Neuroanatomy for the Head and Neck Surgeon

Neuroanatomy for the head and neck surgeon

Property of Taylor & Francis Group - Not for redistribution

PETER C. WHITFIELD

Overview and embryology 279 Optic nerve and chiasm 284 The meninges 280 Internal carotid artery 284 Topography of the brain 280 Cavernous sinus 284 Blood supply 280 Facial nerve 284 Surgical hazards 283 Lower cranial nerves, the foramen magnum Venous sinuses, meningeal vessels and and clivus 285 bridging veins 283 Summary 285 Olfactory tract 284 Further reading 285

OVERVIEW AND EMBRYOLOGY The sagittal, coronal and lambdoid sutures are of major importance, with premature fusion leading An estimated 100 billion neurones are organized to craniofacial deformity. The skull base is formed into the human brain. The 1400 g of tissue com- by endochondral ossification of cartilage leading prising the brain is arranged in a highly complex to formation of the skull base components of the − series of neural networks receiving 750mL/min 1 ethmoid, sphenoid, petrous and occipital bones. of blood. The brain develops from the cephalic The bones of the face are mainly formed from car- end of the neural tube. Three primary vesicles tilages of the first two pharyngeal arches with the form giving rise to the forebrain, midbrain and associated musculature supplied by the mandibu- hindbrain. These give rise to the cerebral hemi- lar and facial nerves, respectively. spheres, thalamus, hypothalamus, midbrain, Common disease processes include neoplas- pons, medulla and cerebellum. The brain com- tic, vascular and traumatic conditions. Surgical municates with the head and neck structures via access to the brain is achieved directly through the cranial nerves; most of these arise from the the calvarium, or via the skull base or a combi- brainstem. Connections with the limbs and trunk nation of approaches. No part of the skull, brain are via the afferent and efferent spinal cord path- or surrounding tissues is immune to pathological ways. The calvarial part of the skull, or skull vault, processes, with many conditions requiring col- is formed by membranous ossification of mes- laborative surgical teams encompassing neuro- enchyme that invests the embryonic brain. The surgery, maxillofacial surgery, otolaryngology and bones of the newborn skull are joined by sutures. plastic surgery.

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THE MENINGES

The brain is invested in a layer of pia mater that is adherent to the gyri and sulci. The subarachnoid space lies between the pial layer and the arachnoid. The arachnoid layer is a transparent membrane that envelops the brain. Cerebrospinal fluid (CSF) cisterns are arachnoidal sacs of CSF that are found in close proximity to the cranial nerves at the skull base. They lie between the arachnoid membrane and the pial surface. Microsurgical opening of the CSF cisterns facilitates drainage of CSF and visual- ization of the vessels and nerves in the area under consideration. TheseProperty cisterns are evident in life but are not well visualized in cadaveric specimens. The dura mater is a thick, fibrous membrane that Lateral view of the right cerebral lines the inner surface of the skull. Folds of dura Figure 28.1 hemisphere. From this view note the Sylvian form structurally important partitionsof within the Taylor (frontotemporal) fissure lying between the frontal cranial cavity. The falx cerebri is a sickle-shaped and temporal lobes. Note the cerebellar hemi- dural membrane located in the midline sagit- sphere inferiorly. (Supplied by Dr D. Hilton.) tal plane, separating the cerebral hemispheres.& Anteriorly, it is attached to the crista galli. It Francisarches over the corpus callosum to a posterior attachment the expressive component of speech. Temporal lobe at the internal occipital protuberance. At this level it functions include memory and auditory processing forms the tentorium cerebelli, dividing the cranial Groupincluding the receptive component of speech. The cavity into the supra and infratentorial compart- parietal lobe is involved in the perception of touch ments. The cerebellum lies beneath the tentorium and the integration of sensory information, and - cerebelli. The superior sagittal venous sinus lies the occipitalNot lobe is responsible for visual process- within a dural enclosed fold along the superior ing. Deeper structures include the basal ganglia, aspect of the falx cerebri. The inferior sagittal sinus thalamus forand hypothalamus. The basal ganglia lies in the free inferior margin of the falx cerebri. contribute to movementredistribution control. The thalamus is a key relay station in sensory and circuits and contrib- TOPOGRAPHY OF THE BRAIN utes to movement control. The nearby hypothalamus is a centre for autonomic nuclei that subserve The brain comprises the cerebral hemispheres which sympathetic and parasympathetic functions and is lie in the supratentorial compartment (Figures 28.1 directly connected to the posterior lobe of the pitu- and 28.2). They are connected by the commissural itary gland. The adjacent anterior pituitary is cru- fibres of the corpus callosum. The cerebellum and cial in orchestrating the body’s hormonal milieu, brainstem (midbrain, pons, medulla) are located in secreting growth hormone, thyroid stimulating the infratentorial compartment. The midbrain lies hormone, adrenocorticotrophic hormone, gonado- at the level of the tentorial hiatus acting as a conduit trophic hormones and prolactin. The cerebellum is for information between the hemispheres and the concerned with the control of posture and muscle brainstem. Most of the cranial nerves originate from coordination. the brainstem. The hemispheres are subdivided into the frontal, temporal, parietal and occipital lobes. BLOOD SUPPLY The different lobes undertake important functions; language functions are usually lateralized to The arterial blood to the brain is supplied by the dominant hemisphere. Frontal lobe functions bilateral internal carotid and bilateral vertebral include motor, personality, executive functions and arteries. These supply the anterior and posterior

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Property Figure 28.2 Topography of the brain – the cerebellum and brainstem. The cerebellum has been divided in the sagittal planeof and splayed open like the pages of a book. ThisTaylor demonstrates the arboreal architecture of the cerebellum. Figure 28.3 Circle of Willis. This is a 3D comput- Note the cerebellar tonsil projecting inferiorly. erized tomography angiogram. In the posterior This impacts against the craniocervical junction& circulation, the vertebral arteries unite forming if ‘coning’ occurs. The lower panel illustratesFrancis the basilar artery. Anteriorly, the bilateral internal axial sections of the midbrain, pons and medulla carotid arteries bifurcate into the medially pro- (from left to right). In the midbrain note the jecting anterior cerebral arteries and the laterally projecting middle cerebral arteries. The distal small diameter of the Aqueduct of Sylvius, the Group pigmented substantia nigra and the cerebral segments of the anterior cerebral arteries travel peduncles projecting anteriorly. The pons is char- in the interhemispheric plane in close apposition acterized by the prominent transverse pontine to each- other. In this case the posterior commu- fibres – these are fibres connecting the motor nicatingNot arteries are not well visualized. (Supplied cortex to the cerebellum allowing the cerebellum by Dr W. Mukonoweshuro.) to modulate coordination. The medulla contains for several cranial nerve nuclei subserving autonomic redistribution functions. Compression of these during coning branch that often supplies the descending motor ultimately causes brainstem death. (Supplied by fibres located within the confines of the internal Dr D. Hilton.) capsule. The internal carotid artery then bifurcates into the medially directed anterior cerebral artery circulations, respectively. The posterior commu- and the laterally directed middle cerebral artery. nicating arteries arise from the posterior aspect of The anterior cerebral artery crosses the optic nerve the intracranial component of the internal carotid and then abruptly turns in an anterosuperior arteries and join the posterior cerebral arteries direction to supply the frontomedial aspect of the providing an anastomotic link (of variable degree) cerebral hemispheres via the frontobasal, perical- between the anterior and posterior circulations. losal and callosomarginal terminal branches. At This was eloquently described as the Circle of Willis the point of inflection, the anterior communicat- in the seventeenth century (Figure 28.3). On entry ing artery, a short (2 mm) branch, provides a direct into the skull, the internal carotid artery (Figure communication between the right and left anterior 28.4) traverses the cavernous sinus. The ophthal- cerebral vessels. This is a key component of the mic artery is a small branch that arises at the point Circle of Willis. The middle cerebral artery trav- of emergence into the subarachnoid space. The els in the frontotemporal (Sylvian) fissure. After next branch is the posterior communicating artery a few centimetres it bifurcates into end arteries followed by the anterior choroidal artery: a small supplying the bulk of the cerebral hemispheres.

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Perforating branches arise from both the anterior and middle cerebral vessels supplying the deep structures including the basal ganglia, thalamus, hypothalamus, internal capsule and insula. The vertebral arteries enter the dura at the level of the foramen magnum (Figures 28.4 and 28.5). The first significant branch is the posterior inferior cerebellar artery which follows a tortuous course, supplying the lateral brainstem and the posterolateral aspect of the cerebellar hemispheres. The vertebral arteries unite on the ventral surface of the brainstem forming the basilar artery. This projects supe- riorly on the ventral surface of the pons, giving rise to the paired anteriorProperty inferior cerebellar arteries, labyrinthine arteries, superior cerebellar arteries

of Taylor Figure 28.5 Oblique view digital subtraction angiogram of the left vertebral artery. In this & case conventional posterior circulation anatomy Francis is evident. The basilar artery can be traced to the basilar bifurcation where the posterior cerebral arteries are formed. The large branches just below the posterior cerebral vessels are superior Groupcerebellar arteries and supply much of the upper part of the cerebellum and upper brainstem. (Supplied- by Dr W. Mukonoweshuro.) Not

and multiplefor pontine perforating branches. At the level of the midbrainredistribution the basilar artery bifurcates, forming the paired posterior cerebral arteries. These course around the midbrain, pass above the tentorium cerebelli and supply the medial aspect of the occipital lobes. They receive important anastomotic connections with the anterior circulation via the posterior communicating arteries. Figure 28.4 Internal carotid artery. This is a 3D The venous drainage of the brain is via super- angiogram of the left internal carotid artery. ficial and deep venous drainage networks (Figure The tortuous course of the cervical, petrous and 28.6). Deep structures (e.g. thalamus) drain via cavernous segments is evident. The first branch the paired basal and internal cerebral veins. These of the supraclinoid segment is the ophthalmic then unite with the superior cerebellar vein from artery: a small vessel in this case. The posterior the upper brainstem and the inferior sagittal communicating artery is then seen to project sinus, forming the Great Cerebral Vein (of Galen) posteriorly. In this case it is a large vessel and that drains into the straight sinus. The straight appears to supply the occipital lobes via the posterior cerebral vessels. This arrangement is an sinus lies in the midline ‘ridge’ of the tentorium embryological variant called a ‘fetal type’ circula- cerebelli. At the internal occipital protuberance, tion and is present in approximately 20 per cent deep venous blood is usually directed into the of cases. (Supplied by Dr W. Mukonoweshuro.) left transverse sinus and thence the sigmoid sinus

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before departing the cranium in the left internal jugular vein. The superficial components of the hemispheres drain into cerebral veins. The middle cerebral vein (or superior Sylvian vein) is located superficially in the Sylvian fissure and enters the cavernous sinus, which then drains posteriorly to the basilar-petrosal sinus confluence and jugular veins. Other superficial cerebral veins pass directly to the superior sagittal sinus and to the transverse/ sigmoid junction. The larger of these have epony- mous names despite the considerable variability in clinical practice: the inferior anastomotic vein of Labbé drains in an postero-inferior direction from the Sylvian fissureProperty to the transverse sinus; the superficial anastomotic vein of Trolard passes superomedially from the Sylvian fissure to the superior sagittal sinus at the level of the precentral (motor) gyrus; the Rolandic veinof drains the super- Taylor ficial cortex to the superior sagittal sinus anterior Figure 28.6 Venous drainage of the brain. This to the vein of Trolard. Venous blood in the superior is an anteroposterior view of the venous phase of sagittal sinus travels posteriorly to the confluence& a cerebral angiogram. Cortical veins can be seen of sinuses at the internal occipital protuberance.Francis It connecting to the midline superior sagittal sinus. then usually enters the right transverse and thence Blood flows posteriorly towards the confluence of sinuses. In this case most of the blood drains the sigmoid sinus on its descent to the internal from the confluence via the right transverse jugular vein. Groupand sigmoid sinuses to the internal jugular vein. (Supplied by Dr W. Mukonoweshuro.) - SURGICAL HAZARDS Not bones meet. This lies just posterosuperior to the Surgical approaches to the brain can be under- mastoid processfor and is located at the same level taken from almost every trajectory. Even though as the zygomaticredistribution arch, an easily palpable surgical intraoperative navigation can facilitate identifica- landmark. tion of anatomical landmarks, knowledge of key The middle meningeal artery arises as a branch structures is necessary to minimize trauma and of the maxillary artery. It traverses the foramen enhance operative safety. Some of the major ana- spinosum, just posterolateral to the mandibular tomical hazards are discussed below. nerve within the larger foramen ovale. The vessel supplies the dura via several branches and can Venous sinuses, meningeal vessels cause bleeding during a pterional craniotomy; and bridging veins it is safely sacrificed during a surgical approach where necessary. Anterior ethmoidal arteries sup- A craniotomy can be safely elevated directly over ply much of the anterior fossa dura. These can be a a venous sinus, although if this can be avoided source of profuse bleeding during surgery for inva- the risk of serious haemorrhage is minimized. sive tumours of this region. The superior sagittal sinus lies in the midline as Once the intracranial compartment is opened, described above. The posterior extent is marked veins may be encountered traversing from the brain externally by the external occipital protuberance. to the dural venous sinuses. These veins include The transverse sinus runs from the posterior occip- bridging veins from the cerebral hemispheres to ital protuberance to the asterion, a junction of the superior sagittal sinus and polar veins from sutures where the temporal, parietal and occipital the tip of the temporal lobe to the spheno-parietal

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sinus. These can be divided to prevent inadvertent Cavernous sinus haemorrhage, though preservation is preferable in the region of the motor cortex. The cavernous sinus comprises a series of inter- linked venous channels and is located immediately lateral to the pituitary fossa. Surgical access Olfactory tract to this area is achieved via trans-sphenoidal, pterional (greater wing of sphenoid) and orbito- The olfactory tract lies on the undersurface of the zygomatic approaches. The cavernous segment frontal lobe. It commences at the olfactory bulb of the internal carotid is located within the sinus immediately superior to the cribriform plate of and usually lies several millimetres lateral to the the ethmoid bone and projects posteriorly to the midline. However, an abnormally tortuous ves- proximal Sylvian fissure. The olfactory nerves are sel can approach the midline and impede trans- vulnerable to traumatic brain injury and is also at sphenoidal surgical access to the pituitary fossa. risk in anterior skull base surgery, particularly as a The lateral wall of the cavernous sinus contains the result of frontal lobe Propertyretraction during a bifrontal oculomotor (III) and trochlear (IV) nerves, and craniotomy or a pterional approach. the ophthalmic (V1) and maxillary divisions (V2) of the trigeminal nerve. The abducent nerve takes a Optic nerve and chiasmof more medial route, traversing the cavernous sinus Taylor in a posterior-anterior direction. The oculomotor, The optic nerve provides a well-recognized land- trochlear, abducent and ophthalmic nerves enter mark during intracranial surgery that facilitates & the orbit via the superior orbital fissure whilst the identification of the internal carotid artery.Francis The maxillary nerve traverses the foramen rotundum. optic nerve exits the orbit via the optic canal and emerges into the parasellar region travelling in a Facial nerve posteromedial direction towards the optic chiasm. Group The supraclinoidal segment of the internal carotid The facial nerve has a complex anatomical course artery is located posterior to the optic nerve. The and may be encountered in the extracranial, intra- - anterior cerebral artery crosses the posterior seg- petrousNot or intracranial segments. The facial nerve ment of the optic nerve. The pituitary stalk is located enters the deep facial structures at the stylomas- immediately posterior to the optic chiasm. toid foramen,for passing superficial to the styloid process and enteringredistribution the posteromedial aspect of the Internal carotid artery parotid gland where it divides into five principal branches. The temporalis branch innervates the The internal carotid artery can be subdivided into frontalis muscle and is vulnerable during retrac- four segments. The cervical segment passes from tion of the temporalis muscle. This can be pre- the common carotid bifurcation to the skull base. vented by division of the temporalis immediately This segment is usually exposed for the purposes anterior to the tragus, extending inferiorly to the of proximal vascular control when operating on zygomatic arch. pathology in this region of the skull base. The The facial nerve is also vulnerable to injury dur- petrous segment is difficult to access and lies in ing transtemporal approaches to the cerebellopon- close proximity to the cochlea and several cranial tine angle (CP angle), the medial skull base and nerves (VII, IX, X, XII). The initial vertical compo- during otological surgery. The facial nerve traverses nent enters the carotid canal before turning at the the CP angle with the superior and inferior ves- genu into an anteromedially projecting horizontal tibular nerves, the cochlear nerve and the nervus segment. The sinusoidal course then continues as intermedius, which subserves taste and secretion of the cavernous segment. On emergence from the tears and saliva. The intracanalicular component of cavernous venous sinus, the internal carotid con- the facial nerve then traverses the internal auditory tinues as the supraclinoid segment that terminates canal: this is exposed during retrosigmoid vestibu- at the carotid bifurcation. lar schwannoma excision. The nerve enters the bony

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facial canal with the initial labyrinthine segment the posterior aspect of the pituitary fossa and the coursing to a location just lateral to the cochlea. The foramen magnum) are usually approached via nerve then angles sharply forwards to the geniculate an anterior midline corridor of access. This may ganglion. Traction on the emerging greater superfi- constitute trans-sphenoidal, transmaxillary or cial petrosal branch (GPSN) can lead to facial nerve trans-oral approaches. Occasionally, a mandibu- weakness during middle fossa surgery. At the genu, lar osteotomy may be performed to improve access the nerve executes a hairpin bend and runs, as the further. These anterior approaches minimize the tympanic segment, along the medial wall of the tym- risk of cranial nerve injury, providing a direct sur- panic cavity, inferior to the labyrinth. At the second gical route to the site of pathology. If dural opening genu the nerve turns sharply inferior on the poste- is required, the vertebral arteries and basilar artery rior wall of the tympanic cavity as the mastoid seg- must be identified and protected. The main com- ment, passing directly to the stylomastoid foramen. plication of such an approach is postoperative CSF Other intrapetrous branches include the nerve to leakage and infection. stapedius and the chordaProperty tympani. Lower cranial nerves, the foramen SUMMARY magnum and clivus of It is wise for the head and neck surgeon to be Taylor cognizant of basic neuroanatomy. The pattern Surgical procedures in the region of the jugular of blood supply is based upon knowledge of the foramen, including far-lateral approaches to the Circle of Willis and its tributaries. Venous anat- foramen magnum, place the lower cranial& nerves omy is important when planning skull openings. Francis and the vertebral artery at risk. The vertebral artery Knowledge of cranial nerve regional anatomy usually hugs the posterior surface of the superior is crucial for the avoidance of collateral damage articular facet of the atlas. The levator scapulae mus- when undertaking combined specialty approaches Group cle also provides a useful landmark since the vessel to the skull base. Such rudimentary knowledge lies immediately medial to the upper attachments of will enhance patient safety and increase the surgi- the muscle (posterior tubercles of C1 to C4 trans- cal satisfaction- achieved when operating on com- Not verse processes). The glossopharyngeal, vagus and plex, challenging pathological conditions of the spinal accessory nerves all exit the skull via the jug- brain and forsurrounding structures. ular foramen. Postoperative injury causes swallow- ing impairment and may necessitate tracheostomy. redistribution These nerves are soon joined by the hypoglossal FURTHER READING nerve before each nerve pursues an individual route. The glossopharyngeal passes forward, superficial to Crossman AR, Neary D. Neuroanatomy. 4th ed. the internal carotid artery, but deep to the external Edinburgh: Churchill Livingstone Elsevier, carotid artery supplying the pharynx and poste- 2010. rior one-third of the tongue. The vagus descends Logan BM, Reynolds P, Hutchings RT. McMinn’s between the internal jugular vein and the internal Colour Atlas of Head and Neck Anatomy. carotid artery. The accessory nerve passes back- 4th ed. Philadelphia: Mosby, 2009. wards to enter the anterior border of the sternomas- Rhoton Jr AL. Cranial Anatomy and Surgical toid muscle 3–6 cm below the mastoid tip, and the Approaches. Philadelphia: Lippincott, hypoglossal passes forwards, anterior to the internal Williams & Wilkins, 2003. and external carotid arteries to reach the tongue. Sadler TW. Langman’s Medical Embryology. 12th Extradural lesions in the region of the foramen ed. Philadelphia: Lippincott, Williams & magnum and clivus (the long bony incline between Wilkins, 2012.

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