Case report

After extensive counselling, an 11-year-old boy affected by auditory neuropathy was implanted in the right ear with a Cochlear Nucleus Freedom CI24RE Contour implant with Contour Advance electrode (Cochlear Pty Ltd, Sydney, Australia), on July 2009. Pre-operative high resolution computed tomography and magnetic resonance imaging of the temporal bone had excluded gross inner ear malformations.

At surgery, the facial nerve was found to lie more anteriorly than normal, concealing the round window niche (Figure 1). Insertion of the electrode array was not possible through the narrow posterior tympanotomy, even though it had been widened as much as possible. Thus, a retro-facial approach was carried out by a skilled surgeon (MAB) familiar with the procedure, in a manner similar to that described by Beltrame et al. Reference Beltrame, Frau, Shanks, Robinson and Anderson⁷ After identification and sufficient exposure of the round window niche, a cochleostomy was performed antero-inferior to it using a 1-mm burr. The electrode array was inserted completely through the cochleostomy (Figure 1).

Fig. 1

Surgical photograph showing electrode array fully inserted into the cochleostomy, via a retro-facial approach. PWC = posterior wall of the external ear canal; FN = facial nerve; TS = tympanic sinus; EA = electrode array; SS = sigmoid sinus

During intra-operative CI testing, impedance results were normal while neural response telemetry was undetectable. The stapedial reflex response could not be evaluated due to the lack of visible anatomical references. Intra-operative plain radiographs were apparently consistent with correct placement of the electrode array (Figure 2).

Fig. 2

Intra-operative plain X-ray image.

After activation, vestibular symptoms were elicited at effective electrical stimulation, leading to suboptimal mapping of the processor and unsatisfactory functional results. A device integrity test and a high resolution computed tomography scan of the temporal bone were planned.

The computed tomography scan (Figure 3) showed that the electrode array was not entering the basal turn of the cochlea because it was entirely coiled within the anterior vestibule.

Fig. 3

Axial, high resolution computed tomography scan showing the electrode array self-curling within the vestibule.

Re-implantation was carried out via the same approach, but was complicated by marked cerebrospinal fluid leakage from the reshaped cochleostomy, which probably had notched the lateral aspect of the internal auditory canal. Several attempts were made to insert a straight probe electrode array into the cochlea, under fluoroscopic control, but the tip of the dummy invariably entered the internal auditory canal.

It was decided to convert to a canal wall down procedure. The resulting broad visualisation of the promontorium and round window enabled a second, safe, promontorial cochleostomy. A straight electrode array was inserted correctly into the second cochleostomy, as confirmed by post-operative high resolution computed tomography (Figure 4). Intra-operative neural response telemetry was still absent during revision surgery. The mastoid and tympanic cavities were obliterated with abdominal fat grafts in order to control cerebrospinal fluid leakage and to prevent infection.

Fig. 4

Axial, high resolution computed tomography scan taken to follow up re-implantation, showing the straight electrode array correctly positioned within the cochlea. The canal wall down cavity is obliterated with fat grafts.

Facial nerve monitoring was uneventful during all surgical procedures.

The post-operative course was unremarkable.

The device was successfully activated four weeks later. At the time of writing, the patient was doing well.

Cochlear perimodiolar electrodes have become popular following research in the normal cochlea which compared the straight electrode to the perimodiolar electrode, and which demonstrated that a more reliable and less traumatic insertion can be obtained using the perimodiolar electrode with a soft insertion technique.Reference Roland³ Misdirection of the electrode array occurs more often in cases of deviated cochleostomy.Reference Tange, Grolman and Maat⁴ Jain and Mukherji reported that the electrode array can be misplaced into the middle-ear cavity, mastoid bowl, cochlear aqueduct, petrous carotid canal or eustachian tube, or may be only partially inserted into the cochlea.Reference Jain and Mukherji¹ The electrode array can also be inserted in the vestibular system, e.g. into the superior or lateral semicircular canal.Reference Tange, Grolman and Maat^4, Reference Mecca, Wagle, Lupinetti and Parnes^9–Reference Sorrentino, Coté, Eter, Laborde, Cochard and Deguine¹¹ Vestibular symptoms have sometimes been reported as complications of CI, and should arouse suspicion of electrode array misplacement.Reference Mecca, Wagle, Lupinetti and Parnes^9, Reference Sorrentino, Coté, Eter, Laborde, Cochard and Deguine¹¹

Intra-operative neural response telemetry recordings may suggest CI failure.Reference Sorrentino, Coté, Eter, Laborde, Cochard and Deguine¹¹ However, neural response telemetry results cannot determine whether the electrode array is placed within the cochlea rather than in the vestibule, because cochlear and vestibular action potentials are similar.Reference Pau, Parker, Sanli and Gibson⁶ On the other hand, the absence of a detectable neural response telemetry threshold is observed in some patients intra-operatively, without precluding successful functional results. In such cases, intra-operative electrically evoked auditory brainstem response testing can be useful to monitor appropriate activation of brainstem auditory pathways; however, responses may be undetectable in some forms of auditory neuropathy.Reference Pau, Parker, Sanli and Gibson⁶

Intra-operative plain radiographs are not routinely requested in uncomplicated CI cases.Reference Copeland, Pillsbury and Buchman⁵ Occasionally, X-ray imaging may enable intra-operative repositioning of the electrode array, avoiding the need for revision surgery.Reference Ramalingam, Ramalingam and Padmaja¹⁰ However, in Copeland and colleagues' prospective analysis, intra-operative plain radiographs appeared of negligible value in assessing correct electrode array placement.Reference Copeland, Pillsbury and Buchman⁵ These authors reported that intra-operative plain radiographs changed intra-operative management in only one out of 79 CI cases, despite requests for multiple X-ray examinations in a considerable proportion of cases (23 per cent).

Stenver's transorbital view of the temporal bone is the most common projection performed to study the position of the electrode array during post-operative plain X-ray imaging. This projection is not obtainable in the operating theatre, as the patient needs to lie prone with the sagittal plane of the head at a 45° angle and the side being radiographed close to the film cassette.

Intra-operative radiographs are often obtained at less than optimal angles, given the constraints of patient positioning, the need to avoid contact with the operative field, and the technical limitations of portable equipment.Reference Copeland, Pillsbury and Buchman⁵

In the intra-operative plain radiographs taken in the presented case, the self-curling electrode array had a radiological appearance consistent with appropriate positioning within the cochlea. Bearing in mind this pitfall, at revision surgery the electrode array was inserted under fluoroscopic guidance. Intra-operative fluoroscopy has been advocated as a simple, safe and effective technique in CI cases with difficult insertion and/or cochlear malformation, because it can provide a real-time view of electrode array insertion.

High resolution computed tomography of the temporal bone is indicated if plain radiographs are unreliable, if the patient experiences post-operative complications or if there is clinical evidence of CI failure.Reference Mecca, Wagle, Lupinetti and Parnes⁹