As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

The contribution of air conduction auditory brainstem response (AC-ABR) testing in the paediatric population is widely accepted in clinical audiology. However, this does not allow for differentiation between conductive and sensorineural hearing loss. The purpose ofthis paper is to review the role of bone conduction auditory brainstem responses (BC-ABR). It is argued that despite such technical difficulties as a narrow dynamic range, masking dilemmas, stimulus artifact and low frequency underestimation of hearing loss, considerable evidence exists to suggest that BC-ABR testing provides an important contribution in the accurate assessmentof hearing loss in infants. Modification of the BC-ABR protocol is discussed and the technical difficulties that may arise are addressed, permitting BC-ABR to be used as a tool in the differential diagnosis between conductive and sensorineural hearing. Two relevant case studies are presented to highlight the growing importance of appropriate management in early identification of hearing loss. It can be concluded that BC-ABR should be adopted as a routine clinical diagnostic tool.

The objective of this study was to present aspects of the current treatment protocol, such as patient evaluation and selection for therapy, multimodality monitoring for optimal auditory brainstem implant (ABI) positioning and radiological evaluation, that might have an impact on the functional results of ABI. Out of a series of 145 patients with bilateral vestibular schwannomas 10 patients received an ABI, eight of which are reported here. Patient selection was based on disease course, clinical and radiological criteria (according to the Hannover evaluation and prognosis scaling of neurofibromatosis type 2 (NF2)), extensive otological test battery and psycho-social factors. ABI placement was controlled by multimodality electrophysiological monitoring in order to activate the auditory pathway and to prevent false stimulation of the cranial nerve nuclei or long sensory or motor tracts. Results of hearing function were correlated with patients’ ages, duration of deafness, tumour extension, tumour-induced compression or deformation of the brainstem, and numbers of activated electrodes without any side-effects. Out of 59 patients with pre-operative deafness eight patients received an ABI of the Nucleus 22 type. All these patients became continuous users without any side effects and experienced improved quality of life. Speech reception in combination with lip-reading was markedly improved, with further improvement over a long period. A short duration of deafness may be favourable for achieving good results, while age was not a relevant factor. Lateral recess obstruction may necessitate a more meticulous dissection, but did not prevent good placement of the ABI in the lateral recess. Pre-existing brainstem compression did not prevent good results, but brainstem deformation and ipsi- and contralateral distortion were followed by a less favourable outcome. Among the factors that can be influenced by the therapy management are the selection of patients with a slow progressing NF2 disease, a short duration of deafness, a careful analysis of brainstem deformation and consideration of either side for implantation. Long-standing brainstem deformation might not lead to recovery, but instead lead to a low number of active electrodes and possibly only moderate results. ABI treatment is a safe procedure that can increase a patient’s quality of life considerably. ABI placement along with neurophysiological control helps to prevent side effects and to improve acoustic activation. Further studies on structural and functional changes of the brainstem after previous tumour compression and distortion should increase our understanding and facilitate a decision on the best side for ABI implantation.

A deaf female patient was diagnosed with bilateral acoustic neurinomas. Diagnosis incorporated the standard audiological battery for sensorineural hearing loss, computed tomography and magnetic resonance imaging. The left side had been operated on four years previously in another clinic using the suboccipital approach. The auditory brainstem implant surgery was performed on the ‘second side’ using the same approach. It was an uneventful operation with good anatomy and no serious post-operative complications. Post-operatively, the patient performed exceptionally well, with up to 50 per cent of words recognized in the opened set and 85 per cent in the closed set, both without lip-reading.

A knowledge of the microanatomy of the cochlear nucleus complex and its variations is essential for successful implantation and for the design of stimulation devices. One hundred cerebellopontine angle specimens were dissected under surgical conditions using the Zeiss NC31 surgical microscope. The topographical anatomy of the exit of the vestibulocochlear nerve, the cochlear nucleus and the surface of the medulla and their relation to the surrounding structures was recorded and measured. The mean distances between the exits of the VIIth and VIIIth cranial nerves were 4.7 ± 0.9 mm, between the VIIth and IXth 6.3 ± 1.2 mm and between the VIIIth and IXth 5.5 ± 1.0 mm. The visible area of the cochlear nucleus covered a square of 10.0 ± 2.9 by 3.3 ± 1.0 mm. A major AICA-loop had to be re-routed in 17 per cent of specimens. The taenia of the choroid plexus was present in 92 per cent and had to be cut in 51 per cent in order to enter the foramen of Luschka, that had a mean size of 3.5 by 2.0.mm. It was wide open in 24 per cent , open only after incision of the arachnoid in 53 per cent, functionally closed but opened by extensive dissection in 18 per cent and anatomically occluded in five per cent of the specimens. The typical straight vein at the cochlear nucleus leading to the entrance of the foramen of Luschka was found in 76 per cent of specimens. Constant anatomical landmarks are very helpful for finding the cochlear nucleus, but variations may endanger dissection and implantation in a remarkable number of cases.

Patients with bilateral total deafness due to lesions of the vestibulocochlear nerve can be treated by electrical stimulation of the second auditory neuron. A 22-channel multi-electrode implant with transcutaneous transmission was developed that allows the selection of the most useful electrodes. Acoustic neuromas were removed from 49 out of 58 patients by ENT surgeons and neurosurgeons working in collaboration and using either a translabyrinthine or suboccipital approach. The central electroauditory prosthesis was implanted in the same procedure. Six patients were deaf after previous acoustic neuroma surgery without recurrence, three had diagnoses other than neurofibromatosis type 2 (NF2). There were no complications due to the implantation procedure. Side effects could be excluded by stimulation of the auditory electrodes alone. Most of the patients used their ABI daily. They reported perception of different sounds and frequencies, enhancement of lip-reading ability, and three of the patients were able to use the telephone.

The present paper reports our experience with the surgical retrosigmoid-transmastoid (RS-TM) technique for implanting auditory brainstem implants (ABIs). From April 1997 to August 1998, four patients with neurofibromatosis type 2 (NF2) were operated on for vestibular schwannoma removal with ABI implantation. The subjects (three men and one women) ranged in age from 22 to 31 years. Tumour size ranged from 12 to 30 mm. A classical RS-TM approach was performed. After tumour excision, identification of landmarks (VIIth, VIIIth and IXth cranial nerves, choroid plexus) to the foramen of Luschka was carefully carried out. The choroid plexus was partially removed and the tela choroidea divided and deflected. The floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleus became visible. The electrode array was then inserted into the lateral recess and placed in the correct position with the help of electrically-evoked auditory brain stem responses. Auditory sensations were induced in all patients with various numbers of electrodes. Different pitch sensations could be identified with different electrode stimulation. Details of the results are presented. In our series, the RS-TM approach represents the elective route for ABI insertion.

An auditory brainstem implant (ABI) is indicated for patients suffering from bilateral neural deafness. The most affected patients are those with neurofibromatosis type 2 (NF2). An implantation is possible either at the same time as, or after, surgical removal of an acoustic neuroma. This paper demonstrates the results of eight out of 11 patients with NF2, seven of whom received an ABI after tumour removal. Pre-operatively, all of them were deaf. Post-operatively, the first fitting served to determine the individual stimulation parameters for each electrode. The stimulation-dependent side-effects were eliminated by reducing the stimulus intensity without causing negative effects on the hearing with the ABI. Only in one case was an open set understanding achieved within the first year. However, all patients had a better speech understanding when they combined their hearing with the ABI and their lip-reading abilities. There is no correlation between the performance with ABI and the tumour size or the duration of deafness.

In patients with reduced auditory nerve function, for example due to tumour removal or an accident, hearing rehabilitation can be elicited by an auditory brainstem implant (ABI). The electrode array of the ABI manufactured by Cochlear Ltd., Sydney, consists of 21 circled contacts in a silicon carrier. This is inserted in the lateral recess of the fourth ventricle. Since 1996, in Hannover eight patients have been implanted with a cochlear ABI Nucleus 21 + 1. All of them were profoundly deaf on both sides due to neurofibromatosis type 2 (NF2). To find the optimal electrode position during surgery, a multimodal monitoring by auditory evoked potentials (AEP), electromyography (EMG) and somatosensory evoked potentials (SEP) was performed. When monitoring AEPs, the function of the implant can be checked first by the stimulus artefact. By analysing the AEPs in more detail, the optimal positioning of the electrode on the cochlear nucleus can be found. If systems other than the auditory system are stimulated this will be revealed in one or more of the AEP, EMG and SEP recordings. According to the literature, AEPs stimulated by an ABI consist of three vertex positive peaks with latencies shorter than 4 ms. Typical AEPs are correlated with good postoperative hearing sensation. Comparing these AEPs with AEPs stimulated acoustically or electrically at different sites of the auditory system, it can be assumed that the first peak corresponds to J3, the second to J4 and the last to J5. From this comparison it can also be concluded that no potentials should occur later than 5 ms. This corresponds to our findings. Post-operatively, side-effects occurred when areas of the electrode array were stimulated that showed potentials with latencies longer than 5 ms intra-operatively. Our results indicate that monitoring is an essential aid for the surgeon in finding the optimal electrode position. Positioning solely with reference to anatomical landmarks may not be enough to find the optimal functional position.

Auditory brainstem implants (ABIs) are a modern method of treatment of total bilateral deafness in cases of extracochlear origin. In most cases therapy is applied in patients with neurofibromatosis type 2 (NF2). This paper presents the results of surgical treatment and rehabilitation in a 28-year-old woman with bilateral, multiple tumours of the central nervous system causing total deafness. Simultaneous removal of the tumours and implantation of ABI allowed treatment of the potentially lethal pathology and hearing restoration. Improving auditory skills and excellent tests results were noted in the year following implantation.

The multichannel auditory brainstem implant (ABI) provides the potential for hearing restoration in patients with neurofibromatosis type 2 (NF2). Programmes for auditory brainstem implantation have been established in two Australian centres. Eight patients have been implanted under the protocol of an international multi-centre clinical trial. Three patients had ABI insertion at the time of first side tumour removal, four at second side tumour removal and one after previous bilateral surgery where there was some residual tumour. The translabyrinthine approach was used in all cases. Successful positioning of the electrode array was achieved in seven of eight patients, all of whom achieved auditory perception with electrical stimulation. Intra-operative electrically evoked auditory brainstem response testing was successful in four patients and was useful in confirming correct electrode position. In six cases postoperative psychophysical and auditory perception testing demonstrated that useful auditory sensations were achieved. Five of these patients regularly used the implant. In one patient electrode placement was unsuccessful and only non-auditory sensations occurred on stimulation. In the remaining patients nonauditory sensations were minimal and avoidable by selective electrode programming. Auditory brainstem implantation should be considered in patients with NF2. The greatest benefit is seen in patients without debilitating disease who have non-aidable hearing in the contralateral ear.

Since 1994, a US Food and Drug Administration clinical trial evaluated the multichannel auditory brainstem implant (ABI) on 92 subjects with neurofibromatosis type 2 (NF2). The trial has shown that 85 per cent of patients receive auditory sensations. A small number of patients demonstrate a clinically significant degree of open-set sentence recognition in the sound-alone condition; however, when the ABI is combined with lip-reading cues, 93 per cent of patients demonstrate improved sentence understanding at three to six months. In addition, the majority of recipients report daily use of their devices, and satisfaction with the decision to receive the ABI.

We report on the surgical technique for surface electro-auditory prosthesis (EAP) implantation, pathological changes occurring at the cochlear nucleus complex (CNC), and its relation with electrical stimulation. Fourteen Macaca fascicularis were operated upon for a translabyrinthine bilateral auditory neurectomy, and simultaneous unilateral EAP implantation. Six animals were not stimulated, and the remaining eight were connected to an external active device. Stimulation was planned for 1000 hours. Biotolerance to the materials was adequate without significant reactions in the CNC surface, but an ependymal reaction. Lesions attributed to surgical trauma were also found. Two animals being stimulated could not complete the planned course due to cable break or EAP extrusion. One stimulated animal developed an asymptomatic brainstem abscess. A good knowledge of CNC topography is required to avoid surgical trauma. Externally connected devices may facilitate extrusion of the EAP or ascending infections.

Neurofibromatosis type 2 (NF2) causes bilateral hearing loss due to tumour growth in the cerebellopontine angle. We report the results of promontory testing and transtympanic electrocochleography on subjects with deafness due to NF2 referred for an auditory brainstem implant. All 19 ears tested revealed loss of cochlear microphonics. Nine ears (mainly without previous treatment) revealed auditory perception during promontory stimulation, indicating cochlear deafness. One of these subjects has been successfully provided with a cochlear implant. The other 10 ears (mainly after previous surgery) revealed negative promontory stimulation, indicating additional retrocochlear deafness. These findings indicate that neurofibromas initially cause a cochlear deafness, so that a cochlear implant can be used if the auditory nerve can be preserved. This option has to be considered in rehabilitating patients with bilateral tumours due to NF2.