Where is the expected location of the fissula ante fenestram?
The fissula ante fenestram is a cleft of fibrocartilaginous tissue located in between the inner and middle ears just anterior to the oval window. Fenestral otospongiosis begins at the anterior margins of the oval window before spreading to involve all of the margins of the oval and round windows.
Why does this patient have mixed hearing loss?
There is both fenestral and retrofenestral otospongiosis. In fenestral otospongiosis, there is ankylosis of the stapes footplate resulting in conductive hearing loss. Retrofenestral otospongiosis causes sensorineural hearing loss. This is thought to be due to either compromise of the spiral ligament of the cochlea or due to toxic proteases affecting the cochlea nerve cells.
Axial and coronal temporal bone CT shows a "halo" of radiolucency surrounding both cochleae in keeping with retrofenestral otospongiosis.
There is also radiolucency at the expected location of the fissula ante fenestram on both sides. This represents fenestral otospongiosis.
There is a right stapes prosthesis.