Pectus carinatum (PC), commonly called “pigeon chest” or “keel chest,” is a structural deformity of the anterior chest wall. The condition involves the sternum (breastbone) and the attached costal cartilage protruding outward, giving the chest a prominent appearance. This congenital deformation is the second most common chest wall abnormality, following Pectus Excavatum. The outward bulge is typically not present at birth but develops and becomes more noticeable as a child grows, often causing cosmetic concern.
Defining Pectus Carinatum and Its Classifications
Pectus carinatum is characterized by the forward projection of the sternum and ribs. This contrasts with Pectus Excavatum, which involves an inward, sunken appearance. PC is estimated to affect approximately one in every 1,000 to 1,200 teens and is observed more often in males than in females, typically at a 4-to-1 ratio.
Pectus Carinatum is classified based on the specific area of the sternum that protrudes. The most common form, accounting for roughly 95% of cases, is the Chondrogladiolar type, also known as keel chest. This type involves the middle and lower sections of the sternum (the gladiolus) being pushed forward.
The less common subtype is the Chondromanubrial type, sometimes called Pouter Pigeon breast. This variation features the upper part of the sternum (the manubrium) as the most prominent area. In some cases, this type presents as a mixed deformity where the upper sternum protrudes while the lower sternum is mildly sunken.
The Structural Mechanism of Protrusion
The physical mechanism underlying Pectus Carinatum is the abnormal growth of the costal cartilage. Costal cartilage connects the ribs to the sternum, forming a flexible bridge in the front of the rib cage. The issue arises when this cartilage overgrows or develops disproportionately compared to the surrounding bony structures.
This excessive cartilage growth exerts constant internal pressure on the sternum. Since the sternum is rigid, the pressure gradually forces it to buckle and project outward, creating the characteristic protrusion. Therefore, the deformity is considered a problem of the cartilage-bone junction rather than a primary bone defect.
The condition most frequently becomes apparent during the rapid growth spurts of late childhood and early adolescence. Although the underlying cause is congenital, the outward bulge typically worsens significantly between the ages of 11 and 18. The sudden increase in bone and cartilage growth during puberty accelerates the protrusion, making the deformity noticeable.
Primary Genetic and Syndromic Causes
The precise cause of Pectus Carinatum is often considered idiopathic, meaning it arises spontaneously without a known trigger. However, a strong pattern of familial occurrence suggests a clear genetic predisposition. A family history of a chest wall deformity (PC or PE) is present in approximately 25% to 33% of affected individuals.
When PC is not an isolated finding, it frequently manifests as a broader, systemic issue, particularly a disorder affecting connective tissue. Connective tissues provide the body’s internal structure. Defects in these tissues can lead to various skeletal and organ-related problems, and the cartilage overgrowth in PC is thought to be related to this underlying systemic weakness.
Marfan Syndrome is strongly associated with PC. It is caused by a mutation in the FBN1 gene, which produces the protein fibrillin-1. Fibrillin-1 is a component of the extracellular matrix that provides structural support. A defect in this protein leads to systemic connective tissue weakness, including the cartilage connecting the ribs, and PC is recognized as a significant feature in the diagnosis of Marfan Syndrome.
Other genetic syndromes involving connective tissue also feature PC, including Ehlers-Danlos Syndrome and Noonan Syndrome. Ehlers-Danlos Syndrome is characterized by joint hypermobility and skin laxity due to defects in collagen. Noonan Syndrome is a genetic disorder that often presents with congenital heart disease and skeletal malformations like PC.
Beyond these genetic syndromes, PC often co-occurs with other musculoskeletal issues. A notable association is with scoliosis, an abnormal curvature of the spine. Congenital heart defects are also sometimes found in individuals with the deformity, particularly when PC is part of a larger syndromic presentation.