education The meninges and common pathology Understanding the anatomy can lead to prompt identification of serious pathology The meninges are three membranous of the skull and extends into folds that arterial blood has sufficient pressure to layers that surround the structures of the compartmentalise the skull.1 2 The large separate the dura from the bare bone of central nervous system. They include the midline fold separates the two hemispheres the skull.4 The classic example of this is dura mater, the arachnoid mater, and and is called the falx.1 A smaller fold a severe blow to the temple that ruptures the pia mater. Together they cushion the separates the cerebral hemispheres from the middle meningeal artery, which brain and spinal cord with cerebrospinal the cerebellum and is known as the has part of its course between the skull fluid and support the associated vascular tentorium cerebelli usually abbreviated as and the dura at a weak point called the structures.1 2 Although they are usually “tentorium” (fig 1).1‑3 pterion.1 2 4 This creates an extradural mentioned as a trio, there are subtle but Where the edges of the falx and tentorium haematoma,2 a potentially lifethreatening important differences to the arrangement of meet the skull, the dura encloses large injury that classically presents with the meninges in the spine and cranium. The venous sinuses that are responsible for decreased consciousness and vomiting aim of this introduction to the meninges is draining venous blood from the brain.1 4 after a lucid interval (an initial period of to clarify the anatomy and link these details These are not to be confused with the air apparently normal consciousness). The to relevant clinical situations. filled cavities in the skull, which are also lucid interval can be a falsely reassuring called sinuses. phenomenon, which can cause a fatal Dura mater Although the brain is insensitive to pain, extradural haematoma to go unrecognised. The outermost of the meninges (singular: the dura is well innervated by branches of With prompt identification and surgical meninx) is the dura mater, commonly called the trigeminal nerve, which carries pain decompression, however, the prognosis is the dura.3 As its name suggests, the dura is sensations experienced as headache.1 2 usually good.2 a tough thick membrane composed of dense In the spine (but not the cranium), there collagen fibres.2‑4 Outside the dura is a physiological space around the dura, In the cranium, the dura is continuous In the normal cranium there is no space known as the epidural space. This space is with the periosteum of the inner surface between the dura and the skull. However, only a few millimetres wide but surrounds the dura all the way down the spine from Bone (calvarium) the foramen magnum to the tip of the Dura mater sacrum. It contains predominantly adipose tissue but also has a plexus of large, thin Arachnoid walled veins, the epidural venous plexus. Subarachnoid space In turn, the epidural space is surrounded by the bones of the vertebral canal.1 Pia mater Insertion of a catheter into the epidural Grey matter (brain) space, usually in the mid‑lumbar region, allows anaesthetic drugs to be delivered close to the spinal cord and its nerve Bone (calvarium) roots. This technique is known as epidural analgesia and is a common method of pain Dura mater relief in labour and after surgery.1 3 Venous sinus Arachnoid mater Beneath and in direct contact with the Grey matter dura, is the arachnoid mater. The arachnoid follows the dura closely throughout the cranium and spine down to the level of S2 White matter where the two layers fuse and terminate. Arachnoid granulation The arachnoid encloses the cerebrospinal Venous sinus (blood) fluid, which circulates around the brain and spinal cord. The space occupied by the Falx cerebrospinal fluid is therefore known as the subarachnoid space.4 The dura and the Dura mater Direction of CSF flow arachnoid can together be referred to as the Trabeculae theca. The intrathecal space is an alternative term for the subarachnoid space (fig 2). The arachnoid is a thin, semi‑transparent Fig 1 The meninges in the cranium membrane that gives off a very fine network student bmj | volume 19 | december 2011 student.bmj.com | 27 education Vertebral body is sometimes needed, the prognosis is usually more guarded than for an Pia Subarachnoid extradural haematoma. mater space Spinal Cerebrospinal fluid cord Cerebrospinal fluid, which circulates in the subarachnoid space between the Dura mater Denticulate arachnoid and pia mater, surrounds and plus arachnoid ligament supports the delicate structures of the central nervous system.3 Although an Transverse adult brain weighs 1400 g, its apparent weight when cushioned by cerebrospinal process fluid is only about 50 g. Cerebrospinal Epidural fluid is produced by a specialised tissue space Ligamentum called the choroid plexus, which is present 4 flavum inside the ventricles of the brain. The Facet joint ventricles are in direct communication with the subarachnoid space, which allows the cerebrospinal fluid to flow freely around the central nervous system.3 4 Anterior The total volume of cerebrospinal fluid in an adult is about 150 mL. Cerebrospinal Spinous process fluid is produced at about 500 mL/ day. Cerebrospinal fluid is resorbed into the bloodstream by the arachnoid Posterior granulations, outpouchings of arachnoid thinly encased by dura mater that extend into the venous sinuses.3 4 Cerebrospinal fluid is a transparent, Fig 2 The meninges in the spine at the mid-thoracic level colourless ultrafiltrate of plasma. Its electrolyte levels, glucose level, and pH of filaments (arachnoid trabeculae), which techniques to place thin coils of metal into are very similar to those of plasma, but it traverse the subarachnoid space from the the aneurysm cavity to cause thrombosis. contains little protein and few cells.3 The inner surface of the dura to the outer surface presence of blood in cerebrospinal fluid is of the brain, where the pia mater lies.1 3 Spinal anaesthesia always abnormal. These trabeculae create the impression The technique of spinal anaesthesia The technique of lumbar puncture of cobwebs, which gives rise to the name involves injecting a small volume of local involves inserting a needle into the “arachnoid.”3 anaesthetic into the subarachnoid space in midline of the lumbar spine, into the In adults, the spinal cord typically ends the lumbar region.3 This produces a rapid subarachnoid space, to obtain a sample of at the L1-2 interspace. In neonates, it can onset of numbness of the lower abdomen cerebrospinal fluid for diagnostic analysis. extend a segment or two further. In both and legs. This anaesthesia is sufficient for The composition of cerebrospinal fluid adults and neonates, the dural sac extends the performance of many types of surgery, varies in different illnesses, including down to S2. There is a degree of normal such as knee joint replacement or caesarean malignancy, infection, and some variation in these levels. section, without the need for general neurological diseases such as multiple anaesthesia. Alternatively this technique sclerosis. Circle of Willis can be used as an adjuvant to general The brain receives its arterial supply anaesthesia to provide postoperative pain Intracranial pressure from the circle of Willis. This arterial relief for similar procedures. Intracranial pressure is the pressure circuit, formed by the union of the basilar inside the cranium. Normal intracranial artery with the two internal carotid Between the dura and the arachnoid pressure is about 7-17 mm Hg when supine. arteries, lies within the subarachnoid Throughout the cranium and the spine of Intracranial pressure can be increased by space at the base of the brain. This a healthy individual, the arachnoid and several pathologies: meningitis, tumour, can be a site of aneurysm formation, the dura are never separated. A potential abscess, or haematoma. Above 25-30 which carries a risk of spontaneous space, the subdural space, exists between mm Hg, raised intracranial pressure can rupture resulting in a subarachnoid the two.1 Sandwiched between the dura interfere substantially with arterial blood haemorrhage. Subarachnoid haemorrhage and arachnoid lie veins that connect the entering the skull, reducing blood flow to is a crucial differential diagnosis to brain’s venous system with the venous the brain. consider in anyone with a sudden and sinuses encased by the dura mater. The The optic nerves are invested in a sleeve severe headache. It can be associated force needed to separate the arachnoid of arachnoid and dura. The optic nerves join with vomiting, seizures, and loss of from the dura is comparatively minor, the retina at the optic disc (the “blind spot”), consciousness owing to the rise in and venous bleeding can create enough which is the only part of the central nervous intracranial pressure, and it is frequently pressure to cause a subdural haematoma system that is visible from the outside, fatal. In those who survive the initial between the arachnoid and the dura.2 by fundoscopy. If intracranial pressure is haemorrhage, treatment is focused on Older people and people with alcoholism raised, the increased pressure is transmitted stabilising the aneurysm to reduce the are particularly at risk of chronic subdural along the optic nerves to the optic disc, risk of rebleeding by applying clips to the haematomas owing to their vulnerability which could cause bulging of the optic disc. aneurysm directly, or by using keyhole to falls. Although surgical decompression This appearance is known as papilloedema, 28 | student.bmj.com student bmj | volume 19 | december 2011 education Cryptococcus neoformans and tuberculous meningitis. Bacterial meningitis is associated with a grave prognosis, and the risk of death or brain damage is very high.6 Meningococcal meningitis refers to disease caused by the meningococcus.