Flushing out your nasal cavity is a common method for clearing sinuses, but it could also be a vector for deadly, brain-eating amoeba. At least, that's what can happen if you don't carefully follow the instructions, as a new case report has made all too clear.

Using a neti pot or another kind of personal irrigation device helps many people to breathe easier by washing out and moistening their nasal passages, but the fatal experience of one Seattle woman highlights the potentially extreme danger of not using sterile water with these kinds of health aids.

"When I operated on this lady, a section of her brain about the size of a golf ball was bloody mush," neurosurgeon Charles Cobbs from Swedish Medical Centre told The Seattle Times.

"There were these amoeba all over the place just eating brain cells. We didn't have any clue what was going on, but when we got the actual tissue we could see it was the amoeba."

About a year before this, the 69-year-old woman in question had developed a chronic sinus infection.

When medicine didn't relieve her symptoms, her doctor suggested using a saline irrigation device to clear her sinuses. These devices flush the sinus system with a saltwater solution that cleans out the nasal passage, but as guidelines advise, it's important to only use water free of any infectious organisms.

This means using purchased distilled or sterile water, or using boiled and cooled tap water that has been boiling for between 3 to 5 minutes. Aside from this, the neti pot or other device used also has to be sterile, which means scrubbing down in between uses.

Unfortunately, the 69-year-old woman in this case didn't comply with these instructions. Instead of using sterile water with her device, she cleaned her sinuses out with filtered tap water, which hadn't been boiled.

After a month of treating herself this way, her sinus infection had developed into a large rash on the bridge of her nose, along with raw red skin at the nasal opening.

Despite several visits to her dermatologist, the mystery behind these strange symptoms – initially suspected to be a kind of rosacea – remained unsolved, although a sudden decline in her health a year later did provide some answers.

About a year after the nose rash showed up, the patient experience a seizure, with the woman losing cognition, and the left side of her body shaking.

A CT scan revealed what looked like a tumour about the size of a small coin in the motor cortex on the right side of her brain. A biopsy showed necrosis consistent with such a tumour, but when the patient returned only days later with new symptoms, further analysis revealed lesions were spread throughout her brain.

"On postoperative day 19, the consulting neuropathologist at Johns Hopkins University suggested the possibility of amoebic infection," the case note explains.

"Subsequent histopathological evaluation from the second resection revealed clear evidence of amoebic infection and dramatic haemorrhagic necrosis."

While it's not known for sure how the woman contracted the amoeba infection, the researchers suspect the organism – Balamuthia mandrillaris – got into the woman's brain via "improper nasal lavage", first entering her bloodstream, before setting up home in her brain.

What's not in doubt is that infections like this are very rare. In fact, if the team's hypothesis is correct, it represents the first first case of B. mandrillaris brain infection from nasal lavage, although another kind of amoeba – Naegleria fowleri – has previously been reported infiltrating humans this way.

"The pathologist was able to look at it under a microscope and see the characteristic, actually the amoeba, in the tissue," Cobbs told Q13 FOX.

"This is extremely rare. This amoeba was not even known 20 years ago hardly. There's been about 200 cases world-wide."

Sadly, once the woman in the case had her culprit identified, it was too late. Despite aggressive anti-amoebic therapy, her condition deteriorated, and in litte over a week, she was dead.

While the rarity of these kinds of infections means we shouldn't unduly panic about them, at the same time, researchers are reminding people that if they're going to use devices like neti pots to clean their sinuses, it's imperative to do so safely, by following all the guidelines.

"The reason that you can get brain infections by nasal irrigation, as opposed to swallowing tap water or bathing in tap water, is that the roof of the nose is one of the only parts of the human body where there's a direct extension of the brain and central nervous system into the outside world," otolaryngologist Ben Bleier from Harvard Medical School, who wasn't involved with the case, told TIME.

"We still think it's very, very safe to use. You just have to do it in a clean way."

The findings are reported in the International Journal of Infectious Diseases.

Editor's note (11 Dec 2018): The headline of this article has been updated to more sensitively reflect the tragic nature of this case. We apologise for any offense caused.