Surgical approach to the superior vestibular nerve in guinea pigs

Hearing

Research,

31 (1987)

193-196

193

Elsevier

HRR

01001

Short Communication

Surgical approach

to the superior vestibular

M. Hildesheimer, School

for

Commumcalion

Disorders,

Speech

and Hearing,

(Received

The structure meatus. several relative Superior

C. Muchnik Sackler

and M. Rubinstein

School of Medicine,

9 April 1987: accepted

nerve in guinea pigs

16 August

Tel Hashomer

Hospital,

Tel Hashomer.

Israel

1987)

superior vestibular nerve carries homo- and contra-lateral efferent fibers to the cochlea. The subarcuate fossa. a tube-like in the temporal bone of the guinea pig, can be used to reach the superior vestibular nerve at the level of the internal acoustic Normally, this structure accommodates the dorsal and ventral floccular extension of the cerebellum. This technique has advantages. Firstly. a reduced cranial opening is necessary; secondly, less cerebellar tissue is sacrificed. Then there is the insulation of the operative field, and finally, it presents a straight guide to the internal auditory meatus and vestibular nerve. vestibular

nerve;

Efferent

fiber

Introduction

The cochlea-vestibular efferent innervation is one of the most intensively studied structures in the ear. However, despite the time and energy which has been invested, the results are still unand many questions remain unsatisfactory, answered. One problem in this type of research is the experimental model used. The internal acoustic meatus. a deeply located structure with its nerves, is reached with difficulty, and during the procedure damage to the surrounding structures cannot be completely avoided. As a result, the interpretation of the experimental findings is more difficult, and the conclusions which should be reached are less accurate. The efferent cochlea-vestibular fibers, on exiting the brain, can easily be followed through the superior vestibular nerve up to the internal acoustic meatus. They constitute a small fascicle known as the Fascicle of Ort, or the vestibulo-cochlear anastomosis, which carries fibers of the homoand contra-lateral efferent innervation. Exposure

Correspondence

to: M. Hildesheimer. School for Communication Disorders, Speech and Hearing, Sackler School of Medicine. Tel Hashomer Hospital. Tel Hashomer, Israel.

0378-5955/X7/$03.50

‘Q 1987 Elsevier Science Publishers

of the superior vestibular nerve, before its entrance into the temporal bone, allows for stimulation or interruption of the efferent innervation of the cochlea, according to experimental needs. The purpose of the present report is to describe a surgical approach to the vestibular nerve which is safer and less traumatic than the conventional method. In guinea pigs this is possible by utilizing the subarcuate fossa, a short and large canal which normally accommodates the dorsal and ventral para-floccular extension of the cerebellum (Figs. 1, 2). Exposure of the superior vestibular nerve at the internal auditory meatus was used by many authors in order to interrupt the crossed and uncrossed olivo-cochlear bundle (Carlier and Pujol. 1982; Morrison et al., 1975). The same anatomical structure, the subarcuate fossa, was used by Gummer and Klinke (1983) to reach the internal acoustic meatus for implanting electrodes in the cochlear nucleus. Due to the still large approach and the excision of the dura the procedure remained traumatic. By the technique presently described the dural excision is avoided and the whole procedure becomes almost an extra-dural intervention. In Fig. 2 we can see that the inferior bony rim of this subarcuate fossa overhangs the internal acoustic meatus at a distance of only 1-2 mm.

B.V. (Biomedical

Division)

Fig. 1. Hindbrain of guinea pigs. Redrawn from Cooper and Schuller, 1975. (1) Dorsal paraflocculus; (2) spinal cord; (3) cerebellum; (4) temporal area of cerebral cortex.

6 2

skin at the lateral end of the nuchal crest, where the occipital parietal and temporal bone fuse together. The periosteum was elevated and the bone closing the subarcuate fossa drilled out (Fig. 3), exposing the dura covering the suprafloccular lobe. The dura was separated from the bone. During this step one should remember that the antero-inferior wall of the superficial section of the subarcuate fossa is common with the ear cavity. Thus, faulty ossification of this structure could cayse opening of the middle ear during this step. The periosteum and dura connecting the floccular lobe with the bony walls of the subarcuate fossa having been separated, the sac was penetrated with a large needle and its neural content aspirated. The remaining empty dural sac was pushed dorsally and medially into the cranial cavity, thus exposing the angle rim between the dorsal and medial aspect of the petrous part of the temporal bone. It should be noted that the inferior or ventral wall of the subarcuate fossa covers the dorsal aspect of the petrous bone, which, with its medial aspect containing the porous acousticus, creates an angle between them. Occasionally, looking parallel to the inferior wall of the subarcuate fossa, in the direction of the internal acoustic meatus, one can miss seeing the very slight bulging of the superior vestibular nerve. In such cases, when this occurred, one of two procedures was undertaken.

3

Fig. 2. Internal aspect of temporal bone. Redrawn from Cooper and Schuller, 1975. (1) Dorsal bulla; (2) facial foramen; (3) transverse crest of internal auditory canal; (4,5) cochlear nerve opening; (6) subarcuate fossa; (7) superior vestibular nerve.

Procedure

With the guinea pig under sedation of Nembutal (30 mg/kg body wt) and local anesthesia of Procaine, an incision of 2-24 cm was made in the

Fig. 3. Internal aspect of the temporal bone in guinea pigs. (1) Superior vestibular nerve; (2) facial nerve; (3) cochlear nerve; (4) subarcuate fossa - the bone which closes the subarcuate fossa from outside was drilled.

195

(1) The first procedure involved enlarging the external opening of the subarcuate fossa by drilling out from the superior border. This allowed a more direct view of the region of the internal acoustic meatus. (2) The second procedure was used in more difficult cases. Here, the point of entry of the superior vestibular nerve into the bone was first found - with a bent hook. Then the nerve was caught on the hook and pulled upward until it ruptured. Generally, there was no bleeding involved and the general condition of the animals remained good. The compound auditory potential, l-2 weeks following surgery, did not show any difference when compared with the pre-operative recording. Conclusions The surgical approach efferent fibers by exposure

for elimination of the superior

of the vestibu-

lar nerve, at the internal acoustic meatus, represents several clear advantages over the conventional techniques. (1) A reduced cranial opening is created; (2) dural opening reduced to a needle puncture; (3) less cerebellar tissue is sacrificed: (4) relative insulation of the operative field from the rest of the cranial cavity is maintained; (5) most importantly, the subarcuate fossa guides us straight to the internal acoustic meatus with the favorable position of having the superior vestibular nerve nearest to the surgeon. References Carlier, E. and Pujol, R. (1982) Sectioning the efferent bundle decreases cochlear frequency selectivity. Neurosci. Lett. 28. 101-106. Gummer, A.W. and Klinke, R. (1983) Influence of temperature on tuning of primary like units in the guinea pig cochlear nucleus. Hear. Res. 12, 367-380. Morrison, D., Schindler, R.A. and Wersall, J. (1975) A quantitative analysis of the efferent innervation of the organ of Corti in guinea pig. Acta Oto-Laryngol. 79, 11-23.