Pain below the sternum
Disclaimer: The following explanation is focused on vascular compression syndromes. Of course, and more frequently other explanations may apply. In any case, please turn to your doctor or ask here.
The sternum is a fixation point for the diaphragm. This muscle plate lies horizontally and separates the thorax from the abdominal cavity. The center of the diaphragm is a fibrous plate its periphery consists of muscle fibers which radiate from the fibrous center towards the anchoring points. These anchoring points are the sternum, the ribs, and the spine.
Pain below the sternum might be elicited by the organs under the dome of the diaphragm. From the perspective of vascular compressions, the squeezing of the largest sympathetic nervous plexus, the coeliac plexus, is a pre-dominant cause. This is found in median arcuate ligament syndrome. The reason for this compression is the dragging of the diaphragmatic muscle fibers, the so-called diaphragmatic crura. These form a slit-like opening to let pass the aorta from the thorax into the abdominal cavity. This opening is called the aortic hiatus. Since the left and the right diaphragmatic crus (singular of crura) are attached on the left and the right side of the lumbar vertebra, between them free space is left for the aorta. Both crura unite in front of the aorta and form a sharp angle, similar to a gothic church window. This sharp angle is held together by a sickle-like tight fibrous band, called the arcuate ligament.
When the lordosis of the lumbar spine increases, it drags the arcuate ligament downwards and backwards. The ligament then cuts onto the aorta. But between the aorta and the arcuate ligament lies the coeliac trunk which is surrounded by the coeliac plexus. Thus, not the aorta is directly squeezed by the tightly straddling arcuate ligament, but the origin of the coeliac trunk is squeezed, and the surrounding nervous plexus is jammed under the sharp margin of the arcuate ligament. This permanent cutting pressure causes pain in the epigastric area, which is the triangular area between the ribs and sternum.
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