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the cerebellopontine angle can be a room full of spinal fluid. it has the brain stem as its medial boundary, the cerebellum as its roof and posterior boundary, and also the posterior surface of your temporal bone as its lateral boundary. the ground of the cerebellopontine angle is formed by the reduce cranial nerves (ix-xi) and their bordering arachnoid investments. the flocculus of the cerebellum may possibly lie inside the cerebellopontine angle and may be closely related with cranial nerves viii and vii as they cross the cerebellopontine angle to enter the internal auditory canal. 
the facial nerve arises 2-3 mm anterior to the root entry zone with the vestibulocochlear nerve. the foramen of luschka (ie, the opening of the lateral recess with the fourth ventricle) is located just inferior and posterior to the root entry zones of the facial and vestibulocochlear nerve. a tuft of choroid plexus can frequently be observed extruding from it. inferior and a bit anterior to the foramen of luschka is the olive,Office Professional 2010, and just posterior to the olive lie the rootlets of origin for cranial nerves ix, x, and xi. the hypoglossal nerve exits the brain stem through a series of small rootlets anterior to the olive.
vascular structures within the cerebellopontine angle
the most important vascular structure inside of the cerebellopontine angle is the anterior inferior cerebellar artery (aica). it arises most commonly as a single trunk from the basilar artery but can arise as 2 separate branches. in rare cases, it originates as a branch of the posterior inferior cerebellar artery (pica). as the aica moves from anterior to posterior, it first follows the ventral floor from the brain stem, but in the cerebellopontine angle it takes a long loop laterally to the porus acousticus. in 15-20% of cases, the aica actually passes into the lumen of the inner auditory canal before turning back on itself toward the posterior surface area with the brain stem. the aica can thus be divided into the premeatal, meatal, and postmeatal segments.
the main branch with the aica passes over cranial nerves vii and viii in only 10% of cases. the remainder with the time, it either passes below the vii and viii cranial nerves or, in 25-50% of individuals, actually passes between them. three branches that regularly arise from the meatal segment from the aica can be identified. small perforating arteries supply blood to the brain stem. the subarcuate artery passes through the subarcuate fossa into the posterior surface area with the temporal bone, and also the third regular branch is the inner auditory artery (labyrinthine artery). cranial nerves vii and viii receive their blood supply from small branches of aica.
two venous structures must be kept in mind during surgical procedures involving the cerebellopontine angle. the petrosal vein (of dandy) brings returning venous blood from the cerebellum and lateral brain stem to the superior or inferior petrosal sinus. it is generally encountered in the area of your trigeminal nerve anterior to the porus acousticus. the petrosal vein often carries enough venous blood that its obstruction can lead to venous infarction and cerebellar edema, and it should be preserved if at all possible. additional venous blood reaches the superior petrosal sinus through a series of bridging veins that cross the cerebellopontine angle. although every attempt should be made to preserve these veins, their sacrifice is generally inconsequential.
the vein of labbé carries returning venous blood from the inferior and lateral surface area of the temporal lobe to the superior petrosal sinus, tentorial venous lakes,Microsoft Office 2010 Key, or the transverse sinus. its configuration and anatomy is quite variable. however, obstruction, obliteration, or occlusion with the superior petrosal sinus could, in some cases, result in occlusion from the vein of labbé. sudden occlusion from the vein of labbé carries with it high risk of venous infarction of the temporal lobe and rapid life-threatening cerebral edema.
nerves
the facial nerve leaves the brain stem anterior to the foramen of luschka. as it leaves the brain stem, the fibers are sheathed in oligodendroglia derived from the central nervous system. inside of a few millimeters of leaving the brain stem, however, the nerve loses its oligodendroglial ensheathment and becomes ensheathed instead by schwann cells. throughout the remainder of its peripheral course, it remains inside of its schwann cell investment. it passes directly across the cerebellopontine angle for about 15 mm, accompanied through the vestibulocochlear nerve. it consistently enters the inner auditory canal by crossing the anterior superior margin of the porus acusticus.
the vestibulocochlear nerve arises from the brainstem slightly posterior to the facial nerve. it remains sheathed in oligodendroglia for approximately 15 mm (almost to the point at which it passes into the internal auditory canal). it's the longest oligodendroglial investment of any peripheral nerve. the junction between oligodendroglia and schwann cells (ie, the obersteiner-redlich zone) occurs just medial to the porus acousticus. because acoustic neuromas arise from schwann cells, they arise most commonly within the most lateral portions of the cerebellopontine angle or the inner auditory canal.
the nervus intermedius (nerve of wrisberg) leaves the brain stem together with the vestibulocochlear nerve. at some point inside the cerebellopontine angle, the nervus intermedius crosses over to become related with the facial nerve. it may possibly do so as several separate rootlets. the point where the nervus intermedius crosses to become connected with the facial nerve shows considerably variation,Office 2010 Pro Plus Key, but in 22% of individuals, it is adherent to the vestibulocochlear nerve for 14 mm or more. as the vestibulocochlear and facial nerve reach the porus acousticus (medial opening of the inner auditory canal) they pass together with the nervus intermedius and sometimes a loop of aica.
internal auditory canal
the inner auditory canal is approximately 8.5 mm in length (range 5.5-10.5 mm), lined with dura, and full of spinal fluid. its medial end is oval in shape and is referred to as the porus acousticus. its lateral end is actually a complicated structure referred to as the fundus or lamina cribrosa. the fundus is divided into a superior and inferior half from the transverse crest. the upper half is further subdivided into an anterior and posterior segment by a vertical crest, often referred to as bill’s bar, named after william house, who popularized its importance as a surgical landmark. the vertical crest separates the macula cribrosa superior, a series of very small openings through which the terminal fibers from the vestibular nerve pass in order to reach the cupula from the superior semicircular canal, from the meatal foramen,Windows 7 X86, which marks the point at which the facial nerve leaves the inner auditory canal and enters the fallopian canal as the labyrinthine segment.
because the most lateral portion with the internal auditory canal is 4-5 mm inferior to the level of your geniculate ganglion, the labyrinthine segment from the facial nerve must take a vertically oriented course upward to reach it. the labyrinthine segment could be less than a millimeter wide as it passes between the cochlea and the anterior end with the superior semicircular canal. the inferior portion of your fundus can be a single oval-shaped room, the anterior portion of which is occupied by a rounded depression (tractus spiralis foraminosus) stuffed with small openings to accommodate the terminal branches of the cochlear nerve. the posterior portion is stuffed with a macula crista inferior through which pass the terminal ends from the inferior vestibular nerve.
temporal bone
the anatomy of the superior floor with the temporal bone must be mastered if middle fossa approaches are to be undertaken successfully. laterally, the irregular superior floor of your temporal bone transitions relatively smoothly to the temporal squamosa. the free edge of your tentorium and the superior petrosal sinus attach to the medial edge of the superior surface of the temporal bone. the arcuate eminence, a bony prominence that is perpendicular to the petrous ridge and lies two centimeters medial to the squamous temporal bone, often overlies the superior semicircular canal. the arcuate eminence is often difficult to identify, especially in well-pneumatized temporal bones.
the geniculate ganglion generally lies in the substance with the temporal bone just medial to and a few millimeters anterior to the head from the malleus. the geniculate ganglion might be dehiscent, or alternatively, it could lie several millimeters beneath the superior floor of the bone. the head from the malleus is generally easy to identify if the thin bone with the tegmen tympani is removed so as to enter into the middle ear space. in difficult surgical situations,Office 2010, the head with the malleus can be used to identify the geniculate ganglion. the greater superficial petrosal nerve originates from the geniculate ganglion and courses anteromedially, passing over the superior floor with the temporal bone at the facial hiatus. the facial hiatus is generally 4-8 mm anterior to the geniculate ganglion. the greater superficial petrosal nerve can generally be identified in this area. it can then be followed retrograde to the geniculate ganglion.
the middle meningeal artery and connected veins traverse the foramen spinosum, which is located approximately 1 cm anterolaterally to the greater superficial petrosal nerve. the mandibular division with the trigeminal nerve traverses the foramen ovale, which lies a few millimeters anterior and medial to the foramen spinosum. the horizontal portion with the carotid canal courses through the anterior temporal bone medial to the foramen spinosum and foramen ovale. the cochlea cannot be identified from the surface appearance with the superior temporal bone. it lies just anterior and inferior to the labyrinthine segment from the facial nerve but is deep to the geniculate ganglion.