Operative Technique An anatomical assessment of the supracerebellar midline and paramedian approaches to the inferior colliculus for auditory midbrain implants using a neuronavigation model on cadaveric specimens Giles H. Vince a, * , Christian Herbold a , Jan Coburger a , Thomas Westermaier a , Detlev Drenckhahn b , Ansgar Schuetz c , Ekkehard Kunze a , Laszlo Solymosi c , Klaus Roosen a , Cordula Matthies a a Department of Neurosurgery, University of Wuerzburg, Josef-Schneider-Str. 11, 97080 Wuerzburg, Germany b Department of Cell Biology and Anatomy, University of Wuerzburg, Wuerzburg, Germany c Department of Neuroradiology, University of Wuerzburg, Wuerzburg, Germany article info Article history: Received 25 June 2009 Accepted 29 June 2009 Keywords: Cerebellopontine angle Inferior colliculus Neural deafness abstract The inferior colliculus (IC) is an alternative site for electrode placement in neural deafness due to its sur- gical accessibility and its well-known tonotopic stratification. In patients where tumor surgery has already occurred and the cerebellopontine angle contains scar tissue or tumor-remnants, midline and paramedian supracerebellar approaches are alternative routes. They are often avoided due to concerns regarding the venous drainage of the cerebellum, the electrode trajectory and the course of the electrode cable. We studied these surgical routes in five neuronavigated fixed cadaveric specimens. For paramedian and midline approaches, the transverse sinus was exposed 5.8 mm on average. A mean of 1.6 cerebellar veins, with an average diameter of 2.0 mm, draining to the tentorium were transected to reach the ten- torial notch. Only 0.4 arterial branches were met. We conclude that the supracerebellar midline and par- amedian approaches provide a good exposure of the IC and offer safe and viable alternative routes to the IC. Additionally, they provide a wider angle of action for optimal electrode placement. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Many patients with severe sensory hearing loss are able to re- gain a useful level of speech perception using cochlear implants (CI). However, for deaf individuals suffering from tumors in the cerebellopontine angle (CPA) such as meningiomas, schwannomas or neurofibromas, CI is rarely possible and treatment options be- come limited. Most will be patients with neurofibromatosis 2 (NF2), a genetically determined condition with an incidence rate of 1/40,000 newborns. Patients with NF2 typically progress to neu- ral deafness brought on by the development of bilateral vestibular schwannomas. 1–5 To date, the only treatment solution for patients with these con- ditions has been the auditory brainstem implant (ABI), a central auditory prosthesis that stimulates the auditory pathway proximal to the damaged nerve. 2,5–8 However, in patients with NF2, the achievable degree of hearing using a two-dimensional electrode ar- ray is rarely more than an augmentation of lip-reading capacity and the perception of environmental sounds; a result that com- pares to single-channel CI. 3,9–11 Only a few patients will achieve open-set speech recognition without lip reading. Penetrating ABI that can be placed more securely along the tonotopic gradient of the cochlear nucleus showed only slightly improved hearing re- sults in patients with NF2. 12,13 These unsatisfactory results are pos- sibly explained by a direct lesioning of the cochlear nucleus by the progressing tumor. A finding that supports this theory is that in pa- tients with cochlear aplasia or traumatic damage to the cochlear nerve but who harbor no tumor, ABI are able to secure a remark- able level of speech perception. In these patients the inferior colliculus (IC) is a feasible alterna- tive to ABI at the cochlear nucleus due to its surgical accessibility and its well-known tonotopic stratification in frequency ranges. The IC can be reached by lateral or median supracerebellar suboc- cipital approaches, both of which have been established as surgical routes for the removal of pontine lesions and cerebellopontine tu- mors. 14 Auditory midbrain implants (AMI) have been developed for the stimulation of the IC with a surface or penetrating array along the tonotopic gradient of the IC. AMI are specially designed to acti- vate specific frequency regions required for the fluent perception of speech. 1,12,15,16 1.1. Rationale for an alternative surgical approach The implantation of the electrode will usually be performed along a lateral infratentorial route in the CPA. This approach allows 0967-5868/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jocn.2009.06.034 * Corresponding author. Tel.: +49 931 201 24805; fax: +49 931 201 24534. E-mail address: Vince.g@nch.uni-wuerzburg.de (G.H. Vince). Journal of Clinical Neuroscience 17 (2010) 107–112 Contents lists available at ScienceDirect Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn