Definition / Overview
Otosclerosis is a disease of the otic capsule characterised by disordered endochondral bone remodelling, in which abnormal resorption of normal lamellar bone is followed by deposition of spongy, vascular, disorganised new bone. This process most commonly affects the region anterior to the oval window (the fissula ante fenestram), leading to progressive ankylosis of the stapes footplate and conductive hearing loss. Otosclerosis is the leading cause of progressive conductive hearing loss in adults without middle ear pathology.
Epidemiology
- Prevalence of histological (subclinical) otosclerosis in the white population is approximately 7-10%; clinical disease manifests in roughly 1% of the general population.
- Female-to-male ratio is approximately 2:1.
- Onset of symptomatic hearing loss typically in the third decade; 90% present before age 50.
- Bilateral in approximately 70% of cases, though asymmetric presentation is common.
- Rare in people of East Asian, African, and Indigenous descent.
Pathophysiology
Bone Remodelling and the Otic Capsule
Normal otic capsule bone is uniquely resistant to remodelling throughout adult life. In otosclerosis, this resistance is lost. The initiating event triggers focal areas of osteoclastic resorption, producing a hypercellular, vascular, spongy lesion termed otospongiosis. Over months to years, this is replaced by dense sclerotic bone that progressively encases the annular ligament and footplate.
Aetiology and Genetics
- Autosomal dominant inheritance with variable penetrance (approximately 25-40% penetrance); thus, a substantial proportion of gene carriers never develop clinical disease.
- Multiple genetic loci implicated; no single dominant gene identified.
- Measles virus RNA has been detected in otosclerotic foci; viral provocation of disease in genetically susceptible individuals is the leading environmental hypothesis.
- Oestrogen accelerates disease activity, explaining female predominance and the well-recognised deterioration with pregnancy.
- Fluoride has historically been proposed to inhibit osteoclastic activity at the foci of disease.
Sites of Involvement and Hearing Consequences
| Site of Otosclerotic Focus | Predominant Effect |
|---|---|
| Fissula ante fenestram (anterior oval window) | Stapes fixation; conductive hearing loss |
| Entire annular ligament / footplate | Progressive conductive loss; obliterative footplate |
| Round window niche | Persistent air-bone gap post-stapes surgery |
| Cochlear endosteum (pericochlear) | Sensorineural hearing loss; "Schwartze sign" on otoscopy |
| Entire cochlear capsule | Profound SNHL; cochlear implantation territory |
The severity of conductive loss reflects the degree of footplate ankylosis. Mild ligamentous stiffening causes early low-frequency loss; complete bony fixation produces a maximal air-bone gap approaching 60 dB.
Clinical Features and Diagnosis
History
- Gradual, progressive, bilateral (often asymmetric) hearing loss, typically commencing in the late teens or twenties.
- Paracusis Willisii: the paradoxical phenomenon of hearing speech more clearly in noisy environments. In noise, speaking companions raise their voices, compensating for the patient's conductive loss while background ambient noise remains attenuated by the fixed stapes. This is pathognomonic of significant conductive hearing loss.
- Difficulty hearing one's own voice while chewing (bone-conducted sound is attenuated by the fixed ossicular chain).
- Tinnitus: low-frequency, bilateral; present in a significant minority.
- Vestibular symptoms (mild imbalance, rarely frank vertigo) occur when otosclerotic foci encroach on the vestibular apparatus.
- Family history of conductive hearing loss or stapes surgery in a first-degree relative.
- Acceleration of hearing loss with pregnancy.
Examination
- Otoscopy: tympanic membrane is normal and translucent in the vast majority. A reddish-pink blush visible through the tympanic membrane in the region of the promontory, the Schwartze sign (flamingo pink sign), indicates active, highly vascular otospongiosis and is present in a minority of cases.
- No middle ear effusion, no perforation, no cholesteatoma.
- Normal external ear canal.
Tuning Fork Tests
| Test | Expected Finding in Otosclerosis |
|---|---|
| Rinne (512 Hz) | Negative (BC $>$ AC); positive Rinne at 256 Hz, negative at 512 Hz suggests mild-moderate loss |
| Weber | Lateralises to the worse hearing ear (more severely affected stapes) |
| Absolute Bone Conduction (ABC) | Normal (distinguishes from SNHL) |
| Bing | Absence of occlusion effect (reduced or absent in stapes fixation) |
Tuning fork findings help confirm conductive loss and exclude sensory impairment prior to surgery.
Investigation
Audiological Assessment
- Pure tone audiometry (PTA): characteristic ascending conductive pattern; air-bone gap (ABG) widest at low frequencies in early disease. The Carhart notch: a mechanical artefact producing an apparent depression in bone conduction at 2 kHz (approximately 5 dB) due to loss of the stapes inertial component of bone-conducted sound. The notch resolves after successful stapes surgery and should not be mistaken for sensorineural loss.
- Speech audiometry: speech discrimination is preserved and excellent in pure conductive otosclerosis, unlike SNHL.
- Tympanometry: type A or, characteristically, type As (reduced compliance/shallow peak) reflecting increased stiffness of the ossicular chain. Tympanogram is normal in very early disease.
- Acoustic stapedial reflexes: absent (ipsilateral and contralateral) due to stapes fixation. This is a highly sensitive indicator; absent reflexes with normal tympanogram and normal-appearing ear strongly suggests otosclerosis.
Imaging
- High-resolution CT of the temporal bones (HRCT): not routinely required to diagnose otosclerosis but indicated to:
- Exclude superior semicircular canal dehiscence (SSCD) mimicking otosclerosis.
- Assess the degree of footplate thickening and obliterative disease pre-operatively.
- Identify aberrant anatomy (dehiscent facial nerve, high jugular bulb, persistent stapedial artery) that substantially alters surgical risk.
- Evaluate pericochlear lucency (the "halo sign") in cochlear otosclerosis.
- MRI is not routinely indicated but may be requested to exclude retrocochlear pathology if SNHL predominates.
Differential Diagnosis of Conductive Hearing Loss
| Condition | Key Distinguishing Features |
|---|---|
| Otosclerosis | Normal otoscopy; absent reflexes; Carhart notch; positive family history |
| Ossicular discontinuity | ABG often $\geq$ 40-50 dB; type Ad tympanogram (increased compliance) |
| Tympanosclerosis | Abnormal otoscopy; white plaques; history of chronic ear disease |
| Superior semicircular canal dehiscence (SSCD) | Autophony; Tullio phenomenon; negative Rinne may coexist with type A tympanogram and present acoustic reflexes |
| Chronic otitis media with effusion | Type B tympanogram; conductive loss; possible history of upper respiratory illness |
| Congenital ossicular anomaly | Lifelong; unilateral often; CT essential |
| Malleus fixation | Stapes mobility at surgery; may coexist with otosclerosis |
SSCD must be excluded before stapes surgery as it mimics otosclerosis audiologically and stapes surgery in SSCD will not improve hearing.
Management
Conservative Management
- Observation: appropriate for mild asymptomatic loss, patient preference, or where surgical risk is unacceptable.
- Hearing amplification: conventional air-conduction hearing aids are effective in conductive otosclerosis due to excellent speech discrimination scores. Bone-anchored hearing devices (BAHDs/BCDs) are an option for patients who decline surgery or where surgery is contraindicated.
- Sodium fluoride: historically prescribed to stabilise cochlear otosclerosis and slow sensorineural deterioration. Evidence of benefit is limited; not universally endorsed or available in the Australian/New Zealand setting. May be considered in cochlear otosclerosis with progressive SNHL.
Surgical Management: Stapes Surgery
Stapes surgery is the definitive treatment for symptomatic otosclerotic conductive hearing loss and offers immediate, reproducible hearing rehabilitation. The decision to operate requires:
- Confirmed conductive hearing loss with ABG $\geq$ 25-30 dB.
- Absent stapedial reflexes confirming stapes fixation.
- Adequate residual cochlear reserve (bone conduction thresholds within implantable range where cochlear implant may later be needed).
- Informed patient who understands the risk of sensorineural hearing loss.
- In bilateral disease: operate on the worse-hearing ear first; the better-hearing ear may be operated 6-12 months later.
Pre-operative Counselling and Consent
Key risks to discuss explicitly:
- Improvement in hearing in approximately 90-95% of appropriately selected cases (closure of ABG to within 10 dB in the majority).
- Permanent sensorineural hearing loss in approximately 1% of cases (total/dead ear risk approximately 0.5-1%): the most significant risk and must be clearly communicated.
- Temporary vertigo and disequilibrium (common in the immediate post-operative period, usually resolves within weeks).
- Perilymph fistula (PLF): rare but important cause of delayed SNHL.
- Dysgeusia / altered taste from chorda tympani manipulation (temporary in most; permanent in a minority).
- Tinnitus: may improve with hearing restoration or occasionally worsen.
- Tympanic membrane perforation.
- Facial nerve injury: extremely rare but catastrophic; risk heightened by anatomical variants.
- Failure to improve hearing; need for revision surgery (approximately 5-10% lifetime revision rate).
Surgical Procedure: Stapedotomy (Small Fenestra Technique)
Small fenestra stapedotomy is the current gold standard, largely supplanting the earlier total stapedectomy.
- Approach: transcanal under local anaesthesia with sedation (preferred, as patient feedback on hearing and vestibular symptoms is possible) or general anaesthesia. Postauricular or endaural approach for difficult canals.
- Tympanomeatal flap elevation: exposure of the posterior middle ear space.
- Confirmation of stapes fixation: palpation of the ossicular chain; mobility of the malleus and incus confirmed.
- Chorda tympani: preserved where possible; may require mobilisation.
- Curettage of the posterior bony canal wall (scutum) to improve oval window visualisation if needed.
- Division of the stapedial tendon and posterior crus.
- Measurement of prosthesis length: measured from the undersurface (inferior surface) of the long process of the incus to the footplate; typically 4.25-4.75 mm.
- Fenestration of the footplate: using a laser (KTP, CO$_2$, or Er:YAG) or calibrated microdrill to create a fenestra of approximately 0.6-0.8 mm in diameter in the posterior footplate.
- Prosthesis insertion: a piston prosthesis (Teflon, titanium, or similar MRI-compatible material) is positioned through the fenestra and crimped onto the long process of the incus.
- Tissue seal: a blood clot, fat, or perichondrium plug is placed around the piston to seal the fenestra.
- Hearing check (if local anaesthesia): patient reports improvement intraoperatively in the majority.
Total Stapedectomy vs Stapedotomy
| Feature | Total Stapedectomy | Small Fenestra Stapedotomy |
|---|---|---|
| Footplate management | Entire footplate removed | Fenestra 0.6-0.8 mm created; footplate retained |
| Tissue graft | Fat, perichondrium or vein graft over oval window | Perichondrium/fat plug around piston |
| SNHL risk | Higher (greater perilymph disturbance) | Lower (preferred technique) |
| Current preference | Retained for obliterative footplate | Standard technique |
| Obliterative footplate | More applicable | Partial stapedectomy or stepwise drillout |
Management of the Obliterative (Biscuit) Footplate
Extensive otosclerosis may replace the entire footplate with dense bone ("biscuit footplate"). Surgical strategies:
- Careful stepwise drillout of the centre of the footplate with a microdrill under constant irrigation.
- CO$_2$ or KTP laser vaporisation in graduated steps.
- A floating footplate (unintended mobilisation into the vestibule) is a recognised intraoperative complication requiring immediate conversion: remove prosthesis and footplate fragments; seal with connective tissue graft; defer re-exploration.
Prosthesis Options
- Piston prostheses: Teflon-platinum (Shea type), titanium, stainless steel variants.
- All modern prostheses should be MRI compatible at 1.5T and 3T; titanium is preferred.
- Clip prostheses (e.g. nitinol) allow self-crimping and reduce the need for manual crimping force.
Complications and Their Management
Intraoperative
| Complication | Management |
|---|---|
| Floating footplate | Gentle retrieval with suction; convert to stapedectomy; connective tissue graft |
| Obliterative footplate | Stepwise laser or drill fenestration; patience |
| Persistent stapedial artery | Cauterise and divide; enlarges surgical field requirement |
| Dehiscent facial nerve | Identify and protect; proceed with care or abandon |
| Perilymph gusher | Pack with connective tissue; bed rest; avoid Valsalva; may require lumbar drain if persistent |
Post-operative
| Complication | Features | Management |
|---|---|---|
| Conductive hearing loss (immediate) | Short prosthesis; prosthesis displacement; unrecognised malleus fixation | Re-exploration when appropriate |
| Conductive hearing loss (delayed) | Prosthesis migration; fibrosis; incus necrosis | HRCT; re-exploration and revision |
| Sensorineural hearing loss (early) | Surgical trauma; labyrinthitis | Systemic corticosteroids; monitor |
| Sensorineural hearing loss (delayed) | Perilymph fistula; post-traumatic hydrops | PLF: conservative then re-exploration; graft seal |
| Perilymph fistula | Fluctuating or progressive SNHL; vertigo | Conservative (bed rest, avoid straining); surgical re-exploration and sealing |
| Persistent vertigo | Labyrinthitis; PLF; prosthesis too long | Vestibular suppressants initially; investigate PLF |
| Chorda tympani injury | Taste disturbance; metallic taste | Observe; resolves in most within 3-6 months |
| Reparative granuloma | Pain; conductive loss; vertigo days post-op | Systemic steroids; re-exploration and removal of prosthesis |
| Tympanic membrane perforation | Visible on otoscopy | Myringoplasty when healed |
Special Considerations
Otosclerosis and Sensorineural Hearing Loss
Cochlear otosclerosis produces SNHL through enzymatic and toxic effects on the cochlear endosteum, disruption of the blood-labyrinth barrier, and direct encroachment on the spiral ligament and stria vascularis. Patients with mixed hearing loss require careful counselling: stapes surgery can restore the conductive component, but the sensorineural component is not reversible. When bone conduction thresholds are severely reduced, cochlear implantation may be the preferred or adjunctive option.
Bilateral Disease: Surgical Sequencing
- Operate on the worse ear first; the better-hearing ear is the patient's functional reserve.
- The second ear may be offered surgery after a minimum 6-month interval to confirm stable hearing in the operated ear.
- Never operate on both ears simultaneously.
Otosclerosis in Pregnancy
- Pregnancy accelerates the course of disease; oestrogen promotes otosclerotic activity.
- Surgery is deferred until after delivery and cessation of breastfeeding in the elective setting.
Cochlear Implantation in Otosclerosis
- Profound bilateral SNHL from cochlear otosclerosis is an established indication for cochlear implantation.
- Otosclerosis was among the most common aetiologies in the earliest cohorts of cochlear implant recipients.
- Pre-operative HRCT is essential to assess cochlear patency; extensive pericochlear otosclerosis may narrow or obliterate the scala tympani, requiring specialised electrode arrays (e.g. compressed, double array).
- Intraoperative electrical impedance monitoring helps anticipate difficulties with electrode insertion.
Revision Stapes Surgery
Indications: failure of primary surgery; incus necrosis; prosthesis displacement; fibrosis at the oval window.
- Revision carries higher risk of SNHL than primary surgery (approximately doubled).
- HRCT assists in identifying the cause of failure before revision.
- Incus necrosis may require interposition of cartilage or bone between the prosthesis and the manubrium.
Malleus Fixation
Coexistent malleus fixation (from tympanosclerosis or, rarely, a second focus of otosclerosis) may account for persistent conductive hearing loss after apparently successful stapes surgery. Intraoperatively, the surgeon should palpate the malleus independently after stapes mobilisation or prosthesis insertion to exclude this before closing.
Long-term Care and Outcomes
- Successful stapedotomy closes the ABG to within 10 dB in 85-95% of patients at experienced centres.
- Long-term durability is excellent; hearing gain is sustained in the majority over decades.
- Sensorineural component may progress independently of stapes fixation and should be monitored with annual audiometry.
- Patients with progressive SNHL despite patent stapes surgery should be counselled regarding hearing aids or cochlear implantation as disease evolves.
- Patients who decline surgery or are not candidates should be offered appropriate hearing rehabilitation (air-conduction aids, bone-anchored devices) with regular audiological follow-up.
- Contact sports and activities with significant barotrauma risk (deep-sea diving) carry a theoretically elevated risk of prosthesis displacement and are generally discouraged in the post-operative period; long-term restrictions are individualised.