vaccine induced primary anhidrosis. What would an MRI-PET scan with glucose radio tracer expect to show

Short answer: an FDG-PET would most likely be normal, because glucose PET does not image sweat glands or the small nerves that drive them; no retrieved study has ever scanned vaccine-induced anhidrosis.

Nothing in today's retrieval covers vaccine-induced anhidrosis at all. The closest records are indirect. A drug-induced case (zonisamide) showed postganglionic sweating failure with structurally normal sweat glands on biopsy and normal nerve terminals, recovering after the drug stopped [2]. In diabetes, anhidrosis appears as widespread patchy loss on thermoregulatory sweat testing, a test that does not involve PET [3]. The practical bedside tests for sudomotor nerve loss are sweat tests, skin biopsy of sweat-gland nerve fibers, and electrochemical skin conductance, which only modestly tracks biopsy findings (predictive power around 0.73, not a replacement) [4]. None of these is a glucose PET finding.

FDG-PET measures glucose use, and its validated use here is the brain. In a mouse model of autonomic failure, altered glucose uptake appeared in autonomic control regions: reduced uptake in the brainstem, basal forebrain, and amygdala, with increased uptake in the hypothalamus [1]. If a vaccine injury targeted central autonomic centers, that is the pattern you might hypothesize: hypothalamic and brainstem metabolic changes. But that is extrapolation from mice, not human evidence. If the injury is postganglionic, at the sweat gland nerve endings, the brain PET would be expected to look normal, and the informative tests would be the sweat tests and biopsies above [2][4].

My call: expect a normal or non-diagnostic FDG-PET; the scan that answers this question is thermoregulatory sweat testing plus skin biopsy, not glucose PET. Confidence: moderate, and the retrieval gap on vaccine-induced anhidrosis specifically is total.