The endoscopic anatomy of the middle ear approach to the fundus of the internal acoustic canal

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OBJECTIVE

The application of the endoscope in the lateral skull base increases the importance of the middle ear cavity as the corridor to the skull base. The aim of this study was to define the middle ear as a route to the fundus (lateral end) of the internal acoustic canal and to propose feasible landmarks to the fundus.

METHODS

This was a cadaveric study; 34 adult cadaveric temporal bones and 2 dry bones were dissected with the aid of the endoscope and microscope to show the anatomy of the transcanal approach to the middle ear and fundus of the internal acoustic canal.

RESULTS

In the middle ear cavity, the cochleariform process is one of the key landmarks for accessing the fundus of the internal acoustic canal. The triangle formed by the anterior and posterior edges of the overhang of the round window and the cochleariform process provides a landmark to start drilling the bone to access the fundus of the internal acoustic canal.

CONCLUSIONS

The external acoustic canal and middle ear cavity combined, using endoscopic guidance, can provide a route to the fundus of the internal acoustic canal. A triangular landmark crossing the promontory has been described for reaching the meatal fundus. This transcanal approach requires an understanding of the relationship between the middle ear cavity and the fundus of the internal acoustic canal and provides a potential new area of cooperation between otology and neurosurgery for accessing pathology in this and the bordering skull base.

Article Information

INCLUDE WHEN CITING Published online September 2, 2016; DOI: 10.3171/2016.5.JNS16261.

Correspondence Noritaka Komune, Department of Otorhinolaryngology, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi Higashi-ku, Fukuoka 812-8582, Japan. email: norikomu007@gmail.com.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A: Superior view of the right middle cranial base. The yellow square in the inset shows the area of view. B: Right lateral view. A mastoidectomy has been completed and the mastoid tip, parotid gland, and posterior wall of the external acoustic canal and tympanic membrane were removed to show the relationships of the middle ear. C: Right lateral view. The anterior wall of the external canal, styloid process, facial nerve, chorda tympani, incus and malleus, and jugular bulb were removed to show the relationships among the middle ear, jugular fossa, and carotid artery. D–F: Enlarged views showing the anatomy of the middle ear and related structures. D: The middle ear structures were exposed after removal of the posterior wall of the external acoustic canal and the tympanic membrane. E: Removal of the promontory, ossicles, and chorda tympani exposed the basal turn of the cochlea. F: Removing the bone inferior to the cochleariform process exposed the middle and apical turns of the cochlea. A. = artery; Ac. = acoustic; Ant. = anterior; Car. = carotid; CN = cranial nerve; Coch. = cochlea; Cochlear. = cochleariform; Cond. = condylar; Emin. = eminence; Endolymph. = endolymphatic; Eust. = eustachian; Ext. = external; For. = foramen; Gang. = ganglion; Gen. = geniculate; Glen. = glenoid; Gr. = greater; Int. = internal; Jug. = jugular; Lat. = lateral; M. = muscle; Mast. = mastoid; Memb. = membrane; N. = nerve; Pet. = petrosal, petrous; Post. = posterior; Proc. = process; Pyramid. = pyramidal; Rec. = recess; Seg. = segment; Sig. = sigmoid; Spher. = spherical; Staped. = stapedius; Stylomast. = stylomastoid; Subarc. = subarcuate; Sulc. = sulcus; Sup. = superior; Temp. Mandib. = temporomandibular; Tens. = tensor; Tymp. = tympani, tympanic; V. = vein; V1, V2, V3 = segments 1–3 of CN V.

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    Dissection of a dry, right temporal bone. A: Lateral view of the temporal bone after basic mastoidectomy. The external acoustic canal is formed by 3 parts of the temporal bone. The tympanic part forms the anterior and inferior parts of the external acoustic canal; the mastoid part forms the posterior wall; and the squamous part forms the roof. B: Superior view showing the relationships of the middle ear. The middle ear cavity (asterisk) and mastoid cavity (double asterisks) are shown. C: Right lateral view along the long axis of the anterior wall of the external acoustic canal. The round window is exposed through the external canal (red arrowhead). D: View along the long axis of the posterior wall. The opening of the eustachian tube has been exposed through the external canal. E: Superior view of the labyrinthine, carotid canal, internal acoustic canal, tympanic sulcus, and cochleariform process. The internal acoustic canal passes between the cochlea and the semicircular canals. The cochlea and the semicanal of the tensor tympani muscle are located just above the genu of the internal carotid artery. F: Lateral view of the labyrinthine. The fundus of the internal acoustic canal is located just inferomedial to the tympanic part of the facial nerve. The position of the cochlea (green dashed line) in the posterior wall of the middle ear is shown. The red triangle outlines the area below the cochleariform process through which the fundus of the meatus can be exposed. Ac. = acoustic; Car. = carotid; Coch. = cochlea; Cochlear. = cochleariform; Digast. = digastric; Eust. = eustachian; Glen. = glenoid; Gr. = greater; Impress. = impression; Jug. = jugular; Mast. = mastoid; Pet. = petrosal, petrous; Proc. = process; Rec. = recess; Sig. = sigmoid; Squam. = squamous; Subarc. = subarcuate; Sulc. = sulcus; Tens. = tensor; Trigem. = trigeminal; Tymp. = tympani, tympanic; Zygo. = zygomatic.

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    Endoscopic transcanal approach to the right internal acoustic canal. A: Normal external acoustic canal with intact tympanic membrane. The tympanomeatal incision and flap are made on the external acoustic canal 2 cm away from the tympanic membrane. B: The middle ear cavity after removal of the tympanomeatal flap, which includes the external canal mucosa and tympanic membrane. The tympanic notch is positioned at the top of the tympanic ring. C: The osseous tympanic sulcus is drilled to provide sufficient exposure of the medial wall of the middle ear cavity. The sharp upper edge of the sulcus is referred to as the scutum. D: Enlarged view of the medial wall of the middle ear cavity. Note the position of the cochleariform process and round window, key landmarks in the approach. Annul. = annular; Aud. = auditory; Auriculotemp. = auriculotemporal; Br. = branch; Cochlear. = cochleariform; Emin. = eminence; Eust. = eustachian; Fiss. = fissure; Inc. = incudo; Mall. = malleolar; Memb. = membrane; Petrotymp. = petrotympanic; Proc. = process; Pyramid. = pyramidal; Stap. = stapedial; Staped. = stapedius; Sulc. = sulcus; Tens. = tensor; Tymp. = tympani, tympanic; Tympanomast. = tympanomastoid.

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    A: Removal of the scutum exposes the epitympanum containing the incudomalleal joint. B: Enlarged view of panel A. C: Structures around the stapes footplate. The stapes sits in the oval window niche, the depression in which the oval window is positioned. D: Removing the incus and malleus exposes the cochleariform process and the tensor tympani tendon. The anterior canaliculus of the chorda tympani is exposed (yellow star). E: Removing the stapes and chorda tympani nerve exposes the triangular landmark formed by the anterior and posterior edges (pillars) of the round window and the cochleariform process (green triangle). The spherical recess is exposed through the oval window. F: Enlarged view of panel E. The triangular landmark for promontory drilling is outlined (green triangle). Car. = carotid; Cochlear. = cochleariform; Eust. = eustachian; Inc. = incudo; Mall. = malleolar; Plex. = plexus; Proc. = process; Rec. = recess; Spher. = spherical; Staped. = stapedius; Tend. = tendon; Tens. = tensor; Tymp. = tympani, tympanic.

  • View in gallery

    A: Removal of the bone covering the triangular landmark exposes the membranous basal turn of the cochlea and vestibule. B: Extending the bone removal inferior to the cochleariform process opens the middle and apical turn of the cochlea. C: Removing the bone laterally through the triangular landmark exposes the dura lining the internal acoustic canal. D and E: Opening the dura of the internal acoustic canal exposes the facial, cochlear, and superior and inferior vestibular nerves. The green square in panel D outlines the area exposed in panel E. F: View of the fundus of the right internal acoustic canal after drilling the posterior meatal wall from medially. The endoscope has been advanced along the anterior surface of the nerves to show the opening through the middle ear into the fundus of the meatus. The opening from the middle ear cavity is located just below the vertical crest (red arrow at the meatal fundus) separating the facial and cochlear nerves. The superior and inferior vestibular nerves are located behind the facial and cochlear nerves. Ac. = acoustic; Car. = carotid; Coch. = cochlea; Cochlear. = cochleariform; Intermed. = intermedius; Mid. = middle; Proc. = process; Tens. = tensor; Transv. = transverse; Tymp. = tympani, tympanic; Vest. = vestibular.

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