Cancer pain (classification and pain syndromes).

Inspite the new informations about the physiology and biochemistry of pain, it remains true that pain is only partially understood. Cancer pain is often experienced as several different types of pain, with combined somatic and neuropathic types the most frequently. If the acute cancer pain does not subside with initial therapy, patients experience pain of more constant nature, the characteristics of wich vary with the cause and the involved sites. Chronic pain related to cancer can be considered as tumor-induced pain, chemotherapy-induced pain, and radiation therapy-induced pain. Certain pain mechanisms are present in cancer patients. These include inflammation due to infection, such as local sepsis or the pain of herpes zoster, and pain due to the obstruction or occlusion of a hollow organ, such as that caused by large bowel in cancer of colon. Pain also is commonly due to destruction of tissue, such as is often seen with bony metastases. Bony metastases also produce pain because of periostal irritation, medullary pressure, and fractures. Pain may be produced by the growth of tumor in a closed area richly supplied with pain receptors (nociceptors). Examples are tumors growing within the capsule of an organ such as the pancreas. Chest pain occurring after tumor of the lung or the mediastinum due to invasion of the pleura. Certain tumors produce characteristic types of pain. For example, back pain is seen with multiple myeloma, and severe shoulder pain and arm pain is seen with Pancoast tumors.