With a conductive hearing loss and normal otologic examination the most likely diagnosis is that of otosclerosis.
A normal tympanic membrane and middle ear cleft
An audiogram showing bilateral significant conductive hearing loss
A CT scan is arranged mainly to exclude other causes of conductive hearing loss. In otosclerosis the CT scan is often normal, but in some cases subtle signs of otosclerosis can be seen at the anterior aspect of the stapes superstructure.
In a subset of patients with otosclerosis there is evidence of retrofenestral otosclerosis or cochlear otosclerosis which causes an accelerated sensrorineural hearing loss. This can be seen in this scan with demineralisation of bone around the cochlear (the halo sign)