Subarachnoid Space Martin M

80 Subarachnoid space Martin M. Mortazavi1, Nimer Adeeb2, Fareed Rizq2 and R. Shane Tubbs3 1University of Washington School of Medicine, Seattle, Washington, United States 2Children’s of Alabama, Birmingham, Alabama, United States 3Seattle Science Foundation, Seattle, Washington, USA St George’s University, School of Medicine, St Georges, Grenada University of Dundee, Dundee, UK Subarachnoid space size Cerebral subarachnoid space Variations the size of the subarachnoid space have been revealed by ultrasonographic (US) measurements mainly in neonates, infants, and children (Libicher and Troger 1992; Frankel et al. 1998; Lam et al. 2001; Narli et al. 2006; Sabouri et al. 2011; Okur et al. 2013); computed tomography (CT) and magnetic resonance (MR) studies have also been conducted. The width of the subarachnoid space correlates positively with weight, height, and head circumference; there is no significant gender difference (Narli et al. 2006; Sabouri et al. 2011). The width also correlates positively with age, peaking at 7 (Lam et al. 2001) Figure 80.1 Anatomic landmarks and sonographic variables of or 13 (Okur et al. 2013) months and declining thereafter. This the subarachnoid space in the coronal plane. C = cerebral cortex; has been related to the development of the arachnoid villi and SSS = superior sagittal sinus; CCW = craniocortical width; improved cerebrospinal fluid (CSF) drainage at 6–18 months of IHW = interhemispheral width; SCW = sinocortical width. age (Sabouri et al. 2011). A wide subarachnoid space is therefore Source: Lam et al. (2001) considered an anatomical variation during the first year of life. The site of measurement is also important: the craniocortical (CC) distance (between the cranium and cerebral hemisphere), and the shortest width of the anterior horn of the lateral ventri- sinocortical (SC) area (between the cerebral hemisphere and the cle at the foramen of Monro are calculated (Okur et al. 2013). superior sagittal sinus), or interhemispheric (IH) area (between the two hemispheres), measured from the narrowest to the Spinal subarachnoid space widest point (Lam et al. 2001) (Fig. 80.1). Libicher and Troger The subarachnoid space dimensions measured between the (1992) reported the upper limits of the normal range as 3, 4, and arachnoid and the pia on the anterior and posterior sagittal 6 mm at the CC, SC, and IH respectively. Sabouri et al. (2011) diameters and the right and left transverse diameters are sym- reported higher upper limits: 5, 5.8, and 8 mm at the CC, SC, metrical between the right and left sides. In contrast, they are and IH, respectively. These authors also suggested that race, asymmetrical and vary widely on the anterior and posterior socioeconomic conditions, and dietary regime could affect the sides over the range 1–5 mm, and are larger on the posterior width of the subarachnoid space. Okur et al. (2013) reported side. These measurements also vary and decrease monotonically the narrowest CC width of the subarachnoid space, over the from the cervical to the lumbar spine (Zaaroor et al. 2006). range 0.5–6 mm (Okur et al. 2013). Frankel et al. (1998) and Sabouri et al. (2011) reported narrower ranges of 1.9–5.7 mm Others and 1–4 mm, respectively. However, measurement of the suba- The subarachnoid space can be absent in locations where the rachnoid space to ventricular width ratio (SAS:VW ratio) could brain is in close proximity or adherent to the arachnoid, and be a more accurate method for determining a normal value; the where the nerves and blood vessels exit the brain (Adeeb et al. ratio between the shortest CC width of the subarachnoid space 2013). Bergman’s Comprehensive Encyclopedia of Human Anatomic Variation, First Edition. Edited by R. Shane Tubbs, Mohammadali M. Shoja and Marios Loukas. © 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc. 959 960 Bergman’s Comprehensive Encyclopedia of Human Anatomic Variation Hodges (1970) denied the presence of a sheet‐like suba- The origin of the frontopolar arteries and the median artery rachnoid space over the cerebral hemispheres and considered of the corpus callosum (CC) (Yaşargil 1984; Wang et al. 2011a), the arachnoid to be in direct contact with the gyri. Instead, he and several (15–40) small subcallosal arteries, might be found believed this space to be formed where the arachnoid bridges within the lamina terminalis cistern in addition to its normal over the sulci, creating a space within each sulcus about 1–2 mm content. The subcallosal arteries have diameters of 0.1–0.3 mm across and 5–10 mm deep (Hodges 1970). and can arise from the anterior communicating artery (ACoA), the A2 segment of the anterior cerebral artery (ACA), or the median artery of the CC (Wang et al. 2011a). Subarachnoid cisterns Carotid cistern The basilar cisterns underlie and partially surround the struc- The medial wall of the carotid cistern can be absent, unilaterally tures on the floor of the skull, and are named according to or bilaterally. In such cases the cistern communicates freely with the major anatomical structures they bathe. The first detailed the chiasmatic cistern. In other cases, the arachnoid membranes description and naming of most cisterns was provided by Key separating the carotid, interpeduncular, and crural cisterns are and Retzius (1875) and the cisterns have received more atten- absent, creating a confluent area through which cerebrospinal tion since. In the following text, deviations from the normal fluid can pass easily via the posterior part of the carotid cistern anatomy of the subarachnoid cisterns will be described. (Yaşargil 1984; Brasil and Schneider 1993; Froelich et al. 2008). Chiasmatic cistern Olfactory cistern The location of the chiasmatic cistern in relation to the sellae The anterior part of this cistern is usually high and broad. Its can vary. It usually overlies the diaphragma sella and the sella highest point reaches 3.0–4.0 mm above the olfactory bulb and turcica, but it sometimes lies more posterior over the dorsum its most lateral point extends 2.0–3.0 mm beyond the bulb. Its sella (post‐fixed chiasm) or, less commonly, more anterior over posterior part is usually wide, reaching a maximal width of 1 the tuberculum sella (prefixed chiasm) (Gulsen et al. 2010). In cm (Wang et al. 2008). There may be a slit‐like extension of cases of incompetent diaphragma sella, the chiasmatic cistern the olfactory cistern (5–13 mm) if the olfactory sulcus is deep can extend into the sella turcica (Yaşargil 1984). (Yaşargil 1984; Wang et al. 2008). The size of the cistern’s cavity also varies. It can be very nar- Subdiaphragmatic cistern row, with walls attaching to the olfactory structures and insuffi- The size of this cistern is determined by the length of the sub- cient communication with the surrounding subarachnoid space diaphragmatic portion of the pituitary stalk, which can be com- (Wang et al. 2008). pletely supradiaphragmatic in cases where the opening of the There are usually openings at the inferior wall of the olfac- diaphragma sellae is huge and the pituitary dome herniates tory cistern through which it communicates with the adjacent upwards, resulting in a large cistern. It also varies according to: carotid and sylvian cisterns. They might be big (reaching up to (1) the shape of the diaphragma sellae (flat, concave, or convex); 5 mm in diameter), small (less than 0.1 mm in diameter), or (2) the size of the sellar cavity; (3) the size of the pituitary gland; absent (Wang et al. 2008). (4) the location of the pituitary stalk; and (5) the size of the pitu- There can also be up to four small olfactory arterial branches, itary stalk (Di Ieva et al. 2012). 0.1–0.35 mm in diameter. They arise from the main olfactory artery, anterior olfactory artery, posterior olfacorty artery (most Lamina terminalis (LT) cistern common), or recurrent artery of Heubner (least common). The The anteroposterior (AP) length of the cistern’s floor ranges latter does not usually enter the cistern but the small arterial from 14.0 to 28.0 mm. Its anterior boundary can extend as far as branches do. The course of these branches within the cistern 5.0 mm anterior to the limbus sphenoidalis. Occasionally there depends on their origin, since they can begin anteriorly or is no significant anterior boundary, and the LT cistern commu- posteriorly if they arise from the anterior or posterior olfac- nicates directly with the interhemispheric cistern (Wang et al. tory artery, respectively. Infrequently, the arterial branches are 2011a). derived from the orbital artery; these often have a more anterior The LM cistern can also extend inferoanteriorly to form a origin and divide repeatedly within the cistern. Olfactory veins tent‐shaped recess above the interspace anterior to the OC, with within the cistern are usually 1–3 in number and 0.3–0.5 mm in an AP length of 4.0–12.0 mm. Its superolateral wall is formed diameter. They drain into the frontopolar vein or the origin of by the pia mater under the gyrus rectus (GR), and its inferior the superior sagittal sinus (Wang et al. 2008). wall by the arachnoid between the optic nerves (ON). There are sometimes dividing membranes within this recess (Yaşargil Sylvian cistern 1984; Wang et al. 2011a). The size and shape of this cistern depends on the relationship In some cases, the lateral walls partially adhere to each other between the frontal and temporal lobes. In most cases it narrows in the middle part of the cistern (Wang et al. 2011a). superiorly as the frontal and temporal lobes approach each other Chapter 80: Subarachnoid space 961 over a length of 15–20 mm. At this level, the width of the cistern is usually about 0.5–1.0 cm.

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