Researchers used the world's strongest MRI, the ultra-high field 7 Tesla - one of two in Australia - to see whether the conditions mirrored the same effects on brain structure.

Scans showed the brainstem was significantly larger in myalgic encephalomyelitis/chronic fatigue syndrome patients and long COVID patients, compared with those who did not have the same ailments, lead study author Kiran Thapaliya said.

"It also showed similar volumes of the brainstem in patients which could be the reason long COVID patients exhibit all common core symptoms of ME/CFS," Dr Thapaliya said.

"We also discovered smaller midbrain volumes were associated with more severe breathing difficulty in (both sets of) patients.

"Therefore, brainstem dysfunction in encephalomyelitis/chronic fatigue syndrome and long COVID patients could contribute to their neurological, cardiorespiratory symptoms, and movement disorder."

The researchers, from Griffith University, used the MRI to investigate the brainstem and its sub regions as it could discover abnormalities that other scans were unable to detect, National Centre for Neuroimmunology and Emerging Diseases director Sonya Marshall-Gradisnik said.

Previous studies showed that up to 43 per cent of people infected by COVID developed long COVID, and symptoms of long COVID and chronic fatigue syndrome were similar.

The Griffith University research findings were published in Frontiers in Neuroscience.

Australian Associated Press