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Christian Strauss

Object. Functional results after surgery for acoustic neuromas that have little or no growth within the internal auditory canal are controversial, because these medial tumors can grow to a considerable size within the cerebellopontine angle (CPA) before symptoms occur.

Methods. A prospective study was designed to evaluate the surgical implications of the course of the facial nerve within the CPA on medial acoustic neuromas. This study included a consecutive series of 22 patients with medial acoustic neuromas (mean size 32 mm, range 17–52 mm) who underwent surgery via a suboccipitolateral approach between 1997 and 2001. All patients underwent pre- and postoperative magnetic resonance imaging and preoperative electromyography (EMG). Evaluation was based on continuous intraoperative EMG monitoring and video recordings of the procedure. All patients were reevaluated at a mean of 19 months (6–50 months) postsurgery.

Preoperative evaluation of facial nerve function revealed House—Brackmann Grade I in six, Grade II in 14, and Grade III in two patients. During surgery a distinct splitting of the nerve at the root exit zone through its intracisternal course was seen in eight patients and documented by selective electrical stimulation. The facial nerve was separated into a smaller portion that ran cranially and parallel to the trigeminal nerve, and a larger portion on the anterior tumor surface. Both components joined anterior to the porus without major spreading of the nerve bundle. In two cases the nerve was found on the posterior surface of the cranial tumor. In one case the facial nerve entered the porus of the canal at its lower part, obtaining the expected anatomical position proximally within the middle portion of the canal. An anterior cranial, middle (five cases each), or caudal course (two cases) was seen in the remaining patients. After surgery, facial nerve function deteriorated in most cases; on follow-up evaluation House—Brackmann Grade I was found in 11, Grades II and III in 10, and Grade V in one patient.

Conclusions. The facial nerve requires special attention in surgery for medial acoustic neuromas, because an atypical course of the nerve can be expected in the majority of cases. A split course of the nerve was found in 36% of the cases presented. Meticulous use of intraoperative facial nerve stimulation and continuous monitoring ensures facial nerve integrity and offers good functional results in patients with medial acoustic neuromas.

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Johann Romstöck, Christian Strauss, and Rudolf Fahlbusch

Object. Electromyography (EMG) monitoring is expected to reduce the incidence of motor cranial nerve deficits in cerebellopontine angle surgery. The aim of this study was to provide a detailed analysis of intraoperative EMG phenomena with respect to their surgical significance.

Methods. Using a system that continuously records facial and lower cranial nerve EMG signals during the entire operative procedure, the authors examined 30 patients undergoing surgery on acoustic neuroma (24 patients) or meningioma (six patients). Free-running EMG signals were recorded from muscles targeted by the facial, trigeminal, and lower cranial nerves, and were analyzed off-line with respect to waveform characteristics, frequencies, and amplitudes. Intraoperative measurements were correlated with typical surgical maneuvers and postoperative outcomes.

Characteristic EMG discharges were obtained: spikes and bursts were recorded immediately following the direct manipulation of a dissecting instrument near the cranial nerve, but also during periods when the nerve had not yet been exposed. Bursts could be precisely attributed to contact activity. Three distinct types of trains were identified: A, B, and C trains. Whereas B and C trains are irrelevant with respect to postoperative outcome, the A train—a sinusoidal, symmetrical sequence of high-frequency and low-amplitude signals—was observed in 19 patients and could be well correlated with additional postoperative facial nerve paresis (in 18 patients).

Conclusions. It could be demonstrated that the occurrence of A trains is a highly reliable predictor for postoperative facial palsy. Although some degree of functional worsening is to be expected postoperatively, there is a good chance of avoiding major deficits by warning the surgeon early. Continuous EMG monitoring is superior to electrical nerve stimulation or acoustic loudspeaker monitoring alone. The detailed analysis of EMG-waveform characteristics is able to provide more accurate warning criteria during surgery.

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Christian Strauss, Johann Romstöck, Christopher Nimsky, and Rudolf Fahlbusch

✓ Intraoperative electrical identification of motor areas within the floor of the fourth ventricle was successfully carried out in a series of 10 patients with intrinsic pontine lesions and lesions infiltrating the brain stem. Direct electrical stimulation was used to identify the facial colliculus and the hypoglossal triangle before the brain stem was entered. Multichannel electromyographic recordings documented selective stimulation effects. The surgical approach to the brain stem was varied according to the electrical localization of these structures. During removal of the lesion, functional integrity was monitored by intermittent stimulation. In lesions infiltrating the floor of the fourth ventricle, stimulation facilitated complete removal. Permanent postoperative morbidity of facial or hypoglossal nerve dysfunction was not observed. Mapping of the floor of the fourth ventricle identifies important surface structures and offers a safe corridor through intact nervous structures during surgery of brain-stem lesions. Reliable identification is particularly important in mass lesions with displacement of normal topographical anatomy.

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Christian Strauss, Elke Lütjen-Drecoll, and Rudolf Fahlbusch

✓ A safe paramedian approach to the rhomboid fossa for surgical treatment of intrinsic brainstem lesions is based on detailed knowledge of the morphometric anatomy of superficially located motor structures. The morphometry of the rhomboid fossa is described in this report on the basis of histological studies conducted in six human brainstem specimens, with special emphasis on the colliculus facialis and the trigona nervi hypoglossi and vagi. Morphometric data include analysis of shrinkage factors in each specimen. The colliculus is a landmark for the nervus facialis, oculomotor nuclei, and the paramedian pontine reticular formation. In the surgeon's view from the posterior approach, the colliculus covers an area of 5.7 mm in the mediolateral and 6.8 mm in the craniocaudal direction and is located 0.6 mm lateral to the median sulcus. The fibers of the nervus facialis come as close as 0.2 mm to the surface of the fourth ventricle. The colliculus is located 15.7 mm above the obex. The trigona nervi hypoglossi and vagi cover a rectangular area measuring 3.1 by 6.5 mm and serve as a landmark for lower cranial nerve nuclei. These nuclei are located 0.3 mm lateral to the midline. An area with a maximum extension of 0.9 cm between the colliculus and trigona can be used for an infracollicular paramedian approach. The same applies to a supracollicular approach cranial to the colliculus and caudal to the fibers of the nervus trochlearis within the medullary velum, with a craniocaudal extension of 4 mm. Superficial motor nuclei and fibers can be identified by neurophysiological mapping, which helps to define safe surgical corridors into the rhomboid fossa, thus reducing functional morbidity caused by the operative approach in intrinsic pontine and pontomedullary lesions.

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Christian Strauss, Johann Romstöck, and Rudolf Fahlbusch

Object. The authors describe their technique of electrophysiological mapping to assist pericollicular approaches into the rhomboid fossa.

Methods. Surgical approaches to the rhomboid fossa can be optimized by direct electrical stimulation of superficially located nuclei and fibers. Electrophysiological mapping allows identification of facial nerve fibers, nuclei of the abducent and hypoglossal nerves, motor nucleus of the trigeminal nerve, and the ambiguous nucleus. Stimulation at the surface of the rhomboid fossa performed using the threshold technique allows localization above the area that is located closest to the surface. Simultaneous bilateral electromyographic (EMG) recordings from cranial motor nerves obtained during stimulation document the selectivity of evoked EMG responses. With respect to stimulation parameters and based on morphometric measurements, the site of stimulation can be assumed to be the postsynaptic fibers at the axonal cone. Strict limitation to 10 Hz with a maximum stimulation intensity not exceeding 2 mA can be considered safe. Direct side effects of electrical stimulation were not observed.

Conclusions. Electrical stimulation based on morphometric data obtained on superficial brainstem anatomy defines two safe paramedian supra- and infracollicular approaches to the rhomboid fossa and is particulary helpful in treating intrinsic brainstem lesions that displace normal anatomical structures.

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Christian Strauss, Julian Prell, Stefan Rampp, and Johann Romstöck

Object

The facial nerve in vestibular schwannomas (VSs) is located on the ventral tumor surface in more than 90% of cases; other courses are rare. A split facial nerve course with two distinct bundles has thus far been described exclusively for medial extrameatal tumors.

Methods

Between 1996 and 2005, 16 consecutive cases of 241 surgically treated VSs were observed to have distinct splitting of the facial nerve. The mean tumor size measured 27 mm. In one third of the cases, intrameatal tumor extension with obliteration of the fundus was documented. All patients underwent extensive intraoperative neurophysiological monitoring using multichannel electromyography recordings. Patients were reevaluated 12 months after surgery.

In all 16 patients, distinct splitting of the facial nerve was demonstrated. The major portion of the facial nerve followed a typical course on the ventral tumor surface. The smaller nerve portion in all cases ran parallel to the brainstem up to the level of the trigeminal root exit zone and crossed on the cranial tumor pole to the internal auditory canal. The two nerve portions rejoined at the level of the porus acusticus. The smaller portion carried fibers exclusively to the orbicularis oris muscle, whereas the major portion supplied all three branches of the facial nerve.

Conclusions

In VSs, an aberrant course with distinct splitting of the facial nerve adds considerably to the surgical challenge. Long-term facial nerve results are excellent with extensive neurophysiological monitoring, which allows the differentiation and identification of aberrant facial nerve fibers and avoids additional risks to facial nerve preservation.

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Christian Scheller, Jens Rachinger, Julian Prell, Malte Kornhuber, and Christian Strauss

The intermediate nerve is seldom identified as the site of tumor origin in cerebellopontine angle schwannomas. A 29-year-old man presented with a 6-month history of slowly progressive hearing loss and dizziness; facial nerve weakness was not observed clinically. Magnetic resonance imaging revealed a tumor in the left cerebellopontine angle region extending up to the geniculate ganglion and along the course of the superficial petrosal nerve. A CT scan showed enlargement of the facial nerve canal. Microsurgery was performed via an extended retrosigmoid approach. Intraoperative and electrophysiological findings identified the intermediate nerve as the site of tumor origin.