Pulsing ultrasound waves through the brain could improve survival after haemorrhagic strokes by clearing inflammatory dead blood cells, a study in mice suggests. The non-invasive approach enhanced lymphatic drainage and reduced brain damage. Researchers plan to test it in people with Alzheimer's disease next year.
Haemorrhagic strokes, which account for about 15 per cent of all strokes, occur when a blood vessel in the brain ruptures, causing bleeding that disrupts oxygen supply and damages cells, often leading to movement and cognitive issues.
Current treatments involve sealing the vessel with a metal clip and invasively clearing dead red blood cells via catheter, which can cause further brain damage or infections, says Raag Airan at Stanford University in California.
Airan explored ultrasound after accidentally leaving a device on too long during mouse experiments, observing enhanced drug distribution in cerebrospinal fluid that clears brain waste. His team mimicked haemorrhagic strokes by injecting blood into mice brains. Over three days, half the mice received 10-minute daily pulses of ultrasound through their skulls; the others got none.
In a behavioural test, ultrasound-treated mice turned left 39 per cent of the time in a tank, compared to 27 per cent for controls, and gripped a metal bar more strongly, indicating less brain damage confirmed by post-euthanasia brain analysis.
One week later, about half the control mice had died, versus a fifth in the ultrasound group. "We increased survival by about 30 [percentage points] with just three 10-minute sessions of ultrasound," says Airan.
The pulses activated pressure-sensitive proteins on microglia, reducing inflammation and boosting clearance of dead cells to neck lymph nodes via increased cerebrospinal fluid flow.
The method could extend to neurodegenerative diseases by clearing smaller toxic proteins like tau in Alzheimer's and Parkinson's. "If ultrasound can drive clearance of red blood cells, which are fairly large, from the brain, it should be able to clear out toxic proteins that are much smaller," says Airan.
"It’s a really impressive study with a tremendous potential for future translation, as it’s non-invasive," says Kathleen Caron at the University of North Carolina at Chapel Hill. Ultrasound is considered safe, though further safety research is needed. Due to the urgency of stroke treatment, trials will first focus on Alzheimer's, starting next year.
The study appears in Nature Biotechnology (DOI: 10.1038/s41587-025-02866-8).