ARACHNOIDITIS IS AN IATROGENIC CONDITION. This means it is CAUSED by a medical procedure. Today, most cases of Arachnoiditis are causes by Epidural Steroid Injections, whether as a treatment for lower back pain or as a spinal block during childbirth.
The spinal cord is intended to be the most pristine part of the body. Any foreign matter introduced into the spinal cord, even your own blood, can cause issues. So if that needle used during Epidural Steroid Injections goes even ONE MILLIMETER too far into the sura and enters the spinal cord, the medication used (no matter WHAT it is) can cause Arachnoiditis.
Other causes include myelograms, surgeries, meningitis, and extreme blunt trauma to the spine.