Relentless multi-dermatomal neuropathic pain and fasciculations causes?

[timsdeece]

I have a 22 month history of unmanageable and unremitting, subjective, severe bilateral (left > right) multi-dermatomal pain which radiates from my lumbosacral area, as well as fasciculations and intense axial lumbar spine pain. This all came upon me acutely in Mar. 2018 following a year of manageable and mild, seemingly mechanical, back pain issues (early 2017 – Mar. 2018; with an MRI in Nov. 2017 showing a moderately degenerative, mildly bulging disc at L4/L5 with no neuro-compression). I had an updated MRI in Mar. 2018 following the acute massively exacerbating event which showed only the same diseased disc at L4/L5 apparently basically unchanged; there was no evidence of neuro-compression or in general no structural lesions which could explain the complaints of very severe neurological pain. The rest of my spine was unremarkable.                                                            
                                                                      
Despite this, initially all of the symptoms were suspected to be of lumbar spine aetiology, however multiple tests and treatments under this assumption have failed to ameliorate the neurological symptoms specifically. As well as that, the described symptoms do not seem to correspond completely with the level where there was spinal disease (there is, for example, fasciculations at the bottom of the foot, pain along the left sided S1 dermatome and diffuse stinging and burning in the entire lumbosacral area).

I spent ten months (Mar. 2017 – Jan. 2019) effectively bed-bound as I had no sitting, and very little standing, tolerance. I had initially two caudal epidural injections in Sept. And Dec. 2018, respectively which did not affect the neurological symptoms. I then had an artificial disc replacement of L4/L5 (Jan. 2019) which interestingly resolved my axial lumbar spine pain but had no effect on my neurological symptoms. Finally, I had targetted nerve blocks and a pulsed radiofrequency ablation of the dorsal root ganglion at L4/L5 in Sept. 2019 to definitely check if the pain was generated by L4/L5 radiculopathy but this examination and treatment again had no effect - that is, they showed L4/L5 exiting nerve roots are presumably not the (sole) problem.                                                                                            
                                                                                        
We (any doctor I can get to listen) are now investigating underlying peripheral nerve disease as a superimposed issue with a different aetiology. In Aug. 2019 I had an EMG / NCS which was unremarkable. However my clinical picture (very functionally debilitating, unremitting pain which leaves me mostly home-bound) suggests further testing is necessary. Normal MRI of the lumbar, thoracic, cervical spine and brain, normal CT of the lumbar spine and flexion/extension x-rays have been unrevealing (Aug. 2019) as have several blood tests (Mar. 2018: CRP, ESR, FBC, Glucose, PSA June 2019: ACE, Albumin, C(3,4), CCP, CK, Comp. Metabolic Profile, CRP, CTD Screen, eGFR, Electrophoresis, FBC, Ferritin, Free T4, HbA1c DCCT Derived, HbA1c IFCC, HBSAG, Hep. C, HIV 1+2, Ig(A, G, M),  Lyme, Neurtophil Cytoplasmic Ab,  Rheumatoid Factor,  Serum Folate,  Total Protein, TSH, tTG, Urate, Vitamin (B12, D) Aug 2019: HLA-B27, FBC, Comp. Metabolic Profile, C3, C4.

Due to the acuity, the lack of mechanical dependence of symptoms (present when completely recumbent), anatomical distribution (asymmetric, non-length dependent) and results of various blood tests, a single-organ inflammatory issue is suspected, potentially microvasculitic. I have taken 1 mg / kg of oral prednisone for over 9 weeks now with no marked improvement of symptoms. However in systematic reviews of non-systemic vasculitic neuropathies the authors warn that resolution of nerve ischaemia can lead to more pain and that pain in general is not a good measure of disease activity. So I'm unsure what to think.

I feel the following tests are needed:                  
                                                                                                                                    
    1. A magnetic resonance neurography of the lumbosacral exiting nerve roots, plexus(es) and peripheral nerves with analysis.                                                      
    2. A nerve biopsy of clinically involved nerves at a proximal and distal location with analysis.
    3. A skin punch biopsy at two locations to be sent for intra-epidermal fibre density analysis.
    4. A robust diagnostic nerve block procedure.
    5. Autonomic testing.
    6. A SPEC/CT scan.

My queston is this: does anyone out there have any idea what might be going on and can you point me in the direction of a disease or test I may not be aware of?
I feel like some auto-immune inflammatory problem selectively affected mostly small fibers (but also affecting large hence the fasciculations) is most likely. Perhaps a self-limiting variant of non-diabetic lumbosacral radiculoplexus neuropathy. The pain is really absolutely woeful and high dose opioids barely keeps it in check.