Several layers of musculature must be traversed during the exposure of the occiput and cervical vertebrae. The occipitofrontalis muscle is found superior to the line of attachment at the superior nuchal line than the trapezius, and sternocleidomastoid muscles are disconnected from the superior nuchal line. The sternocleidomastoid muscle is attached also to the posterolateral mastoid process. The deeper muscular layer includes the semispinalis capitis, splenius capitis, and longissimus capitis muscles. The first two muscles attach either on or below the superior nuchal line, while the longissimus capitis muscle, located most laterally of the three muscles, attaches to the mastoid process. The obliquus capitis superior, obliquus capitis inferior, rectus capitis posterior minor, and rectus capitis posterior major muscles form the deepest layer of musculature closest to the cervical vertebrae. To gain access to the occipitocervical junction and vertebral artery, their attachments to the occipital bone, to the spinous processes of C1 and C2, and to the transverse process of C1 must be resected.

The suboccipital triangle is a region bounded 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. The floor of this space is occupied by the posterior atlantooccipital membrane and the posterior arch of the atlas and within the triangle run both the extradural terminal segment of the vertebral artery and the first cervical nerve [8–10].

On the posterior side of the external carotid artery, at the level of the posterior belly of the digastric muscle, the occipital artery originates. It runs along the muscle to the mastoid where it passes under the longissimus and splenius capitis muscles forming two terminal branches. The lateral branch carries on vertically upwards in the subcutaneous layer. The medial branch continues horizontally as far as the union before turning through a right angle to carry on vertically across the trapezius muscle, ending in the subcutaneous tissue. A paramedian skin flap hinged at the bottom to expose the posterior fossa can become necrotic if the stalk is too narrow.

The anatomy of the vertebral artery, especially the V3 and V4 segments, must be kept in mind when approaching lateral posterior fossa and the craniovertebral junction.

The V3 segment, also known as the “suboccipital segment,” extends from the C2 transverse process to the dura mater of the foramen magnum. Its course can be subdivided in three parts: a vertical segment, between the transverse processes of C2 and C1; a horizontal segment in the groove of the posterior arch of the atlas; and an oblique segment up to the dura mater [11]. The V3 segment is enclosed by a periosteal sheath that invaginates into the lateral dura at the level of the foramen magnum, thus forming a double furrow for 3–4 mm: indeed, the periosteal sheath is in continuity with the outer layer of the dura. The VA adventitia, adherent to the double furrow, forms sort of distal fibrous ring, so that, despite being the most mobile part, the V3 segment is fixed at its ends. Anatomical relations of the VA are modified by head movements of rotation, as well as during surgical positioning.

The V4 segment runs from the dura up to the anterior side of the pontomedullary sulcus where it joins the contralateral one to form the basilar artery. Initially, the V4 segment faces posteriorly and medially the occipital condyle, the hypoglossal canal, and the jugular tubercle. Later, the V4 segment lies on the clivus and runs in front of the hypoglossal and the lower cranial nerves rootlets. From the V4 segment originate several important branches: the PICA, the anterior spinal artery, and the anterior and posterior meningeal arteries. The PICA origin can be variable: at, above, or below the FM level [12].

Despite the wide use of modern imaging guidance technology, the knowledge of superficial landmarks and their relationship to deeper intracranial structures are crucial in surgical planning of a lateral approach, and it is one of the keys to access the posterior fossa. The transverse sinus, the transverse-sigmoid junction, and the sigmoid sinus are deeply placed within a large bony groove which makes the first burr hole quite risky if wrongly planned. The precise placement of burr holes has several advantages allowing an accurate and small craniectomy laterally enough to decrease the risk of significant cerebellar retraction during approaches to the cerebellopontine angle. The asterion, a horizontal line from the superior aspect of the zygomatic arch (“zygomatic line”) and a vertical line from the mastoid notch (“mastoid line”), was thought to be the most important surface landmarks to localize the transverse and sigmoid junction [13].

The glossopharyngeal (CN IX), vagus (CN X), and spinal accessory (CN XI) nerves start as rootlets from the postolivary sulcus and go into the jugular foramen, passing ventral to the choroid plexus protruding from the foramen of Luschka and dorsal to the vertebral artery.

The accessory nerve is composed of rootlets originating from both the spinal cord and the medulla. Spinal rootlets join together to form the main trunk, which ascends through the foramen magnum running behind the dentate ligament and unites with the upper medullary rootlets.

The hypoglossal nerve (CN XII) arises from rootlets of the preolivary sulcus. It runs anterolateral through the subarachnoid space and pass behind the vertebral artery to reach the hypoglossal canal. Rarely, the VA separates the CN XII rootlets [14, 15].

The greater occipital nerve is formed by the medial branch of the dorsal ramus of C2 that runs between the posterior arch of the atlas and lamina of the axis. The greater occipital nerve ascends between the inferior oblique and the semispinalis capitis muscles. It pierces the semispinalis capitis and the trapezius between the superior and inferior nuchal lines (2 cm below and lateral to the external occipital protuberance [16]. This nerve supplies the skin of the back of the scalp as far forward as the vertex of the skull. Obviously, transverse incision in this region could damage the nerve.

Preoperative clinical evaluation of the patient with lesions in the lateral part of the posterior fossa or close to the craniocervical junction is crucial. The clinical examination, as well as the radiologic information, will often help to decide from which direction to approach the ventral midline lesion in these lateral approaches (usually on the side with the pre-existing neurologic deficits). A careful preservation of the contralateral lower cranial nerves during tumor removal prevents devastating functional deficits with the airway speech, and oromotor functioning, often related to bilateral lower cranial nerve damage.

Valuable information can be gained from neuroimaging. CT allows assessment of the bony components of the craniocervical junction, mostly the occipital condylar-atlas-axis joint interfaces, and a view through the bony aperture of the foramen magnum. MRI indicates the extent of the lesion in relation to the brainstem and the upper spinal cord within the posterior fossa. MRI angiography (MRA) helps to study vertebral artery anatomy and patency. If the surgeon is forced or plans to sacrifice one of the vertebral arteries, conventional angiography can provide more detailed anatomic information, as well as the functional status of the contralateral vertebral artery by a balloon occlusion test.

Neurophysiological monitoring is recommended during lateral suboccipital exposures. It includes somatosensory-evoked potentials, auditory-evoked responses, the monitoring of the facial nerve, and often cranial nerves IX through XII. Motor-evoked potentials are monitored when intraparenchymal tumors are resected.

The lateral suboccipital approaches have been usually performed with the patients in a lateral, park-bench, or sitting position. Even if the sitting position is ideal, most anesthetists reject its routine use for the lateral suboccipital approach because of a higher risk of air embolism. In the lateral or park-bench position, the shoulder of the pediatric patient, especially children whose neck is short, sometimes restricts both the surgeon’s operative maneuvers and the direction of view of the microscope. Care must be taken to minimize tension on the cervical spine and brachial plexus with use of shoulder or axillary rolls where applicable.

Possible indications for unilateral paramedian approach include hemispheric tumors of the cerebellum, angiomas, hemorrhages, space-occupying infarcted areas of the cerebellar hemisphere, malformations, and inflammatory processes.

The patient can be positioned both in the sitting and in the prone position. Use of the sitting position for surgery requires use of precordial ultrasonography and a central venous access at the level of the superior vena cava/right atrium for detection and treatment of air embolism. In both cases, the head is fixed in a three-pin head holder and carefully rotated about 30° toward the side of the lesion. Either an S-shaped or a straight skin incision may be used.

The greater occipital nerve and the accompanying artery and vein sometimes cannot be spared. As far as possible, the musculature is entered along the natural borders of adjoining muscles while the deeper muscles are divided by monopolar coagulation.

The first burr hole is planned in the occipital squama and the bone defect is extended to the desired size osteoclastically by forceps or by the craniotome forming a paramedian flap. The dura over the cerebellar hemisphere can be carefully opened in a cruciate fashion. At this point in case of intracerebellar lesions, neuronavigation or ultrasound can be very helpful in localizing the lesion. Sometimes it can be useful to insert a ventricular catheter under ultrasound guidance until the lesion (e.g., an hemorrhage) and then to follow the catheter into the cerebellar parenchyma until the lesion.

The dura is closed with interrupted sutures. Elevation sutures of the dura are placed prior to closure of the dura and are tied after the closure. Covering of the bone gap created by the osteoclastic craniotomy is not necessary. The wound is closed in layers.

Common errors during this approach can be overlooked: blood loss during the operation from skin, muscle, and/or bone especially in small children; lesions of the dura or cerebellar cortex; lost of orientation inside the cerebellar hemisphere; and postoperative bleeding from the cerebellar surface, from bridging veins, or due to inadequate dural elevation sutures.

The retrosigmoid approach to the cerebellopontine angle is still an evolution of the lateral suboccipital craniectomy described by Dandy in 1929 and in 1934 [5, 6], which was later made through a straight lateral incision as proposed by Adson in 1941 [3] and then extended laterally and inferiorly as was already emphasized by Bucy in 1951 [4].

The retrosigmoid approach allows access to posterolateral pons, lateral middle cerebellar peduncle, superior lateral medulla, and cerebellopontine angle.

Typical indications for retrosigmoid approach include space-occupying processes in the cerebellopontine angle, arteriovenous malformations, aneurysms, compression syndromes of cranial nerves, inflammatory processes, and tumors [17].

Cerebellopontine angle can be described as a pyramid-shaped space but inclined anteromedially with its apex extending to the posterior clinoid process and the base facing the inner surface of the squamosal part of the temporal and occipital bones. The superior side corresponds to the retrosellar area including the III cranial nerve and the inferior surface of the tentorium. The inferomedial side is close to the foramen magnum, including the vertebral artery and XII cranial nerve. The anterior side is represented by the posterior surface of the petrous bone, while the posterior side is limited by the petrosal surface of the brainstem and cerebellum. The medial third of the pyramid has the highest density of vital structures as the SCA, AICA, and PICA and the third to twelfth of cranial nerves.

Different variations of the shape, placement, and size of the retrosigmoid craniotomy can be planned to reach different target structures in the area of cerebellopontine angle.

The intradural compartments of the cerebellopontine angle are divided into (1) an inferior petroclival space with the medulla and the foramen magnum, (2) a middle space, and (3) a superior petroclival space.

These three compartments contain different neurovascular complexes: the upper around the trigeminal nerve, the middle around the vestibulocochlear-facial complex, and the lower around the lower cranial nerves [18, 19].

For a retrosigmoid approach the patient is placed in the horizontal position, either in the so-called park-bench (three quarters prone) position or in the supine position with the head turned away by about 30–40° from the surgeon. Each position offers different advantages and drawbacks.

We perform this approach with the patient fixed in the prone position with the neck rotated 25–30° to the side of the lesion and with the operating table rotated approximately 20° in the same direction. The surgeon is in a sitting position beside the contralateral shoulder of the patient so he can easily access the operative filed, because the patient’s shoulder contralateral to the lesion is shifted downward. In this way, the height of the patient’s shoulder ipsilateral to the lesion is much lower than that in the lateral position, and it is also advantageous for a far-lateral approach.

The auricle is retracted with adhesive tape (or a suture). A slightly S-shaped skin incision is preferred. After soft tissue dissection, the key anatomical landmarks of the lateral temporo-occipital bones are precisely defined: the zygomatic arch, external auditory meatus, suprameatal crest, mastoid process and incisura, asterion, and external occipital protuberance. The asterion, defined as the junction of the lamboid, the occipitomastoid, and the parietomastoid sutures, has be proven a reliable marker in order to localize the junction between the transverse and sigmoid sinuses. The superior nuchal line is also identified by drawing a line from the root of the zygoma to the inion and the transverse-sigmoid junction is avoided by planning the burr holes below this line (1 or 2 cm below the asterion).

A craniotomy measuring approximately 3–5 cm in diameter is completed, bordering anteriorly the sigmoid sinus and superiorly the transverse sinus. Venous hemorrhages into the bone are controlled with bone wax, and emissaries are likewise sealed. If retrosigmoid mastoid air cells are entered, they must be exenterated of mucosa and then packed with muscle and iodinated bone wax.

The dura is subsequently widely opened in a curved fashion following the borders of the craniotomy, and the cerebellopontine angle is brought into view. Bridging veins can be coagulated and divided, to avoid they are not torn during placement of retractors and dissection. The cerebellum normally tends to fall away from the tumor once the subarachnoid space is opened and so it needs only to be very lightly retracted (Fig. 9.8). At this point, all the neurovascular structures in the cerebellopontine angle are identified, and if available, neuromonitoring is used to look for cranial nerves. The facial nerve can be identified at or near the brainstem, and it is important to limit electrocautery as much as possible, depending on gentle blunt dissection to dissect blood vessels away from the lesion and the eighth cranial nerve, while using Gelfoam to stop light venous oozing. If the tumor is bulky, it may be decompressed with the use of an ultrasonic aspirator at this point.

At the end of the procedure, the intradural space is filled with Ringer solution at body temperature. The dura is closed with interrupted or continuous sutures in a watertight fashion, if necessary with the aid of a dural patch graft. The craniotomy and overlying soft tissues are then closed in standard fashion.

Common pitfalls can be wrong patient positioning (compression of the main cervical vessels or severe venous congestion in the posterior fossa), inaccurate placement of the burr holes (injury to venous sinuses), inadequate removal of CSF (severe cerebellar contusion due to spatula pressure), injury to sensitive vessels, and nerves in the cerebellopontine angle due to microsurgical dissection.