Older siblings, tonsillectomy linked to odds of developing AS

Removing tonsils before age 16 increased chance by 36%

Margarida Maia PhD avatar

by Margarida Maia PhD |

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A child draws on a piece of paper.

Children with one or more older siblings are up to 34% as likely as those who don’t have older siblings to develop ankylosing spondylitis (AS), a large Swedish study found.

Having a tonsillectomy, a surgery to remove the tonsils when they repeatedly become infected before age 16, increased the odds of developing AS by 36%.

“While genetic predisposition is the leading cause of the disease, [early life] environmental factors are also thought to have a role,” according to a press release.

These early-life “environmental factors and particularly infections deserve continued research focus, to elucidate the mechanisms by which they contribute to AS development,” the researchers wrote in “Association of childhood infections and perinatal factors with ankylosing spondylitis: a Swedish nationwide case–control and sibling study,” which was published in RMD Open.

AS occurs when the sacroiliac joints, where the base of the spine meets the pelvis, became inflamed, causing a range of symptoms, which often emerge before age 45.

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Studying AS in Sweden

Besides genetic factors, early-life experiences may contribute to developing AS. “Some studies reported season of birth, birth order among siblings, and childhood infections to be associated with disease development in adulthood,” wrote the researchers, who drew on data from the Swedish National Patient Register to compare exposure to various early-life environmental factors in adults with or without AS.

The study included 5,612 patients (43% women) born in Sweden from 1973 onward who received a diagnosis of AS between 2001 and 2022.

More than one in four patients (27.1%) also had a diagnosis of anterior uveitis, which is an inflammation in the inside of the front of the eye; 7.5% had inflammatory bowel disease; and 7.1% had psoriasis, an autoimmune disease that causes skin rashes with itchy, scaly patches.

Each patient was matched for sex, year of birth, and region of residence to an average number of four controls from the general population (22,042 people total) who didn’t have an AS diagnosis.

Links to AS and older siblings, removing tonsils

Several early-life environmental factors were linked to heightened odds of an AS diagnosis.

For example, having one older sibling increased the odds of developing AS by 12%. Having two or more older siblings increased the odds by 15% compared with having no older siblings.

Children who had a tonsillectomy were 30% as likely to develop AS compared with those who didn’t have the surgery. The link was stronger for those who had a tonsillectomy when they were ages 5 to 9.

To account for potential environmental factors shared within families, the researchers then carried out a sibling analysis where they compared 3,965 patients against their 6,070 siblings who didn’t have AS.

The effect of having older siblings became more pronounced, with 18% heightened odds for one older sibling compared with none. This increased to 34% for two or more older siblings, after adjusting for sex, mother’s age, and year of birth.

“We found that the risk of AS increased with the number of older siblings. This result also held in the sibling comparison, suggesting that the association is not a result of confounding from familial factors,” the researchers wrote. “The mechanism behind this risk increase cannot be determined from our data, but it has been shown that infants with older siblings are more exposed to infections early in life than infants without siblings.”

In the sibling analysis, having a tonsillectomy in childhood increased the odds of developing AS by 36% compared with having no tonsillectomy, after adjusting for potential confounding factors.

Contracting a serious childhood infection or having a twin sibling also increased the likelihood of developing AS, but the link became weaker when considering other factors.

“Having older siblings and a history of tonsillectomy in childhood were independently associated with development of AS, even after adjustment for family-shared factors in a sibling comparison analysis,” the researchers said.