ROLE OF RADIOLOGY IN OTOSCLEROSIS
The imaging tests are not routinely necessary and the diagnosis is confirmed at surgery.
Imaging tests are useful in atypical cases to rule out other causes of hearing loss and also to rule out postoperative complications. It also allows us to identify factors of poor prognosis and anatomical alterations that hinder surgery and increase the risk of surgical complications,
such as an obliterative otosclerosis or facial nerve canal dehiscence.
High resolution CT of the temporal bone using 1-mm (or less) thick sections without contrast is the imaging modality of choice.
For correct evaluation of the labyrinthine windows and cochlear capsules,
axial and coronal reformats should be made in the plane of and perpendicular to the lateral semicircular canal.
MRI has a limited role in the evaluation of the labyrinthine capsule,
but it is useful for the evaluation of cochlear light before the cochlear implant in cases of advanced retrofenestral otosclerosis
NORMAL ANATOMY OF THE INNER EAR
Fig. 1: Axial reconstruction of the temporal bone where the normal anatomy is visualized with the fissula ante fenestram (blue arrow), the oval window (white arrow), cochlea (#), horizontal segment of the facial nerve (blue lines), internal auditory canal (*), posterior semicircular canal (yellow arrow). References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
Fig. 2: Coronal reconstruction of the temporal bone where the normal anatomy is illustrated, visualizing the oval window (yellow line), horizontal segment of the facial nerve (white arrow), basal turn of the cochlea (#), vestibule (*), stapes head (yellow arrow), cochlear promontory (blue arrow), lateral semicircular canal (yellow arrowhead), upper semicircular canal (blue arrowhead). References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
RADIOGRAPHIC FEATURES
Otosclerosis is classified into two subtypes according to the affected area:
- Fenestral: it is the most frequent subtype.
Involves the anterior region of the oval window.
- Retrofenestral: cochlear involvement.
Fenestral otosclerosis
The replacement of the enchondral bone with an "ivory" appearance by areas of vascularized spongy vascular bone that manifest as hypodense or radiolucent foci in CT.
The most frequently affected area is the fissula ante fenestram, which is a cleft of fibrocartilagenous tissue between the inner and middle ear,
located in the anterior part of the oval window.
The disease gradually extend along the oval window and footplate of the stapes.
Involvement of the annular ligament leads fixation of the stapedo-vestibular joint,
which conditions conductive hearing loss.
Fenestral otosclerosis can also involves the promontory,
tympanic segment of the facial nerve and round window.
During the evolution of the disease,
these regions become sclerotic.
In advanced stages the oval window is completely occupied (obliterative otosclerosis)
Fig. 3: Axial CT images showing osteolytic areas (arrows) in the fissula ante fenestram, it is typical of fenestral otosclerosis. References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
Fig. 4: Axial CT images showing osteolytic areas (arrows) in the fissula ante fenestram, it is typical of fenestral otosclerosis. References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
Fig. 5: Obliterative otosclerosis. A and B) Coronal CT images of the temporal bone showing spots of fenestral otosclerosis that obliterate the oval window. The affectation of the stapes (arrow) is also observed. C) Normal anatomy in coronal CT images of the oval window.
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
Retrofenestral or cochlear otosclerosis
It is usually associated with fenestral otosclerosis.
Extension of the ostosclerotic foci to the promontory,
internal auditory canal,
pericholecule,
facial nerve duct.
When there is pericoclear involvement,
it can be accompanied by sensorineural hearing loss.
Fig. 6: A) and B) Axial CT images C) Coronal CT images of the temporal bone show pericochlear osteolysis foci (arrow). D) The “double ring” or “halo” sign due to presence of hypodense rings in the pericochlear otic capsule due extensive area of osteolysis that surrounds the cochlea as a sign of advanced otosclerosis.
References: Department of Radiology, Hospital Universitario de Donostia. San Sebastián.
DIFFERENTIAL DIAGNOSIS
Tympanosclerosis Fig. 7 :
- Medical history of middle ear infections.
- It usually shows signs of chronic inflammation of the middle ear and alteration of mastoid pneumatization.
- It is characterized by more irregular calcifications and with greater attenuation than in otosclerosis.
It usually affects structures of the middle ear (stapes footplate,
tympanic membrane,
ossicular chain ...
Other diseases:
In advanced phases of otosclerosis the differential diagnosis with other diseases that produce osteolysis of the otic capsule (osteogenesis imperfecta,
neurosyphilis,
Paget's disease,
fibrous dysplasia Fig. 8 ) is practically impossible,
however the diagnosis is usually possible thanks to the clinical manifestations and involvement of other bones.
PREOPERATIVE CT EVALUATION OF THE TEMPORAL BONE IN OTOSCLEROSIS
Discard anatomical alterations or factors of poor prognosis that increase surgical complications:
- Oval window niche height : if decreased increases operative difficulty and subjective discomfort during otosclerosis surgery.
- Obliteration of the round window may result in a poor result after stapedectomy
- Dehiscent jugular bulb
- Facial nerve canal dehiscence
- Otosclerosis obliterative: requires milling oval window to place stapes prosthesis.
Discard other causes of hearing loss:
- Dehiscence of upper semicircular canal.
- Ossicular chain fixation,
fusion or fracture.
- Secretory otitis media and primary cholesteatoma.
POST-STAPEDECTOMY COMPLICATIONS
Causes for recurrent hearing loss and vertigo after surgery:
- Prosthesis displacement.
- Perilymphatic fistula
- Reparative granuloma
- Labyrinthitis.
CT:
- Imaging modality of choice for evaluate position of the prosthesis.
MRI:
- Imaging modality of choice for evaluate labyrinthitis or granulomas.