The brainstem nucleus that fails first
There is a small nucleus in the brainstem — the locus coeruleus — that is the brain’s main source of norepinephrine. It is also unusually vulnerable.
In Parkinson’s disease, the locus coeruleus is affected before the dopamine-producing substantia nigra, and human imaging now uses neuromelanin-sensitive MRI to track its integrity; one Parkinson’s subtype shows early degeneration of both nuclei (Zhou et al. (2023)).
The ME/CFS parallel — and the crucial difference. The selective norepinephrine deficit of ME/CFS shows a metabolite pattern consistent with locus-coeruleus involvement (norepinephrine down, dopamine preserved) — the same kind of pattern seen early in Parkinson’s. But no one has examined the ME/CFS locus coeruleus at the resolution needed to detect degeneration: no imaging or post-mortem study has looked. So degeneration has not been ruled out — it simply has not been demonstrated. If the failure is functional (underperforming rather than dying), that would be a different and more hopeful situation.
Why the distinction matters. Degeneration is largely irreversible; functional failure is, in principle, reversible. Whether the ME/CFS failure is functional or degenerative is untested — the test would be 7T neuromelanin imaging, which should show normal locus-coeruleus volume if the failure is functional, unlike in Parkinson’s.
The caveats. This parallel is an inference; no one has imaged the locus coeruleus in ME/CFS at this resolution. And the underlying norepinephrine finding is single-centre and unreplicated.