Arachnoiditis

Excerpt

The meninges are intracranial and intraspinal membranes that protect the central nervous system (CNS) and provide a supportive framework for the brain and spinal vascular structures (see Images. Relationship of the Meninges to the Skull and Brain; Spinal Cord and Meninges). The meninges consist of 3 layers: dura mater (dura), arachnoid mater (arachnoid), and pia mater (pia). The cerebrospinal fluid (CSF) circulates in the subarachnoid space between the arachnoid and pia, delivering nutrients to neural tissues and maintaining electrolyte and extracellular fluid balance within the CNS. The choroid plexuses, located in the ventricular system, produce the CSF, which fills the ventricles and subarachnoid space, providing cushioning and mechanical protection against brain displacement.

The dura is adherent to the skull and the internal surface of the vertebrae and consists of 2 layers: the periosteal layer externally and the meningeal layer internally. The periosteal layer lines the inner surface of the skull, while the fibrous meningeal layer forms a strong membrane continuous at the foramen magnum, where the cranial and spinal dural layers meet. The epidural space is a potential space between the bone and dura that expands when fluid or blood accumulates.

The dural venous sinuses are endothelium-lined cavities separating the periosteal and meningeal layers of the dura. Collections of arachnoid villi, termed "arachnoid granulations," project through the meningeal layer into these sinuses, facilitating CSF absorption into the venous system. The subdural space is a potential space between the dura and arachnoid.

The arachnoid and pia are closely apposed membranes derived from a single mesenchymal layer enveloping the embryonic brain. These layers are collectively referred to as the "pia-arachnoid," "leptomeninx," or "leptomeninges." Fluid-filled intervals within the leptomeningeal layers form the subarachnoid space. Arachnoid trabeculae extending across this space create a web-like appearance, which gives the arachnoid mater its name.

The arachnoid contains collagen, elastic fibers, and fibroblasts. CSF pressure maintains contact between this avascular membrane and the meningeal layer of the dura. The pia is a thin, highly vascularized membrane that adheres closely to the surface of the brain and spinal cord.

The subarachnoid space lies between the arachnoid and pia and, unlike the epidural and subdural spaces, constitutes a true anatomic space containing trabecular cells and blood vessels. Freely circulating CSF within this space supports and cushions the CNS.

Arachnoiditis is a chronic inflammatory disorder involving the arachnoid mater and subarachnoid space, most commonly within the spinal canal. The condition is poorly understood but is frequently associated with prior spinal surgery, infection, subarachnoid hemorrhage (SAH), lumbar epidural injections, exposure to oil-based myelographic contrast agents, or chemical irritation. Idiopathic forms have also been described.

Inflammation produces leptomeningeal thickening, dural adhesions, fibrosis, and nerve root clumping. Fibrotic tissue may envelop the nerve roots, obstruct CSF flow, and progress to chronic adhesive arachnoiditis. Arachnoiditis ossificans, characterized by ossification of the arachnoid following prolonged inflammation, may cause progressive neurological impairment (see Image. Arachnoiditis Ossificans). Advanced disease can manifest with spinal cord swelling or syringomyelia.