Osseous and Muscular Relationships, Transverse and Sigmoid Sinuses, and External Surgical Landmarks

When dealing with suboccipital and retrosigmoid approaches, surgeons initially confront the occipital and temporal bones and their related structures. Careful exposure, particularly of the bony sutures and other osseous prominences and depressions, enables these sites to be used as important landmarks for surgical orientation and for more restricted and appropriate approaches.

The occipital bone surrounds the foramen magnum and is divided into a squamosal part above and behind the foramen magnum, a basal portion in front of the foramen magnum, and paired condylar portions lateral to the foramen magnum.3 The squamous portion is an internally concave plate, and its upper margins articulate with the parietal bones at the lambdoid sutures. Its lower margins articulate with the mastoid portion of the temporal bones at the occipitomastoid sutures. The convex external surface has several prominences on which the muscles of the neck attach. The largest prominence, the external occipital protuberance, or inion, is situated at the central portion of the external surface and over the inferior margin of the confluence of the sagittal and transverse sinuses. Two parallel ridges radiate laterally from the protuberance. The highest nuchal line is the upper and thinner ridge; the superior nuchal line is lower and more prominent. The rough and irregular area below the nuchal lines serves as the site of attachment of numerous muscles. A vertical ridge, the external occipital crest, descends from the external occipital protuberance to the midpoint of the posterior margin of the foramen magnum. The inferior nuchal lines run laterally from the midpoint of the crest ( Fig. 33.1 ).

The basilar portion of the occipital bone, also referred to as the clivus, is a thick quadrangular plate of bone that extends anteriorly and superiorly at about a 45-degree angle from the foramen magnum. It joins the sphenoid bone at the spheno-occipital synchondrosis just below the dorsum sellae.4 The superior surface of the clivus is concave from side to side and is separated on each side from the petrous portion of the temporal bone by the petroclival fissure. This fissure has the inferior petrosal sinus on its upper surface and ends posteriorly at the jugular foramen. On the inferior surface of the basilar part in front of the foramen magnum, a small elevation called the pharyngeal tubercle attaches to the fibrous raphe of the pharynx.

The paired lateral or condylar portions are situated at the sides of the foramen magnum. The occipital condyles, which articulate with the atlas, protrude from the external surface of this part. These condyles are located lateral to the anterior half of the foramen magnum. They are oval, convex downward, and face downward and laterally. Their long axes are directed anteriorly and medially. A tubercle that attaches to the alar ligament of the odontoid process is situated on the medial side of each condyle. The hypoglossal canal, which transmits the hypoglossal nerve, is situated above the condyle and is directed anteriorly and laterally from the posterior cranial fossa. The canal may be partially or completely divided by a bony septum.

The superior nuchal lines, which extend laterally and horizontally from the external occipital protuberance, are the boundary between the scalp and neck. These lines are often sharp.5 Each is located at the level of an imaginary line formed by the inion and external acoustic meatus.6 Along this imaginary line, the trapezius inserts medially. The sternocleidomastoid muscle, which covers the semispinalis and splenius capitis muscles, inserts laterally. The highest nuchal lines, which are more arched than the superior nuchal lines, are eventually identified. Medially, the galea aponeurotica inserts, and laterally the occipitofrontalis muscle inserts. The slightly arched inferior nuchal lines are located below the external occipital protuberance. The semispinalis capitis and the superior oblique muscles are inserted medially and laterally between the inferior and superior nuchal lines. The rectus capitis posterior minor and major muscles are inserted medially and laterally, respectively, below each inferior nuchal line ( Fig. 33.2 ).

The suboccipital triangle is a region bound superiorly and medially by the rectus capitis posterior major muscle, superiorly and laterally by the superior oblique muscle, and inferiorly and laterally by the inferior oblique muscle. It is covered by the semispinalis capitis muscle medially and by the splenius capitis muscle laterally. The floor of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. The structures in the triangle are the terminal extradural segment of the vertebral artery and the first cervical nerve.3

The external occipital crest descends from the external occipital protuberance, with the nuchal ligamentum attached. The lambdoid, occipitomastoid, and parietomastoid sutures3,7–11 are united at the asterion and separate the occipital, parietal, and temporal bones.

The lambdoid suture begins at the lambda, where it meets the sagittal suture. Along its oblique course, it separates the squamous part of the occipital bone from the parietal bone. It is particularly evident superiorly due to its deeper and prominent serrations.11 Inferior to the asterion, the lambdoid suture continues as the occipitomastoid suture, which separates the lower portion of the occipital squamous from the petromastoid portion of the temporal bone, ending at the jugular foramen. The parietomastoid suture separates the mastoid temporal portion from the posteroinferior portion,11 or mastoid angle, of the parietal bone. The suture is horizontal to the skull base. Occasionally, sutural bones are present, usually along the lambdoid suture. An isolated bone at the lambda is named the Inca bone.

Laterally and parallel to the occipitomastoid suture, along and medial to the mastoid process, the deep groove of the mastoid notch is where the posterior belly of the digastric muscle inserts. Parallel and between this notch and the occipitomastoid suture, the occipital artery lies in a shallow occipital groove.11 The sulcus of the transverse sinus extends laterally from the protuberance on each cranial side; the tentorium cerebelli attaches to its margins.12 Frequently, the larger sulcus, usually the right one, is continuous with the sulcus of the superior sagittal sinus. The smaller sulcus is usually better related with the straight sinus ( Fig. 33.3 ).11,13

The transverse sinuses are sites of frequent anatomical variations,11,14–21 but they usually communicate along the confluence of the sinuses, which is indicated by a depression on one side of the internal occipital protuberance.11 Each transverse sinus is situated posteriorly over the squamous portion of the occipital bone and anteriorly over the posterior and inferior portion of each parietal bone. This sinus ends at the posterolateral extremity of the petrous portion of the temporal bone from where it extends inferiorly as the sigmoid sinus. Along their course, the transverse sinuses can receive occipital, temporal, cerebellar, and tentorial veins.12,13,22–26

The transition of the transverse sinus into the sigmoid sinus occurs at the point where the former receives the superior petrosal sinus, at the level of the so-called sinodural angle of Citelli. The sigmoid sulcus lies over a deep, curved groove on the inner surface of the mastoid portion of the temporal bone, which is anteriorly separated from the mastoid air cells by a thin lamina of bone. It ends at the jugular fossa, where the sinus enters the jugular foramen.4,27,28

The relationships between the cranial sutures and venous sinuses can be used in surgical planning.29,30 The asterion corresponds to the meeting point of the lambdoid, occipitomastoid, and parietomastoid sutures. This important anthropological point is usually located over the lower aspect of the transverse sinus at its distal margin. The midpoint of the inion-asterion line is particularly related to the bottom of the transverse sinus. This relationship has led to the observation that the inion-asterion line usually corresponds to the inferior aspect of the transverse sinus.29,30

The occipitomastoid suture always crosses the posterior margin of the sigmoid sinus at the level of the superior aspect of the mastoid notch. This crossing point coincides with the intersection of the occipitomastoid suture and an imaginary line between the inion and mastoid tip. The relationships among these external landmarks can be specified relative to the transverse and sigmoid sinuses. In particular, (1) the asterion and the midpoint of the inion-asterion line are related to the inferior half of the transverse sinus; (2) the superior and inferior points of the transverse and sigmoid sinus junction, respectively, are situated above and below the posterior portion of the parietomastoid suture; (3) the intersection of the parietomastoid and squamous sutures is located at the level of the posterior aspect of the superior surface of the petrous bone; and (4) the occipitomastoid suture and the posterior margin of the sigmoid sinus crossing point is situated at the level of the superior and posterior aspect of the mastoid notch. This point corresponds to the intersection of the occipitomastoid suture with the inionmastoid tip line.29,30

These relationships can be used intraoperatively to plan the placement of burr holes during suboccipital and retromastoid exposures. An initial burr hole placed just anterior to the asterion can expose the transition between the transverse and sigmoid sinuses. A second burr hole placed over the occipitomastoid suture posterior to the mastoid process at the most posterior level of the mastoid notch demarcates the posterior margin of the sigmoid sinus.

Retrosigmoid Approach

The retrosigmoid approach is most commonly selected for approaching the structures in the cerebellopontine angle ( Figs. 33.4 and 33.5 ). A detailed understanding of the complex microsurgical anatomy of this area is essential to optimize operative results.3,25,31–33 The retrosigmoid exposure is performed with the patient in the three-quarter prone position with the midsagittal plane roughly parallel to the floor, with the top of the head tilted slightly downward to the floor to reduce the possibility that the shoulder will obstruct access to the operative field ( Fig. 33.5 ). The table is tilted so that the head is slightly above the chest and abdomen. The vertical lateral suboccipital scalp incision for the retrosigmoid approach is situated in the lateral half of the middle third of the distance between the midline and the mastoid process so that it crosses the asterion at the junction of the parietomastoid, occipitomastoid, and lambdoid sutures. The surgeon is seated above the head of the patient for operations in the upper part of the cerebellopontine angle and behind the head for operations in the middle and lower part of the cerebellopontine angle.

The muscle opening is carried inferiorly to just lateral to the foramen magnum, with care taken to avoid injury to the vertebral artery as it courses behind the atlanto-occipital joint and posterior arch of the atlas. A bone flap medial to the sigmoid sinus is elevated, and the opening is enlarged to the lower margin of the transverse sinus and the posterior margin of the sigmoid sinus ( Fig. 33.5 ). The osteotome cut around the margin of the bone flap is made after placing a burr hole medial to the sigmoid sinus and stripping the dura from the inner table. The foramen magnum does not routinely need to be opened to remove a tumor in the cerebellopontine angle. The mastoid air cells entered in the lateral part of the craniectomy are closed with bone wax. The dura is opened with the pedicle medially. The dural cuff bordering the transverse and sigmoid sinuses is tacked up to the muscles and fascia bordering the craniectomy margin with sutures to minimize the need for cerebellar retraction. The cerebellum, which commonly bulges outward upon opening the dura, usually relaxes after opening the arachnoid membrane over the cisterna magna or the superolateral margin of the cerebellum and allowing cerebrospinal fluid (CSF) to escape. Cerebellar resection is infrequently needed, even when removing large tumors.

All of the intradural part of the procedure is conducted with the operating microscope. The surface of the cerebellum facing the posterior surface of the temporal bone is elevated to expose a tumor in the cerebellopontine angle ( Fig. 33.6 ). Self-retaining retraction rather than handheld retraction is used. A wide retractor blade that will cover most of the lateral margin of the cerebellum and does not need to be moved as the operation progresses causes less damage than smaller blades that must be shifted repeatedly. A tapered blade 15 to 25 mm wide at its base and 10 to 15 mm at its tip is commonly used.

The operation for a cerebellopontine angle tumor should be planned so that the tumor surface is allowed to settle away from the neural tissue rather than the neural structures being retracted away from the tumor. No attempt is made to see the whole tumor on initially elevating the cerebellum when removing meningiomas. The surface of the tumor is then opened and biopsied, and the intracapsular contents are removed. As the intracapsular contents are evacuated, the tumor shifts laterally, making it possible to remove more of it through the small exposure. Carefully applied, fine bipolar coagulation is preferred for controlling bleeding and shrinking small deposits of tumor. In the final step, the last thin sheet of tumor capsule is removed from the neural and vascular structures using fine dissecting instruments. The most common reason for the tumor appearing to be tightly adherent to neural structures is not adhesions between the capsule and surrounding tissue; rather, it is residual tumor in the capsule wedging the tumor into position. Only rarely are tumors so densely adherent that they defy easy removal after their intracapsular contents are removed. A remnant of tumor capsule may be left if it is so firmly adherent to vital neural or vascular structures that removing it would damage these structures.

If the pia-arachnoid is adherent to the tumor capsule or a mass of tumor in the capsule prevents collapse of the capsule away from the pia-arachnoid, there is a tendency to apply traction to both the capsule and pia-arachnoid and to tear vessels running on the neural structures. Gentle irrigation directed into the plane between the tumor capsule and brain will often open the appropriate cleavage plane. Under magnification, individual adhesions between vital structures and tumor can be divided with microinstruments. Prior to separating the pia-arachnoid from the capsule, it is important that the entire tumor be removed so that the capsule is so thin that it is almost transparent. If one is uncertain about the margin between the capsule and the pia-arachnoid, several sweeps of a fine dissector through the area while gently irrigating the area will help clarify the appropriate plane for dissection.

Any vessel that stands above or is stretched around the tumor capsule should be dealt with initially as if it were a vessel that runs over the tumor surface to supply the brain. An attempt should be made to displace the vessel off the tumor capsule using a small dissector after the tumor has been removed from within the capsule. When dissected free of the capsule, vessels that initially appear to be adherent to the capsule often prove to be neural vessels. Occlusion of a cerebellar artery is one of the most common causes of morbidity and mortality in removing cerebellopontine angle tumors.32 One of the most dangerous aspects of surgery in the cerebellopontine angle is removing a tumor from an encased vertebral artery. The site of encasement is commonly near the site of passage of the artery through the dura. The cutting loop should be avoided. A remnant of tumor may be left on an artery if removing it might lead to major hemorrhage or occlusion of the artery.

The number of veins sacrificed should be kept to a minimum because of the undesirable consequences of their loss.25 Obliteration of the petrosal veins, which pass from the surface of the cerebellum and brainstem to the superior petrosal sinus, is inescapable in reaching and removing some cerebellopontine angle tumors. Occlusion of these veins, which drain much of the cerebellum and brainstem, may infrequently cause hemorrhagic edema of the cerebellum and brainstem. Some of these veins will need to be sacrificed if the tumor extends into the area above the internal acoustic meatus; however, small tumors located in the middle or lower part of the cerebellopontine angle may be removed without sacrificing a petrosal vein.

Preserving the arachnoidal walls of the cisterns bordering the cerebellopontine angle aids in protecting adjacent neural and vascular structures during the removal of some tumors. Meningiomas commonly arise outside the outer arachnoidal membrane and compress the cisterns without extending directly into them. Some cerebellopontine angle meningiomas can be removed without opening the outer arachnoid membrane because the membrane is displaced around the inner surface of the tumor.

In removing meningiomas, the initial intradural approach is directed toward obliterating the blood supply of the tumor at the point that it comes through the dura ( Fig. 33.6 ).34 In the initial approach, avoid entering the dome of the tumor opposite its vascular supply because this will allow bleeding throughout the operation. Meningiomas in the upper part of the cerebellopontine angle receive their predominate blood supply from the meningohypophysial branch of the intracavernous segment of the internal carotid artery, and those in the lower part of the posterior fossa receive their predominate supply from the branches of the external carotid artery that pass through the hypoglossal canal and jugular foramen. Only the dural base of the tumor is exposed initially, and the bipolar forceps are used to coagulate the base of the tumor and separate it from its blood supply and site of attachment. As the dissection crosses the dural base of a meningioma, the outer circumference is composed of soft tumor that separates easily from the dura. As the dissection and coagulation of the base are carried deeper, the hyperostosis and fibrous attachment are encountered. Finally, on the opposite side of the base, soft tumor is encountered again. It may not be possible to completely cross the base of the tumor until some tumor has been removed from within the capsule near the base.

Meningiomas and other tumors that arise lateral to the nerves in the cerebellopontine angle and displace them medially are easier to remove than those that arise medial to the cranial nerves and displace them laterally ( Figs. 33.6 and 33.7 ). In exposing and removing medially placed tumors in the upper part of the cerebellopontine angle, the approach can be directed through the interval between the lower margin of the tentorium and the trigeminal nerve. Care is needed to protect the trochlear nerve and superior cerebellar artery in this area. Further inferiorly, the medially placed tumor is approached through the interval between the trigeminal nerve above and the facial and vestibulocochlear nerves below. If the tumor has an even lower attachment near the jugular foramen, it can be approached through the interval between the facial and vestibulocochlear nerves above and the glossopharyngeal nerve below or through the interval between the lower rootlets of the vagus nerve and the upper part of the spinal accessory nerve, or between the widely separated accessory rootlets. The intervals between the glossopharyngeal nerve above and vagus nerve below and between the individual vagal rootlets are too small to work through unless they have been widened by the tumor.

Meningiomas may infrequently extend into the internal acoustic meatus, in which case it may be necessary to remove the posterior wall of the internal auditory canal. Greater care is required in opening the meatus with a meningioma than with an acoustic neuroma because the facial nerve is more commonly displaced posteriorly on the back side of the tumor than with an acoustic neuroma, which commonly displaces the nerve around the anterior half of the tumor.

During the craniectomy and removal of the posterior meatal lip or a hyperostosis, the mastoid air cells may be opened and must then be sealed to prevent CSF leakage and meningitis. The air cells that are opened in removing the posterior meatal lip are carefully closed with bone wax. A patch of fat taken from a small transverse incision on the lower abdomen will settle into the depression created by the drilling and is then laid over the drilled meatal margin. The patient′s own dura is carefully closed. If some openings are left in the dura along the suture line, a small patch of suboccipital muscle or fat can be sutured over the opening.

The operation for removal of an acoustic neuroma differs from that for a meningioma because the acoustic neuroma has its firmest attachment to the area around the internal acoustic meatus rather than to the dura on the posterior surface of the petrous bone ( Fig. 33.7 ). After opening the dura to expose an acoustic neuroma, the lateral margin of the tumor is exposed using gentle self-retaining retraction, and the posterior wall of the internal auditory canal is removed using an irrigating drill. No attempt is made to see the whole tumor on initial retraction of the cerebellum. The posterior wall of the internal canal is removed as an initial step if the tumor is small. The intracapsular contents of the tumor are removed before opening the meatus if the tumor is large and deforms the brainstem. Care is required to avoid injury to the anterior inferior cerebellar artery if it is adherent to the dura covering the posterior wall of the internal auditory meatus. After removing the posterior wall of the meatus, the dura that lines the meatus is opened to expose its contents. The facial nerve is identified near the origin of the facial canal at the anterior superior quadrant of the meatus rather than in a more medial location where the direction of displacement is variable. The tumor within the meatus is separated from the posterior surface of the facial and vestibulocochlear nerves. There are several landmarks that are helpful in identifying the facial and vestibulocochlear nerves at the brainstem on the medial side of the tumor. These nerves, although distorted by the tumor, can usually be identified on the brainstem side of the tumor at the lateral end of the pontomedullary sulcus, just rostral to the glossopharyngeal nerve and just anterosuperior to the foramen of Luschka, flocculus, and choroid plexus protruding from the foramen of Luschka. After the facial and vestibulocochlear nerves are identified on the medial and lateral sides of the tumor, the final remnant of the tumor is separated from the intervening segment of the nerves.

Epidermoid tumors in the cerebellopontine angle are easily recognized on computed tomography and magnetic resonance images by their characteristic appearance. They occasionally may be confused with arachnoid cysts or cystic acoustic neuromas. Epidermoid tumors are removed by completing an intracapsular removal and then by excising the portions of the capsule that are not firmly adherent to the brainstem, cranial nerves, and vascular structures. The intracapsular contents are usually soft and easily removed with gentle suction; however, portions of the capsule, although thin, may need to be left in place because they are firmly adherent to the cranial nerves or brainstem. The surface of the adherent capsule should be carefully cleaned of squamous debris, which may cause aseptic meningeal reaction if it mixes with the CSF. Epidermoid tumors are often situated medial to the nerves in the cerebellopontine angle and must be removed by working between the nerves. Subarachnoid cysts in the cerebellopontine angle are treated by removing enough of their wall that they communicate freely with the adjacent subarachnoid cisterns.

Gliomas may grow into the cerebellopontine angle and mimic other tumors there. The senior author has operated on several exophytic gliomas that displaced but did not infiltrate the nerves in the cerebellopontine angle. After completing the intracapsular removal of the exophytic portion of the tumor, it became obvious that the medial part of these tumors infiltrated the side of the pons near the facial and vestibulocochlear nerves. With the use of gentle traction on the remaining tumor and microdissection, these tumors were delivered from the side of the pons, thus achieving gross total removal. Ependymomas may extend into the cerebellopontine angle from their origin along the lateral recess or from the area surrounding the foramen of Luschka. The tumor protrudes into the area behind the glossopharyngeal and vagus nerves and below the facial and vestibulocochlear nerves.

At the time of the craniectomy and the removal of the posterior meatal lip or a hyperostosis, the mastoid air cells may be opened and must then be sealed to prevent CSF leakage and meningitis. The air cells that are opened in removing the posterior meatal lip are closed with bone wax, after which a pledget of fat taken from a small incision on the abdomen is laid over the drill site and onto the dura surrounding the opening. The craniotomy margin is closed by bone wax that is carefully and heavily applied ( Fig. 33.5 ). In most cases, a watertight dural closure is obtained using the patient′s own dura. A dural graft, commonly from cadaveric dura or fascia lata, is infrequently needed. The patient′s pericranium or fascia lata may also be used. Small openings in the dura may be closed with pledgets of muscle or fat taken from the wound margin.

There are several other variants of the retrosigmoid approach in addition to those used for tumor removal ( Figs. 33.8 and 33.9 ).1,35 The superior variant, in which a small cranial opening is centered adjacent to the upper part of the sigmoid sinus, is commonly selected for vascular decompression operations for trigeminal neuralgia. The surgeon is positioned at the head of the patient for this type of retrosigmoid approach ( Fig. 33.8 ). Another variant, in which a small opening is centered behind the lower part of the sigmoid sinus, is used to approach lesions in the lower part of the cerebellopontine angle or for a decompression operation for hemifacial spasm. For operations in the mid and lower part of the cerebellopontine angle, the surgeon is seated behind the head of the patient ( Fig. 33.7 ).