Low-Dose CT Better Measures Spondyloarthritis Bone Formation

Ingrid Hein

October 08, 2018

GHENT — Low-dose CT scan is more effective for the evaluation of new bone formation in progressive spondyloarthritis than x-ray, new data show.

"You can see much more bone formation over a shorter period of time than with conventional radiography," said Désirée van der Heijde, MD, from the Leiden University Medical Center in the Netherlands, here at the International Congress on Spondyloarthritides 2018.

"The main advantage is that low-dose CT scans offer a much clearer look at changes in the thoracic spine," she told Medscape Medical News. It "doubles the number of vertebrae you can assess, and it's in 3D and it's high resolution."

Although radiography has been the gold standard until now, it's important to realize that it does not capture everything, she explained. There is a lot more that rheumatologists can evaluate to gain better insight into how to treat axial spondyloarthritis more effectively.

For example, "we need to be able to study whether a novel treatment can slow bone formation. If it can, it would mean the drug has the capacity to modify structure," said van der Heijde. Otherwise, the medication might very well be acting simply as a pain killer to improve signs and symptoms.

New Scoring Method

To date, the modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS) for conventional radiography has been the most reliable and sensitive method to monitor disease progression. However, because ankylosing spondylitis progresses slowly, it takes a minimum of 2 years to get enough information to make a measurement with this method.

To understand whether biologic drugs can inhibit progression and to have a more precise understanding of disease progression, that's just too long, said van der Heijde.

Recently, she and her colleagues developed a new method to score disease progression using low-dose CT (Ann Rheum Dis. 2018;77:371-377). The CT Syndesmophyte Score (CTSS) uses measurements of the lower half of a vertebra, the intervertebral disc space, and the upper half of the next vertebra to assess disease progression.

Low-dose CT picks up changes in all three parts of the spine — something a radiograph can't do, said van der Heijde. With conventional radiographs, the inflammation and fat metaplasia characteristic in the sacroiliac joints of the cervical and lumbar spine can be assessed reliably, but you can only assess the anterior corners of the vertebrae, she explained.

With low-dose CT scan, we can see the entire rim of the vertebrae.

"With low-dose CT scan we can see the entire rim of the vertebrae," she reported. "And you can see more than four times the syndesmophytes. You see them growing."

Syndesmophytes — bony growths originating inside a ligament — are important because they are characteristic components of the spine pathology of ankylosing spondylitis. Imagery showing their growth has the potential to clarify the pathogenesis of the disease, and can lead to more effective treatment, said van der Heijde.

She and her colleagues assessed 50 patients with ankylosing spondylitis, and compared the CTSS scoring method for low-dose CT with mSASSS for conventional radiography over a 2-year period.

With CTSS, scoring is based on the amount of intervertebral disc space the syndesmophytes occupy. Patients who have both new syndesmophytes and growing existing syndesmophytes receive a higher score.

Bridging syndesmophytes were identified in more patients with CTSS than with mSASSS (30% vs 6%).

With low-dose CT, it became clear that syndesmophytes have preferred locations around the vertebral rim, and were not randomly distributed, the researchers report.

"I am convinced that this method gives us much more specificity," said van der Heijde.

We are really working on developing imaging methods that have a high sensitivity to show change, she said. "The next step will be to look at facet joints using CT scans."

The drawback of the CTSS method is that CT requires a high dose of radiation, although that is changing as technology improves. Already, multislice scanners are producing a good-quality image of the entire spine in three planes with a maximum radiation dose of 4 mSv, "and this can be reduced to about 2.6 mSv," van der Heijde explained.

But CTSS cannot be widely adopted for clinical practice "until we can lower radiation dose further," she added.

Even low-dose CT exposes patients to more radiation than rheumatologists are comfortable with. The thyroid gland should not be exposed to too much radiation, she said.

A "Promising Tool"

The challenge is to measure disease progression more quickly, said Robert Landewé, MD, PhD, from the Amsterdam Rheumatology and Clinical Immunology Center.

"In clinical trials with placebo, 2 years is just enough to measure progression; 1 year doesn't offer enough contrast," he told Medscape Medical News. A more precise signal and better images will offer more statistical power, which means fewer patients will be needed to do clinical trial research.

Researchers are now starting to use low-dose CT with a subgroup of patients in trials, he said.

"We hope that low-dose CT can reduce the time to 1 year," he added. "We are just seeing the first study results, but this looks like a promising tool for the future."

International Congress on Spondyloarthritides (SPA) 2018. Presented October 6, 2018.

Follow Medscape Rheumatology on Twitter @MedscapeRheum and Ingrid Hein @ingridhein

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