July 31 (HealthDay News) -- People whose blood relatives have the joint disease ankylosing spondylitis (AS) are at increased risk not just for AS, but for inflammatory bowel disease (IBD) as well, an Icelandic study shows.

Looking at data from six generations of Icelanders, the researchers discovered a familial link between IBD and AS, an inflammatory disease that primarily affects the spine.

People with AS often suffer from chronic bowel disease and gastrointestinal distress, and both AS and IBD are known to run in families. However, prior to this study, no link had been identified between the two conditions.

According to the researchers, Icelanders provide an ideal study population, because the environmental and lifestyle factors are the same throughout the country. Iceland also maintains an extensive genealogical registry, as well as a half century of records on both AS and IBD sufferers, providing a mine of data for researchers.

Researchers at Landspitali University Hospital, Reykjavik, analyzed the databases for the occurrence of IBD and AS among relatives and the risk of inheriting either and both disorders.

Health information from more than 200 people with AS and more than 1,350 people diagnosed with IBD, as well as genealogic data from more than 790,000 Icelanders, revealed extensive clustering of AS and IBD in families.

First-, second- and third-degree relatives of people with either disorder had increased risks of developing the same disorder, with the risk decreasing as people moved further out on the family tree. First-degree relatives are immediate blood relatives; second-degree relatives are aunts, uncles, nieces, nephews and grandparents; and third-degree relatives are those who share one-eighth of the genetic code, such as first cousins.

The researchers write in the August issue of Arthritis & Rheumatism that the key finding was the increased cross-risk for close relatives of AS and IBD sufferers. In other words, blood relatives of an IBD sufferer also had a greater risk of having AS, and blood relatives of an AS sufferer were at greater risk of IBD. This is the first study to demonstrate a possible hereditary pattern between the two conditions, wrote the researchers, although the specific genetic mechanism has yet to be identified.

"Our results provide strong evidence that a molecular-genetic approach should be utilized in patients with these diseases," the study's lead author, Dr. Bjarni Thjodleifsson, said in a prepared statement. "If this approach proves successful, this will open the possibility of identifying a common early pathophysiologic event in both AS and IBD that may be amenable to new and selective treatments."

The Spondylitis Association of America estimates that there are at least 500,000 people in the United States with AS, although many cases go undiagnosed. Inflammatory bowel disease is thought to affect up to 600,000 people every year.

SOURCE: Arthritis & Rheumatism, news release, July 30, 2007