List six causes of pneumatosis intestinalis.

1. Bowel necrosis
2. Emphysematous gastritis
3. Mucosal disruption (e.g., as a result of peptic ulcer disease)
4. Immunocompromise (e.g., as a result of AIDS, steroid therapy, or lymphoma)
5. Pulmonary disease (e.g., COPD, asthma, cystic fibrosis)
6. Overdistention as a result of obstruction or endoscopy