PATHOLOGY

 1. CONGENITAL ANOMALIES OF THE MIDDLE EAR

There is a great variety of malformed parts in the middle ear and they can be unilateral or bilateral.

- Minor anomalies (only involvement of the middle ear): changes in configuration or size of the tympanic cavity, reduction of the distance between structures, fixated ossicles...

- Major anomalies (associated with an involvement of the tympanic membrane and external ear): there may be aplasia, hypoplasia, thickening or thinning of ossicles. It may also exist fusion or excessive separation of joints inside the ossicular chain (Images 3-4)

- Other anomalies: dehiscence of the facial nerve, apneumatized mastoid and aberrant courses of arteries and veins.

Recommendations for the radiology report

• Existence (or not) of tympanic bone (external auditory canal).
• Presence (or not) of tympanic membrane.
• Mandibular condyle’s position.
• Form and size of the tympanic cavity.
• Analysis of the ossicular chain and its anomalies.
• Presence of round window and oval window in a correct position.
• Facial nerve trajectory (useful for preoperative studies).
• Carotid artery and yugular vein’s location.

2.  INFLAMMATORY LESIONS

2.a) ACUTE OTITIS MEDIA

It is primarily a disease of infants and young children. Patients present with fever, otalgia, and a red bulging tympanic membrane. The infection is usually caused by bacteria such and imaging is usually not necessary in uncomplicated acute otitis media.

2.b) ACUTE MASTOIDITIS

In a clinical situation that courses with postauricular erythema and edema, imaging is crucial to exclude complications, such as:

• Coalescent mastoiditis: Destruction of the mastoid trabeculae, with the possible development of a subperiosteal abscess.
• Petrous apicitis (image 5): It occurs in the setting of a pneumatized petrous apex (present in 30% of the population). It is characterized by septal and cortical destruction, osteitis, meningeal inflammation.
• Osteomyelitis.
• Intracranial complications: dural venous sinus thrombosis, epidural abscess, subdural empyema, meningitis, brain abscess...

2.c) CHRONIC OTITIS MEDIA (image 6)

It may be due to underlying eustachian tube dysfunction or tympanic membrane perforation. Some of the important sequelae of chronic otitis media may have very similar appearances on CT images:

Granulation tissue it does not destroy or displace the structures. Cholesterol granuloma: Common locations include the middle ear and the petrous apex.

Cholesteatoma.

2.d) CHOLESTEATOMA (Images 7-8-9-10)

It is characterized by accumulation of desquamated keratin epithelium in the middle ear cavity or in other pneumatized portions of the temporal bone.

Most cholesteatomas are acquired (98%; 80% associated with the pars flaccid and 20% associated with pars tensa), approximately 2% are congenital (child with no previous history of otorrhea, tympanic membrane perforation or otologic procedures).

CT is the imaging mainstay of temporal bone inflammation.

a)    Acquired pars flaccida cholesteatoma: Expansile lobulated lesion in the Prussak space eroding the scutum, with medial displacement and erosion of the ossicles.

b)   Aquired pars tensa cholesteatoma: It is usually medial to the ossicles and displace them laterally.

c)    Congenital cholesteatoma: It is commonly located just above the opening of the eustachian tube.

Complications of Cholesteatoma

If only one cholesteatoma cell is missed and is not removed, the cholesteatoma may re-occur (residual cholesteatoma).

The primary goal of surgery is eradication of disease and prevention of recurrence. Preservation or reconstruction of hearing is an important but secondary consideration.

2.e) TYMPANOSCLEROSIS

A chronically scarring process which can lead to conductive hearing loss due to sclerotic fixation of the ossicles.

It should be distinguished from otosclerosis that is a noninflammatory process characterized by demineralization in the region of the fissula ante fenestram at CT and, in the late phase, there may be sclerotic thickening around the oval window, at the stapes footplate and around the cochlear capsule.

3. TRAUMA

CT is the modality of choice for evaluating temporal bone trauma, particularly fractures.

Traditional system for classifying temporal bone fractures:

-  Longitudinal fracture (80%–90%): parallel to the long axis of the petrous portion of the temporal bone. It results in ossicular and tympanic membrane injury.

-  Transverse fracture (10%–20%): perpendicular to the petrous pyramid. It is associated with a higher risk of facial nerve injury.

There is a newer system that shows better correlation with clinical outcome and complications:

- The otic capsule–sparing fracture is much more common (94%–97%), results from a temporoparietal blow, and has an increased incidence of conductive hearing loss due to ossicular injury.

- The otic capsule–-violating fracture (3%–6%) results from an occipital blow and has a higher incidence of facial nerve paralysis (30%–50%), sensorineural hearing loss and CSF fistula.

It may be more important to describe the vital structures involved (ossicles, otic capsule, facial nerve, tegmen, external auditory canal, internal auditory canal, carotid canal) rather than classifying the fracture in a type or another.

Complications of Temporal Bone Trauma

• Conductive hearing loss is caused by the disruption of the conductive chain, which may be due to ossicular luxation (more common) or fracture. Sensorineural hearing deafness could be caused by intracranial lesions or middle/inner ear pathology. CT can detect pneumolabyrinth and signs of perilymphatic fistulae but it can’t detect subtle lesions within the inner ear.
• CSF Leak: CT depicts a bone defect in 70-93% of cases with clinically active leaks, depending on series.
• Facial Nerve Injury: On CT we should search for a fracture line coursing through the facial nerve.
• Labyrinthitis Ossificans: the fluid-filled lumen of the otic capsule is replaced by bone (or fibrous tissue if in the early stages). On CT images, osseous attenuation is noted within the inner ear.
• Vascular Injury (arterial injury or venous sinus thrombosis).

4. IMAGING OF POSTOPERATIVE TEMPORAL BONE (images 11-12-13)

It is important to be familiar with some of the more commonly performed procedures and their corresponding imaging findings.

4.a) Myringotomy and Tympanostomy Tube

Tympanostomy tubes are commonly inserted into the tympanic membrane via an incision in the tympanic membrane (myringotomy) for treating chronic or recurrent otitis media recalcitrant to medical management.

The tubes usually fall out on their own after a few months, but medial migration occasionally occurs and can result in conductive hearing loss.

4.b) Mastoidectomy

The different types of mastoidectomy essentially consist of resecting variable portions of the mastoid air cells and adjacent structures, which may be performed for treatment of mastoiditis, cholesteatoma resection, cochlear implantation, or endolymphatic surgery, among other indications.

-    Canal-wall-up mastoidectomy: exenteration of the mastoid air cells.

-  Canal-wall-down mastoidectomy: in addition the posterior wall of the external auditory canal is resected.

-  A radical mastoidectomy includes removal of the tympanic membrane, malleus, and incus, with attempted preservation of the stapes.

-   Tympanomastoidectomy: mastoidectomy performed in conjunction with a middle ear procedure.

The mastoid bowl remains clear and the presence of soft-tissue attenuation material on CT images may represent soft tissue grafts, granulation tissue or, in the appropriate scenario, recurrent cholesteatoma. On CT images, recurrent cholesteatoma is suspected when there is new bone erosion associated with a soft-tissue focus.

4.c) Ossicular Reconstruction

-   Stapes prostheses are used in patients with conductive hearing loss due to otosclerosis or congenital anomalies.

-   A partial ossicular replacement prosthesis substitutes the malleus and incus, and thus extends from the tympanic membrane to the head of the stapes.

-    A total ossicular replacement prosthesis extends from the tympanic membrane to the stapes footplate or oval window and is utilized if the stapes is also diseased.

Ossicular prostheses can be composed of various materials including hydroxyapatite, metal, and plastic. Ossicular prosthesis failure most commonly results from migration or dislocation (it can also result from prosthesis fracture, recurrent cholesteatoma, pressure erosion of the ossicles, and perilymphatic fistula).

4.d) Cochlear Implantation

Temporal bone CT imaging is useful for preoperative planning and avoiding complications and failure of cochlear implantation.