Arachnoiditis occurs when the arachnoid is damaged. The arachnoid is part of a group of membranes that cushion the brain and spinal cord nerves. When this cushioning is damaged, nerves may not function correctly. The nerves in the spinal cord are surrounded by several thin tissues, called meninges, which provide cushioning and protection. The arachnoid mater is the middle tissue of the meninges. Damage to the arachnoid mater can cause swelling and inflammation. This swelling can cause nerves in the spine to stick together, altering nerve function and triggering intense pain. Arachnoiditis is almost always caused by an injury. There are some reports of hereditary arachnoiditis, but most people with arachnoiditis develop the condition because of an injury to the spine. Arachnoiditis is a progressive disorder, which means that it tends to get worse over time if not treated. In some people, it can be debilitating.