Endoscopic Lateral Skull Base Surgery

1 Anatomy of the Lateral Skull Base

Mustafa Kapadia, Livio Presutti, Alejandro Rivas, and Marchioni Daniele

Abstract

The lateral skull base is a very complex anatomical region separating the brain from the ear and the upper neck. It is composed by the temporal, sphenoid, and occipital bones, and contains vital neurovascular structures. An advanced understanding of its threedimensional architecture is therefore mandatory for the surgeon approaching this area. In particular, a detailed knowledge of the anatomy of the temporal bone is the cornerstone to correctly perform lateral skull base surgery. In fact, it occupies a central position in the lateral skull base and contains several noble structures having a winding course such as the internal carotid artery, the sigmoid sinus with the internal jugular vein, the internal auditory canal with the acoustic-facial bundle, and the facial nerve. In the same way, the knowledge of the anatomical entities located in close relationship with the temporal bone plays a key role: (1) the jugular foramen, anterolaterally bounded by the petrous temporal bone and posteromedially by the basioccipital bone, transmitting the sigmoid sinus, the jugular bulb, the inferior petrosal sinus, the lower cranial nerves (IX, X, XI) with their ganglia, and the meningeal branches of the occipital artery and the ascending pharyngeal artery; (2) the infratemporal fossa, a complex three-dimensional nonfascial bound space located inferomedial to the zygomatic arch and the ramus of the mandible, acting as a conduit for the neurovascular structure entering and leaving the skull base; and (3) the cerebellopontine angle, the anatomic space between the petrous bone and the petrosal cerebellar surface folding around the pons and the middle cerebellar peduncle, containing the posterior cranial fossa nerves.

Keywords: lateral skull base anatomy, temporal bone, jugular foramen, infratemporal fossa, cerebellopontine angle, petrous apex

1.1 Introduction

The skull base forms the floor of the cranial cavity and separates the brain from the ear, the paranasal sinuses, and the upper neck. This anatomic region is complex and poses surgical challenges for otolaryngologists and neurosurgeons alike. In the skull base, there are numerous foramina that transmit cranial nerves, blood vessels, and other structures. The skull base foramina are openings located in the inferior portion of the cranium. They allow for the passage of several vascular and nervous structures. From an inferior view, there are 10 conventionally described skull base foramina: the greater palatine, lesser palatine, lacerum, ovale, spinosum, external opening of the carotid canal, stylomastoid, jugular, mastoid, and the external opening of the hypoglossal canal (see ? Fig. 1.1, ? Fig. 1.2, ? Fig. 1.3, ? Fig. 1.4). Working knowledge of the anatomy of the skull base is essential for effective surgical treatment of diseases in this area.

Fig. 1.1 Skull base foramina: exocranial and endocranial surfaces.

Fig. 1.2 Left exocranial surfaces of the skull base: the skull foramina are shown.

Fig. 1.3 Cranial nerves and vascular structures in the left exocranial surfaces of the skull base. ica: internal carotid artery; ijv: internal jugular vein; mma: middle meningeal artery.

Fig. 1.4 Cranial nerves and vascular structures in the endocranial surfaces of the skull base. afb: acoustic-facial bundle; ica(h): horizontal portion of internal carotid artery; ica: internal carotid artery; ips: inferior petrosal sinus; jb: jugular bulb; mma: middle meningeal artery; sps: superior petrosal sinus.

The five bones that make up the skull base are the ethmoid, sphenoid, occipital, frontal, and temporal bones. The skull base can be subdivided into three regions: the anterior, middle, and posterior cranial fossae (see ? Fig. 1.1).

The anterior cranial fossa is formed by the anterior and cribriform plate of the ethmoid bone, the lesser wings of the sphenoid, and the jugum sphenoidale. The middle cranial fossa is composed of the body and the greater wing of the sphenoid, the anterior surface of the temporal pyramid, and parts of the temporal squama. The posterior cranial fossa is bordered by the clivus, the pyramid of the temporal bone, and the occipital bone.

Irish and coworkers in 1994 reviewed 77 skull base malignancies from a clinical point of view. From this work, they developed a classification system of three regions based upon anatomic boundaries and tumor growth patterns. Region I is composed of the anterior cranial fossa. Tumors of this region are commonly resected via an anterior approach. Region II includes the infratemporal and pterygopalatine fossae, with a possible tumor extension into the middle cranial fossa. Region III involves the temporal bone with a possible tumor extension into the posterior or middle cranial fossa. From a clinical point of view, the "lateral skull base" is defined as the anatomical compartments resulting from the combination of Regions II and III. Anatomically, Region II extends from the posterior wall of the orbit to the petrous temporal bone and it is formed by the infratemporal and pterygopalatine fossae and the overlying part of the middle cranial fossa. In this region, there are several important neurovascular structures which include the internal carotid artery (ICA), the facial nerve, the vestibulocochlear nerve, and the maxillary (V2) and the mandibular (V3) divisions of the trigeminal nerve. Region III is located mainly in the posterior cranial fossa and also includes the posterior segment of the middle cranial fossa. Vital structures located in this region include the internal jugular vein and the vagus, the glossopharyngeal, the spinal accessory, and the hypoglossal nerves.

The lateral skull base has very noble and complex anatomical structures (see ? Fig. 1.5 and ? Fig. 1.6). Lateral skull base surgery demands an advanced anatomical knowledge of the temporal bone and a three-dimensional animated perception of the related surrounding structures. The surgical procedures are technically challenging because the pathological site is concealed deep within, which requires extensive bone drilling and tissue retraction, and because vital neurovascular structures are located in a relatively small area.

Fig. 1.5 Skull base bones. cc: carotid canal; eac: external auditory canal; fla: foramen lacerum; fo: foramen ovale ; fs: foramen spinosum; hyp: hypoglossal canal; jf: jugular foramen; petrous a: petrous apex; sty: styloid process.

Fig. 1.6 Endocranial view of skull base bones.

1.2 The Temporal Bone

The temporal bone occupies the central position in the lateral skull base and is anteriorly bounded by the zygomatic bone, and the greater wing and pterygoid plate of the sphenoid bone, superiorly bythe parietal bone, posteriorly and posteromedially by the occipital bone, and medially by the clivus (see ? Fig. 1.7). There are various vital structures related to the lateral skull base like the acoustic-facial bundle, trigeminal nerve, cochlea, semicircular canals, ICA, sigmoid sinus, internal jugular vein, lower cranial nerves, and brain parenchyma. Most of the times diseases in this area are benign, so we have to protect and preserve most of the vital structures.

Fig. 1.7 Temporal bone in the skull. Lateral view. Eac: external auditory canal.

The temporal bone has five different parts: the squamous, mastoid, tympanic, petrous, and styloid process (see ? Fig. 1.8). These parts are arranged the around external auditory canal and the tympanic cavity so that the tympanic part is directed downwards, the squamous part upwards and forwards, the mastoid part backwards, and the petrous part directed medially and inwards. The petrous temporal bone is a three-cornered pyramid with the base directed laterally and its long axis directed anteriorly and medially forming an angle of about 45 degrees with the median plane of the skull. The petrous apex is rough and uneven, having an anterior opening for the carotid canal, and it forms the posterolateral boundary of the foramen lacerum along with the greater wing of the sphenoid and the basioccipital bone. The labyrinth and the internal auditory canal (IAC) are located within the petrous temporal part. The petrous part of the ICA enters temporal bone through the carotid canal situated on its inferior surface. The other important structure related to the temporal bone is the jugular foramen (JF), which is located in the petro-occipital region.

Fig. 1.8 Temporal bone: (a) posterior fossa surface of the temporal bone; (b) base view of the temporal bone.

Fig. 1.9 Left ear. Drawing showing the acoustic-facial nerves from behind inside the internal auditory canal (IAC). The eighth nerve, upon passing into the IAC under the facial nerve, divides into two branches: the cochlear nerve and the vestibular nerve. The cochlear nerve runs into the fundus of the IAC, attaching to the foraminous tract of the fundus, forming the tractus spiralis foraminosus. The vestibular nerve divides into two branches: the upper branch which divides into the utricular nerve (which attaches on the elliptical recess) and the superior ampullary nerve (for the superior membranous ampulla), and the lower branch which divides into the saccular nerve (which attaches on the spherical recess) and the posterior ampullary nerve, which passes into the singular foramen. ivn: inferior vestibular nerve; lsc: lateral semicircular canal; psc:...