@esouppain Thank you for the detailed explanation. The solution the surgeon is proposing is suggesting that there is spinal cord compression and compression of the nerve roots between L3 and L4. Severe thecal sac compression indicates severe spinal cord compression. All of these findings are consistent with your symptoms. I don't know why this happened for you, but if it is a disc herniation where the contents of the jelly like nucleus inside the disc is extruded, that will cause inflammation and likely cause bone spurs to grow. Bone also remodels due to pressure, and when a disc herniates it is tipped one way or another and that changes the pressure on the "end plates" of the vertebra bone (the ends) also encouraging bone grown. If a disk herniates the material into the foramen (the space where nerve roots exit the spinal cord between vertebrae), it causes inflammation there and bone growth that will begin to compress the nerves. Another factor in spinal cord compression can come from a ligament on the back of the spine running the length of it that can become enlarged. Sometimes that is removed during surgery.

I do think second opinions are important because there is no going back after you have surgery. In saying that, you also have to consider what benefit you may receive from surgery with a good surgeon vs the risk you'll face of any complications that pertain to your situation. Another very important question to ask is what will happen if you do not do surgery? How will your present condition progress as you age? Will you become permanently incontinent or wheelchair bound? Will you require a paid caregiver to assist with daily living or end up in assisted living? How much will that cost? Paying for caregiver help is very expensive and it's hard work with a lot of burnout so many caregivers quit, and it is hard to find them. Caregivers do not do lifting, as that is a different category of more costly hired help. This I know because I was a caregiver for my parents with my dad at end stage heart disease who could no longer get in and out of bed or get to the bathroom. I had to use a Hoyer lift because I didn't have the strength to lift him, and this was also during the time that I needed spine surgery myself with a herniated cervical disc.

Cleveland Clinic is similar to Mayo and would be a good place for a second opinion. Get as many opinions as you need to make an informed decision. That may take time, and you are already in an urgent situation, so choose your surgeon wisely. Surgeons will not be able to tell you how much time you have before incontinence becomes permanent because the nerves have died from compression.

Cleveland Clinic does recognize arachnoiditis and here is a link to the information about it. It is important to let the surgeon be the expert and give you the diagnosis. It's very easy to read something online and believe that you have it because many conditions have overlapping symptoms, and it is the job of a specialist to figure out which condition is producing which symptom. It takes many years of training for that knowledge and carefully listening to the patient.
https://my.clevelandclinic.org/health/diseases/12062-arachnoiditis
All surgery creates scar tissue. My cervical surgery created some scar tissue that will tighten up, and I periodically stretch it out with my hands. I do a technique I learned from my physical therapist called myofascial release. This is a way to stretch overly tight fascia that binds everything together in our bodies. It can help break up surgical scar tissue. I have another spot I work on on my shin because of surgery for a fractured ankle that created scar tissue that pulls through the joint. I couldn't figure out why my ankle was sometimes collapsing in pain until I figured out that link and stretched it out and it stopped that from happening. So there is hope. You may want to speak with a physical therapist who is an expert in myofascial release to ask what can be done after spine surgery. A consult with MFR specialist could answer questions and aid in your surgical decision. They also may know about various spine surgeons after treating patients post operatively.

You can search for a MFR provider at this link: https://www.mfrtherapists.com/

Here is our discussion on Myofascial Release Therapy (MFR) :

Neuropathy - "Myofascial Release Therapy (MFR) for treating compression and pain"
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
Have you heard of Myofascial Release Therapy before?