The subarachnoid space is the compartment within the spinal column which contains the cerebrospinal fluid (CSF). CSF is produced in the ventricular system of the brain. It communicates freely with the subarachnoid space via the foramina of Luschka and Magendie near the brainstem. CSF entering the subarachnoid space circulates caudad toward the lumbar cistern. Most of this is returned to the venous system via arachnoid villi which are present on the dural sleeves of spinal nerves although a small amount returns to the brain.

The subarachnoid space is bounded by the spinal meninges. These form a fluid filled dural sac which surrounds the spinal cord. The pia mater is a delicate, vascular membrane which closely invests the surface of the spinal cord. The subarachnoid space lies between the pia mater and the arachnoid mater. The latter is a thin, avascular membrane which adheres closely to the fibrous dura mater, creating between them a potential space of small capacity, called the subdural space (Chapter 15). Inadvertent administration of local anaesthetic solution into this space can result in a subdural block (Figure E.2). This is one of the mechanisms postulated for the complication of epidural blockade referred to as a 'total spinal block'

The subarachnoid space extends from the basal cisterns surrounding the brainstem superiorly, to its termination opposite the level of the second sacral vertebra (Figure 16.1). The meninges continue caudad from this point fused as the filum terminale which anchors the spinal cord to its attachment on the coccyx.

The dimensions of the subarachnoid space are determined by the cross-sectional shape of the spinal cord at each segmental level. The sac widens in the cervical and lumbar regions which correspond to the enlargement of the spinal cord by nerves subserving the limbs. Above the level of the first lumbar vertebra it exists as a ring shaped recess. Here, the distance between the arachnoid mater and undivided spinal cord is 2-3 mm. The caudal end of the spinal cord, the conus medullaris, usually lies opposite the disc between the first and second lumbar vertebrae. Below this point the subarachnoid space expands as a large lumbar cistern which contains the mobile cauda equina. The diameter at this point approaches 9 mm (Figure 16.2).

The subarachnoid space is anatomically a single compartment which contains the body of the spinal cord and CSF. The space is trabeculated by extensions of the pia mater in several planes. Laterally these form the dentate ligament - a toothed longitudinal membrane which anchors the spinal cord to the arachnoid and dura mater. Anteriorly a pial extension runs along the length of the anterior median fissure as the linea splendens. Posteriorly, in the mid-line, it extends from the posterior median sulcus to the dura as the posterior subarachnoid septum.