Normal imaging anatomy
Key structures of the external and middle ear are visualized on axial CT scans of the temporal bone. The external acoustic meatus consists of a fibrocartilaginous part and a bony isthmus. The boundary between the external and middle ear is the tympanic membrane (pars tympanica), which is inclined at an angle of about 140 degrees. The membrane is divided into the upper flaccid part (pars flaccida) and the lower taut part (pars tensa).
The tympanic cavity is topographically divided into the epitympanum (attic), mesotympanum, and hypotympanum. Inside are the auditory ossicles: the malleus (malleus), incus (incus), and stapes (stapes). Important anatomical landmarks evaluated on CT include the jugular bulb, the intratemporal segment of the internal carotid artery, the foramen spinosum and foramen ovale, as well as the facial nerve canal.
Pathologies of the external acoustic meatus
Changes in the external acoustic meatus are often associated with infections, benign or malignant neoplasms:
- Osteoma and exostoses: bony growths narrowing the lumen. Osteomas usually grow on a stalk, whereas exostoses have a broad base.
- Cholesteatoma of the external meatus: causes erosion of the adjacent bony wall. The CT appearance can be indistinguishable from squamous cell carcinoma, so otoscopy with biopsy is required for accurate differentiation.
- Necrotizing otitis externa (NOE): aggressive inflammation in elderly immunocompromised patients (especially with diabetes). The infection can spread through the fissures of Santorini, causing skull base osteomyelitis and cranial nerve involvement (VII–XII). Important diagnostic marker: the earliest sign on CT is the effacement of fat planes (especially the retrocondylar fat pad), which appears even before visible bone destruction.
Middle ear pathologies: cholesteatoma and sclerosis
Among middle ear diseases in diagnostic imaging, special attention is paid to destructive and sclerotic processes.
Cholesteatoma is an accumulation of ectopic desquamated epithelium rather than a true neoplasm. It is most often acquired, forming from a retraction pocket of the tympanic membrane due to negative pressure. The process leads to bone destruction: the lateral wall of the attic (scutum) and the ossicles are destroyed, and upon progression, the otic capsule and the roof of the tympanic cavity (tegmen tympani). MRI (Non-EPI DWI sequence) is used to detect recurrences after surgery, where cholesteatoma lights up due to diffusion restriction.
Otosclerosis (otospongiosis) manifests as the replacement of dense bone with spongy bone:
- Fenestral type: affects the region of the fissula antefenestram anterior to the oval window. Leads to stapes fixation and conductive hearing loss.
- Retrofeneestral (pericochlear) type: foci surround the cochlea, causing sensorineural hearing loss. In severe cases, the "double ring" sign of the cochlea is visible on CT.
Tympanosclerosis is characterized by calcification foci in the middle ear cavity against the background of chronic otitis. If only the membrane becomes calcified, the condition is called myringosclerosis.
Traumatic injuries of the temporal bone
Disruption of the ossicular chain most commonly occurs in the setting of a temporal bone fracture. The incus is the most vulnerable ossicle, as the malleus and stapes are firmly fixed by ligaments and muscles.
Radiologically, several types of injuries are distinguished (in descending order of frequency):
- Incus-stapes joint disruption: the lenticular process of the incus is displaced anteriorly to the stapes head.
- Incus-malleus joint disruption: the normal contour of the articulation disappears on axial CT slices.
- Complete dislocation of the incus.
Isolated fractures of the ossicles are rare, but a fracture of the malleus handle, for example, is possible from cotton swab trauma.
Briefly: answers to topical questions
What passes through the foramen spinosum and foramen ovale, which are important landmarks on temporal bone CT?
The branches of the trigeminal nerve, as well as meningeal arteries and veins, pass through the foramen spinosum and foramen ovale, which are important landmarks on computed tomography.
- Foramen ovale (foramen ovale) — transmits the mandibular nerve and an accessory branch of the meningeal artery.
- Foramen spinosum (foramen spinosum) — transmits the middle meningeal artery (a. meningea media), a vein, and a branch of the mandibular nerve.
What branches does the facial nerve give off as it courses through its canal inside the pyramid of the temporal bone?
Within its canal inside the pyramid of the temporal bone, the facial nerve gives off three main branches.
- Greater petrosal nerve (n. petrosus major) — heads toward the foramen lacerum and joins the deep petrosal nerve.
- Stapedial nerve — innervates the stapedius muscle (m. stapedius).
- Chorda tympani (chorda tympani) — provides taste sensation to the anterior two-thirds of the tongue and joins the lingual nerve.
Which specific muscles and ligaments fix the malleus and stapes, making them less vulnerable to trauma?
Special muscles and ligaments provide fixation and mobility regulation of the malleus and stapes within the tympanic cavity.
- Tensor tympani muscle (m. tensor tympani) — tenses the tympanic membrane, to which the handle of the malleus is attached.
- Stapedius muscle (m. stapedius) — attaches via its tendon to the neck of the stapes and reduces its mobility.
- Annular ligament — surrounds the footplate of the stapes, securely fixing it to the margins of the vestibular window.
Which anatomical structures form the bony labyrinth of the inner ear?
The bony labyrinth of the inner ear is a system of communicating cavities made of compact bone and consists of three parts.
- Vestibule (vestibulum) — the central oval cavity communicating with the cochlea and semicircular canals.
- Cochlea (cochlea) — the anterior part, representing a spiral bony canal that makes two and a half turns.
- Bony semicircular canals (canales semicirculares ossei) — three arcuate passages located posteriorly and laterally in three mutually perpendicular planes.
Through what anatomical pathways can infection spread to the skull base in necrotizing otitis externa?
In necrotizing otitis externa, infection can spread to the skull base through specific anatomical defects.
- Fissures of Santorini (fissures of Santorini) are anatomical defects in the lower part of the fibrocartilaginous external acoustic meatus.
Infection spreads inferiorly through these fissures, leading to the development of skull base osteomyelitis with involvement of the underlying soft tissues and potential lower cranial nerve palsy.
Normally on axial CT, the incudomalleolar joint resembles an "ice cream cone" (malleus head = scoop, incus body = cone). If this contour is deformed, look for joint disruption.
Class and exam questions
What is the earliest CT sign of necrotizing external otitis?
Disappearance of the retrocondylar fat pad. This soft-tissue sign appears before bone demineralization and erosion develop.
How to differentiate cholesteatoma from granulation tissue postoperatively?
The optimal method is MRI with diffusion-weighted imaging (Non-EPI DWI). Cholesteatoma demonstrates persistent diffusion restriction, unlike granulation tissue.
Where is fenestral otosclerosis most commonly localized?
In the lateral wall of the otic capsule, in the region of the fissula antefenestram, anterior to the oval window, leading to fixation of the anterior crus of the stapes.
What else to study on this topic
- Features of squamous cell and basal cell carcinoma of the external acoustic meatus
- Congenital atresia and stenosis of the external ear: pre-surgical evaluation
- Anatomical variations of the facial nerve course in the temporal bone
- Sigmoid sinus thrombosis as a complication of mastoiditis
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