Neurosurgical anatomy - Clinical Tree

This chapter contains an overview of the surgical anatomy of the brain that will be amplified in later chapters. The use of cisternal anatomy to guide microsurgical procedures in the supratentorial and infratentorial compartments is emphasized. The pterional, transcallosal and telovelar approaches are described because they are standard procedures used to access a variety of intra-axial or intraventricular lesions.

Surgical surface anatomy

The pterion is the point where the frontal, parietal and temporal bones and the greater wing of the sphenoid meet. It is located 2.5 cm above the zygomatic arch and 1.5 cm behind the frontal process of the zygomatic bone. The asterion is the junction of the lambdoid, occipitomastoid and parietomastoid sutures and overlies the junction of the transverse and sigmoid sinuses.

The lateral sulcus (Sylvian fissure) is marked by a line drawn from the lateral canthus to a point 75% of the distance from the nasion to the external occipital protuberance. There is variability in the location of the central sulcus, which lies between the motor strip (anteriorly) and the primary sensory cortex (posteriorly): it lies approximately 4–5 cm posterior to the coronal suture. This is also at a point approximately 2 cm posterior to the mid position of the arc joining the nasion and the external occipital protuberance.

Placement of a ventricular catheter is usually via Kocher's point ( Fig. 20.1 ). This is a burr-hole, usually on the right side, in the mid-pupillary line, 1 cm anterior to the coronal suture. The catheter is inserted by aiming toward the medial canthus of the ipsilateral eye in the coronal plane and toward the external auditory canal (external acoustic meatus) in the anterior–posterior plane. The ventricle is normally at a depth of around 5–5.5 cm. Keen's point lies 2.5–3 cm behind and 2.5–3 cm above the auricle (pinna) and may be used to access the trigone of the lateral ventricle. The Frazier burr-hole lies 3–4 cm from the midline and 6 cm above the external occipital protuberance: it can be used to tap the occipital horn.

Fig. 20.1, Landmarks for Kocher's point, the most commonly used entry point for ventricular access.

The superior sagittal sinus runs posteriorly from the nasion to the external occipital protuberance and is a midline structure. The transverse sinus passes horizontally forward from the level of the external occipital protuberance toward the mastoid at the same level as a line projected posteriorly from the zygomatic arch.

Clinical anatomy

Surface anatomy provides useful landmarks for operative planning. The intracranial approaches require knowledge of neuroanatomy, including detailed knowledge of the hemispheres, the diencephalon, the brainstem, the cerebellum, the ventricles and the blood supply. The complex relationships between structures requires three-dimensional understanding. In the supratentorial compartment, the lateral sulcus (Sylvian fissure) and the central sulcus are key landmarks in identifying structures on the lateral aspect of the hemisphere. During a pterional approach, opening of the lateral sulcus enables identification of the optic nerve and carotid artery, providing a base for onward dissection. In the posterior fossa the midline structures are useful landmarks for initial orientation. In the cerebellopontine angle the relationship between the choroid plexus and cranial nerves VII and VIII enables accurate orientation.

Subarachnoid cisterns

Arachnoid mater invests the brain and forms a series of partially interconnecting cerebrospinal fluid (CSF) cisterns around the base of the brain ( Fig. 20.2 ). Each cistern contains cranial nerves and/or blood vessels. Knowledge of the subarachnoid cisterns is of immense practical value during microsurgical approaches. The principal cisterns of operative importance, and their contents, are listed in Table 20.1 .

Fig. 20.2, A schematic representation of the basal cerebrospinal fluid cisterns (Arabic numbers) as observed during microsurgical procedures. Note the intimate association with the cranial nerves (Roman numerals). Many of the cisterns are also intimately associated with cerebral arteries. Key: 1, olfactory cistern; 2a, callosal cistern; 2b, lamina terminalis cistern; 3, chiasmatic cistern; 4, carotid cistern; 5, Sylvian cistern; 6, crural cistern; 7, interpeduncular cistern; 8, ambient cistern; 9, prepontine cistern; 10, superior cerebellar–pontine cistern; 11, inferior cerebellar–pontine cistern (lateral cerebellomedullary); 12, anterior spinal cistern; 13, posterior spinal cistern. I, olfactory bulb; II, optic nerve; III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; VI, abducens nerve; VII, facial nerve; VIII, vestibulocochlear nerve; IX, glossopharyngeal nerve; X, vagus nerve; XI, accessory nerve; XII, hypoglossal nerve.

TABLE 20.1

The principal cisterns of operative importance and their contents

Cistern	Content
Lamina terminalis	Anterior communicating artery and branches
Chiasmatic	Proximal anterior cerebral artery; optic nerves
Carotid	Internal carotid artery and origins of posterior communicating artery and anterior choroidal artery
Sylvian	Middle cerebral artery
Crural	Anterior choroidal artery, medial posterior choroidal artery
Interpeduncular	Bifurcation of basilar artery and posterior cerebral arteries; CN III
Ambient (with supratentorial and infratentorial components)	Posterior cerebral artery, superior cerebellar artery (SCA), basal veins; CN IV
Quadrigeminal	Vein of Galen, distal pericallosal arteries, distal posterior cerebral arteries and SCA; CN IV
Prepontine	Basilar artery, anterior inferior cerebellar artery (AICA), SCA; CN VI
Premedullary	Vertebral arteries, posterior inferior cerebellar artery (PICA); CN XII
Cerebellopontine	AICA; CN V, VII, VIII
Cerebellomedullary	Vertebral artery, PICA; CN IX, X, XI, XII
Cisterna magna	Distal PICA, craniospinal junction

Lobes of the brain

The frontal lobe lies anterior to the central sulcus (of Rolando) and comprises the precentral gyrus and the superior (F1), middle (F2) and inferior (F3) frontal gyri, as viewed laterally. The neocortex of the precentral gyrus gives rise to the descending motor projection fibres from the pyramidal cells of Betz in layer five of the primary motor cortex. The inferior frontal gyrus contains the fronto-orbital, triangular and frontal opercular regions: the latter two comprise the motor speech cortex in the dominant hemisphere. The medial aspect of the frontal lobe comprises mainly the F1 gyrus and part of the cingulate gyrus as it envelops the genu and rostrum of the corpus callosum. The basal aspect of the frontal lobe comprises the gyrus rectus medially and the orbital gyri laterally.

The lateral sulcus separates the frontal and temporal lobes. The sulcus is deep and is characterized by short anterior and ascending rami that demarcate the apex of the triangular portion of the inferior frontal (F3) gyrus. The superior (T1), middle (T2) and inferior (T3) temporal gyri extend posteriorly toward the inferior parietal lobule. The superior temporal gyrus has a considerable surface area that forms the floor of the lateral sulcus and includes the primary auditory cortex located within Heschl's transverse temporal gyri. The medial surface of the temporal lobe comprises the parahippocampal gyrus, with the dentate gyrus and fimbria lying more superiorly. The posterior end of the parahippocampal gyrus is intersected by the calcarine sulcus, with the upper part merging with the cingulate gyrus and the lower part with the lingula. The collateral sulcus is fairly constant and defines the lateral margin of the lingula and the parahippocampal gyrus.

The parietal lobe lies posterior to the central sulcus. A line drawn from the parieto-occipital sulcus (best identified from the medial aspect of the hemisphere) to the inferiorly located preoccipital notch defines the posterior limit of the parietal lobe. A second line, drawn from the first line to the lateral sulcus, defines the boundary between the temporal and parietal lobes. The parietal lobe comprises superior and inferior lobules. The latter includes the supramarginal gyrus, which lies anterior to the angular gyrus. These gyri surround the posterior aspects of the lateral sulcus and the superior temporal sulcus (between T1 and T2); they are involved in language control.

The lateral aspect of the occipital lobe shows considerable variability. Medially, the occipital lobe is separated from the parietal lobe by the parieto-occipital sulcus. The calcarine sulcus begins near the occipital pole and courses forwards, joining the parieto-occipital sulcus. The cuneus lies above and the lingula lies below the sulcus. The primary visual cortex is located both above and below the calcarine sulcus.

The insula lies deep to the lateral sulcus. Although triangular in shape, it is bounded by the circular sulcus. The central sulcus of the insula demarcates a large anterior component containing several short gyri and a posterior part comprising two long gyri. The insula overlies many deep structures, including the lentiform nucleus and the internal capsule. The M ₂ segments of the middle cerebral artery cross the insula. During resection of tumours involving the insula, the lenticulostriate perforators coming off the M ₁ are vulnerable to injury and their damage can result in motor deficits. The most lateral lenticulostriate perforator acts as the medial margin of such a resection. The central grey matter of the hemispheres is considered in Chapter 29 ( Fig. 20.3 ).

Fig. 20.3, The lateral aspect of the left cerebral hemisphere, indicating the major gyri and sulci.

Posterior fossa

The posterior fossa contains the cerebellum, brainstem, cranial nerves VI to XII and much of the posterior circulation. The cerebellum comprises a midline vermis and two cerebellar hemispheres including the tonsils and the flocculi. Lesions within the cerebellar hemisphere are generally approached via a transcortical dissection. Lesions on the undersurface of the tentorium or in the region of the quadrigeminal cistern are usually approached via a supracerebellar, infratentorial route. The structures contained within the quadrigeminal cisterns are hazards to avoid. Lesions in the cerebellopontine angle are approached by retraction of the petrous aspect of the cerebellar hemisphere. Opening the cerebellopontine cistern and the cerebellomedullary cistern will enable identification of the anterior inferior cerebellar artery (AICA), posterior inferior cerebellar artery (PICA) and cranial nerves VII to XII. Identification of choroid plexus protruding through the foramen of Luschka assists in identifying cranial nerves VII and VIII. The ascending fibres of the spinal accessory nerve are easily identified. The hypoglossal rootlets arise anterior to the olive, from the preolivary sulcus, while the rootlets of IX and X arise posterior to the olive.

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