Arachnoiditis can occur after spine surgery as it can after a number of events. See the section on arachnoiditis on the web site.
Normally, arachnoiditis symptoms occur with a slow onset and are chronic. It is unusual for flair-ups and remissions although I have seen that on rare occasion.
I assume your arachnoiditis is located at L4-5 where your prior surgery was. Numbness from the waist down does not fit the symptoms expected for this disorder unless in the unlikely event that the root clumping is located high up in the lumbar spine.
Numbness at the waist level would start with the L1 and L2 nerve roots. Again, assuming this arachnoiditis is at the L4-5 level, the L1 and L2 roots would not be involved. I would have a neurologist work with you to determine the real diagnosis is.
Dr. Corenman
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.