Bullous disease of the lung and cannabis smoking: insufficient evidence for a causative link

Abstract

Cannabis smoking appears on teaching slides as a cause of bullous lung disease and consequent pneumothorax. This seems to have become received wisdom—`A striking feature of cannabis smoking is that it is associated with bullous lung disease in young people'.1 The authors cite a report of four cases.2 The suggestion appears, albeit in more muted terms, with the same citation in a UK Parliamentary memorandum on adverse effects of smoking marijuana on lung health.3 What is the evidence that cannabis smoking causes bullous lung disease and what is its association, directly or indirectly, with pneumothorax? The characteristic clinical condition of acute pneumothorax is of a patient who presents with symptoms of rapid onset and is found to have free air in the pleural space. There is a widely accepted taxonomy.4 Patients presenting acutely are classified as having spontaneous pneumothorax to distinguish them from those caused by trauma (including iatrogenesis from various interventions with needles in the neck vessels, breast, pleura or lung). Spontaneous pneumothorax is then subdivided dichotomously into primary and secondary. Secondary spontaneous pneumothorax includes all of those where the pneumothorax occurs in the presence of any underlying lung disease. It is most frequently seen against a background of the common lung diseases that cause enlargement and weakening of the air spaces such as emphysema and chronic obstructive pulmonary disease. Occasional patients have one or more very large bullae. They are usually distinguished from the patient group with chronic obstructive pulmonary disease and emphysema on the basis of younger age and characteristic radiological findings. Is cannabis a cause of these large bullae? Secondary pneumothorax also occurs in other common diseases such as lung cancer, tuberculosis and lung abscess which may necrose, cavitate and breach the pleura and occasionally present with pneumothorax. There are many much less common diseases such as lymphangioleio-myomatosis, Wegner's granulomatosis, pneumocystis and Langerhans' cell histiocytosis that may present for the first time with pneumothorax. This long and effectively endless list of possible causes is not surprising when one notes that the visceral pleura is thin and vulnerable and arguably any lung disease can result in pneumothorax occasionally.4 Primary spontaneous pneumothorax is the characteristic condition of young people, predominately males (3:1) in their teens to mid thirties (Figure 1). They tend to be tall and lean; a family history is not infrequent. They fall into a well characterized diagnostic frame.5 The words `primary' and `spontaneous' have a tendency to be conflated with idiopathic or of unknown aetiology but there is a very definite pathological lesion. In most cases there are small bullae apparent at the apex of the lung. Now that surgeons can obtain excellent magnified views at video assisted thoracic surgery computerized tomography6,7 and radiologists inspect them with high resolution CT there is no mystery about it. Might cannabis smoking contribute to this group? After all bullae must start small and progress over time to become larger. Figure 1 A consecutive series of 117 cases of pneumothorax managed surgically in Guy's Hospital during 2003. It shows the typical bimodal distribution and that males (darker grey bars) outnumber women in both distributions There is a quite separate way in which cannabis smokers are reported to present with abnormal air in the chest (pneumothorax and/or pneumomediastinum). Instances in case reports are attributed to extreme breath holding, Valsalva, and Muller's manouevres.8-11 This is a form of barotrauma which is a well-recognized cause of pneumothorax in intensive care units. If there is a cause-and-effect relationship between cannabis smoking and acute rupture of previously normal lung air spaces due to extreme pressure change, it falls under the diagnostic heading not of spontaneous but of traumatic pneumothorax. The focus in our investigation was on bullous lung disease in cannabis smokers.