My son is 17 yrs old and thin and about 5'9" tall. His chest is uneven, the left side is higher than his right. It is not muscle, it is clearly bone. It has always been like that but was not as noticeable as he is thinner now. He has no symptoms other than yesterday when he was lifting weights (something he recently started doing) and he felt some "pulling" pain on the left side of his chest. It does not fit into the pigeon chest or anything I have read about since its just one side. You can feel the difference and also can see it. It's not a huge difference, but enough to notice. He has no medical conditions and is healthy.
I can't say for sure because I cannot examine your son, but it sounds like he may have pectum excavatum, in which the growth of bone and cartilage in the anterior chest wall is abnormal, typically affecting 4-5 ribs on each side of the sternum. The defect can range form very mild to severe, and there can be a great deal of asymmetry between the right and left sides (like in your son). The exact mechanism involved in this abnormal bone and cartilage overgrowth is not known.
Some patients with pectus excavatum experience chest and back pain that is usually musculoskeletal in origin. The exact cause of the pain is poorly understood, but is probably contributed to by the poor posture noted in many patients with pectus deformities.
You should take your son to his physician for diagnosis- if it appears that this may be what is going on, he should have Xrays, and possible CT scan. Also, some patients with pectum excavatum have cardiac and respiratory issues, so he may need testing for these if his physician deems necessary.
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