Unlike isolated pectus excavatum or carinatum, Poland's syndrome and other similar but rarer syndromes are caused not by overgrowth of the costal cartilages resulting in a 'sinking' or 'protrusion' of the chest but by what doctors call aplasia or hypoplasia (no or under development) of the chest wall. Presenting at birth this congenital condition affects boys 3 times more commonly than girls. It causes under development or 'missing' chest wall muscles such as pectoral muscles and bones such as ribs as well as affecting the arm and hand.
Other syndromes that can be seen with pectus deformities include a group of disorders collectively called 'connective tissue' disorders. These disorders cause systemic (global) weakness of the connective tissues (the building blocks or scaffold of the body) leading to poor musculoskeletal development. The common associated syndrome is Marfan's syndrome, a genetic condition more common in boys that can cause a cluster of problems including problems with the eyes, heart, lungs and central nervous system as well as pectus deformities. Marfan's patients often have a particular appearance (often described as 'Marfanoid') including being very tall, with disproportionally long slender limbs and long fingers and toes.