Rarely, individuals present with multiple poromas either in one or widespread areas; these cases are termed poromatosis.Poromas present more commonly in: pregnancy; patients treated with electron therapy for mycosis fungoides; sites of chronic radiation dermatitis caused by long-term radiation exposure; patients who received chemotherapy with or with radiation therapy (these patients have often presented with poromatosis); individuals with underlying skin conditions such as hypohidrotic ectodermal dysplasia and Bowen’s disease (i.e. a form of squamous cell carcinoma that is localized to the outermost layer of the skin); and skin plaques of individuals with the congenital disease, nevus sebaceous.Poromas are usually slow growing and asymptomatic but some individuals report that their lesions are itchy, mildly tender, or  painful. An existing poroma that develops spontaneous bleeding, ulceration, sudden itching, pain, or rapid growth over a short period of time may indicate that it has become a cancerous porocarcinoma. These cancers may metastasize to local lymph nodes, nearby or distal skin sites, bones, bladder, breast, retroperitoneum, ovary, liver, lung, brain, or stomach. However, the progression of a poroma to a porocarcinoma is very rare, i.e. less than 1% of cases. Histopathology
Microscopic histopathological examinations of the tumor tissues of all poroma variants stained with hematoxylin and eosin dyes reveal: a) basophilic "poroid cells" (i.e. small, cuboid-shaped cells with oval nuclei which resemble cells in the peripheral layer of the distal portion of eccrine sweat gland ducts) that may form cords and broad columns extending downward from the epidermis; b) larger cuticular cells (i.e.
For this reason, beta blockers that selectively block β1 adrenergic receptors (termed cardioselective or β1-selective beta blockers) produce fewer adverse effects (for instance, bronchoconstriction) than those drugs that non-selectively block both β1 and β2 receptors. In a laboratory experiment conducted on biopsied heart tissue, nebivolol proved to be the most β1-selective of the β-blockers tested, being approximately 3.5 times more β1-selective than bisoprolol. However, the drugs receptor selectivity in humans is more complex and depends on the drug dose and the genetic profile of the patient taking the medication. The drug is highly cardioselective at 5 mg. In addition, at doses above 10 mg, nebivolol loses its cardioselectivity and blocks both β1 and β2 receptors, while the recommended starting dose of nebivolol is 5 mg, sufficient control of blood pressure may require doses up to 40 mg. Furthermore, nebivolol is also not cardioselective when taken by patients with a genetic makeup that makes them "poor metabolizers" of nebivolol (and other drugs) or with CYP2D6 inhibitors. As many as 1 in 10 Caucasian people and even more black people are poor CYP2D6 metabolizers and therefore might benefit less from nebivolols cardioselectivity although currently there are no directly comparable studies.Nebivolol while selectively blocking beta(1) receptor acts as a beta(3)-agonist. β3 receptors are found in the gallbladder, urinary bladder, and in brown adipose tissue. Their role in gallbladder physiology is unknown, but they are thought to play a role in lipolysis and thermogenesis in brown fat.