Otosclerosis (otospongiosis) is a temporal bone disease characterized by abnormal bone remodeling of the otic capsule. The otic capsule normally forms through endochondral ossification and shows minimal remodeling in the postnatal period — in otosclerosis this quiescence is disrupted and an active bone resorption/formation cycle begins. Fenestral type (80-90%) affects the stapes footplate and fissula ante fenestram region, causing conductive hearing loss. Retrofenestral (cochlear) type shows spread around the cochlea and adds sensorineural hearing loss (mixed type). Thin-section HRCT (0.5-0.6mm) is the gold standard for diagnosis; focal lucency (demineralization) at the fissula ante fenestram is the early diagnostic finding. In advanced stages, a lucency halo around the cochlea (fourth turn sign) indicates retrofenestral involvement.
Age Range
20-50
Peak Age
35
Gender
Equal
Prevalence
Common
Otosclerosis is a process of abnormal resorption and remodeling in the endochondral bone layer of the otic capsule. The normal otic capsule is a unique endochondral bone structure resistant to bone remodeling in the postnatal period — this resistance is maintained by enzyme inhibitors called glob interresi. In otosclerosis this protective mechanism fails and active osteogenesis begins. In the early active phase (otospongiosis), intense osteoclastic activity with bone resorption and increased vascularization is seen — this phase appears as lucency (demineralization) on CT and shows T2 hyperintensity and enhancement on MRI. In the late phase (otosclerosis), new bone formation and sclerosis develop — appearing as normal or increased density on CT. The fissula ante fenestram (cartilage remnant area anterior to the oval window) is the most common starting point because endochondral bone is thinnest and most vulnerable here. Stapes footplate fixation is the mechanism of conductive hearing loss — footplate thickening and sclerosis restrict stapes movement at the oval window. In cochlear spread (retrofenestral type), spiral ligament and endosteal bone involvement adds a sensorineural component.
Focal lucency at the fissula ante fenestram, immediately anterior to the oval window, on thin-section HRCT is the earliest and most specific finding of otosclerosis. This focal low density in the normally homogeneous high-density otic capsule is abnormal and pathognomonic.
On thin-section HRCT (0.5-0.6mm), a focal low-density area at the fissula ante fenestram (immediately anterior to the oval window) is seen. This reflects active resorption and demineralization of the endochondral bone layer of the otic capsule. The normal otic capsule is a very dense and homogeneous bone structure; any lucency in this region is pathological. This finding is pathognomonic for early fenestral otosclerosis.
Report Sentence
Focal lucency at the bilateral/right/left fissula ante fenestram is seen, consistent with fenestral type otosclerosis.
In advanced retrofenestral otosclerosis, a band-like lucency is seen around the cochlea — called 'halo sign' or 'double ring sign'. The outer ring represents normal otic capsule bone density, the inner ring represents the cochlear lumen, and the lucency band between them represents otospongiotic tissue. This finding correlates with a sensorineural hearing loss component and affects treatment planning (stapedectomy alone may not suffice, cochlear implant may be needed).
Report Sentence
Band-like pericochlear lucency (halo sign) is seen, consistent with retrofenestral type otosclerosis.
Normal stapes footplate thickness is 0.1-0.3mm; in fenestral otosclerosis, thickening (>0.5mm) and irregularity are seen. In advanced stages, the footplate may become completely obliterated. This finding is the direct cause of conductive hearing loss and provides critical information for surgical planning (stapedectomy/stapedotomy). The oval window and stapes are best evaluated in the coronal plane.
Report Sentence
Stapes footplate thickening/irregularity and oval window narrowing are seen, consistent with fenestral type otosclerosis.
In the active otospongiosis phase (early stage), enhancement may be seen on MRI — reflecting increased vascularization. T2-weighted sequences show hyperintense signal. This active phase can be used to assess treatment response (sodium fluoride therapy) and determine disease activity. Enhancement disappears in the mature sclerotic phase.
Report Sentence
Enhancement in the otic capsule is seen, suggestive of active otospongiosis phase.
In advanced retrofenestral otosclerosis where cochlear implant is planned, cochlear lumen patency must be assessed. High-resolution T2 sequences (3D CISS, FIESTA, DRIVE) show cochlear fluid as bright signal; if the lumen is obliterated, signal is lost. This finding determines whether implant electrode can be placed and is critical for surgical planning.
Report Sentence
The cochlear lumen appears patent/obliterated on high-resolution T2 sequences, to be considered in cochlear implant planning.
Criteria
Fissula ante fenestram and stapes footplate involvement. Conductive hearing loss. Most common type (80-90%).
Distinct Features
Fissula ante fenestram lucency, footplate thickening, treated with stapedectomy.
Criteria
Spread around cochlea. Mixed hearing loss (conductive + sensorineural). Less common (10-15%).
Distinct Features
Pericochlear halo sign, cochlear implant may be needed, sodium fluoride therapy may be tried.
Criteria
Cochlear lumen completely obliterated by bone/fibrous tissue. Profound sensorineural hearing loss. Rare.
Distinct Features
No cochlear fluid signal on 3D CISS, difficulty in cochlear implant electrode placement, drill-out may be needed.
Distinguishing Feature
COM has middle ear opacification and mastoid sclerosis, otic capsule intact; otosclerosis has otic capsule lucency, middle ear may be clear
Distinguishing Feature
Vestibular schwannoma is enhancing mass in IAC; otosclerosis has no mass in IAC, changes in otic capsule
Distinguishing Feature
Cholesteatoma has soft tissue mass + bone erosion in middle ear; otosclerosis has lucency in otic capsule, no mass in middle ear
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
12-monthFenestral type otosclerosis is very successfully treated with stapedectomy/stapedotomy (>90% success). Retrofenestral type may require hearing aid or cochlear implant. Clinical diagnosis can be made even if CT is negative (CT has 80-85% sensitivity). Sodium fluoride therapy may be tried in active otospongiosis phase. Preoperative CT is mandatory for surgical planning (facial nerve variation, oval window anatomy, cochlear involvement degree).
Otosclerosis is an important cause of treatable hearing loss. Fenestral type is treated with stapedectomy/stapedotomy with excellent outcomes. Retrofenestral type can cause sensorineural hearing loss and may require cochlear implant. Clinical diagnosis can be made even if CT findings are negative.