Pectus excavatum: This is a congenital condition characterized by a sunken chest, which can affect one or both sides of the chest. It is the most common congenital deformity of the chest wall and can be present at birth or develop over time.

Muscular weakness or paralysis: Conditions affecting the muscles of the chest wall, such as muscular dystrophy, can lead to a sunken appearance of the chest on one side if the weakness is asymmetric.
Scoliosis or kyphoscoliosis: Abnormal curvature of the spine can cause an uneven appearance of the chest, with one side appearing sunken due to the spinal deformity.
Previous thoracic surgery: Surgical interventions, especially those involving the removal of ribs or parts of the chest wall, can result in a sunken appearance of the chest on the affected side.

Pneumothorax: A collapsed lung can cause the chest on the affected side to appear sunken. This is a medical emergency requiring immediate attention.
Diaphragmatic hernia: A congenital or acquired defect in the diaphragm allowing abdominal contents to herniate into the thoracic cavity can cause respiratory distress and a sunken chest appearance on the affected side.
Tumor or cystic lesions: Although rare, tumors or cysts within the chest can cause asymmetry and a sunken appearance if they compress or displace the lung or chest wall structures.

Poland syndrome: A rare congenital condition characterized by underdevelopment or absence of the chest muscle (pectoralis major) on one side of the body, which can also involve the arm and hand. It can cause a sunken appearance of the chest on the affected side.
Jeune syndrome (asphyxiating thoracic dystrophy): A rare genetic disorder that affects the development of the bones, particularly those in the thorax, leading to a narrow, bell-shaped chest that can appear sunken.
Thoracic dysplasia: Various rare genetic conditions that affect the development of the thoracic cage, potentially leading to a sunken or deformed chest appearance.