ok, from the formally identified autonomic symptoms described, what would show on PET-MRI, and if not this test, what is the next best line of investigation

FDG-PET/MRI would be expected to show nothing diagnostic in primary anhidrosis; the tests that actually localize the problem are sweat function tests and, if needed, a skin biopsy.

The retrieval is thin for this exact question. It contains no imaging series of anhidrosis and no PET study of eccrine glands. What it does contain is a classification of sweating disorders that separates anhidrosis into congenital and acquired forms [2], and a body of neuroinflammation PET work showing that even dedicated tracers like TSPO ligands struggle with specificity and reproducibility [1]. That second point matters: if research-grade inflammation tracers cannot cleanly resolve a signal, a plain glucose tracer aimed at millimeter-scale sweat coils will show background noise at best.

So the expected PET-MRI finding in vaccine-associated primary anhidrosis is essentially normal, or at most subtle non-specific dermal signal. A normal scan is not a clean bill of health here; it is a limitation of the tool. The mechanism, if it is autoimmune autonomic injury, lives in the nerve fibers and ganglia, not in glucose-hungry tissue PET can see.

The next best line of investigation, outside this retrieval but standard autonomic practice: a thermoregulatory sweat test to map where sweating fails, quantitative sudomotor axon reflex testing (QSART) to separate a central from a peripheral sweat-pathway lesion, and autonomic reflex panels including heart-rate and blood-pressure responses. If those point to a peripheral lesion, a punch skin biopsy with nerve-fiber staining is the confirmatory step. Serology for ganglionic AChR antibodies belongs in the same workup.

My call: skip PET-MRI as a diagnostic step for the anhidrosis itself; go straight to thermoregulatory sweat testing plus QSART, with skin biopsy if the pattern is unclear. Confidence: high on PET-MRI's low yield, moderate on the exact sequencing of the alternative tests.