14 Transmastoid Approach: Retrolabyrinthine, Translabyrinthine, and Transcochlear Approaches
10.1055/b-0034-63762
14 Transmastoid Approach: Retrolabyrinthine, Translabyrinthine, and Transcochlear Approaches
The retrolabyrinthine, translabyrinthine, and transcochlear approaches offer the surgeon progressively flatter trajectories to the clivus. These approaches are usually combined with a temporal craniotomy and incision of the tentorium cerebelli to increase the scope of the surgery and the surgeon’s working room. Even the retrolabyrinthine approach, which preserves the patient’s hearing, affords the surgeon a very nice view of the lateral brainstem limited by the seventh and eighth cranial nerves. This approach is used to expose extra-axial tumors and small to moderate sized aneurysms of the basilar artery. Larger lesions and lesions that significantly cross the midline are exposed by the translabyrinthine and transcochlear exposures.
Fig. 14.1 (Step 1) Skin incision retroauricular n”C” shaped.The mastoid tip is palpated and outlined on the skin, the internal auditory canal lies along a plane drawn through the external auditory canal. (MT, mastoid tip)Fig. 14.2 (Step 2) Elevating the scalp flap.The scalp flap should be elevated in two layers. The skin, subcutaneous fat, and a thin layer of fibrous tissue are elevated. Two-thirds the thickness of fibrous tissue is left behind.Fig. 14.3 Harvesting the galeofascial patch.The galeofascial fibrous membrane (epicranium aponeurotica) should be harvested to facilitate closing the dura at the end of the surgery. (GFFM, galeofascial fibrous membrane)Fig. 14.4 Harvesting a galeofascial fibrous membrane. (PAM, posterior auricular muscle)Fig. 14.5 Identifying the postauricular and sternocleidomastoid muscles.After the galeofascial fibrous tissue is harvested, the postauricular and sternocleidomastoid muscles are exposed. (PAM, posterior auricular muscle; SCM, sternocleidomastoid muscle)Fig. 14.6 Elevating the myofascial layer to expose the mastoid.The myofascial layer can be incised in a “T” shaped fashion, but incising the fascia in a “C” shape makes for better exposure. The underlying muscle is elevated from the occipital bone.Fig. 14.7 (Step 3) Identification of superficial bony landmarks.The squamosal, parietomastoid, occipitomastoid, and lambdoid sutures and the asterion are identified. Macewen’s triangle is 15 mm superficial to the mastoid antrum and the spine of Henle is 15 mm superficial to the external genu of the facial nerve. The supramastoid crest or ridge that corresponds to the temporal tegmen is identified. (As, asterion; LS, lambdoid suture; MacT, Macewen’s triangle; OMS, occipitomastoid suture; PMS, parietomastoid suture; SMC, supramastoid crest; SqS, squamosal suture)Fig. 14.8 Identifying the asterion.The asterion, the intersection of the parietomastoid, occipitomastoid, and lambdoid sutures, is identified. The asterion is an approximate landmark for the inferior corner of the transverse sinus–sigmoid sinus junction. (As, asterion; LS, lambdoid suture; OMS, occipitomastoid suture; PMS, parietomastoid suture)Fig. 14.9 Identifying Macewen’s triangle and the spine of Henle.Macewen’s triangle is detected as a depressed, triangular area in the mastoid bone posterior and superior to the external auditory canal. The spine of Henle is a bony protrusion on the superoposterior edge of the external auditory canal. In some cases this bony protrusion is not well developed. (MacT, Macewen’s triangle; SH, spine of Henle; SMC, supramastoid crest)Fig. 14.10 Identifying the three corners of the outer triangle.The posterior point of the root of the zygoma, the mastoid tip, and a point just medial to the asterion are the landmarks of the outer triangle. This triangle makes the superficial landmarks for beginning the mastoidectomy.Fig. 14.11 (Step 4) Incising the outer triangle.The sides of the outer triangle are incised with a cutting bur.Fig. 14.12 Drilling the cortical bone and mastoid air cells.The cortical bone within the outer triangle is removed with a cutting bur. The mastoid air cells are drilled away evenly in a planar fashion. The surgeon must avoid digging a hole into the air cells. Another option is a cosmetic mastoidectomy. The cortical bone over the mastoid is removed as a single piece to be replaced at the end of the case. This technique has a greater risk of injuring the sigmoid sinus. The value of the cosmetic mastoidectomy is diminished because the bony defect of a standard mastoidectomy can be restored with titanium plates and hydroxy appetite.Fig. 14.13 (Step 5) Identification of sigmoid sinus plate.With even planar drilling of the posterior temporal bone, the cortical bone over the sigmoid sinus will transmit the blue of the sigmoid sinus. The sigmoid sinus may run very anteriorly when the mastoid sinus is small. In such cases the bone over and behind the sigmoid sinus should be removed so that the sigmoid sinus can be retracted posteriorly. (SSP, sigmoid sinus plate)Fig. 14.14 (Step 6) Identification of the mastoid antrum.The first deep landmark is the mastoid antrum. This lies 15 mm deep to Macewen’s triangle. The antrum is a large air cell. The hard bone of the lateral semicircular canal is seen protruding posteriorly into the mastoid antrum. In addition, the short crus of the incus protrudes inferiorly into the antrum. (MA, mastoid antrum; SSP, sigmoid sinus plate)Fig. 14.15 Identifying the bony protrusion of the lateral semicircular canal.The hard yellow cortical bone of the lateral semicircular canal can be seen bulging into the mastoid antrum. All the important anatomical structures except the sigmoid sinus lie medial to the level of the lateral apex of the lateral semicircular canal. This means that drilling lateral to the lateral semicircular canal is safe. (LSC, lateral semicircular canal; MA, mastoid antrum; SSP, sigmoid sinus plate)Fig. 14.16 Saucerizing the mastoid cavity.The remaining air cells lateral to the lateral semicircular canal are removed. The posterior fossa dura is found medial to the sigmoid sinus. The dura can be exposed without risk to the level of the lateral semicircular canal. Medial to this level the surgeon will encounter the posterior semicircular canal. (LSC, lateral semicircular canal; MA, mastoid antrum; SSP, sigmoid sinus plate)Fig. 14.17 Identifying the incus.The body and short crus of the incus can be seen protruding into the inferior antrum. This important landmark distinguishes the antrum from other air cells. (I, incus; LSC, lateral semicircular canal; SSP, sigmoid sinus plate)Fig. 14.18 Exposing the temporal tegmen and sigmoid sinus.To access to the presigmoid area, the bone covering the lateral and the sigmoid sinuses should be removed completely. The bone is drilled until it is paper thin and then dissected from the venous sinus with a flat sharp dissector. The freed sinus can be retracted posteriorly out of the surgeon’s line of sight. (I, incus; LSC, lateral semicircular canal; SS, sphenoid sinus)Fig. 14.19 Exposing the digastric ridge.The fallopian canal exits the temporal bone through the medial edge of the anterior digastric ridge, making the ridge an important landmark. (DR, dural ring; I, incus; LSC, lateral semicircular canal; SS, sphenoid sinus)Fig. 14.20 Removing the anterior portion of the temporal tegmen.Great care should be taken to avoid perforating the thin dura above the anterior portion of the temporal tegmen that is near the aditus. The midportion of the temporal base slopes inferiorly into the mastoid, making it easy for the surgeon to drill through the underlying dura. A good technique is to use Gelfoam (Pfizer Inc., NY, NY) as a spacer between the middle fossa dura and the temporal bone. ((I, incus; LSC, lateral semicircular canal)Fig. 14.21 Exposing the lateral semicircular canal.The hard bone of the lateral semicircular canal is followed posterior laterally from the antrum. The lateral semicircular canal shares its anterior wall with the fallopian canal. (I, incus; LSC, lateral semicircular canal)Fig. 14.22 (Step 7) Exposure of semicircular canals.The posterior semicircular canal runs perpendicular to the lateral semicircular canal. The endolymphatic sac, which can be seen on the posterior fossa dura, points to the inferior half of the posterior semicircular canal. The superior semicircular canal protrudes into the temporal tegmentum. The common crus connects the superior semicircular canal with the posterior canal. The center of the superior semicircular canal is marked by the subarcuate artery. (LSC, lateral semicircular canal; PSC, posterior semicircular canal; SSC, superior semicircular canal)Fig. 14.23 Removing the opercular bone medial to the posterior semicircular canal.The bone over the posterior fossa dura up to the posterior semicircular canal should be removed. Removing the opercular bone medial to the posterior semicircular canal improves the surgeon’s vision of the lateral cerebellopontine angle. (LSC, lateral semicircular canal; PSC, posterior semicircular canal; SSC, superior semicircular canal)Fig. 14.24 Thinning the bone of the semicircular canals (bony labyrinth).The hard bone overlying the semicircular canals can be gradually thinned until the blue of the underlying membranous canals is seen using a diamond drill and profuse irrigation. Irrigation is important to mitigate the heat generated by the drill and to wash away the dull bone dust, which improves the surgeon’s view. (I, incus; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SSC, superior semicircular canal)Fig. 14.25 (Step 8) Identification of tympanic segment of the facial nerve anterior to the lateral semicircular canal.The bone overlying the tympanic segment of the facial nerve lies immediately anterior to the lateral semicircular canal. This is the most anatomically constant position of the nerve, which can meander through the mastoid. (I, incus; LSC, lateral semicircular canal; SSC, superior semicircular canal; T-VII, tympanic segment of the facial nerve)Fig. 14.26 Skeletonizing the mastoid segment of the facial nerve.The most anatomically constant location of the facial nerve is its external genu, which lies anterior to the lateral semicircular canal. The course of the nerve is then charted to the stylomastoid foramen starting from this point. The nerve is exposed by removing bone first from its lateral and then its posterior surface. It is possible to find the nerve at the stylomastoid foramen by following the digastric groove anteriorly, but at the stylomastoid foramen the nerve is covered by fibrous tissue and the nerve is not distinct. (DR, dural ring; I, incus; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SS, sphenoid sinus; SSC, superior semicircular canal; V-VII, vertical segment of the facial nerve)Fig. 14.27 Exposing the inferior segment of the sigmoid sinus.As the sigmoid sinus is followed inferiorly, the wall of the sinus becomes thinner and more fragile. A thin malleable layer of bone may be left in place over the inferior loop of the sigmoid sinus. Sigmoid sinus bleeding is best controlled by Gelfoam. (JB, jugular bulb; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SS, sphenoid sinus; SSC, superior semicircular canal)Fig. 14.28 (Step 9) Exposure of jugular bulb.In this specimen, the jugular bulb is prominent. The jugular bulb lies anterior to the sigmoid sinus. In this specimen the bulb rises almost to the posterior semicircular canal. Drilling anterior to the posterior semicircular canal risks injury to the cochlea. (JB, jugular bulb; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SS, sphenoid sinus; SSC, superior semicircular canal; V-VII, vertical segment of the facial nerve)Fig. 14.29 Exposing the facial recess.The external genu and mastoid segments of the facial nerve and the chorda tympani are exposed. The facial recess is defined by the incus buttress, the chorda tympani, and the mastoid segment of the facial nerve. The stapes with the long crus of the incus and the promontory with the round window can be observed through the facial recess. (CT, chorda tympani; G, genu [second turn or external genu] of the facial nerve; I, incus; IB, incus buttress; JB, jugular bulb; LSC, lateral semicircular canal; P, promontory; PSC, posterior semicircular canal; S, stapes; V-VII, vertical segment of the facial nerve)Fig. 14.30 Skeletonizing the annulus tympanicus.The lateral portion of the annulus tympanicus can be skeletonized. It is seen as a white band ventral to the chorda tympani. Disrupting the annulus of the tympanic membrane or the skin of the external auditory canal lateral to the membrane risks the formation of a cerebrospinal fluid fistula. (CT, chorda tympani; FR, fibrocartilaginous ring; G, genu [second turn or external genu] of the facial nerve; LC (I), long crus of the incus; LSC, lateral semicircular canal; P, promontory; PSC, posterior semicircular canal; S, stapes; T-VII, tympanic segment of the facial nerve; V-VII, vertical segment of the facial nerve)Fig. 14.31 Examining the middle ear.The annulus tympanicus is exposed. The chorda tympani runs between the malleus and the incus. The manibrium of the malleus can be seen attaching to the tympanic membrane. The cochleari-form process is seen between the incus and the lateral semicircular canal after the buttress of the incus is removed. The cochleariform process marks the location of the geniculate ganglion. (CP, cochleariform process; CT, chorda tympani; FR, fibrocartilaginous ring; G, genu [second turn or external genu] of the facial nerve; LC (I), long crus of the incus; LSC, lateral semicircular canal; S, stapes; SC (I), short crus of the incus; T-VII, tympanic segment of the facial nerve; V-VII, vertical segment of the facial nerve)Fig. 14.32 Demonstrating the middle ear anatomy.The chorda tympani has been removed to better demonstrate the relationship between the annulus tympanicus, the tympanic membrane, and the manibrium of the malleus. (CT, chorda tympani; FR, fibrocartilaginous ring; G, genu [second turn or external genu] of the facial nerve; LC (I), long crus of the incus; LSC, lateral semicircular canal; Man (M), manibrium of the malleus; SC (I), short crus of the incus; TM, tympanic membrane; V-VII, vertical segment of the facial nerve)Fig. 14.33 Identifying Jacobson’s nerve on the promontory.Jacobson’s nerve (the tympanic nerve), a branch of the glossopharyngeal nerve, can be seen passing over the promontorium. The nerve passes through the middle ear to the otic ganglion innervating the parotid gland. Jacobson’s nerve on the promontory can give rise to glomus tympanicum tumors. (CT, chorda tympani; G, genu [second turn or external genu] of the facial nerve; LC (I), long crus of the incus; LSC, lateral semicircular canal; P, promontory; RW, round window; S, stapes; SC (I), short crus of the incus; TN, tympanic nerve [Jacobson’s nerve]; T-VII, tympanic segment of the facial nerve)Fig. 14.34 Identifying Jacobson’s nerve in front of the jugular bulb.Jacobson’s nerve (the tympanic nerve) is identified by drilling the bone anterior to the jugular bulb. The nerve runs in the bony canal between the jugular bulb and the carotid canal. Jacobson’s nerve in this location can be the origin of a glomus jugulare tumor. (C7, C7 portion of the internal carotid artery; CT, chorda tympani; IX, glossopharyngeal nerve; SS, sphenoid sinus; TN, tympanic nerve [Jacobson’s nerve]; V-VII, vertical segment of the facial nerve)Fig. 14.35 Identifying the origin of Jacobson’s nerve.Jacobson’s nerve (the tympanic nerve) originates from the glossopharyngeal nerve. (C7, C7 portion of the internal carotid artery; IX, glossopharyngeal nerve; SS, sphenoid sinus; TN, tympanic nerve [Jacobson’s nerve])Fig. 14.36 Identifying the stapedius muscle.The stapedius muscle, which is innervated by the facial nerve, can be exposed medial to the external genu of the facial nerve in the temporal bone. The short tendon of the stapedius muscle emerges from the pyramidal eminence of the temporal bone and inserts onto the neck of the stapes close to its articulation with the incus. This muscle tilts the head of the stapes away from the oval window, protecting the cochlea from loud noises. (CT, chorda tympani; G, genu [second turn or external guru] of the facial nerve; I, incus; JB, jugular bulb; LSC, lateral semicircular canal; PSC, posterior semicircular canal; S, stapes; SM, stapedial muscle; SSC, superior semicircular canal; V-VII, vertical segment of the facial nerve)Fig. 14.37 Establishing the blue lines of the semicircular canals.The blue lines (arrowheads) of the semicircular canals can be seen by gradually shaving the cortical bone. The blue line of each semicircular canal corresponds to the membranous labyrinth. In the cadaver the lines appear brown. (LSC, lateral semicircular canal; PSC, posterior semicircular canal; SSC, superior semicircular canal)Fig. 14.38 Exposing the cochlear aqueduct (final view of retrolabyrinthine petrosectomy).The cochlear aqueduct, which connects the cerebrospinal fluid with the perilymph, can be seen between the bony labyrinth and the pyramidal fossa of the jugular foramen. The cochlear aqueduct separates the infralabyrinthine triangle and the suprajugular triangle. The infralabyrinthine triangle is defined by the inferior point of the posterior semicircular canal, the cochlear aqueduct, and the posterior fossa dura. An extension of the superior leg of this triangle leads to the basal turn of the cochlea. Great care should be taken not to drill anterior to this line when preserving hearing. The suprajugular triangle is defined by the superior point of the jugular bulb, the cochlear aqueduct, and the carotid canal. An inferior extension of the anterior side of the triangle leads to the glossopharyngeal nerve. (CA, cochlear aqueduct; CT, chorda tympani; ES, endolymphatic sac; G, genu [second turn or external guru] of the facial nerve; ILT, infralabyrinthine triangle; LSC, lateral semicircular canal; PSC, posterior semicircular canal; SJT, suprajugular triangle; SSC, superior semicircular canal; TT, tegmen tympani; V-VII, vertical segment of the facial nerve)Fig. 14.39 Viewing the course of the cochlear aqueduct.The cochlear aqueduct (ductus perilymphaticus) connects the perilymphatic spaces of the cochlea (scala tympani) with the subarachnoid space of the posterior fossa. The cochlear aqueduct permits the free flow of cerebrospinal fluid into the labyrinth. It passes under the ampullary arm of the posterior semicircular canal and runs toward the top of the hairpin curve of the ninth nerve canal. The cochlear aqueduct arises on the posterior wall of the scala tympani through a small orifice. The aqueduct originates from the inferomedial edge of the round window and proceeds under the spiral lamina. From there, it is directed backward and inferiorly toward the top of the hairpin curve of the ninth nerve canal. It empties into the posterior fossa at the level of the inferior border of the petrosa by the pyramidal fossa, a wide orifice that is found 4 to 5 mm below the internal auditory meatus. The cochlear aqueduct is 10 to 12 mm long and transmits a small vein. (CA, cochlear aqueduct; IX, glossopharyngeal nerve; PSC, posterior semicircular canal)
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Jul 19, 2020 | Posted by drzezo in NEUROSURGERY | Comments Off on 14 Transmastoid Approach: Retrolabyrinthine, Translabyrinthine, and Transcochlear Approaches
