The rare phenomenon has only affected five other people in the world

The unidentified woman sought help 10 years ago and was diagnosed with PCOS

Hormonal treatment stopped the eyelash growth for six years

When she came off medication, the hairy mouth worsened

Doctors are still clueless to the exact cause, but say PCOS plays a part

An Italian woman who grows eyelash-like hairs in her mouth is only the fifth case to ever be reported.

Doctors are completely baffled as to why the rare phenomenon occurs, but believe polycystic ovary syndrome (PCOS) may be to blame.

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When the unidentified 25-year-old first sought help 10 years ago, tests showed she had abnormally high levels of testosterone.

It's characteristic of PCOS, and causes excess hair growth. Hormonal treatment at last stopped the eyelashes growing.

However, six years later, the woman's hairy mouth worsened after she came off her PCOS medication.

Even after doctors looked at samples of her gum tissue under the microscope, they were left clueless.

The cells that line the inside of the mouth and skin cells on the body which sprout hair are different, but may cross over, the team suggested.

An Italian woman grows eyelash-like hairs out of her gums which has only affected five other people in the world. Pictured, the hairs in between her teeth

The woman's hairy mouth got worse when she stopped taking medication for PCOS

Doctors at the University of Campania Luigi Vanvitelli in Italy, treated the woman with gingival hirsutism, Science Alert reports.

The woman first went to the doctor in 2009, aged 15. She had hairs in the mouth, as well as on the chin and neck.

Tests showed she had high testosterone levels, as well as numerous cysts on her ovaries. Therefore, she was diagnosed with PCOS.

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The condition causes woman to grow excess hair in places such as their chest, as well as weight gain, acne and sometimes infertility.

Birth control pills can help relieve symptoms, and it did for the Italian woman for six years.

For unknown reasons, she stopped taking medication for her PCOS, at which point her gingival hirsutism came back more severe.

This time, the medical team didn't just remove the hairs, but a small section of tissue from a her gum to take a closer look under the microscope.

WHO ELSE HAS BEEN DIAGNOSED WITH EYELASHES IN THE MOUTH? A 57-year-old man: The eyelash-like hair was discovered during his autopsy. A 45-year-old man: A single hair was growing in the mouth of a man who claimed the hair had been there since adolescence. During investigations, it was found an oil-producing gland was also present. A 13-year-old boy: Multiple black hairs measuring 4-6mm in length were found in the mouth of a boy who had alopecia. They bizarrely stopped growing with the alopecia treatment. An 11-year-old boy: He complained of rapid oral hair growth in the previous five days all over the mouth. Some were on his tongue. A 30-year-old man: The man had one singular hair in the middle of his tongue. He had been plucking it for three years, but it grew back quickly.

They found a hair shaft pushing its way through unusually thickened tissues of her gums.

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The inside of the mouth - as well as the nose, vagina and anus - is made of mucosal tissue, consisting of epithelial cells.

The researchers said the mucosal tissues inside the mouth are closely related to the tissues that build our skin because they are derived from the same place when we are an embryo.

It's possible, therefore, the oral space ends up with hair and oil-producing cells. The latter is 'common', according to the doctors. But hair producing cells is practically unheard of.

This is the first time gingival hirsutism has been reported in a woman in medical literature. Five men have been reported with the condition.

PCOS was ruled out as the 'cause' of gingival hirsutism, although was thought to fuel the severity, because PCOS is not diagnosed in men.

A year on the woman's condition had worsened, with hairs growing between even more teeth on the top and bottom.

It's not clear if the woman recovered or if she was treated.

The authors wrote: 'Although no systemic cause seemed to exist in some cases reported in the literature, in our case, it was important to perform a deep investigation.'

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The case was published in the journal Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology.