the cerebellopontine angle is really a room stuffed with spinal fluid. it has the brain stem as its medial boundary, the cerebellum as its roof and posterior boundary, as well as the posterior surface area of your temporal bone as its lateral boundary. the floor from the cerebellopontine angle is shaped from the reduced cranial nerves (ix-xi) and their bordering arachnoid investments. the flocculus of the cerebellum may lie within the cerebellopontine angle and may be carefully associated 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 of your vestibulocochlear nerve. the foramen of luschka (ie, the opening with the lateral recess from the fourth ventricle) is located just inferior and posterior to the root entry zones with 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, 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 in the cerebellopontine angle
the most important vascular structure within 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 from the posterior inferior cerebellar artery (pica). as the aica moves from anterior to posterior, it first follows the ventral floor of your 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 with the inner auditory canal before turning back on itself toward the posterior surface area from the brain stem. the aica can thus be divided into the premeatal, meatal, and postmeatal segments.
the main branch from the aica passes over cranial nerves vii and viii in only 10% of cases. the remainder of 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 with 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 from the temporal bone, and 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 the 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 with the temporal lobe to the superior petrosal sinus, tentorial venous lakes, or the transverse sinus. its configuration and anatomy is quite variable. however, obstruction, obliteration, or occlusion of the superior petrosal sinus may, in some cases, result in occlusion from the vein of labbé. sudden occlusion with the vein of labbé carries with it high risk of venous infarction from 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. within a few millimeters of leaving the brain stem,
Windows 7 Professional, however, the nerve loses its oligodendroglial ensheathment and becomes ensheathed instead by schwann cells. throughout the remainder of its peripheral course, it remains in its schwann cell investment. it passes directly across the cerebellopontine angle for about 15 mm, accompanied by the vestibulocochlear nerve. it consistently enters the internal auditory canal by crossing the anterior superior margin from 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 inner auditory canal). it's got 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 your 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 in the cerebellopontine angle, the nervus intermedius crosses over to become connected with the facial nerve. it may possibly do so as several separate rootlets. the point where the nervus intermedius crosses to become linked with the facial nerve shows considerably variation, but in 22% of individuals,
Office 2007 Key, it is adherent to the vestibulocochlear nerve for 14 mm or more. as the vestibulocochlear and facial nerve reach the porus acousticus (medial opening from the inner auditory canal) they pass together with the nervus intermedius and sometimes a loop of aica.
internal auditory canal
the internal auditory canal is approximately 8.5 mm in length (range 5.5-10.5 mm), lined with dura, and stuffed with 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 by 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,
Microsoft Office 2010 Product Key, a series of very small openings through which the terminal fibers of your vestibular nerve pass in order to reach the cupula of your superior semicircular canal, from the meatal foramen,
Microsoft Office Professional Plus 2010, which marks the point at which the facial nerve leaves the internal 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 from the geniculate ganglion, the labyrinthine segment with the facial nerve must take a vertically oriented course upward to reach it. the labyrinthine segment might be less than a millimeter wide as it passes between the cochlea and also the anterior end of the superior semicircular canal. the inferior portion from the fundus is a single oval-shaped space, the anterior portion of which is occupied by a rounded depression (tractus spiralis foraminosus) filled with small openings to accommodate the terminal branches with the cochlear nerve. the posterior portion is filled with a macula crista inferior through which pass the terminal ends of your inferior vestibular nerve.
temporal bone
the anatomy from the superior surface area of the temporal bone must be mastered if middle fossa approaches are to be undertaken successfully. laterally, the irregular superior surface of the temporal bone transitions relatively smoothly to the temporal squamosa. the free edge with the tentorium as well as the superior petrosal sinus attach to the medial edge with the superior surface area with 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 within the substance with the temporal bone just medial to and a few millimeters anterior to the head of the malleus. the geniculate ganglion could be dehiscent, or alternatively, it might lie several millimeters beneath the superior floor of the bone. the head of your malleus is generally easy to identify if the thin bone of the tegmen tympani is removed so as to enter into the middle ear area. in difficult surgical situations, the head of 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 of 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 related veins traverse the foramen spinosum,
Office 2010 Key, which is located approximately 1 cm anterolaterally to the greater superficial petrosal nerve. the mandibular division of 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 area appearance of the superior temporal bone. it lies just anterior and inferior to the labyrinthine segment with the facial nerve but is deep to the geniculate ganglion.
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