Hello and Welcome.
For our members that may not know, the abductor pollicis brevis is a muscle in the hand that functions as an abductor of the thumb. Abduction of the thumb is defined as the movement of the thumb anteriorly, actually away from the rest of the digits.
If you've been reading and researching your SX (symptoms) you must have ran across this PubMed study:
"Referred pain patterns of the abductor pollicis longus resemble the C6, 7, and 8 dermatomes, the superficial radial sensory nerve distribution, and are very similar to the area of pain experienced in de Quervain's tenosynovitis. Thus, identification of the abductor pollicis longus trigger point should be considered in pain of the radial aspect of the wrist and thumb, especially when no other neurologic abnormalities or inflammatory conditions are present."
The above conclusion was found following a "test" to determine referred pain patterns of the Abductor Pollicis Brevis area.
Burning in the Abductor Pollicis Brevis - like all burning pain is most likely related to inflammation, irritation or both of which can cause compression of the nerves in that area. This usually is due to overuse - if not now than at some point.
Tenosynovitis is the inflammation of the synovium that surrounds a tendon.
The synovium is a fluid-filled sheath. That's also a possibility as your research indicates - but that usually is not a burning pain. Tenosynovitis is normally due to a bacteria that has entered that sheath surrounding the tendon. Did you have a puncture injury or laceration to the area - even a small one? It only take a tiny break in the skin to introduce bacteria into our system - in the case of Tenosynovitis, penetrating into the tendon sheath.
Either condition, especially the Tenosynovitis can cause pain to the surrounding structures. Your pain does not fit the norm of osteoarthritis - but I'm far from a physician and that's the medical professional that will have the right answers for you.
I encourage you to consult your PCP - or an ortho that specializes in hands. Obviously we can't DX (diagnosis) you.
I'm sorry for your pain and hope you can find answers. Please keep in touch and let us know what you discover, I'll look forward to hearing more from you.
Sorry. I meant to add that you should consider seeing a neurologist too. They are the best at treating nerve disorders or injuries.
Please let us know how you are doing.