9Finding the Lateral Recess: The ABI Operation

FA target you cannot see

The cochlear nucleus is not on the surface of the brainstem waiting to be touched. It sits on the dorsolateral pontomedullary junction, partly tucked under the cerebellar peduncles, occupying a visible surface area of barely one square centimetre. The surgeon never directly sees the nucleus; instead the electrode paddle is slid into the lateral recess of the fourth ventricle so that it lies over the nucleus from outside.

The gateway to that recess is the foramen of Luschka, the lateral opening of the fourth ventricle. Reaching it and recognising it among distorted or, in malformations, unfamiliar anatomy is the central challenge of the operation. Because the goal is to place a flat array on a soft, CSF-bathed structure that moves with respiration and pulsation, the operation is as much about reading landmarks and confirming function as about exposure.[2008][2019]

TTwo roads to the recess: translabyrinthine and retrosigmoid

Two approaches dominate. The translabyrinthine craniotomy, favoured by the House group, drills through the labyrinth to give a wide, direct view of the lateral recess with minimal cerebellar retraction; skeletonising the jugular bulb widens the exposure of the foramen of Luschka. The retrosigmoid (suboccipital) approach, used more widely in Europe and in non-tumour and paediatric cases, opens behind the sigmoid sinus, retracts the cerebellum and follows the plane between the cochlear nerve root and the choroid plexus into the recess.

Each has trade-offs. The translabyrinthine route avoids cerebellar retraction and gives early control of the facial nerve but sacrifices any residual hearing and needs an abdominal fat graft. The retrosigmoid route can preserve a chance at hearing if the cochlear nerve survives, gives direct vision of the lower cranial nerve roots and larger tumours, and is often quicker, but historically carries higher rates of CSF leak and postoperative headache. In non-tumour and paediatric work, where there is no tumour to remove, the retrosigmoid corridor is frequently chosen.[2002][2008]

CReading the brainstem and seating the paddle

Once near the recess, the surgeon identifies it by a constellation of landmarks rather than by direct sight of the nucleus. The ninth cranial nerve, lying in a relatively fixed position, points to the foramen, which sits just above it; the choroid plexus characteristically protrudes from the opening; the taenia (tela choroidea) marks the lip the electrode must cross; and the seventh and eighth nerve roots define the superior border while the IX and X rootlets define the inferior one. A Valsalva manoeuvre that produces egress of CSF confirms the recess.

The paddle is then advanced gently into the recess and its position optimised using electrically evoked auditory brainstem responses recorded from the scalp, because the array placement, not anatomy alone, determines which electrodes will hear. Throughout, the facial nerve and lower cranial nerves are monitored so that any electrode causing non-auditory stimulation can be identified. Once the best position is found the array is held in place with a small piece of muscle, fat or Teflon felt, since there is no bony channel to anchor it on this moving, pulsatile target.[2002][2008]

CComplications of a brainstem operation

Because the operation opens the posterior fossa and places hardware against the brainstem, its complications are neurosurgical. CSF leak is the most frequent, reported at single-digit rates after the translabyrinthine approach and higher, sometimes well into double digits, after the retrosigmoid approach; meningitis, headache and, rarely, hydrocephalus or pseudomeningocele also occur. Lower-cranial-nerve injury and the rare risk of brainstem vascular injury or stroke are the feared events, which is why the lower cranial nerves are monitored and why initial device activation is performed with medical support on hand.

Many of the device’s troublesome effects are not surgical injuries but non-auditory stimulation: a sizeable fraction of electrodes may cause tingling, dizziness, throat or facial sensations and have to be switched off at programming. Reassuringly, large series, including non-tumour and paediatric cohorts, report that adding the implant to the underlying craniotomy does not substantially raise the overall complication rate above that of the exposure itself, and that non-tumour complication rates are comparable to other established skull-base procedures.[2002][2005][2019]