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Ossicular-Chain Dislocation
Introduction
Hearing loss following head injury is a common clinical finding, although typically delayed in its presentation due to the immediate investigation for more significant injuries. Hearing loss can be conductive, sensorineural, or mixed. When conductive hearing loss persists more than 6 weeks after injury, ossicular chain dislocation must be considered.
The ossicular chain consists of three bones; the malleus, incus, and stapes joined by two synovial joints, the incudomallear and incudostapedial joints.[1] Together, they comprise the primary sound-conduction apparatus to transmit vibratory stimulus from the tympanic membrane to the oval window. In addition to simply conducting sound, amplification occurs due to the compound lever effect of the ossicles and their natural resonance. Complete loss of this conductive pathway would result in a conductive hearing loss of around 50-60dB.[2]
The entire ossicular chain is encased within the petrous portion of the temporal bone within the middle ear cavity. While the temporal bone offers significant bony protection, the structure remains vulnerable to damage from varying mechanisms. The relatively fixed position of the malleus and stapes compared to the incus may contribute to the finding that dislocation of the ossicles is far more common than fracture.[3]
Etiology
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Etiology
Ossicular chain dislocation occurs following trauma. Ossicular chain discontinuity can occur due to cholesteatoma or congenital disease, but this is usually due to erosion of the ossicles or a requirement during cholesteatoma surgery.[4][5] This represents a different disease entity, so it will not be discussed in this article.
The most common injury to result in ossicular chain dislocation is temporal bone trauma.[3] However, it is important to note that dislocation can occur with or without a temporal bone fracture.[6]
Other injuries that have been reported to result in dislocation include:
- An indirect blow to the contralateral side[7]
- Blast injuries[3][8]
- Direct traumatization via instrumentation of the external auditory canal[9][10]
- Barotrauma[11]
The most common mechanisms of injury are road traffic collisions (RTC), falls, and interpersonal violence.[3][6][12]
Epidemiology
Ossicular chain dislocation is relatively uncommon, and the exact incidence is unknown. It has been estimated that up to 50% of temporal bone fractures result in ossicular injury.[3][13] Hearing loss is reported in up to 75% of patients in the first week following a head injury, dropping to 37% after 3 to 6 weeks. For those with residual conductive hearing loss at 6 weeks post-injury, the likely cause is ossicular chain dislocation. These patients represented 4% of those presenting with head trauma.[14]
The majority of patients with ossicular chain dislocation are young males aged between 16 and 30.[13] However, they occur across all age groups and sexes.
Pathophysiology
Injuries seen include:
- Incudostapedial dislocation
- Incudomallear dislocation
- Stapediovestibular subluxation
- Luxation of the incus
- Incudomallear subluxation
The most common site for dislocation, found at the time of surgical exploration is the incudostapedial joint.[15][16][17] Incudomallear joint dislocation has been reported as being the most common site for dislocation based on radiographic studies.[18] However, this is likely because the incudostapedial joint is harder to assess via computed tomography (CT).[19]
History and Physical
The majority of patients present following head trauma. The main presenting complaint is hearing loss. A full otoscopy examination should be undertaken, which may reveal blood in the external auditory canal, hemotympanum, perforated tympanic membrane, or cerebrospinal fluid (CSF) leak. Alternative etiologies of hearing loss, as described above, should be excluded.
Associated findings include:
- Facial palsy[3]
- Tinnitus
- Vertigo
- Perforated tympanic membrane
Evaluation
In patients with ossicular discontinuity following trauma, primary investigations are usually performed to rule out a more significant head injury. A CT head will normally be performed, and if a temporal bone fracture is present, it is important to note whether it is otic capsule-sparing or otic capsule-violating, as this is the most reliable predictor of underlying injuries to the middle and inner ear.[20]
Once the patient is medically fit, an audiogram should be obtained as soon as possible to allow for a baseline measurement of hearing loss. Any conductive hearing loss could be related to either ossicular disruption or hemotympanum, and it is challenging to differentiate between them at this point. Therefore, a repeat audiogram should then be performed at least 6 weeks following the injury by which time any hemotympanum should have resolved. Any conductive loss at this time, in the presence of an intact tympanic membrane, should be assumed to be due to ossicular chain dislocation. Tympanometry may show increased compliance, suggesting discontinuity of the ossicular chain.
A standard trauma-sequence head CT is usually inadequate for appropriate imaging of the ossicles, therefore a CT temporal bone scan should be requested. However, not all ossicular chain dislocations are evident on CT scans, and it may be that diagnosis is only possible following surgical exploration of the middle ear.[15]
Treatment / Management
In patients with suspected or proven conductive hearing loss lasting greater than 6 months, the options would be conservative, with or without hearing aid provision, or surgical intervention.[6][15] Surgery is contraindicated if it is the patient's only hearing ear or if the patient is unfit for surgery.(B2)
Surgical intervention can be delayed. The average interval between injury and surgery is over 5 years.[3][6][15] Middle ear exploration is performed under general anesthetic to identify the disrupted ossicular chain and perform ossiculoplasty. A wide range of ossiculoplasty techniques are available and are covered within other articles in this series.(B2)
Differential Diagnosis
There are multiple pathologies that can result in hearing loss following trauma. They can be divided into those that cause conductive loss and those that cause sensorineural loss. In the case of stapediovestibular subluxation, a mixed hearing loss can be seen. More than one pathology can co-exist.
Conductive hearing loss:
- Hemotympanum
- Tympanic membrane perforation[13]
- Ossicular chain dislocation
Sensorineural hearing loss:
- Post-traumatic inner ear hemorrhage
- Perilymphatic fistula
- Endolymphatic hydrops[18]
- Stapediovestibular subluxation
Prognosis
Ossciculoplasty results in hearing improvement in the majority of patients. Following surgery, an average closure of the air-bone gap of 35dB (range 8-60dB) has been reported. Over 70% of patients will have an air-bone gap of less than 20dB, and over 30% of patients will have an air-bone gap of less than 10dB.[3][6][15]
Complications
A number of complications have been reported following ossiculoplasty, these include:
Deterrence and Patient Education
The public health advice to reduce the incidence of head injuries would also reduce the incidence of ossicular chain dislocation. The public should be advised to wear adequate head protection when using bicycles, motorbikes, and partaking in extreme sports. A seatbelt should be worn in motor vehicles.
People should be advised against the instrumentation of the ear canal. As the saying goes, ‘never put anything smaller than your elbow in your ear.’
Enhancing Healthcare Team Outcomes
The majority of patients who present with a head injury will be managed by an interprofessional team. This will include the emergency clinicians who initially assess the patient, specialists such as neurosurgeons, and rehabilitation professionals. The role of the otolaryngologist in the acute setting is to establish the degree and type of any hearing loss, the state of the facial nerve, and to arrange appropriate follow-up. Only immediate and complete facial nerve palsy would require acute surgical intervention. It is vital that patients are not lost to follow-up as this can result in a delayed presentation of many years.
The management of ossicular chain dislocations should be undertaken by an otologist with extensive experience of middle ear surgery. There are many different approaches to the surgical reconstruction of the ossicular chain, and each case should be assessed individually to establish the most appropriate surgical strategy. Post-operative management should involve otologists and audiologists. Patients should be followed-up for a minimum period of one year post-procedure.[3][6][15] [Level 3]
References
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