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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4399_Библиотеки_им_академика_М_И_Перельмана
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Temporal Bone Histology and Radiology Atlas88
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HORIZONTAL SECTION 7
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CLINICAL CAVEAT: The middle cranial fossa
(MCF) surgical approach to the IAC opens the roof
of the IAC, with initial exposure of the facial nerve
medially and the superior vestibular nerve laterally. Care must be taken to identify and preserve the
facial nerve, and avoid injury to its vascularity, in
order to minimize the risk of even temporary facial
nerve dysfunction postoperatively. There is some
evidence of a higher risk of facial palsy following
MCF versus other approaches to the IAC, particularly when the tumor arises from the inferior vestibular nerve and therefore necessitates some FN
traction.
7
SEVENTH AND EIGHTH CRANIAL
NERVE GANGLIA
The geniculate ganglion of the facial nerve is immediately lateral to the fundus of the IAC. It contains the
fused fibers of the motor facial nerve and the sensory
nervus intermedius and from it come off the greater
and lesser superficial petrosal nerves (see Chapter 5).
The cochlear nerve ganglion is located within the modiolus of the cochlea. Scarpa’s ganglion, the ganglion
of the vestibular nerve, lies in the IAC.
For most cranial and spinal nerves, neuroglial cells
extend for only a fraction of a millimeter beyond their
origin from the brainstem or spinal cord. An exception
is the cochleovestibular nerve, which has glia extending for a median length of 9.75 mm. The zone at which
the glial tissue ends and the Schwann cells begin is
termed the glial-Schwann sheath junction, also known
as the Obersteiner–Redlich zone. This transition zone
is typically located at or medial to the porus of the IAC.
CLINICAL CAVEAT: The historical misnomer
“acoustic neuroma” has led to some confusion
regarding nerve and cell of origin of the nowcorrectly named vestibular schwannoma. Cochleovestibular schwannomas account for more than 90%
of cranial nerve schwannomas, and they arise from
Schwann cells from the Obersteiner–Redlich zone
medially up until their terminations in the cochlea
and vestibule (where they are called intralabyrinthine schwannomas). In the eighth cranial nerve,
vestibular nerve schwannomas occur 3.2 times as
frequently as do cochlear nerve schwannomas.
KEY ANATOMICAL LANDMARKS
IN THE MIDDLE EAR
The Tensor Tympani Muscle
and Tympanic Facial Nerve
The tensor tympani muscle semicanal and the horizontal or tympanic fallopian canal transmitting the facial
nerve are along the same horizontal lie in the middle
ear. The cochleariform process transmitting the tensor tympani tendon demarcates the end of the tensor
tympani semicanal and the beginning of the tympanic
segment of the fallopian canal immediately after the
geniculate ganglion. This is illustrated in Horizontal
Section 7. That relationship is important surgically as
it helps identify the facial nerve, particularly in an infected ear.
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6. Horizontal Temporal Bone Sections with Corresponding Computed Tomography Images 91
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The Chorda Tympani Nerve
The chorda tympani nerve is the third intratemporal
branch of the facial nerve. The first two are the superficial petrosal nerve and the nerve to the stapedius muscle. The chorda tympani emerges from the descending
(vertical or mastoid) segment of the facial nerve at an
acute angle and travels back up superiorly to emerge
into the middle ear just medial to the tympanic membrane via the iter chordae posterior. (Iter means tunnel in
Latin.) It then flies in the middle ear air space, traveling medial to the long process of the malleus and lateral to the long process of the incus, before entering
the iter chordae anterior just anterior to the protympanic
opening of the Eustachian tube.
The otologic surgeon should preserve this nerve
as long as possible in order to use this anatomical information to their advantage. In a diseased, inflamed
ear, orientation may be difficult, and when the chorda
tympani is intact, the surgeon can be assured that manipulation of the structures lateral to it poses less risk
than manipulation of structures medial to it, which include the incus, stapes, and facial nerve.
CLINICAL CAVEAT: Minimizing risk of sensorineural hearing loss following stapedectomy or
stapedotomy surgery is accomplished by avoiding
deep insertion of the prosthesis. A good rule of thumb
is to avoid inserting the piston more than 0.25 mm
into the vestibule, so as to avoid any insertion trauma
near the saccular membrane.
CLINICAL CAVEAT: In patients with otosclerosis
and concurrent symptoms of active endolymphatic
hydrops, treatment of the presumed distension of
the utriculosaccular (as well as Reissner’s cochlear)
membrane should occur to an effective level prior
to attempting stapedotomy or stapedectomy, to
minimize the risk of tear of the membranes and
postoperative sensorineural hearing loss.
STAPES FOOTPLATE AND SACCULAR MACULE
The footplate of the stapes is approximately 1.3 mm
directly lateral to the saccular membrane. The normal
saccular membrane is seen as a thin ribbon between
the saccular macule and the stapes footplate in Horizontal Section 8 and the Mid-Modiolar Section of the
Cochlea on pages 110 and 111. Posterio-medial to the
stapes footplate is the utricular membrane—as close as
0.58 mm to the footplate.9 In cases of endolymphatic
hydrops, the membranes of the saccule and utricle
distend and may even contact the undersurface of the
stapes footplate.
RELATIONSHIP OF CAROTID CANAL
TO EUSTACHIAN TUBE
The Eustachian tube (ET) is an anterior inferior projection of the protympanum. The lateral one-third is bony
and the medial two-thirds is cartilaginous. It connects
to the lateral wall of the nasopharynx. It is lined by mucosal epithelium, remains primarily closed and opens
at intervals to equalize pressures between the ambient
environment and the middle ear. Dilatory dysfunction
of the ET is a cause of otitis media and middle ear barotrauma. Balloon, laser, or other dilatation of the ET is
performed at the cartilaginous ET and not in the osseous ET.
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HORIZONTAL SECTION 8
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