Arachnoiditis

Excerpt

The meninges are membranous intracranial and intraspinal structures protecting the central nervous system (CNS) and providing a framework for the brain and spinal cord's vascular structures (see Image. Relationship of the Meniges to the Skull and Brain). The meninges consist of 3 layers: dura mater (dura), arachnoid mater (arachnoid), and pia mater (pia) (see Image. Spinal Cord and Meninges). The cerebrospinal fluid (CSF) circulates in the subarachnoid space—the space between the arachnoid and pia. This fluid delivers nutrients and helps maintain electrolyte and extracellular fluid balance in the CNS. The choroid plexuses of the 4 brain ventricles produce the CSF. The fluid exits the ventricular system and enters the subarachnoid space, where it acts as a cushion.

The dura mater adheres to the skull and vertebrae's internal aspect and has 2 sublayers: the external periosteal and internal meningeal layers. The external periosteal layer lines the internal periosteal surface. The fibrous internal meningeal layer is a strong membrane that is continuous at the foramen magnum, where the brain and spinal cord's dura layers meet. The epidural space is a potential space between the bone and dura mater that can enlarge with fluid or blood accumulation.

The dural venous sinuses are endothelium-lined cavities separating the periosteal and meningeal dural sublayers. Collections of arachnoid villi called "arachnoid granulations" protrude into the dural venous sinuses through the meningeal dural sublayer, where the CSF enters the venous system. The subdural space is a potential space separating the dura and arachnoid.

The arachnoid and pia mater closely adhere to each other, having developed from a single mesenchymal layer wrapping around the embryonic brain. Thus, these coverings are often referred to as "pia-arachnoid," "leptomeninx", or "leptomeninges." Fluid-filled spaces in these layers give rise to the subarachnoid space. Arachnoid trabeculae passing within the leptomeningeal layers resemble a spiderweb, thus the arachnoid mater's name.

The arachnoid matter contains collagen and elastic fibers and fibroblasts. CSF pressure closely applies this avascular membrane to the meningeal dural layer. The pia mater is a thin, highly vascularized membrane adhering tightly to the brain and spinal cord's surface.

The subarachnoid space is the space separating the arachnoid and pia. Unlike the epidural and subdural spaces, the subarachnoid is an actual space, as it contains trabecular cells and blood vessels. Freely flowing CSF suspends the CNS in this space.

Arachnoiditis is a persistent inflammation of the arachnoid mater and subarachnoid space, mostly in the spinal cord. The condition is poorly understood, though it is often related to prior spinal surgery, infection, subarachnoid hemorrhage (SAH), lumbar epidural injections, injection of oil-based myelographic contrast agents, and chemical irritation. Arachnoiditis may also be idiopathic.

The condition induces leptomeningeal thickening, dural adhesions, scar tissue formation, and, often, nerve root clumping. Scar tissue can encapsulate the nerve roots, impede CSF flow, and lead to chronic adhesive arachnoiditis. Arachnoiditis ossificans is arachnoid mater ossification that may arise from prolonged inflammation and produce progressive neurological debility (see Image. Arachnoiditis Ossificans). Spinal cord swelling and syringomyelia are some of the manifestations of advanced arachnoiditis.