The mechanical apparatus for breathing consists of the ribs, intercostals muscles, costochondral junctions & the diaphragm. Diaphragm is a muscle located at the base of the chest wall cavity. The ribs move at the joint formed by them with the vertebrae (costovetebral joint). The intercostals muscles are attached to the ribs & are responsible for the movements of the ribs during respiration.
During inspiration, the diaphragm moves downwards & expands the chest cavity. At the same time, the ribs move up & out to expand the thoracic cavity. This movement is akin to a bucket handle (moving up). The movement of the ribs is brought about by the contraction of the intercostal muscles.
In Ankylosing spondylitis, there is inflammation of the costovertebral joint. Later, as AS progresses, the joint may also get fused. This inflammation/ fusion then restricts the movement at the joint & consequently the chest expansion. Physically, this manifests as restricted chest expansion.
Ankylosing spondylitis also leads to inflammation at insertion of intercostal muscles to the ribs (enthesitis). This leads to pain in the rib cage area & also further difficulty in taking a deep breath.
Sneezing involves a rapid & high intensity contraction of the intercostal muscles. It is very painful due to the enthesitis & is almost felt like a ‘catch’
Tight control of AS with NSAIDs/ DMARDs/ biologics/ diet & deep breathing exercises are helpful in reducing this chest pain.