Question: Which ICD-10-CM code should I report for ankylosing spondylitis of the thoracic region? Texas Subscriber Answer: You should report code M45.4 (Ankylosing spondylitis of thoracic region) for ankylosing spondylitis of the thoracic region. Ankylosing spondylitis is an arthritic disorder of the spine. Although the exact cause of ankylosing spondylitis is not known, experts believe that a patient’s genetics influences this condition. Many patients with ankylosing spondylitis test positive for the gene HLA-B27.