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Hearing Loss: Types, Investigations, and Otosclerosis

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PART 1: TYPES OF HEARING LOSS

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1. Conductive Hearing Loss (CHL)

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2. Sensorineural Hearing Loss (SNHL)

• Usually permanent
• Loudness recruitment (abnormal growth of loudness) is characteristic - helps distinguish from CHL
• Distortion of complex sounds, particularly speech in noise, which cannot be corrected by simple amplification
• The "neural" in SNHL refers only to the cochlear nerve connections from inner hair cells - it does NOT include auditory neuropathy

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3. Mixed Hearing Loss

4. Auditory Neuropathy Spectrum Disorder (ANSD)

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PART 2: INVESTIGATIONS

A. Tuning Fork Tests

1. Weber Test

• Strike the tuning fork (512 Hz) and place on the center of the forehead, nasofrontal suture, or anterior incisors
• Patient reports which ear hears the tone louder, or if it is midline
• Interpretation:
  • Midline = normal or equal bilateral loss
  • Lateralizes to the worse (affected) ear = CHL in that ear (as little as 5 dB of CHL will cause lateralization)
  • Lateralizes to the better ear = SNHL on the opposite (worse) side
• Weber is reliable but its acoustic basis remains unclear; patients with unilateral CHL may hesitate to acknowledge hearing better in the "bad" ear

2. Rinne Test

• Strike a 512 Hz tuning fork; place the base on the mastoid process (BC) - patient indicates when they can no longer hear it, then move the tines 2-3 cm in front of the EAC (AC) and ask if they still hear it
• Alternatively: compare loudness at mastoid vs. at ear canal
• Results:
  • AC > BC (Positive Rinne) = Normal hearing OR sensorineural hearing loss
  • BC > AC (Negative Rinne) = Conductive hearing loss (implies a CHL of ≥20 dB with a 512 Hz fork)
• Note: "Positive" = normal result - a common source of confusion; it is preferable to report as "AC > BC" or "BC > AC"
• False negative Rinne: In profound unilateral SNHL, the fork placed on the mastoid is heard by the opposite (better) cochlea via bone conduction - falsely suggesting CHL. This is detected by masking.

3. Bing Test (Occlusion Test)

• Assesses BC enhancement by canal occlusion
• In normal hearing and SNHL, occluding the EAC with a finger while fork is on the mastoid increases loudness (positive Bing)
• In CHL, no change in loudness (negative Bing) - canal is already functionally occluded

Summary Table (Weber + Rinne):

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B. Audiometry

• Air conduction (AC): Tested via headphones or insert earphones; tests entire auditory pathway
• Bone conduction (BC): Tested via a vibrator on the mastoid; bypasses the outer and middle ear - directly tests cochlear function
• Masking: Needed to prevent the non-test ear from participating (cross-hearing)

• Speech Reception Threshold (SRT): Lowest level at which patient repeats 50% of spondee words; should correlate with PTA
• Word Recognition Score (WRS) / Speech Discrimination: Percentage of monosyllabic words correctly identified at comfortable listening level; typically good in CHL, poor in SNHL (due to distortion)

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C. Impedance Audiometry (Immittance Audiometry)

1. Tympanometry

2. Acoustic Reflex Testing

• Normal reflex: Bilateral (ipsilateral and contralateral) at appropriate levels
• In otosclerosis:
  • Early: Diphasic (on-off) reflex - brief initial compliance change reverses quickly (characteristic of early stapedial fixation)
  • Moderate/advanced: Absent reflex when probe is in the affected ear
  • Advanced: Contralateral reflex also absent due to degree of CHL

3. Static Compliance

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PART 3: OTOSCLEROSIS

Etiology

• Genetics: Autosomal dominant with variable penetrance (~25-40% penetrance). Sporadic cases account for 40-50%. Family history of hearing loss in approximately 2/3 of patients.
• Sex: Women are twice as likely to be affected as men. Pregnancy is known to accelerate disease progression.
• Race: Predominantly affects White (Caucasian) populations; rare in Africans and East Asians.
• Age: Typically presents in the third and fourth decades of life (teens to 20s onset); often not noticed until age 30-40.
• Bilateral: Approximately 75% of cases are bilateral, though often asymmetric.
• Viral trigger hypothesis: Measles virus RNA has been found in otosclerotic foci, suggesting persistent paramyxovirus infection may trigger disease in genetically susceptible individuals (supporting the role of measles vaccination in reducing incidence).
• Fluoride hypothesis: Low fluoride in drinking water is associated with higher prevalence; fluoride may stabilize otic capsule bone.
• Other factors: Hormonal influences (estrogen, pregnancy), autoimmunity, and TGF-β1 signaling pathways are implicated.

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Pathology

1. Active (otospongiotic) phase:
   • Normal enchondral bone replaced by vascular, sponge-like woven bone
   • Increased osteoclastic resorption creates lacunae filled with highly vascular fibrous tissue
   • Blue lakes/haversian spaces filled with blood - responsible for the Schwartz sign (flamingo-pink blush on the promontory seen through the intact TM due to increased vascularity)
   • Histologically: large vascular channels, active osteoblasts and osteoclasts, irregular "mosaic" pattern of bone
2. Inactive (sclerotic) phase:
   • Vascular tissue replaced by dense, sclerotic lamellar bone
   • Ankylosis of the stapes footplate in the oval window
   • Spiral ligament hyalinization if cochlear extension occurs

• Fissula antefenestrum (anterior to the oval window): Most common site - explains why anterior footplate is fixed first
• Fenestral otosclerosis: Focus on the oval window niche - causes CHL by stapes fixation
• Cochlear (retrofenestral) otosclerosis: Extension to the cochlear capsule - causes or contributes to SNHL ("double-ring sign" or halo sign on CT)
• "Biscuit footplate": Total replacement of the annular ligament by otosclerotic bone - complete fixation

• Early: Low-density (demineralized) plaques at the fissula antefenestrum (anterior oval window border)
• Halo sign / double-ring effect: Band of demineralization surrounding the cochlear canal on CT - cochlear otosclerosis
• Fourth ring of Valvassori: Low-density ring around the otic capsule in advanced retrofenestral disease
• Late: Increased density and marked thickening of sclerotic bone around otic capsule
• MRI with gadolinium: Enhancement within active otospongiotic foci (mirrors Schwartz sign)

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Differential Diagnosis of Otosclerosis

• Bilateral, progressive CHL starting in young adulthood
• Normal TM and normal tympanogram (Type A or As)
• Absent acoustic reflexes
• Carhart notch at 2000 Hz on BC
• Paracusis of Willis (hearing better in noise - characteristic of CHL)
• Schwartz sign (flamingo-pink promontory blush)
• Positive family history
• Female sex; worsened with pregnancy

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Medical Management of Otosclerosis

1. Sodium Fluoride

• Mechanism: Fluoride converts hydroxyapatite to fluorapatite, which is more resistant to enzymatic resorption, potentially stabilizing active otospongiotic foci
• Dosage: Typically 20-40 mg/day (as sodium fluoride tablets), sometimes combined with calcium and vitamin D
• Indication: Active cochlear otosclerosis (active Schwartz sign, progressive SNHL, positive CT halo sign)
• Evidence: Controversial - some studies show slowing of SNHL progression; no benefit for established CHL
• Side effects: GI upset, fluorosis with long-term high-dose use; not universally endorsed

2. Bisphosphonates

• Etidronate and risedronate have been explored to inhibit osteoclastic activity in active disease
• Evidence remains limited; not standard of care

3. Hearing Aids

• A valid and effective non-surgical treatment
• Conventional air-conduction hearing aids work well in pure CHL
• Bone-anchored hearing aids (BAHA/Bonebridge) are an option for those unsuitable for surgery
• Patients must be counselled that a hearing aid is an alternative to surgery, not simply an inferior option

4. Cochlear Implants

• For advanced cochlear otosclerosis causing severe-to-profound SNHL
• Technically challenging due to cochlear ossification (may require drill-out)
• Outcomes are generally good but may be inferior to non-otosclerotic implantees due to cochlear fibrosis/ossification

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Surgical Management of Otosclerosis

Indications for Surgery

• CHL with an air-bone gap (ABG) of ≥20-25 dB
• Absent acoustic reflexes confirming stapes fixation
• Normal or near-normal BC thresholds (cochlear reserve adequate)
• Patient preference after counselling of risks and alternatives
• Typically performed on the worse ear first; the better ear is operated 6-12 months later if desired

Contraindications

• Only-hearing ear (relative contraindication; increased risk not justified for most)
• Active infection
• ABR showing absent cochlear function (dead ear)
• Poor general health
• Pregnancy

Surgical Options

• Entire footplate removed and replaced with a vein or fat graft + piston/wire prosthesis
• Higher risk of labyrinthine trauma due to wide opening of vestibule
• Original Shea technique (Teflon stapes replica); evolved to wire-over-Gelfoam or fat graft

• Posterior half of footplate removed; tissue graft placed; piston prosthesis used

• A small circular hole (~0.6-0.8 mm) is made in/near the center of the footplate
• A piston prosthesis (typically 0.4-0.6 mm diameter wire-Teflon or titanium piston) is hooked over the long process of the incus and inserted through the fenestra
• Advantages: Minimal opening of vestibule, lower risk of inner ear trauma, lower rate of postoperative high-frequency SNHL
• No significant difference in PTA, ABG closure, or speech discrimination vs. stapedectomy overall
• Increasingly favored for >35 years

• Micropicks/fine picks
• Microdrill (Skeeter drill)
• Laser stapedotomy: KTP-532, argon, or CO2 laser - precise, non-contact, reduces mechanical trauma; CO2 laser now deliverable via handheld fiberoptic cables; single-shot scanning system available. Debate exists on which laser is optimal.

• Teflon piston with stainless steel wire (Schuknecht)
• Platinum fluoroplastic piston
• Titanium piston (MRI-compatible)
• Shape-memory nitinol (self-crimping)

1. Transcanal approach under general or local anesthesia with microscope (or endoscope)
2. Tympanomeatal flap elevated, middle ear entered
3. Chorda tympani nerve identified and preserved
4. Ossicular chain palpated - stapes fixation confirmed
5. Superstructure of stapes removed (crurotomy)
6. Fenestra created in center of footplate (laser, drill, or pick)
7. Prosthesis hooked to long process of incus, piston placed in fenestra (sometimes covered with vein/fat graft)
8. Tympanomeatal flap replaced

• Air-bone gap closure to ≤10 dB: achieved in ~90% of cases
• Hearing improved to serviceable levels in ~95%
• Results stable over long-term follow-up

Complications

• Dry ear precautions for first 3 weeks
• Swimming and diving permitted after ~3 weeks
• No contact sports initially
• Flying: generally permitted after 2-4 weeks

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Summary Table: Otosclerosis at a Glance

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