Chronic respiratory distress with surfactant metabolism deficiency (SMD) is a rare genetic disorder that affects the lungs. It is caused by a mutation in the gene that codes for the surfactant protein B (SP-B) protein, which is essential for normal lung function. People with SMD have difficulty breathing and are at risk for developing chronic respiratory failure. Symptoms of SMD include difficulty breathing, wheezing, and recurrent episodes of pneumonia. Treatment for SMD includes supplemental oxygen, bronchodilators, and antibiotics. In some cases, a lung transplant may be necessary.