure), best seen on sagittal images. Erosion of the petrous apex (DDx epidermoid, giant cholesterol cyst, schwannoma, meningioma, chordoma, metastases, osteochondroma, chondrosarcoma). Schwannomas more likely arise on sensory nerves, hence rare on CN3,4,6. Trigemial neuralgia

Chordoma

Bony neoplasm from remnants of primitive notochord, anywhere along craniospinal axis. 35% in clivus (20-40yo), 50% in sacrum (40-60yo), 15% in vertebral bodies. Intracranial lesions at spehno-occipital synchondrosis (DDx chondrosarcoma), basiocciput, parasella, rarely paranasal sinuses. Most are histologically benign, but locally invasive with poor prognosis and complete excision rarely possible. May spread into nasopharynx or prepontine space. Midline destructive lesion with soft tissue mass, iso T1, invariably very high T2, enhances, calcification in 50%. DDx chondroma, myeloma, osteochondroma, GCT, sphenoid sinus tumour, chondrosarcoma, metastasis, nasopharyngeal carcinoma, craniopharyngioma, pituitary adenoma, glomus tumour, meningioma, schwannoma, epidermoid.

Chondrosarcoma

Rare malignant tumour from cartilage (skull base preformed in cartilage), commonly parasellar petroclival junction, petrosphenoid fissure, CP angle, convexity. 10s-30s, long history of HA and CN palsies. May be associated with enchondromatosis or Maffucci syndrome. Calcification in 60% (stippled, amorphous, arc/ring-like). Lytic bone destruction. Low/intermediate T1, high T2, heterogeneous, marked heterogeneous enhancement. Rarely haemorrahge.

Osteogenic Sarcoma

Usually from previous radiotherapy or malignant transformation of Paget disease.

Metastasis

Metastases from neutrotropic, subarachonid or haematogenous spread to Meckel’s cave is common. Perineural spread (esp V3) via skull base foramina esp adenoid cystic carcinoma, BCC, SCC, lymphoma, mucoepidermoid carcinoma, melanoma, schwannoma; also certain infections (actinomycosis, Lyme disease, herpes zoster). This causes nerve thickening, enlargement of foramen ovale, replacement of normal CSF in the Meckel’s cave, mass in the cavernous sinus, dennervation myositis/atrophy high T2 and enhancement of ipsilateral masticator muscles.

Metastases to skull base include prostate (osteoblastic, meningioma-like bony reaction), lung and breast (usually lytic), myeloma/plasmacytoma. Chloroma (granulocytic sarcoma) – solid tumour of myelogenous cells in leukemia, esp paranasal sinuses, orbit, sphenoid sinus, may extend through clivus. DDx Wegener granulomatosis, inflammatory pseudotumour.

External Auditory Canal (EAC)

Ectoderm from 1st branchial groove/cleft; endoderm from 1st and 2nd branchial pouch.

Congenital Dysplasias

Common, ranges from atresia, webs, hypoplasia, to stenosis of EAC. Associated microtia (small auricle of ear) correlates with degree of stenosis, pneumatisation of middle ear and mastoid, dysplastic inner ear. Higher risk in thalidomie and rubella in pregnancy. May be associated with dysplasia of the TMJ (also from 1st branchial groove).

  • Stenotic EAC – Slope is more vertical, may have keratinous plugs, cholesteatomas. Facial nerve has aberrant course close to stapes footplate, may be dehiscent.

First Branchial Cleft Cyst

Around ear or in parotid gland. May have fistula to EAC bone-cartilage junction.

Exostoses

(Surfer’s ear). Esp swimming in cold water.

Otitis Externa

  • Acute external otitis (swimmer’s ear) – Usually Pseudomonas. Rarely causes mastoiditis or osteomyelitis.
  • Malignant/necrotising otitis externa – Agressive pseudomonas infection, mostly elderly diabetics, HIV. Putrid purulent discharge. Starts at bone-cartilage junction, spreads along fissures or Santorini (2 horizontal fissures at anteroinferior EAC) to parapharyngeal space/parotid. Irrigation under pressure may spread infection to infratemporal fossa, nasopharynx, parapharyngeal space, bone, TMJ, middle/inner ear, intracranial spaces. May cause venous sinus thrombosis. Soft tissue in EAC, bony erosion, osteitis, obliterated parapharyngeal fat. Takes up Tc99m on bone scans.

Keratosis Obturans

Plugs of keratin in EAC, very painful, usually bilateral in middle-aged adults with bronchiectasis or sinusitis. DDx cholesteatoma (from middle ear which has more bony erosion, unilateral).

Tumours

Benign tumours are soft tissue masses, may expand EAC without bony destruction. May enhance.

  • Benign wax buildup (cerumen impaction)
  • Haemangioma
  • Venous vascular malformation
  • Nevi
  • Ceruminoma (adenoma of the ceruminous gland)
  • Polyps
  • Minor salivary gland tumour

Malignant tumours: Lymphatic drainage to intraparotid, retropharyngeal, occipital and skull base nodes.

  • Squamous cell carcinoma – Most common, higher risk with sun exposure. May invade cartilage, middle ear or temporal bone with poor prognosis. Early pain due to periosteal spread or extension into TMJ. Early facial nerve involvement.
  • Basal cell carcinoma – Similar to SCC.
  • Melanoma
  • Kaposi sarcoma in HIV
  • Rhabdomyosarcoma, lymphoma in children
  • Metastases are rare

Middle Ear and Mastoid

Inflammatory disease thought to be from Eustachian tube dysfunction causing reduced intratympanic pressure. Middle ear effusions may be infectious or noninfections (including pressure phenomenon from air-travel, radiotherapy, long-standing NGT); may lead to mastoid opacification.

Congenital Dysplasias

Not as common as EAC. Range from ossicular fusion, hypoplasia, fixation to attic. Esp long process incus, stapes. May be associated with facial nerve anomalies.

Otitis Media

Opacification of the epitympanic recess, thickened tympanic membrane. Most are viral with serous exudate; suppurative disease usually superinfection with S.pneumoniae, moraxella catarrhalis, H.influenzea. Usually obstructed eustachian tube fom lymphoid hypertrophy (URTI) in children. Occasional pneumolabyrinth from labyrinthine fistula, gas-forming organisms. Rarely causes ossicular disruption from erosion of long process incus, doesn’t erode scutum (cf cholesteatoma).

Chronic otitis media usually from Pseudomonas aeruginosa, S.aureus, fungal or mixed flora. May cause ossicular fixation which may be fibrous or tympanosclerotic (calcification around ossicles or ligaments), perforate eardrum, spread to mastoids, penetrate into middle cranial fossa. Necrotizing disease may occur with P.aeruginosa in diabetic patients (see otitis externa). Normal mastoid development requires normal atmospheric pressure; chronic reduction in pressure causes maldevelopment and an infantile/sclerotic mastoid.

Mastoiditis

From otitis media via the aditus ad antrum (narrow channel connecting the two) or cholesteatoma. Represents bony infection rather than mucositis. Most are β-haemolytic strep or pneumococcus. Bony destruction indicates coalescent mastoiditis. High T2, occasionally fluid levels. Cx sigmoid sinus thrombosis, thrombophlebitis, epidural abscess, meningitis, subperiosteal abscess, fistulas, osteomyelitis. Bezold’s abscess – collection inferior to mastoid tip, may spread down SCM.

Acquired Cholesteatoma

Ingrowth of squamous epithelium of perforated tympanic mebrane, begining as localised retraction pockets. Cystic lesions lined by keratonising squamous epithelium or metaplastic mucus-secreting epithelium, filled with amorphous debris (desquamated epithelium, occasionally cholesterol) and surrounding chronic inflammatory reaction. Tympanic membrane is white (cf red in globus tympanicum). From recurrent otitis media. Most common in Prussak space between the pars flaccida (superior membrane which retracts easily), scutum and neck of malleus. Soft tissue causes mass effect with expansion, displaces ossicles medially. Cholesteatoma sac has a rounded convex contour (cf fluid), surrounding bony sclerosis (cf fluid). Need to mention if there is any extension in to the sinus tympani and facial nerve recess (may be hidden from otoscopic view). Retracted tympanic membrane. Low T1, intermediate T2, diffusion restriction, doesn’t enhance (cf posoperative granulation tissue). CT useful to determine size, status of ossicles, labyrinth, tegmen and facial nerve. Pars tensa cholesteatomas are much less common, from perforation of the posterosuperior pars tensa, expanding or eroding the sinus tympani, pyramidal eminence, facial recess, long process incus, stapes. Complications:

  • Erosion (common) of scutum, head of malleus, body of incus.
  • Spread through aditus ad antrum (expanding this) into mastoid air cells.
  • Semicircular canal fistula (4-25%, esp lateral canal) – Dehiscence of the bony labyrinth.
  • Erosion of the tegmen tympani (roof of epitympanic space) with intracranial invasion.
  • Erosion of lateral or inferior wall, causing dehiscence or skeletization of the facial nerve canal or sinus tympani.
  • SCC arising from cholesteatoma.

Epidermoid

(Congenital/primary cholesteatoma, epidermoidoma). Aberrant epithelial rests in petrous apex, Koerner’s septum (petrosquamosal suture), mastoid air cells, eustachian tube opening, geniculate ganglion, middle ear-epitympanum junction, incudostapedial joint, sinus tympani, facial nerve recess. May also be in IAC, basal and CP angle cisterns. Pearly white, hypodense, low T1, high T2 (as bright as CSF), bright FLAIR and DWI. Scalloped margins, erosive, noninvasive. Not associated with perforation of the tympanic membrane, no history of ear infections or previous surgery. Presents with deafness, vertigo, facial nerve palsy. Scutum usually intact. Does not enhance, may be solid or cystic.

Vascular Variants

On coronal scans ICA is at the level of the cochlea, IJV at the level of the vestibule.

  • Aberrant ICA which deviates posterolaterally near the cochlear.
  • Persistent stapedial artery – Anwhere in middle ear, enters near stapes and exits near geniculate ganglion. Replaces middle meningeal arterry with absent foramen spinosum.
  • Dehiscent jugular bulb – Absent bony plate at the superior jugular foramen.
  • High jugular bulb (7%) – Superior to inferior at tympanic rim, may extend into middle ear or petrous bone near endolymphatic sac.
  • Jugular bulb diverticula into middle ear.

Glomus Tumours

Glomus tumours arise from globus bodies (paraganglioma tissue). Causes tinnitus. Multiple paragangliomas in 15% including jugulare, vagale, carotid body and tympanicum. Rarely metastasizes. May secrete norepinephrine.

  • Glomus tympanicum – From neural crest tissue along tympanic branch of CN IX (Jacobson’s nerve) which forms the tympanic plexus over the chochlear promontory. Middle-aged women. Usually small (Sx early) mass behind tympanic membrane, marked enhancement. Tympanic membrane is red (DDx dihiscence of the ICA). Usually along lateral cochlea in middle ear, at or anterior to the chochlear promontory. Engulfs ossicles (cf erosion in cholesteatoma).
  • Globus jugulare – Usually from adventitia of jugular vein in jugular foramen. May extend superiorly into middle ear similating glomus tympanicum. Salt-and-pepper appearance with flow voids within the mass seen on T2 and post-T1. Erodes jugular foramen of temporal bone. May grow inferior into jugular vein or superior into sigmoid and transverse sinuses. May cause venous thrombosis.

Haemangioma

  • Tympanic membrane haemangiomas – Lie behind the tympanic membrane. Marked enhancement, nonspecific density/intensity.
  • Facial haemangiomas – Along the facial nerve. Bony erosion, ‘honeycomb’ internal bony trabeculation/calcification. Indistinct margins. Haemangioma can be surgically resected (cf schwannoma sacrifices the nerve).

Facial Nerve Enhancement

Facial nerves may enhance in geniculate ganglion, horizontal and descending portions (due to surrounding AV plexus); never normally enhances in IAC, canalicular, labyrinthine portions or parotid gland. Enhancement seen in schwannoma, Lyme disease, lymphoma, haemangioma, sarcoidosis, Bell’s palsy, Ramsay-Hunt syndrome, Guillain-Barre syndrome, perineural tumour spread.

  • Bell’s palsy – Self-limited viral infection with swelling in the narrow distal intrameatal segment of IAC causing palsy. May have labyrinthine segment enhancement proximal to geniculate ganglion.
  • Facial schwannoma – May arise anywhere along facial nerve. Expansion of facial nerve canal. Enhances. DDx vestibular schwannoma, glomus tumour.

Ossicular Disruption

From trauma (most) or infection. Usually incudostapedial dislocation.  Malleus held by tight ligaments in epitympanum and tympanic membraine; stapes is anchored to oval window. Incus is only held by articulartion with malleus/stapes, can dislocate. >1mm between long process incus and head of malleus, ‘icecream falling of the cone’ (actually the ‘cone’ is displaced) on axial, both incus and stapes seen on same slice on coronal (should never both be seen normally).

Surgery

Mastoid cavity usually filled with bone chips, fascia or fat.

  • Canal wall up mastoidectomy (simple mastoidectomy) – Removal of involved mastoid air cells with sparing of posterior wall EAC, ossicular chain.
  • Canal wall down mastoidectomy (modified radical mastoidectomy) – Removal of posterior wall EAC, preservation of ossicles.
  • Radical mastoidectomy – Removal of mastoid cells and ossicular chain, but preservation of stapes (hence partial prosthesis can be used).
  • Ossiculoplasty (tympanoplasty) – Restoration of ossicular chain function damaged by cholesteatoma, chronic otitis media or congenital malformation
    • Type 1 – Graft rests on malleus, ossicles all spared.
    • Type 2 – Graft rests on incus.
    • Type 3 (most common) – Graft rests on head of stapes.
    • Type 4 – Graft connects to footplate of stapes.
    • Type 5 – Stapes removed.
  • Stapedotomy – Tiny wire connects long process incus to stapes footplate
  • Stapedectomy – Removal of footplate with implant placed from incus, through oval window into labyrinth. This should not extend beyond 0.25mm into the oval window and there should be no air-gap between prosthesis and oval window.
  • Ossicular replacement prostheses – Now usually synthetic (Proplast, Plastipore). May be partial ossicular replacement prosthesis (PROP) or total (TORP). Failed ossicular replacement from recurrent otitis media, recurrent cholesteatoma, reparative granuloma (foreign body reaction), ossicular subluxation/dislocation (most), adhesions, prosthetic fracture, perilymphatic fistula, granulation tissue, recurrence of otospongiotic bone, excessive postop bony reaction or extrusion. Soft tissue may be residual/recurrent cholesteatoma (more focal, erosive, DWI restriction), scar tissue, acute inflammation of granulation tissue (usually enhances).

Inner Ear and Petrous Apex

Congenital Labyrinthine Abnormalities

May be from thalidomide exposure, congenital rubella, CMV or genetic disorders. Cochlear develops at 5-8/40, semicircular canals at 8-9/40 (superior -> posterior -> lateral).

Cochlear abnormalities:

  • Complete labyrinthine aplasia – Total absence of inner ear structures with common cavity, absent IAC.
    • Michel’s aplasia – Complete labyrinthine aplasia, may be associated with other skull base deformities (eg platybasia), abnormal course of facial nerve, jugular vein anomalies. Absent stapes and oval window. Bilateral in >50%. IAC narrow or absent.
  • Cock’s deformity (Michel’s dysplasia, common cystic cavity; 2nd most common) – Insult at 4-5/40 with common cavity of vestibule and cochlea. No modiolus. Semicircular canals may be abnormal.
  • Cochlear aplasia (5/40) and hypoplasia (6/40) – Failed development with no or just 1 cochlear turn. Unilateral hearing loss. May have perilymph fistulas (between scala vestibuli and tympani). May have missing ipsilateral cochlear nerve and small IAC.
  • Incomplete partition deformity from abnormal scala development.
    • IP-1 – Unpartitioned chochlea, cystic cochleovestibular malformation. ‘Empty’ cochlear lacking modiolus and interscala septa creating cyst. Grossly dilated vestibule with absent vestibular aqueduct.
    • IP-2 (Mondini’s deformity, most common) – Insult at 7-8/40 causing incompletely partitioned cochlea. Incomplete development of the 2.5 turns of chochlea with middle and apical turns ballooning into a cyst. Normal basal turn. Dilated vestibule. Enlarged endolymphatic sac with large vestibular aqueduct in 20%.
      • Pendred syndrome – Sensorineural hearing loss and thyroid dysfunction, common Mondini’s deformity.
  • Enlarged cochlear aqueduct – Medial orifice >1.5mm, midportion >1.2mm (normally tapers laterally). Communication between scala tympani and subarachnoid space. May cause impaired hearing, recurrent meningitis, ear infections.

Perilymphatic fistula – Abnormal connection between subarachnoid CSF and perilymphatic space; commonly oval and round windows (may be associated with stapes malformations). Middle ear infections may cause meningitis, may cause labyrinthitis ossificans. From congenital (enlarged vestibular aqueducts, Mondini’s/Michel’s/Cock’s deformities) or acquired causes (cholesteatomas, chronic otitis media, trauma).

Semicircular canal abnormalities:

  • Aplasia – May be associated with CHARGE syndrome.
  • Hypoplasia – Mostly lateral canal (last to form). Compensatory enlargement of vestibule. Hearing loss. If isolated implies defect after 8-9/40, after cochlear development.
  • Superior semicircular canal dehiscence – Tullio phenomenon – oscillopsia (perception of stationary objects moving) and noise-induced vertigo (DDx otosyphilis, Meniere disease, perilymphatic fistula, Lyme disease). ?Congenital predisposition and traumatic event. Dehiscence only seen on fine slice CT. May be associated with cochlear anomalies, tegmen tympani defects.

Enlarged vestibular aqueduct syndrome (large endolymphatic sac, LEDS) – Most common congenital sensorineural hearing loss. Enlarged flared aqueduct >1.5-2mm, > size of semicircular canal. Progreessive stepwise permanent sensorineural hearing loss after episodes of minor head trauma. May be associated with abnormal cochlear spiralisation (75%), cystic vestibules (30%) or abnormal semicircular canals (25%). May be associated with CHARGE syndrome, Pendred syndrome, congenital CMV.

Atresia/stenosis of IAC may be associated with absent CN8 or CN7 (less common). CN7 may leave the IAC early or aberrantly.

Achondroplasia causes poorly developed mastoid air cells, upward tilting/towering of the petrous ridges and IAC, rotation of chochlea and ossicles, chronic otomastoiditis, narrowing of skull base and foramen magnum.

Labyrinthitis

Sudden hearing loss and vertigo. May be viral, bacterial, luetic (syphilis), autoimmune (to cochlear antigens) or idiopathic. Enhancement of cochlea and vestibule on MR. May also enhance after schwannoma resection, haemorrahge after trauma, labyrinthine schwannoma, Cogan syndrome.

Petrous Apicitis

Inflammation in previously aerated petrous apex (only pneumatised in 1/3). May be associated with Gradenigo syndrome (pain in CN5, palsy CN6, otorea). Low T1 (may be high if chronic with protein), high T2, no destruction. May have dural enhancement of Meckel’s cave. May cause adjacent sigmoid sinus thrombosis.

Cholesterol Granuloma

(Giant cholesterol cyst, chocalate cyst, blue-domed cysts). Typically petrous apex. Chronic obstruction and negative pressure of an air cell -> small blood vessel rupture with recurrent haemorrhage -> blood resorbed leaving cholesterol. Foreign body reaction with giant cell and fibroblast proliferation, cholesteral deposition, recurrent subclinical haemorrhages. Lined by fibrous connective tissue (cf cholesteatomas with stratified squamous epithelium). Lesions are expansile, may cause CN5/8 abnormality. Prevously pneumatised petrous apex filled with soft tissue being high on all sequences including T1 (from haemorrhagic products and cholesterol). DDx mucocele of petrous apex, petrous apicitis, haemorrhagic metastasis.

Meniere Disease

(Endolymphatic hydrops). Absent visualisation of the endolymphatic sac on high resolution 3D T2WI, becoming visible again with improvement. May show endolymphatic sac enhancement.

Labyrinthine Schwannoma

Rare, close to round window niche or within scala tympani of basal and second turns of cochlea. Marked enhancement on MR. From the cochlear branch of CN8. May be associated with NF2.

Endolymphatic Sac Tumour (ELST)

(Previously adenomatoid papillary tumour). Arises from the top of jugular bulb, mucosa of cells around jugular bulb or mastoid cells. Aggressive bony destruction, calcified matrix, high T1 (haemorrahge). Flow voids from branches of ECA if >20mm. Parallels posterior margin of petrous bone. May be associated with VHL.

Labyrinthitis Ossificans

Inflammation of the inner ear after bacterial meningitis (esp H.infuenzae), chronic middle/inner ear infections, trauma, cholesteatoma, mumps, Cogan syndrome or labyrinthectomy. Inflammation induces fibroblasts in the labyrinth to produce fibrosis, may differentiate into osteoblasts to ossify. Bony replacement of labyrinth with sclerosis, increasing density within the cochlea. Cochlear stenosis, chochlear fibro-ossific changes, cochlear ossification, may obliterate semicurcular canals and vestibule. Obliteration of round window may inhibit cochlear implant insertion.

Otospongiosis and Otosclerosis

Common, young-middle aged women, sensorineural hearing loss, 80% bilateral. Most are slowly progressive over decades. Unknown aetiology.

  • Otospongiosis (hypervascular and fibrotic) phase – Endochondral bone replaced by spongy bone with demineralisation and lytic erosion at margins of oval window, round window, cochlear.
    • Fenestral/retrocochlear otospongiosis affects anterior margin of oval window, or round window niche. Stapes is fixed to oval window via fibrous ankylosis, reducing sound transmission. Plaques of bone anteriorly. Tx stepedotomy or stapedectomy.
    • Cochlear form affects middle and basal turns. Double-ring (lucent) sign – resorption of bone immediately around membranous cochlea.
  • Otosclerosis phase – Recalcification with multifocal sclerosis (?with medical treatment). Stapes may be fixated to oval window reducing sound transmission.

Cochlear Implants

Electrode inserted through round window into cochlea, with tip along basal membrane. Multichannel electrodes must be placed 24mm deeep, the more electrodes placed into the cochlear the better the outcome (>19-22 electrode arrays is good). Most children receive implants due to meningitis, others from congenital lesions, viral infections. Preoperative assessment:

  • Bilateral acoustic schwannomas – As a DDx.
  • Bilateral obliterative labyrinthine ossification – Unable to insert impant deeply. May need to drill out internal walls of cochlear and place a shallow implant.
  • Cochlear patency (esp stenosis of basal turn), round window open?
  • Current middle ear infection – Requires preop antibiotics.
  • Congenital cochlear anomalies, vestibular disease – May cause ‘gusher’ ear.
  • Location and aberrant courses of facial nerve, ICA, sigmoid sinus
  • Fractures
  • Hypoplasia of IAC, cochlear nerve present?
  • Size of middle ear cavity and mastoid cell formation