Pectus carinatum (PC), sometimes called “pigeon chest,” is a relatively uncommon chest wall malformation characterized by an outward protrusion of the sternum. The underlying mechanism involves genetic predisposition and abnormal cartilage growth dynamics, which force the breastbone forward. Understanding the causes requires examining the anatomical structures and biological processes that lead to this structural change.
Defining Pectus Carinatum
Pectus carinatum is a structural deformity where the sternum and associated costal cartilages are displaced anteriorly, giving the chest a bowed or keeled appearance. The condition is often asymmetrical and categorized into two main anatomical types based on the location of the greatest prominence.
The most common form is the chondrogladiolar prominence, affecting approximately 95 percent of cases. This type involves the middle and lower sections of the sternum (the gladiolus) and the lower costal cartilages protruding outward. The rarer form is the chondromanubrial prominence, also known as pectus arcuatum, which affects the upper sternum (the manubrium).
In the chondromanubrial type, the upper part of the breastbone juts out while the lower part may be recessed, creating a distinct, high-arched appearance. These anatomical distinctions reflect which segments of the developing chest wall were most affected by the underlying growth abnormality. The core of the problem lies in the localized malformation of the cartilage framework.
Primary Factors Influencing Development
The direct cause of the sternal protrusion in pectus carinatum is the excessive and disorganized growth of the costal cartilage. When they overgrow, they act like overly long struts that push the sternum forward. This overgrowth is thought to be a manifestation of abnormal signaling within the growth plates, leading to hyper-proliferation of chondrocytes, the cells that form cartilage.
The resulting deformity is a mechanical consequence of this biological overgrowth. Some studies suggest the etiology is complex, involving not only longer costal cartilage but also shorter ribs compared to unaffected individuals. This implies a coordinated developmental problem where the chest wall structures are misaligned due to disproportionate growth rates between bone and cartilage components.
In many cases, the specific factor initiating this abnormal cellular activity is classified as idiopathic, meaning the cause is unknown. However, the timing of its progression suggests a link to hormonal changes and rapid skeletal development. The condition frequently becomes noticeably worse during the pubertal growth spurt. This observation reinforces the theory that the primary mechanism is a localized defect in the regulation of growth plate activity.
Genetic Links and Associated Conditions
While the physical mechanism is cartilage overgrowth, the predisposition to this mechanism is strongly linked to genetics and systemic conditions. Approximately 25 to 30 percent of individuals with pectus carinatum have a family member with a chest wall deformity, such as PC or the related condition, pectus excavatum. This suggests an inherited genetic component that affects connective tissue or bone development.
Pectus carinatum is often found in association with specific genetic syndromes that affect connective tissue throughout the body, such as Marfan syndrome and Ehlers-Danlos syndrome. These conditions involve defects in the proteins responsible for forming strong and flexible connective tissues. The structural weakness or abnormal composition of the cartilage in these syndromes can contribute to the overgrowth and subsequent protrusion.
The deformity is also frequently observed alongside other syndromes like Noonan syndrome and Osteogenesis imperfecta. These associations indicate that the abnormal chest wall development is part of a broader, systemic issue related to underlying genetic mutations. For instance, in cases where PC is linked to a specific syndrome like Marfan, the chance of passing the condition to a child can be as high as 50 percent, reflecting an autosomal dominant inheritance pattern.
Diagnosis and Typical Age of Onset
Pectus carinatum is sometimes present at birth, but it is typically subtle and often goes unnoticed in infancy. The condition usually becomes significantly more pronounced as the child enters a period of rapid skeletal growth, most commonly during the adolescent growth spurt (ages 11 to 15 years).
Diagnosis is primarily clinical, beginning with a physical examination by a physician who visually assesses the chest wall and measures the degree of protrusion. Imaging studies are routinely used to confirm the diagnosis, measure the severity, and rule out other associated conditions. Standard chest X-rays can help identify any spinal curvature, such as scoliosis, which is often found in conjunction with PC.
More advanced imaging, such as a computed tomography (CT) scan or magnetic resonance imaging (MRI), may be utilized to accurately quantify the severity of the deformity and its potential effect on the heart and lungs. These scans provide precise cross-sectional images that help doctors assess the extent of the cartilage abnormality and the degree of sternal displacement.