Ossicular Chain Discontinuity

Definition and epidemiology

The ossicular chain consists of the malleus, attached to the eardrum, the incus in the middle, and the stapes, which sits in the oval window. It transmits vibration of the eardrum to the inner ear and underpins the impedance-matching function of the middle ear. Ossicular chain discontinuity is loss of continuity of this chain through separation of a joint (dislocation), fracture of an ossicle or erosion of part of it. The break may be complete, or partial with only a soft-tissue band remaining between the ossicles (Farahmand et al., 2016).

The incus is most often involved. In traumatic cases the commonest finding is separation of the incudostapedial joint (Yetiser et al., 2008). In chronic middle ear inflammation, the long process and lenticular process of the incus, which have a poor blood supply, are the most vulnerable to erosion. In 72 ears undergoing ossiculoplasty for chronic otitis media, the incus was the most frequently eroded ossicle (43%) (Kotzias et al., 2020).

There are no reliable population figures for ossicular discontinuity, because it is usually recorded as a consequence of the underlying injury or chronic ear disease. Congenital ossicular anomalies can produce a similar picture, but this article focuses on acquired discontinuity.

Symptoms and signs

The main complaint is hearing loss that starts after an injury or ear infection and does not recover. A blocked sensation may accompany it. Because the loss is usually in one ear, people struggle to locate sounds and to follow speech in noise. With bone conduction preserved, their own voice may sound louder (autophony). Tinnitus can occur, but vertigo is not a feature of ossicular discontinuity and should raise suspicion of inner ear injury or a perilymph fistula.

After trauma, the early findings may include an eardrum tear, blood behind the drum (haemotympanum) and bleeding from the ear canal. Persistent hearing loss once these have settled suggests ossicular discontinuity. The eardrum usually looks normal; occasionally it is unusually mobile, or a displaced ossicle can be seen behind it. In a Turkish series of 32 patients with traumatic ossicular dislocation, the mean age was 24.6 years and patients presented for treatment a mean of 5.7 years after the injury (Yetiser et al., 2008). This delay shows how easily a discontinuity behind an intact drum is overlooked.

When chronic otitis media or cholesteatoma is the cause, ear discharge, a perforation or a retraction pocket is also present, and hearing declines slowly over months or years.

Causes and risk factors

Trauma: temporal bone fractures and head injuries without a fracture can dislocate the ossicles. In the series by Yetiser and colleagues, road traffic accidents were the commonest cause (Yetiser et al., 2008). Other traumatic causes include a cotton bud, hairpin or similar object pushed through the eardrum onto the ossicles, a slap to the ear, blast injury and pressure changes. Eardrum perforation and inner ear damage may accompany these injuries.

Chronic inflammation: chronic otitis media, deep retraction pockets and cholesteatoma gradually erode the ossicles. The long process of the incus is the most susceptible because it is slender and has a limited blood supply; the stapes superstructure and the malleus head can also be affected (Kotzias et al., 2020). Previous ear surgery (for example a displaced prosthesis) and, rarely, congenital ossicular anomalies can also cause discontinuity.

Audiological and clinical assessment

The audiological pattern of ossicular discontinuity is distinctive, and the audiologist’s findings are central to the diagnosis. With a complete break behind an intact eardrum, very little sound energy reaches the inner ear, producing a flat air-bone gap of around 60 dB, close to the maximum possible conductive loss (Malouf et al., 2024).

• Pure-tone audiometry: bone conduction thresholds are normal or near normal while air conduction thresholds are markedly raised. In complete discontinuity the air-bone gap is about 50-60 dB and flat. A larger gap at high than at low frequencies suggests partial discontinuity with a soft-tissue band between the ossicles (Farahmand et al., 2016). The large gap means masking is needed for both air and bone conduction, and a masking dilemma may arise.
• Speech audiometry: the speech reception threshold rises in line with the air conduction average; at adequate presentation levels word recognition is good because the cochlea is intact.
• Tympanometry: with an intact, normal drum the tympanogram is type Ad, with normal peak pressure and abnormally high static compliance (Jerger, 1970; Malouf et al., 2024). A perforation gives a flat trace and this distinguishing sign is lost. Multifrequency or wideband tympanometry can add information by showing altered resonance characteristics.
• Acoustic reflexes: with the probe in the affected ear, ipsilateral and contralateral reflexes are absent, because stapedius contraction is not transmitted to the eardrum. When the stimulus is delivered to the affected ear, the large conductive loss reduces its effective level, so reflexes are usually absent as well.
• Otoacoustic emissions and ABR: OAEs are absent because of the conductive loss; if an auditory brainstem response (ABR) is recorded, latencies are prolonged in a conductive pattern. These tests are mainly needed when behavioural testing is not possible.
• Imaging: high-resolution temporal bone computed tomography (CT) can show incudostapedial separation, incus dislocation, fracture lines and inner ear involvement. Because the structures are so fine, partial discontinuity is not always visible on CT, and the diagnosis is often confirmed at surgical exploration.

The differential diagnosis includes otosclerosis and other ossicular fixations, middle ear effusion and inner ear third-window conditions such as superior semicircular canal dehiscence. Ossicular fixation gives a low-compliance (type As) tympanogram, whereas in third-window lesions preserved acoustic reflexes and better-than-normal bone conduction thresholds point to the diagnosis.

Treatment and audiological rehabilitation

After trauma, an eardrum tear and haemotympanum usually heal on their own, and part of the conductive loss may recover during this period. Surgery is therefore considered once the middle ear has healed and hearing tests have been repeated. Signs of inner ear injury, a perilymph fistula or facial palsy call for a different approach and prompt medical assessment.

For persistent discontinuity the treatment is surgical reconstruction of the ossicular chain (ossiculoplasty). Depending on the site of the break and the remaining ossicles, the surgeon may reshape and reposition the incus (incus interposition), or use a partial or total ossicular replacement prosthesis, cartilage or bone cement; the choice rests with the surgeon. In a series of 32 patients with traumatic ossicular dislocation, the postoperative air-bone gap closed to within 10 dB in 37.6% and within 20 dB in 71.9% (Yetiser et al., 2008). After ossiculoplasty for chronic otitis media, success (an air-bone gap of 20 dB or less) was reported in 61% (Kotzias et al., 2020), and a systematic review, based on low-level evidence, found that a preserved malleus predicted better postoperative hearing independently of stapes status (Blom et al., 2015). The severity of middle ear disease, mucosal condition and previous surgery all influence the outcome and can be graded before surgery with staging systems (Dornhoffer and Gardner, 2001).

For patients who do not want surgery or are not fit for it, an air conduction hearing aid works well. Because the cochlea is intact, speech clarity is usually good, but enough gain is needed to overcome the conductive loss and the fitting should be verified with real-ear measurement (REM). If the ear canal or chronic discharge prevents use of an air conduction aid, bone conduction implants can be considered. After surgery, hearing is retested at intervals; a displaced prosthesis or reopening of the gap shows up as a renewed fall in hearing.

Impact on quality of life and advice

Ossicular discontinuity usually affects one ear, so its everyday effects resemble those of unilateral hearing loss. Locating sounds, understanding speech in noise and following group conversations become harder, which can increase listening effort and fatigue. When the loss follows an accident or injury, the event itself may also cause anxiety.

In daily life it helps to turn your better ear towards the speaker and, in noisy places, to sit where you can see the speaker’s face. Do not put cotton buds or sharp objects in your ear; see your doctor about earwax problems. If you have had surgery, avoid forceful nose blowing, flying and heavy lifting for as long as your surgeon advises. For children with a conductive loss in one ear, the teacher should be informed and the child seated in a suitable place in class.