A fourth type known as irregular emphysema involves the acinus irregularly and is associated with fibrosis. Though the subtypes can be seen on imaging they are not well-defined clinically. Only the first two types of emphysema – centrilobular, and panlobular are associated with significant airflow obstruction, with that of centrilobular emphysema around 20 times more common than panlobular. Centrilobular emphysema also called centriacinar emphysema, affects the centrilobular portion of the lung, the area around the terminal bronchiole, and the first respiratory bronchiole, and can be seen on imaging as an area around the tip of the visible pulmonary artery. Centrilobular emphysema is the most common type usually associated with smoking, and with chronic bronchitis. The disease progresses from the centrilobular portion, leaving the lung parenchyma in the surrounding (perilobular) region preserved. Usually the upper lobes of the lungs are affected. Panlobular emphysema also called panacinar emphysema can involve the whole lung or mainly the lower lobes. This type of emphysema is associated with alpha-1 antitrypsin deficiency (A1AD or AATD), and is not related to smoking. Paraseptal emphysema also called distal acinar emphysema relates to emphysematous change next to a pleural surface, or to a fissure. The cystic spaces known as "blebs" or "bullae" that form in paraseptal emphysema typically occur in just one layer beneath the pleura. This distinguishes it from the honeycombing of small cystic spaces seen in fibrosis that typically occurs in layers. This type of emphysema is not associated with airflow obstruction. When the subpleural bullae are significant the emphysema is called bullous emphysema. Bullae can become extensive and combine to form giant bullae. These can be large enough to take up a third of a hemithorax, compress the lung parenchyma, and cause displacement. The emphysema is now termed giant bullous emphysema, more commonly called vanishing lung syndrome due to the compressed parenchyma. A bleb or bulla may sometimes rupture and cause a pneumothorax. Classic lung diseases are a complication of HIV/AIDS with emphysema being a source of disease.