Cerebral hemispheres and white matter tracts

Osseous anatomy and landmarks

Cortical anatomy

The cerebral hemispheres are separated in the midline by the sagittal or interhemispheric fissure, running from the frontal pole to the occipital pole. Viewed laterally, the central sulcus separates the frontal and parietal lobes. It runs perpendicularly on either side of the interhemispheric fissure and passes obliquely, coronally and anteriorly before terminating inferiorly at the Sylvian (lateral) fissure. The primary motor cortex lies immediately anterior to the central sulcus; the primary sensory cortex lies immediately posterior to it. The Sylvian fissure separates the opercula of the temporal lobe (inferiorly) from the frontal opercula (anteriorly) and the parietal opercula (posteriorly). It travels posteriorly and superiorly, and terminates approximately one-third of the way along the horizontal length of the parietal lobe. Broca's area is part of the frontal opercula, located within the inferior frontal gyrus, lying above the Sylvian fissure. Wernicke's area is part of the temporal opercula, found immediately behind and below the posterior termination of the Sylvian fissure at the superior temporal gyrus; traditionally, this area is thought to be involved in enabling speech comprehension. The insula is a deep infolding of cortex lying within the Sylvian fissure and is typically obscured from view unless the Sylvian fissure is ‘opened’.

Cortical and subcortical anatomy of the insula

The insula resembles a cortical shield that covers the external capsule of white matter and protects the deeply located basal ganglia. The insular cortex is involved in visceral sensation, regulation of sympathetic cardio­vascular tone, pain processing and motor planning, among other somatosensory functions. Insular gliomas represent a technical challenge for the neurosurgeon due to difficult-to-reach anatomy, relations of highly eloquent cortex and a complex vascular supply. The insular cortex, when exposed, is divided by the oblique insular central sulcus and bounded by the anterior, superior and inferior peri-insular sulci at its anterior, superior and inferior walls, respectively. Anterior to the central sulcus there are three oblique gyri: the anterior, middle and posterior short insular gyri ( Fig. 25.1A ). Posterior to the central sulcus are the anterior and posterior long gyri. The most anteroinferior polar extremity of the insula is the limen insulae, containing the temporal stem (uncinate and inferior fronto-occipital fasciculi) ( Fig. 25.1B ). Deep to the central insula are the extreme capsule, claustrum, external capsule and lentiform nucleus. Posteriorly, geniculate motor fibres within the internal capsule run deep to the posterosuperior extremity of the circular sulcus.

Vascular supply of the insula

The insular cortex is supplied by perforators given off the M ₂ and M ₃ segments of the middle cerebral artery. Collateral perforators given off by the prefrontal artery, originating from the superior trunk of M2, supply the anterior short insular gyrus. Perforators from the precentral artery, again originating from the superior trunk, supply the middle short insular gyrus. M2 subsequently continues postero­superiorly along the lateral insular surface as the superior trunk, which branches into the central, anterior parietal, posterior parietal, angular and temporo-occipital arteries. These branches give off perforators supplying the posterior insular gyri. The inferior edge of the insula is supplied by either the temporal branch or the inferior trunk of the middle cerebral artery, which bifurcates into the middle and posterior temporal arteries that give off perforators supplying the cortex.

Surgical resection of insular tumours

The M1 segment gives off lenticulostriate perforators prior to its bifurcation into M2 branches. The lenticulostriate perforators represent the medial limit of resection for insular gliomas. Identification of vascular structures is critical during insular surgery because inadvertent damage may produce ischaemic deficits. Surgery involves careful Sylvian fissure dissection and retraction of the temporal and frontal lobes to expose the insular surface. An alternative approach has been developed to avoid Sylvian fissure opening and dissection of middle cerebral artery branches, involving the use of intraoperative cortical mapping to identify eloquent cortex. Non-dominant insular tumours can be removed under general anaesthesia, with mapping of facial somatic responses. Dominant hemispheric tumours must be removed using awake crani­otomy, with concurrent speech mapping to ensure language preservation. Once language and motor cortical regions are localized, safe cortical ‘windows’ are created to access the insular region in a subpial fashion, preserving the pia–arachnoid layering overlying middle cerebral artery branches. With these factors taken into consideration, insular tumours, particularly low-grade gliomas once thought to be inoperable, can be safely removed with significant improvement in oncosurgical outcomes.