Thoracic syndesmophytes are common in ankylosing spondylitis, even in the absence of lumbar syndesmophytes, according to a study published in the Journal of Rheumatology.

Sovira Tan, PhD , and his colleagues from the National Institute of Arthritis and Musculoskeletal and Skin Diseases wrote that the current standard for radiographic assessment of ankylosing spondylitis scores only the cervical and lumbar spine because of difficulties visualizing the thoracic spine.

“Consequently, the involvement of the thoracic spine with syndesmophytes has not been widely studied,” they wrote. “If syndesmophytes differentially develop in the thoracic spine, exclusion of this large region may affect the validity of spinal fusion biomarker studies.”

In this study, 18 patients with ankylosing spondylitis who did not show complete lumbar fusion on radiographs underwent thoracolumbar CT and lumbar radiography (J Rheumatol. 2017 Oct 1. doi: 10.3899/jrheum.170340 ). The patients had ankylosing spondylitis for a mean duration of about 19 years, and 11 were treated with tumor necrosis factor inhibitors.

Researchers found syndesmophytes were common overall in intervertebral disc spaces, ranging from 56% to 89%. The rate of syndesmophytes peaked around the thoracolumbar junction, and bridging was both more evident and more extensive in the superior thoracic levels, compared with the lumbar levels.

In all the patients, thoracic syndesmophytes on the CT scan were at least as common as lumbar, and patients with extensive syndesmophytes at multiple lumbar locations typically also had the same in the thoracic region.

However, there were also patients with no, or very few, lumbar syndesmophytes on CT who still had thoracic syndesmophytes, some of which were extensive.

“Importantly, if syndesmophytes were observed on a lumbar radiograph, one could expect there to be syndesmophytes in the thoracic spine, but not the converse,” the authors wrote. “Several patients with normal lumbar radiographs had substantial thoracic syndesmophytes.”

The authors commented that because of its two-dimensional character, radiography has a relatively low sensitivity for detecting syndesmophytes, compared with CT. This could explain why patients with any syndesmophytes in the lumbar region on radiography also had them in the thoracic CT scans.

“By the time syndesmophytes are detectable on lumbar radiographs, structural damage may already have occurred in large portions of the thoracolumbar spine.”

They acknowledged that their method was a research tool and not intended for clinical practice, but also pointed out that CT technology had improved to allow more extensive spine coverage, at an equal effective dose, without compromising scan quality.

The study was supported by the National Institutes of Health. No conflicts of interest disclosures were available.