Misplaced incus: an unusual complication of a temporal bone fracture

Abstract

A 24-year-old man sustained significant injuries as a result of a motor bicycle accident. CT scans demonstrated multiple intracranial injuries including bilateral temporal bone fractures. On emergence from sedation, the patient reported right-sided hearing loss which was attributed to ossicular chain disruption after the incus was found to be in the external auditory canal. Unexpectedly, the initial conductive hearing loss resolved with conservative management and an ossicular chain reconstruction was not required. This is an unusual case of impressive long-term good hearing outcomes following complete dislocation of the incus and extrusion from the middle ear.

Background

Conductive hearing loss is a well-known complication of temporal bone fractures and results in significant morbidity for patients. Conductive hearing loss that persists >6 weeks post-injury is often due to ossicular chain pathology after resolution of blood or cerebrospinal fluid within the middle ear. The conductive hearing loss can often be surgically corrected with an exploratory tympanotomy and ossicular chain reconstruction. We illustrate a case of severe ossicular chain injury and extrusion of the incus from the middle ear though the tympanic membrane to lie within the external auditory canal. A natural reconstruction of the ossicular chain by formation of a likely spontaneous malleostapedopexy resulting in an improvement in the air–bone gap without surgery has never been previously described in the literature.

Case presentation

A 24-year-old man was transported to a tertiary trauma centre following a motor bicycle accident. On presentation, the patient had a Glasgow Coma Scale of 7 with multiple injuries and was promptly intubated and sedated. CT imaging of the head revealed extensive base of skull fractures with bilateral subdural and subarachnoid haemorrhages worse on the left, which were managed conservatively in the intensive care unit following intracranial pressure monitor insertion. Multiple other injuries sustained were also managed including bilateral pulmonary contusions, open fractures of the right tibia and fibula and a right mandibular fracture.

On emergence from sedation 3 weeks following the injury, the patient reported reduced hearing on the right side and a left unilateral facial nerve paresis was noted. The patient did not report vertigo. It was at this point that the otolaryngology team was contacted. There was clotted blood and debris obscuring the view of the tympanic membrane, which could not be adequately removed without causing the patient discomfort. The facial weakness on the left was noted to be lower motor neuron in nature and graded as a House-Brackmann grade 6. Bedside tuning fork tests showed that the Weber lateralised to the right ear and bone conduction was better than air conduction in the right ear and that air conduction was better than bone conduction in the left ear.

Investigations

In view of the injuries and clinical findings as well as the need to exclude a sensorineural hearing loss (which could be performed independent of the status of the tympanic membrane), an audiogram was requested by the otolaryngology team. This demonstrated a mild low-frequency sensorineural hearing loss on the left and a moderate to severe predominantly conductive hearing loss on the right with an air–bone gap of between 10 dB (at 500 Hz) and 50 dB (at 4000 Hz) (figure 1). The left tympanogram was a type C and right tympanometry was unobtainable.

Figure 1.

Pure tone audiogram 3 weeks postbilateral temporal bone fractures.

A dedicated high-resolution CT of the temporal bones demonstrated bilateral longitudinal and obliquely orientated temporal bone fractures with the fracture line crossing the tympanic segment of the facial nerve and the region of the geniculate ganglion (figure 2). Ossicular disruption was demonstrated on the right with a high-density triangularly shaped foreign body in the shape of the incus located within the external auditory canal (figure 3). However, the exact nature of the foreign body could not be confirmed visually due to the debris present within the ear canal.

Figure 2.

Axial CT scan of the temporal bones demonstrating bilateral longitudinal temporal bone fractures.

Figure 3.

Axial CT image of the temporal bones demonstrating the incus (arrow) within the right temporal bone.

Facial nerve conduction studies indicated that the left facial nerve motor potential was present but reduced in amplitude and electromyographical testing showed evidence of spontaneous activity with markedly reduced recruitment and large polyphasic motor unit potentials in muscles supplied by the left facial nerve, in keeping with severe left facial neuropathy. There was evidence of some re-innervation of both the upper and lower facial muscles.

Initial treatment

The patient was managed with a 2-week course of high-dose oral steroids, dry ear precautions and serial audiometry. Eye care in the form of eye taping and regular lubrication was instituted. A 2-week course of ciprofloxacin ear drops was used in a prophylactic manner to prevent infection given that the status of the tympanic membrane was not certain. Outpatient follow-up was arranged and in view of the initial audiogram, plans for elective ossicular chain reconstruction in the future was discussed with the patient pending a subsequent review.

Outcome and follow-up

Two months after the temporal bone trauma, the patient’s left facial nerve function had improved, with a House-Brackmann grade III paresis of the upper and grade IV of the lower branches. At this stage, the ear canal debris had cleared and under the microscope, there appeared to be a bony foreign body within the external auditory canal corresponding to the bony foreign body seen on the initial CT scan. In view of the fact that the incus was not visible on the initial CT scan within the middle ear, in retrospect, this triangularly shaped foreign body was most likely the incus which had been displaced (figure 4). Surprisingly, the visible tympanic membrane was intact with bilateral type A tympanograms.

Figure 4.

Incus visible in right external auditory canal on otoscopy.

At the 1-year review, the patient did not report any concerns about his hearing. On examination, both tympanic membranes were intact, and the incus was no longer visible in the right external auditory canal. A repeat audiogram demonstrated normal hearing on the left with a mild right-sided conductive loss, with a significantly smaller air–bone gap of between 10 dB (at 500 Hz) and 25 dB (at 4000 Hz) (figure 5). Given the improvement in hearing, the absence of the incus, the patient’s reluctance for a repeat CT scan or exploratory surgery, the status of the ossicular chain could not be directly confirmed. The most likely explanation of the patient’s hearing in the absence of incus is the creation of a spontaneous malleostapedopexy. In fact, the initial CT scan demonstrated articulation between in the malleus handle and the head of the stapes (figure 6).

Figure 5.

Pure tone audiogram 1 year postbilateral temporal bone fractures.

Figure 6.

Axial CT scan of the temporal bones demonstrating articulation between the handle of malleus and the stapes superstructure.

Discussion

In cases of conductive hearing loss following head trauma, the incudostapedial joint is the most common location of ossicular chain disruption.^1–3 Compared with the malleoincudal joint, the stapedial arch and the annular ligament, the incudostapedial joint is the joint least tolerant to external physical stresses.⁴ The incus is the most commonly dislocated of all the ossicles,⁵ owing to the absence of a muscular attachment and the heavy weight of the incus as compared with the malleus and stapes.^{2 6}

Incus dislocation has been well reported in the literature, with exploratory tympanotomy revealing the incus to be displaced and located either within the middle ear,⁷ within the antrum⁸ and partially or completely within the external auditory canal.^9–13 In all these cases, dislocation of the incus was associated with conductive hearing loss.

In addition to the case we have described, a single similar case of incus dislocation with preservation of hearing has been described. Zou et al ¹⁴ report a case of a 20-year-old woman with a traumatic right temporal bone fracture and consequential facial nerve paresis. An audiogram 2 months post-trauma demonstrated normal hearing bilaterally. During elective endoscopically assisted facial nerve decompression via a mastoid approach, the incus was discovered within the mastoid cavity. The authors describe that ‘continuity of the ossicular chain had been maintained by smooth fibrous interconnections of the stapedic head, tympanic membrane and malleus handle’.

This is the first case of complete incus dislocation and extrusion externally into the auditory canal with largely preserved conductive hearing that has been reported in the literature.

Learning points.

• The incus is the most vulnerable of the three ossicles to dislocation.

• Trauma can result in dislocation of the incus and partial or total extrusion into the external auditory canal.

• Satisfactory hearing outcomes are possible with conservative management, even with complete incudal dislocation.