How We Do It: Our approach to theossified cochlea

GARETH WILLIAMS, Department of Otolaryngology, University Hospital ofWales, Heath Park, Cardiff, UK

Radiological

Pathological and clinical evidence indicates that the reparative process within thecochlea following various insults can occur to varying degrees and at varying rates.Some causes of deafness are more likely to result in a loss of the cochlear lumen, asa result of either fibrosis or ossification. Although meningitis is recognized as beinga common cause of cochlear ossification, there are many other pathologicalprocesses that can result in the same process. Published and unpublished datasuggest that in approximately 10% of cases where reparative changes haveoccurred, a computed tomography (CT) scan alone will miss these. Magneticresonance imaging (MRI) alone would seem more sensitive in identifying cochlearduct obliteration, but cannot differentiate fibrosis from ossification. Having bothMRI and CT scans of the cochlea will provide complementary images that willinform the surgical approach. An ossified cochlea will appear so on CT scan, withloss of fluid signal from the cochlear duct on the MRI. A fibrosed or ossifyingcochlear may appear clear or ‘fuzzy’ on a CT scan but will also return no signal onMRI, whereas a patent cochlea on CT will appear clear as well as showing a highfluid signal within the cochlea on MRI. Suspecting fibrosis as opposed to ossifi-cation beforehand will influence my surgical management.

The extent of cochlear obliteration will vary. The complementary detailsprovided by MRI and CT maximize the ability to identify apical sparing, whichmight allow a retrograde insertion of the electrode. The integrity of the cochlearnerve is best evaluated by MRI scan, although fracture lines will only be demon-strated by CT.

In summary, both MRI and CT are useful in cases of suspected ossification. Incases of acquired deafness, MRI alone may suffice if there is no evidence of ossifi-cation or fibrosis.

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The decision to operate

The decision to operate is based on a clinical risk assessment whereby the likelybenefits are weighed up against the possible risks. As well as all the usual risksassociated with cochlear implantation, surgery on the ossified cochlea poses specificadditional risks. The outcome is not usually as favourable compared to patients witha patent cochlea, in terms of both speech recognition and the electrophysiologicalinterface. This is an important fact when considering marginal cases. Most patientsand families whose expectations are based on media exposure or anecdotal evidenceof successful ‘implantation’ in other patients need to be made fully aware of thereduced benefits associated with cochlear ossification.

The presence of an intact and functioning cochlear nerve is a pre-requisite.Electrical stimulation tests may confirm the presence of a functioning nerve butmay give conflicting results. Higher stimulation thresholds in patients with anossified cochleae can make it difficult to differentiate auditory from non-auditorystimulation, especially when performing ear canal stimulation. If there is anydoubt, a trans-tympanic promontory stimulation test is performed. This is also auseful guide as to which ear to implant if there are no other overriding lateralizingindicators. In adult patients, the duration of deafness, as always, is an importantpredictor of outcome, with patients deafened for over 20 years unlikely to benefitgreatly.

Surgical approach

The initial approach is as for any standard implant operation, dealing with the softtissues and flap, and the initial extended cortical mastoidectomy with posteriortympanotomy as usual. The key to a successful ‘drill-out’ in an ossified cochlea isan understanding of the surface projection of the cochlea onto the medial wall ofthe middle ear.

Anatomical concepts

The surgical anatomy of the cochlea deserves close study before embarking on anyimplant surgery, but this is particularly important when dealing with an ossifiedcochlea. Details are available in anatomical and surgical texts, as well as throughthe study of temporal bone sections (some useful on-line sites can be found onhttp://oto.wustl.edu/bbears/tb-intro.htm and http://www.ear-anatomy.com/).There is no substitute for temporal bone dissection though, which gives the bestappreciation of the cochlea relationships.

One useful ‘rule of thumb’ I’ve used to help describe the basic relationships isbased on the positions adopted by the fingers and thumbs of clasped hands. Hold yourhands gently together with interlocking fingers, as if praying with the left thumb ontop of the right. Extend both index fingers, the index finger of the right hand(pointing to the left) represents the approximate direction of the internal auditory

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meatus (which is the same as the external auditory meatus) of the right ear. Thenatural position adopted by the left index finger (pointing to the right) represents theapproximate position of the axis of the modiolus of the cochlea. Another analogy Iuse is the position adopted by the left thumb which, if gently flexed so that the tip liesin the anatomical ‘snuff box’ of the right hand, represents the hook of the cochleawith the round window membrane being near the metacarpo-phalyngeal joint.

When viewed through the posterior tympanotomy, the surface projection of theupper (superior) part of the cochlea is hidden from view by the stapes super-structure and incus, both of which need to be removed. Before embarking on thismanoeuvre though, a basal cochleostomy is drilled at the site normally used for astandard insertion. If the round window niche is visible it is used as a guide,drilling just anterior to the anterior lip of the niche. Obliteration of the nicherequires a degree of judgement based on anatomical and surgical ‘memory’ of thenormal morphology. An important feature to look for and follow during surgery isthe whiter bone laid down during ossification. It is possible to differentiate thisfrom the harder, slightly ivory-coloured bone of the bony labyrinth. Between theround window membrane and the lower basal turn of the cochlea is the hook ofthe cochlea. If the drill-out begins close to the round window membrane (or itsoriginal position), the channel will cut across the hook thereby drilling throughlabyrinthine bone as well as whiter ossified bone. This may be a source of a littleconcern until the ossified cochlear duct beyond the hook is entered.

Another useful anatomical concept to have is that of the plane of the basal turnof the cochlea. This is perpendicular to the axis of the modiolus, of course. Theapproach through the facial recess by way of the posterior tympanotomy aligns theplane of the lower basal turn within the field of surgical access. This plane extendsout laterally directly towards the operator and passes from the mastoid process ofone ear to the temple of the opposite side. It is tilted slightly so that the anteriorsurface of this imaginary two-dimensional plane is facing upwards. I am assisted atthe time of surgery by another personal observation, that the axis of the modiolusis in the same direction as the stapedius tendon when viewed through the posteriortympanotomy, and its surface projection is parallel but inferior to the stapediustendon when viewed this way.

Drilling the first channel

Using a 1 mm diamond burr, the first cochleostomy channel is made. This liesinferior to the modiolus. The direction of drilling is based on the normal cochlearanatomy and the anatomical concepts I have described. When performing a drill-out on an ossified cochlea the position of the internal carotid artery must beappreciated. It lies medial to the cochlea and should be just anterior to the plane ofthe basal turn of the cochlea. Anatomical variations mean that it may be directlyin line with the channel being drilled. Another useful ‘rule of thumb’ I haveadopted is a ‘3–6–9’ guide. The ‘9’ refers to the maximum depth in millimetres of

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the first basal channel being drilled. Various studies and personal observation haveshown that the normal carotid artery will not be damaged as long as the channelbeing drilled remains less than 9 mm. I check this distance constantly by lining upthe drill shaft to a measuring tape and marking the shaft at 9 mm and doing sorepeatedly during the drill-out.

Drilling the second channel

Various methods for drilling a second channel have been described. I use themethod in which I drill out of the upper basal turn, on the cephalic side of themodiolus as opposed to drilling into the second turn on the caudal side of themodiolus. Drilling into the upper basal turn can only be achieved after removingthe incus and stapes superstructure.

After removing any remaining bony bridge, the incus is removed. Although thisrequires care, it can be achieved using conventional fine instruments. The removalof the stapes superstructure is more of a challenge and quite different to its removalin otosclerosis, where there is a fixed footplate to assist. I have used both theSkeeter drill (Xomed), with a fine cutting tip drilling against a supported stapes,and the KTP laser. I find the laser much easier and therefore safer. It can also beused to divide the stapedius tendon. The KTP laser is used down a 0.2 mm fibre ona setting of 1 watt for 0.5 s. Four or five applications are usually enough to burnthrough the crura, although charred bone usually remains. This can be removedcarefully using a fine sucker. Care is needed to direct the laser away from the facialnerve, which can be damaged by heat energy. Once the superstructure is removed,the surface projection of the upper part of the cochlea is accessible. Using theanatomical concepts described, the second channel is drilled. I have found the linebetween the oval window and the processus cochleariformis to be a useful guide forpositioning the second channel. This channel is drilled above the modiolus, thusresulting in two channels that straddle it. At this stage I refer to the ‘3–6–9’ guidementioned earlier. I make sure there is at least a 3 mm bridge of bone between thetwo channels. This is a minimum, not a recommended distance. Any less than thiswill mean that the modiolus may be damaged, with the consequent loss of ganglioncells. A 1 mm diamond burr is used again, the channel being drilled in the plane ofthe basal turn, which means directing the burr parallel to the previous channel inthe direction of the opposite temple, slightly away from the horizontal part of thefacial nerve. This channel is drilled to a distance of 6 mm, the ‘6’ of the ‘3–6–9’guide. Once again the whiter ossified bone is followed medially. Bleeding signifiespossible deviation towards the modiolus with a risk of damaging ganglion cells.

Choice of electrode

A split electrode is used if two parallel channels have been fashioned. The onlyexception to this has been a patient in which the lower of the two channels was

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extended into a slot, which allowed a cephalic trajectory of the burr such that itwas possible to enter the medial end of the upper channel through the lowerchannel. This allowed the introduction of a conventional electrode around themodiolus.

When inserting the split electrodes, it is important to notify the audiologistwhich electrode array is in which channel. The package is placed in its bed andsecured before the electrodes are inserted into the channels. Once one electrode isin a channel, a plug of muscle or fascia is used to surround the electrode within themouth of the channel. This will help prevent an irritating tendency for an insertedelectrode to become dislodged as the second electrode is being inserted. Securingthe package beforehand also reduces this tendency. The second electrode issimilarly secured with a muscle fascia plug. Additional fixation of both electrodesto the temporal bone using cement or glue is necessary to reduce the risk ofaccidental dislodgement, a problem that is more likely than in conventionalimplantation.

Address correspondence to: Mr Gareth Williams, Department of Otolaryngology, UniversityHospital of Wales, Cardiff CF14 4XW, UK.

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