I am wondering this myself.
Was diagnosed with childhood migraines, then shingles at age 7, costochondritis, then fibromyalgia, then central sensitization, then idiopathic small fiber polyneuropathy (without biopsy) and occipital neuralgia, bruxism and burning mouth syndrome. I have extreme anxiety, often completely unprovoked.
Lastly was finally diagnosed as AuDHD at age 68.

Have many, many foods intolerances and sensitivities (burning, deep agitated itching, rashes, flushing), temperature regulation issues, extreme sensory hypersensitivities to anything touching my skin, dry eyes/mouth/nose. Even my ears itch inside.

Autoimmune testing has been fully tested twice, negative results.

Now they are testing for histamine and/or MCAS as food intolerances have become more prominent and awful.
I always feel better if I don't eat anything, except for the hunger part. Have lost weight and muscle mass, now age 72.
Many GI issues through the years (leaky gut, extreme bloating, fatty liver, nausea, vomiting, constipation with occasional diarrhea). Colonoscopy showed inflamed gastric lining, inflamed esophageal sphincter, and fatty liver). With each new finding, I have adjusted diet, avoiding more foods or sun or household products.

I have become a recluse as I cannot tolerate anyone's perfumes or lotion. I stopped going to church or any function.
The anxiety issue has become a bigger factor in triggering all this, even when I don't know what could be causing the anxiety; it often just appears with heart pounding, body spasm and pain, jaw clenching, electric zaps and often, migraines which causes fever and widespread fatigue and burning, itching, rashes.

Ice packs reduce the burning a bit and I sleep with softly covered ice packs to reduce burning in low back, pudendal area, elbows, feet and hands.

Since my SFN was never biopsied, I often wonder if perhaps this was severe MCAS all along. Perhaps we could treat the MCAS with better results, as something HAS to get better.

Doctors just seem to think I'm a complaining older woman, pay little attention. However, my symptoms began in my preschool years, so ?
I keep searching for answers.