Otosclerosis

A primary disease of the otic capsule bone in which normal endochondral bone is replaced by irregular, vascular, immature bone, producing stapes fixation and conductive hearing loss, and in some patients cochlear involvement with sensorineural loss.

Pathology

The process begins with an active otospongiotic phase featuring osteocytic resorption, vascular proliferation and a bluish hyperaemic focus, progressing to a sclerotic phase of dense irregular bone. The commonest site, in over 80 percent, is the fissula ante fenestram, immediately anterior to the oval window, which is why the anterior footplate fixes first. Other sites include the round window niche, which causes greater conductive loss and poorer surgical outcome, and the pericochlear region, which causes sensorineural loss.

Epidemiology and inheritance

• Histological otosclerosis is present in around 10 percent of white adults; clinical disease in 0.3 to 1 percent.
• Rare in Africans and East Asians.
• Female to male ratio approximately 2 to 1.
• Presentation typically between 20 and 40 years.
• Autosomal dominant with penetrance of 25 to 40 percent, so a family history is present in around half.
• Pregnancy is associated with acceleration of hearing loss in a proportion of women, and measles virus antigen has been identified within otosclerotic foci, supporting a viral contribution.

Progression

Untreated stapes fixation produces a gradually increasing conductive loss, typically stabilising around a maximum gap of 50 to 60 dB. Cochlear otosclerosis adds a sensorineural component through toxic enzymes and hyalinisation of the spiral ligament, producing a mixed then predominantly sensorineural loss over decades.

• Progressive bilateral, often asymmetric, conductive hearing loss beginning in early adulthood. Bilateral in 70 to 80 percent.
• Paracusis of Willis: better hearing in noisy surroundings, because others raise their voices while the patient's masking threshold is elevated by the conductive block.
• Tinnitus in around 75 percent, usually low pitched and more prominent in active disease.
• Speech is characteristically soft and well modulated, since the patient hears their own voice loudly through bone conduction and therefore does not raise it.
• Mild unsteadiness or vestibular symptoms in a minority.
• Family history in around half.
• Deterioration during and after pregnancy.

Examination

• Normal tympanic membrane in the great majority. A normal drum with a conductive loss in a young adult is otosclerosis until proven otherwise.
• Schwartze sign: a reddish pink blush over the promontory visible through the drum, seen in around 10 percent, indicating an active hyperaemic focus.
• Rinne negative in the affected ear, with the fork threshold indicating the size of the gap: negative with a 256 Hz fork suggests a gap of 15 dB or more, 512 Hz around 25 dB, and 1024 Hz around 35 dB.
• Weber lateralising to the worse hearing ear.
• Gelle test negative, indicating stapes fixation, though it is rarely used now.

• Pure tone audiometry with masked bone conduction: a conductive loss maximal in the low frequencies in early disease, becoming flatter as fixation progresses.
• Carhart notch: an apparent depression of bone conduction of roughly 10 to 15 dB centred at 2000 Hz. It is an artefact of the loss of the ossicular contribution to bone conduction, not true cochlear damage, and typically recovers after successful stapedotomy. Failing to appreciate this leads to patients being wrongly denied surgery.
• Tympanometry: type As with normal pressure and reduced compliance, or a normal type A trace. A normal tympanogram does not exclude otosclerosis.
• Acoustic reflexes: absent bilaterally in established fixation. In early disease an on off effect with a biphasic deflection at the beginning and end of the stimulus is characteristic. Present reflexes with an air bone gap should raise suspicion of a third window lesion rather than otosclerosis.
• Speech audiometry with word recognition scores, which must be adequate to justify surgery.
• High resolution computed tomography of the temporal bone is not required for classical disease but is valuable for atypical presentations, revision surgery, suspected cochlear otosclerosis with a pericochlear lucent halo, and to exclude congenital ossicular fixation, superior semicircular canal dehiscence and enlarged vestibular aqueduct. It has a sensitivity of around 80 to 95 percent for fenestral disease.

Differential diagnosis and the discriminating feature

• Superior semicircular canal dehiscence: air bone gap with present acoustic reflexes, autophony, sound and pressure induced vertigo, and lowered vestibular evoked myogenic potential thresholds. Operating on this ear as otosclerosis is a serious and avoidable error.
• Ossicular discontinuity: larger flat gap approaching 50 to 60 dB with a hypermobile type Ad tympanogram.
• Congenital ossicular fixation: non progressive loss present from childhood.
• Tympanosclerosis: visible chalky plaques on the drum.
• Malleus head fixation: gap that fails to close after an otherwise successful stapedotomy.
• Paget disease of bone: older patient, raised alkaline phosphatase, characteristic skull radiology, mixed loss.
• Osteogenesis imperfecta with van der Hoeve syndrome: blue sclerae, fractures, conductive or mixed loss.

Three options exist and each is legitimate: observation, hearing aids and surgery. The choice belongs to the patient after realistic counselling.

Observation and hearing aids

• Hearing aids work extremely well in otosclerosis because the cochlea is normal, high gain is achievable without distortion, and speech discrimination is excellent once audibility is restored.
• Indicated for patients who decline surgery, those with a poor risk profile, those with an only hearing ear, and those with far advanced disease where discrimination is poor.
• There is no obligation to operate. A patient managed with aids for life comes to no harm from the conductive component.

Stapes surgery

Indications:

• An air bone gap of at least 15 to 20 dB, ideally 30 dB or more, with a bone conduction threshold indicating useful cochlear reserve and speech discrimination generally above 60 percent.
• Operate on the worse hearing ear first.
• Never operate on an only hearing ear.

Technique:

• Small fenestra stapedotomy is preferred to stapedectomy, using a 0.6 to 0.8 mm fenestra created with a microdrill or laser, with a piston prosthesis of 0.4 to 0.6 mm diameter crimped to the long process of the incus and passing 0.25 to 0.5 mm into the vestibule.
• Laser stapedotomy, using potassium titanyl phosphate, argon or carbon dioxide, reduces mechanical trauma to the footplate and the risk of a floating footplate.
• Local anaesthesia with sedation allows intraoperative hearing assessment and immediate detection of vertigo, but general anaesthesia is equally acceptable.
• Endoscopic stapedotomy improves visualisation of the oval window niche and avoids curetting the scutum in some ears.

Intraoperative problems and their management:

• Floating footplate: do not attempt to fish it out with a hook. Use a control hole or the laser, or abandon and reschedule.
• Obliterative otosclerosis with a thick footplate: drill or laser through carefully, accepting the higher sensorineural risk.
• Perilymph gusher, which occurs with an enlarged vestibular aqueduct or an abnormally patent cochlear aqueduct: elevate the head, seal with tissue around the prosthesis, place a lumbar drain if persistent, and abandon in favour of a hearing aid where uncontrollable.
• Dehiscent or overhanging facial nerve obscuring the footplate.
• Persistent stapedial artery.

Outcomes and complications

• Closure of the air bone gap to within 10 dB in around 90 to 95 percent in experienced hands.
• Profound sensorineural hearing loss in around 0.5 to 1 percent, and higher in revision surgery, which is the risk that must be explicitly discussed and which is the reason for never operating on an only hearing ear.
• Vertigo, usually transient in the first days. Persistent or delayed vertigo suggests a long prosthesis intruding into the vestibule, a perilymph fistula or reparative granuloma, and requires urgent exploration.
• Dysgeusia and tongue numbness from chorda tympani stretching or division, usually resolving over months.
• Tympanic membrane perforation, prosthesis displacement, incus necrosis and refixation causing late deterioration.
• Facial nerve injury, rare but recognised.

Postoperative care

• Avoid nose blowing, straining, heavy lifting, flying and diving for 4 to 6 weeks.
• Keep the ear dry until reviewed.
• Packing removed at 1 to 2 weeks.
• Audiometry at 6 to 12 weeks and again at 12 months.
• Any postoperative deterioration in bone conduction, vertigo or fluctuating hearing requires immediate reassessment for fistula or a displaced prosthesis, not reassurance.

Cochlear otosclerosis and far advanced disease

• When the loss becomes severe to profound with poor discrimination, cochlear implantation is effective. Anticipate difficult electrode insertion through an ossified or narrowed cochlear lumen and facial nerve stimulation from current spread through demineralised bone, which is managed by deactivating offending electrodes.
• Sodium fluoride 20 to 60 mg daily with vitamin D and calcium, and bisphosphonates, have been used to arrest otospongiotic activity, particularly where there is a Schwartze sign or documented sensorineural progression. Evidence is weak and adverse effects including gastric irritation and skeletal fluorosis limit enthusiasm. It is not a substitute for surgery or amplification.