28 - routledgestructures including the basal ganglia, thalamus, hypothalamus, internal capsule and...

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28Neuroanatomy for the head and neck surgeon

PETER C. WHITFIELD

OVERVIEW AND EMBRYOLOGY

An estimated 100 billion neurones are organized into the human brain. The 1400 g of tissue com-prising the brain is arranged in a highly complex series of neural networks receiving 750mL/min−1 of blood. The brain develops from the cephalic end of the neural tube. Three primary vesicles form giving rise to the forebrain, midbrain and hindbrain. These give rise to the cerebral hemi-spheres, thalamus, hypothalamus, midbrain, pons, medulla and cerebellum. The brain com-municates with the head and neck structures via the cranial nerves; most of these arise from the brainstem. Connections with the limbs and trunk are via the afferent and efferent spinal cord path-ways. The calvarial part of the skull, or skull vault, is formed by membranous ossification of mes-enchyme that invests the embryonic brain. The bones of the newborn skull are joined by sutures.

The sagittal, coronal and lambdoid sutures are of major importance, with premature fusion leading to craniofacial deformity. The skull base is formed by endochondral ossification of cartilage leading to formation of the skull base components of the ethmoid, sphenoid, petrous and occipital bones. The bones of the face are mainly formed from car-tilages of the first two pharyngeal arches with the associated musculature supplied by the mandibu-lar and facial nerves, respectively.

Common disease processes include neoplas-tic, vascular and traumatic conditions. Surgical access to the brain is achieved directly through the calvarium, or via the skull base or a combi-nation of approaches. No part of the skull, brain or surrounding tissues is immune to pathological processes, with many conditions requiring col-laborative surgical teams encompassing neuro-surgery, maxillofacial surgery, otolaryngology and plastic surgery.

Overview and embryology 279The meninges 280Topography of the brain 280Blood supply 280Surgical hazards 283Venous sinuses, meningeal vessels and

bridging veins 283Olfactory tract 284

Optic nerve and chiasm 284Internal carotid artery 284Cavernous sinus 284Facial nerve 284Lower cranial nerves, the foramen magnum

and clivus 285Summary 285Further reading 285

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280 Neuroanatomy for the head and neck surgeon

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. These cisterns are evident in life but are not well visualized in cadaveric specimens.

The dura mater is a thick, fibrous membrane that lines the inner surface of the skull. Folds of dura form structurally important partitions within the cranial cavity. The falx cerebri is a sickle-shaped dural membrane located in the midline sagit-tal plane, separating the cerebral hemispheres. Anteriorly, it is attached to the crista galli. It arches over the corpus callosum to a posterior attachment at the internal occipital protuberance. At this level it forms the tentorium cerebelli, dividing the cranial cavity into the supra and infratentorial compart-ments. The cerebellum lies beneath the tentorium cerebelli. The superior sagittal venous sinus lies within a dural enclosed fold along the superior aspect of the falx cerebri. The inferior sagittal sinus lies in the free inferior margin of the falx cerebri.

TOPOGRAPHY OF THE BRAIN

The brain comprises the cerebral hemispheres which lie in the supratentorial compartment (Figures 28.1 and 28.2). They are connected by the commissural fibres of the corpus callosum. The cerebellum and brainstem (midbrain, pons, medulla) are located in the infratentorial compartment. The midbrain lies at the level of the tentorial hiatus acting as a conduit for information between the hemispheres and the brainstem. Most of the cranial nerves originate from the brainstem. The hemispheres are subdivided into the frontal, temporal, parietal and occipital lobes. The different lobes undertake important func-tions; language functions are usually lateralized to the dominant hemisphere. Frontal lobe functions include motor, personality, executive functions and

the expressive component of speech. Temporal lobe functions include memory and auditory processing including the receptive component of speech. The parietal lobe is involved in the perception of touch and the integration of sensory information, and the occipital lobe is responsible for visual process-ing. Deeper structures include the basal ganglia, thalamus and hypothalamus. The basal ganglia contribute to movement control. The thalamus is a key relay station in sensory and circuits and contrib-utes to movement control. The nearby hypothala-mus is a centre for autonomic nuclei that subserve sympathetic and parasympathetic functions and is directly connected to the posterior lobe of the pitu-itary gland. The adjacent anterior pituitary is cru-cial in orchestrating the body’s hormonal milieu, secreting growth hormone, thyroid stimulating hormone, adrenocorticotrophic hormone, gonado-trophic hormones and prolactin. The cerebellum is concerned with the control of posture and muscle coordination.

BLOOD SUPPLY

The arterial blood to the brain is supplied by bilateral internal carotid and bilateral vertebral arteries. These supply the anterior and posterior

Figure 28.1 Lateral view of the right cerebral hemisphere. From this view note the Sylvian (frontotemporal) fissure lying between the frontal and temporal lobes. Note the cerebellar hemi-sphere inferiorly. (Supplied by Dr D. Hilton.)

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Blood supply 281

circulations, respectively. The posterior commu-nicating arteries arise from the posterior aspect of the intracranial component of the internal carotid arteries and join the posterior cerebral arteries providing an anastomotic link (of variable degree) between the anterior and posterior circulations. This was eloquently described as the Circle of Willis in the seventeenth century (Figure 28.3). On entry into the skull, the internal carotid artery (Figure 28.4) traverses the cavernous sinus. The ophthal-mic artery is a small branch that arises at the point of emergence into the subarachnoid space. The next branch is the posterior communicating artery followed by the anterior choroidal artery: a small

branch that often supplies the descending motor fibres located within the confines of the internal capsule. The internal carotid artery then bifurcates into the medially directed anterior cerebral artery and the laterally directed middle cerebral artery. The anterior cerebral artery crosses the optic nerve and then abruptly turns in an anterosuperior direction to supply the frontomedial aspect of the cerebral hemispheres via the frontobasal, perical-losal and callosomarginal terminal branches. At the point of inflection, the anterior communicat-ing artery, a short (2 mm) branch, provides a direct communication between the right and left anterior cerebral vessels. This is a key component of the Circle of Willis. The middle cerebral artery trav-els in the frontotemporal (Sylvian) fissure. After a few centimetres it bifurcates into end arteries supplying the bulk of the cerebral hemispheres.

Figure 28.2 Topography of the brain – the cerebellum and brainstem. The cerebellum has been divided in the sagittal plane and splayed open like the pages of a book. This demonstrates the arboreal architecture of the cerebellum. Note the cerebellar tonsil projecting inferiorly. This impacts against the craniocervical junction if ‘coning’ occurs. The lower panel illustrates axial sections of the midbrain, pons and medulla (from left to right). In the midbrain note the small diameter of the Aqueduct of Sylvius, the pigmented substantia nigra and the cerebral peduncles projecting anteriorly. The pons is char-acterized by the prominent transverse pontine fibres – these are fibres connecting the motor cortex to the cerebellum allowing the cerebellum to modulate coordination. The medulla contains several cranial nerve nuclei subserving autonomic functions. Compression of these during coning ultimately causes brainstem death. (Supplied by Dr D. Hilton.)

Figure 28.3 Circle of Willis. This is a 3D comput-erized tomography angiogram. In the posterior circulation, the vertebral arteries unite forming the basilar artery. Anteriorly, the bilateral internal carotid arteries bifurcate into the medially pro-jecting anterior cerebral arteries and the laterally projecting middle cerebral arteries. The distal segments of the anterior cerebral arteries travel in the interhemispheric plane in close apposition to each other. In this case the posterior commu-nicating arteries are not well visualized. (Supplied by Dr W. Mukonoweshuro.)

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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 cerebel-lar artery which follows a tortuous course, sup-plying the lateral brainstem and the posterolateral aspect of the cerebellar hemispheres. The vertebral arteries unite on the ventral surface of the brain-stem forming the basilar artery. This projects supe-riorly on the ventral surface of the pons, giving rise to the paired anterior inferior cerebellar arteries, labyrinthine arteries, superior cerebellar arteries

and multiple pontine perforating branches. At the level of the midbrain 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 anasto-motic connections with the anterior circulation via the posterior communicating arteries.

The venous drainage of the brain is via super-ficial and deep venous drainage networks (Figure 28.6). Deep structures (e.g. thalamus) drain via the paired basal and internal cerebral veins. These then unite with the superior cerebellar vein from the upper brainstem and the inferior sagittal sinus, forming the Great Cerebral Vein (of Galen) that drains into the straight sinus. The straight sinus lies in the midline ‘ridge’ of the tentorium cerebelli. At the internal occipital protuberance, deep venous blood is usually directed into the left transverse sinus and thence the sigmoid sinus

Figure 28.4 Internal carotid artery. This is a 3D angiogram of the left internal carotid artery. The tortuous course of the cervical, petrous and cavernous segments is evident. The first branch of the supraclinoid segment is the ophthalmic artery: a small vessel in this case. The posterior communicating artery is then seen to project posteriorly. In this case it is a large vessel and appears to supply the occipital lobes via the posterior cerebral vessels. This arrangement is an embryological variant called a ‘fetal type’ circula-tion and is present in approximately 20 per cent of cases. (Supplied by Dr W. Mukonoweshuro.)

Figure 28.5 Oblique view digital subtraction angiogram of the left vertebral artery. In this case conventional posterior circulation anatomy 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 cerebellar arteries and supply much of the upper part of the cerebellum and upper brainstem. (Supplied by Dr W. Mukonoweshuro.)

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Surgical hazards 283

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 fissure 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 vein drains the super-ficial cortex to the superior sagittal sinus anterior to the vein of Trolard. Venous blood in the superior sagittal sinus travels posteriorly to the confluence of sinuses at the internal occipital protuberance. It then usually enters the right transverse and thence the sigmoid sinus on its descent to the internal jugular vein.

SURGICAL HAZARDS

Surgical approaches to the brain can be under-taken from almost every trajectory. Even though intraoperative navigation can facilitate identifica-tion of anatomical landmarks, knowledge of key structures is necessary to minimize trauma and enhance operative safety. Some of the major ana-tomical hazards are discussed below.

Venous sinuses, meningeal vessels and bridging veins

A craniotomy can be safely elevated directly over a venous sinus, although if this can be avoided the risk of serious haemorrhage is minimized. The superior sagittal sinus lies in the midline as described above. The posterior extent is marked externally by the external occipital protuberance. The transverse sinus runs from the posterior occip-ital protuberance to the asterion, a junction of sutures where the temporal, parietal and occipital

bones meet. This lies just posterosuperior to the mastoid process and is located at the same level as the zygomatic arch, an easily palpable surgical landmark.

The middle meningeal artery arises as a branch of the maxillary artery. It traverses the foramen spinosum, just posterolateral to the mandibular nerve within the larger foramen ovale. The ves-sel supplies the dura via several branches and can cause bleeding during a pterional craniotomy; it is safely sacrificed during a surgical approach where necessary. Anterior ethmoidal arteries sup-ply much of the anterior fossa dura. These can be a source of profuse bleeding during surgery for inva-sive tumours of this region.

Once the intracranial compartment is opened, veins may be encountered traversing from the brain to the dural venous sinuses. These veins include bridging veins from the cerebral hemispheres to the superior sagittal sinus and polar veins from the tip of the temporal lobe to the spheno-parietal

Figure 28.6 Venous drainage of the brain. This is an anteroposterior view of the venous phase of a cerebral angiogram. Cortical veins can be seen connecting to the midline superior sagittal sinus. Blood flows posteriorly towards the confluence of sinuses. In this case most of the blood drains from the confluence via the right transverse and sigmoid sinuses to the internal jugular vein. (Supplied by Dr W. Mukonoweshuro.)

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sinus. These can be divided to prevent inadvertent haemorrhage, though preservation is preferable in the region of the motor cortex.

Olfactory tract

The olfactory tract lies on the undersurface of the frontal lobe. It commences at the olfactory bulb immediately superior to the cribriform plate of the ethmoid bone and projects posteriorly to the proximal Sylvian fissure. The olfactory nerves are vulnerable to traumatic brain injury and is also at risk in anterior skull base surgery, particularly as a result of frontal lobe retraction during a bifrontal craniotomy or a pterional approach.

Optic nerve and chiasm

The optic nerve provides a well-recognized land-mark during intracranial surgery that facilitates identification of the internal carotid artery. The optic nerve exits the orbit via the optic canal and emerges into the parasellar region travelling in a posteromedial direction towards the optic chiasm. The supraclinoidal segment of the internal carotid artery is located posterior to the optic nerve. The anterior cerebral artery crosses the posterior seg-ment of the optic nerve. The pituitary stalk is located immediately posterior to the optic chiasm.

Internal carotid artery

The internal carotid artery can be subdivided into four segments. The cervical segment passes from the common carotid bifurcation to the skull base. This segment is usually exposed for the purposes of proximal vascular control when operating on pathology in this region of the skull base. The petrous segment is difficult to access and lies in close proximity to the cochlea and several cranial nerves (VII, IX, X, XII). The initial vertical compo-nent enters the carotid canal before turning at the genu into an anteromedially projecting horizontal segment. The sinusoidal course then continues as the cavernous segment. On emergence from the cavernous venous sinus, the internal carotid con-tinues as the supraclinoid segment that terminates at the carotid bifurcation.

Cavernous sinus

The cavernous sinus comprises a series of inter-linked venous channels and is located immedi-ately lateral to the pituitary fossa. Surgical access to this area is achieved via trans-sphenoidal, pterional (greater wing of sphenoid) and orbito-zygomatic approaches. The cavernous segment of the internal carotid is located within the sinus and usually lies several millimetres lateral to the midline. However, an abnormally tortuous ves-sel can approach the midline and impede trans-sphenoidal surgical access to the pituitary fossa. The lateral wall of the cavernous sinus contains the oculomotor (III) and trochlear (IV) nerves, and the ophthalmic (V1) and maxillary divisions (V2) of the trigeminal nerve. The abducent nerve takes a more medial route, traversing the cavernous sinus in a posterior-anterior direction. The oculomotor, trochlear, abducent and ophthalmic nerves enter the orbit via the superior orbital fissure whilst the maxillary nerve traverses the foramen rotundum.

Facial nerve

The facial nerve has a complex anatomical course and may be encountered in the extracranial, intra-petrous or intracranial segments. The facial nerve enters the deep facial structures at the stylomas-toid foramen, passing superficial to the styloid pro-cess and entering the posteromedial aspect of the parotid gland where it divides into five principal branches. The temporalis branch innervates the frontalis muscle and is vulnerable during retrac-tion of the temporalis muscle. This can be pre-vented by division of the temporalis immediately anterior to the tragus, extending inferiorly to the zygomatic arch.

The facial nerve is also vulnerable to injury dur-ing transtemporal approaches to the cerebellopon-tine angle (CP angle), the medial skull base and during otological surgery. The facial nerve traverses the CP angle with the superior and inferior ves-tibular nerves, the cochlear nerve and the nervus intermedius, which subserves taste and secretion of tears and saliva. The intracanalicular component of the facial nerve then traverses the internal auditory canal: this is exposed during retrosigmoid vestibu-lar schwannoma excision. The nerve enters the bony

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Further reading 285

facial canal with the initial labyrinthine segment coursing to a location just lateral to the cochlea. The nerve then angles sharply forwards to the geniculate ganglion. Traction on the emerging greater superfi-cial petrosal branch (GPSN) can lead to facial nerve weakness during middle fossa surgery. At the genu, the nerve executes a hairpin bend and runs, as the tympanic segment, along the medial wall of the tym-panic cavity, inferior to the labyrinth. At the second genu the nerve turns sharply inferior on the poste-rior wall of the tympanic cavity as the mastoid seg-ment, passing directly to the stylomastoid foramen. Other intrapetrous branches include the nerve to stapedius and the chorda tympani.

Lower cranial nerves, the foramen magnum and clivus

Surgical procedures in the region of the jugular foramen, including far-lateral approaches to the foramen magnum, place the lower cranial nerves and the vertebral artery at risk. The vertebral artery usually hugs the posterior surface of the superior articular facet of the atlas. The levator scapulae mus-cle also provides a useful landmark since the vessel lies immediately medial to the upper attachments of the muscle (posterior tubercles of C1 to C4 trans-verse processes). The glossopharyngeal, vagus and spinal accessory nerves all exit the skull via the jug-ular foramen. Postoperative injury causes swallow-ing impairment and may necessitate tracheostomy. These nerves are soon joined by the hypoglossal nerve before each nerve pursues an individual route. The glossopharyngeal passes forward, superficial to the internal carotid artery, but deep to the external carotid artery supplying the pharynx and poste-rior one-third of the tongue. The vagus descends between the internal jugular vein and the internal carotid artery. The accessory nerve passes back-wards to enter the anterior border of the sternomas-toid muscle 3–6 cm below the mastoid tip, and the hypoglossal passes forwards, anterior to the internal and external carotid arteries to reach the tongue.

Extradural lesions in the region of the foramen magnum and clivus (the long bony incline between

the posterior aspect of the pituitary fossa and the foramen magnum) are usually approached via an anterior midline corridor of access. This may constitute trans-sphenoidal, transmaxillary or trans-oral approaches. Occasionally, a mandibu-lar osteotomy may be performed to improve access further. These anterior approaches minimize the risk of cranial nerve injury, providing a direct sur-gical route to the site of pathology. If dural opening is required, the vertebral arteries and basilar artery must be identified and protected. The main com-plication of such an approach is postoperative CSF leakage and infection.

SUMMARY

It is wise for the head and neck surgeon to be cognizant of basic neuroanatomy. The pattern of blood supply is based upon knowledge of the Circle of Willis and its tributaries. Venous anat-omy is important when planning skull openings. Knowledge of cranial nerve regional anatomy is crucial for the avoidance of collateral damage when undertaking combined specialty approaches to the skull base. Such rudimentary knowledge will enhance patient safety and increase the surgi-cal satisfaction achieved when operating on com-plex, challenging pathological conditions of the brain and surrounding structures.

FURTHER READING

Crossman AR, Neary D. Neuroanatomy. 4th ed. Edinburgh: Churchill Livingstone Elsevier, 2010.

Logan BM, Reynolds P, Hutchings RT. McMinn’s Colour Atlas of Head and Neck Anatomy. 4th ed. Philadelphia: Mosby, 2009.

Rhoton Jr AL. Cranial Anatomy and Surgical Approaches. Philadelphia: Lippincott, Williams & Wilkins, 2003.

Sadler TW. Langman’s Medical Embryology. 12th ed. Philadelphia: Lippincott, Williams & Wilkins, 2012.

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28 - routledgestructures including the basal ganglia, thalamus, hypothalamus, internal capsule and...

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