A bleb or bulla may sometimes rupture and cause a pneumothorax. The cause as to why blue rubber bleb nevus syndrome occurs is currently unknown. In medicine, a bleb is a blister (often hemispherical) filled with serous fluid. In some cases the bleb may be associated with an adjacent blocked sebaceous cyst. Glaucoma medications are usually discontinued to improve aqueous humor flow to the bleb. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb. Envenomation can cause pain, swelling, hemorrhagic bleb formation and ecchymosis ("i.e.", bruising). Some clinicians may also include milk blisters as a type of bleb. A blocked milk duct can result from a nipple bleb. The COPD patient with a large bleb represents a relative contraindication for similar reasons. An example is a bleb of sylvite within chlorite. Bleb forming procedures include ab externo trabeculectomy and deep sclerectomy. Blue Rubber Bleb Nevus Syndrome affects males and females in equal numbers.