6.2.1 Normal Anatomy

The meninges that surround the brain are divided into three layers, the dura mater, arachnoid mater, and pia mater. The dura mater is thick, leathery, and adherent to the inner surface of the skull, acting as an inner layer of periosteum. The pia mater is densely adhesive to the brain surface and can harbor no potential space beneath it (entering the “subpial space” means entering the brain parenchyma). The arachnoid mater lies between the dura mater and pia mater. These three membranes create three spaces, including the epidural and subdural spaces (which are potential spaces) and the subarachnoid space (which contains cerebrospinal fluid [CSF]). The arachnoid mater derives its name from its spider weblike appearance, observed by anatomists in the eighteenth century. The arachnoid mater is avascular, easily separated from the dura mater by the subdural potential space, surrounding the entire brain and spinal cord, but unlike the pia mater, it traverses sulci and fissures.14 The subarachnoid space varies in thickness and may be absent when the pia mater and arachnoid mater are in direct contact at sites where nerves and vessels enter or exit the brain. This space is filled with CSF, blood vessels, cranial nerves, and solid or porous trabeculated fibers connecting the arachnoid mater and the pia mater. The subarachnoid space receives CSF via communications with the ventricles, principally at the foramina of Luschka and Magendie from the fourth ventricle. Focal enlargements within the subarachnoid space occur and are termed subarachnoid and basal cisterns. Trabeculations within the subarachnoid space can either divide the cisterns discretely or allow communication with various sized pores.4 The trabeculated membranes and their relationships to the locations of the cisterns help direct CSF flow from its production by the choroid plexus within the ventricles to its absorption by the arachnoid villi and granulations over the convexity.

6.2.2 Classification

The extent of the subarachnoid space surrounding the brain shows local variations. Cisterns can be divided into those that lie over the convex surface where the space is narrow, and those that lie near the skull base, where more voluminous CSF chambers are encountered. The basal cisterns surround the structures lying on the floor of the skull and mid-brain and communicate with the convex surface via three different routes:

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  1. Anteriorly: between the hemispheres and along the rostrum of the corpus callosum.

  
  
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  2. Anterolaterally: along the sylvian fissure over the insula.

  
  
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  3. Posteriorly: from the quadrigeminal cistern up around the splenium of the corpus callosum, and from the quadrigeminal cistern posteriorly over the vermis and dorsal cerebellar surfaces.

Basal cisterns are named and classified according to the major anatomical structure they bathe or anatomical location.14 Yaşargil divided the subarachnoid cisterns into two categories, supratentorial and infratentorial, and then subdivided those into anterior, lateral, posterior, and superior (Table 6.1, Animation 6.1). Fig. 6.1 is a midline sagittal illustration of the brain with the major subarachnoid cisterns labeled, and Fig. 6.2 shows the relevant vascular contents of the subarachnoid cisterns at the skull base.

6.3.1 Chiasmatic Cistern

The chiasmatic cistern is a midline structure that surrounds the optic nerves and chiasm; it borders the lamina terminalis cistern superiorly, the carotid cistern laterally, and the interpeduncular cistern posteriorly (Fig. 6.3). The medial carotid membrane forms the lateral wall, the outer arachnoid membrane forms the anteroinferior wall, and the diencephalic membrane forms the posterior wall. Anteriorly, the cistern is limited by the limbus sphenoidale, which is positioned at the posterior edge of the planum sphenoidale. The chiasmatic cistern contains the optic nerves and chiasm, pituitary stalk, and optic venous plexus and perforating carotid branches including the superior hypophyseal and infundibuar arteries.4,15 It continues anterosuperiorly as the cistern of the lamina terminalis.14

6.3.2 Lamina Terminalis Cistern

The lamina terminalis (LT) cistern is located at the midline of the deep cerebrum adjacent to the lamina terminalis, situated above the optic chiasm. The lamina terminalis forms this cistern′s posterior and posterioinferior walls, and the optic chiasm forms its inferior wall (Fig. 6.3). The septal area and medial surface of the posterior gyrus rectus form the lateral walls. The anterior boundary is formed by the pia and arachnoid mater in front of the anterior communicating arteries. The cistern is bordered by the pericallosal cistern superiorly, the chiasmatic cistern inferiorly, the carotid cistern laterally, and the olfactory cistern anterolaterally. It contains the anterior cerebral arteries (A-1 and proximal A-2 segments), the anterior communicating artery, the recurrent branch of Heubner′s artery, the hypothalamic arteries, the origin of the orbitofrontal arteries, and the venous system of the lamina terminalis.4,15,16 Understanding this cistern′s anatomy and its relationship with the anterior cerebral artery complex is crucial to avoid complications during endoscopic fenestration of the LT.17 (See Chapter 26 Lamina Terminalis Fenestration.)

Abbreviations: AICA, anterior inferior cerebellar artery; CN, cranial nerve; MCA, middle cerebral artery; PICA, posterior inferior cerebellar artery.

Source: Data from Yasargil et al 1976.3