Ossicular Chain Disruption

Ossicular chain disruption is a break in the continuity of the malleus, incus and stapes (discontinuity or fixation). It causes conductive hearing loss, with an intact or perforated tympanic membrane.

Causes:

• Head trauma (temporal bone fracture, or a blow to the ear and barotrauma). This is the commonest cause with an intact drum. The incudostapedial joint is dislocated in about 80 percent of cases (the weakest link, because of the thin lenticular process). Other injuries are incudomalleolar dislocation, stapes fracture, and incus subluxation.
• Chronic otitis media and cholesteatoma (erosion of the long process of the incus).
• Iatrogenic injury during ear surgery or ear cleaning (cotton bud, ear syringing).
• Penetrating injury, and loud noise or explosion.
• Congenital malformations: isolated ossicular anomalies, in about 15 percent of congenital conductive hearing loss, with the long process of the incus and stapes superstructure abnormalities.

Other causes: necrosis and fixation by tympanosclerosis, otosclerosis (fixation, not disruption), and osteogenesis imperfecta.

In traumatic disruption, hearing loss may be accompanied by bleeding into the middle ear (haemotympanum), tympanic membrane perforation, perilymph fistula and facial nerve injury.

• Sudden or persistent conductive hearing loss after trauma, with a maximum air bone gap (50 to 60 decibels with complete interruption with an intact drum).
• Tinnitus, aural fullness, and autophony; sometimes vertigo, especially with perilymph fistula or concurrent inner ear injury.
• Otoscopy: usually normal tympanic membrane (a hearing loss with a normal looking ear), or haemotympanum (blue black drum), perforation, or a retraction pocket; in chronic disease, otorrhoea.
• Associated findings of temporal bone fracture (Battle sign, bleeding from the canal, cerebrospinal fluid otorrhoea, facial palsy).
• Pneumatic otoscopy shows hypermobility of the drum with ossicular discontinuity.

• Pure tone audiometry: conductive hearing loss with air bone gap of 40 to 60 decibels (a gap above 50 dB suggests complete discontinuity); speech discrimination normal.
• Tympanometry: type Ad (hypercompliant, with a deep peak on the tympanogram) in discontinuity with an intact membrane; acoustic reflexes absent.
• High resolution CT of the temporal bone: dislocation or fracture of the ossicles (incudostapedial joint widening, "Y-shaped" appearance and loss of the "ice cream cone" sign for the incudomalleolar joint), associated temporal bone fracture, and middle ear soft tissue; ossicular disruption may not be identifiable on CT, and exploratory tympanotomy is diagnostic.
• Laser Doppler vibrometry (research and specialist use) measures ossicular mobility.
• Exclude perilymph fistula, facial nerve injury, cerebrospinal fluid leak, and sensorineural component.

• Observation for 3 months after trauma with an intact drum and a mild or improving loss, since haemotympanum and oedema resolve and some dislocations improve. Repeat audiometry at 6 to 12 weeks.
• Surgical exploration (exploratory tympanotomy) and ossicular reconstruction for persisting conductive loss above 30 dB with a significant air bone gap, after at least 3 months of stable loss. Use a transcanal or endoscopic approach under general or local anaesthesia with a tympanomeatal flap:
  • Incudostapedial joint dislocation: re-establish continuity by interposing a bone cement (glass ionomer) or repositioning of the incus. Alternatively, place a sculpted incus interposition between the malleus and stapes head, or a prosthesis (titanium partial ossicular replacement prosthesis) from the malleus handle to the stapes head, with a cartilage shoe.
  • Incus necrosis or fracture: autograft incus transposition (sculpted and repositioned) or titanium prosthesis.
  • Stapes fixed or fractured: stapedotomy with piston prosthesis (0.4 mm), or total ossicular replacement prosthesis from the drum to the footplate.
  • Malleus handle fracture: malleus handle replacement or reconstruction with a prosthesis.
  • Congenital anomalies: ossiculoplasty in children from age 5 to 6 years, or a bone anchored hearing aid or middle ear implant as alternatives.
• Success: closure of the air bone gap to within 20 dB in about 70 to 85 percent; lower for total reconstruction (about 50 to 60 percent).
• Hearing aids (air conduction or bone anchored) for patients unsuitable for surgery, bilateral losses and parents electing for later surgery.
• Treat associated problems: tympanoplasty for perforation (type I with temporalis fascia graft), cholesteatoma surgery, facial nerve decompression, repair of cerebrospinal fluid leak and perilymph fistula.
• Counselling: avoid ear cleaning and water entry in the early recovery. Avoid straining and nose blowing (for 2 to 4 weeks) after surgery. Follow up audiometry at 6 weeks, 3 and 12 months.
• Complications: sensorineural hearing loss (2 to 5 percent), extrusion or displacement of prosthesis, graft failure, taste disturbance (chorda tympani injury), and vertigo.