Necrotizing sialometaplasia (NS) is a benign, ulcerative lesion, usually located towards the back of the hard palate. It is thought to be caused by ischemic necrosis (death of tissue due to lack of blood supply) of minor salivary glands in response to trauma. Often painless, the condition is self-limiting and should heal in 6–10 weeks. Although entirely benign and requiring no treatment, due to its similar appearance to oral cancer, it is sometimes misdiagnosed as malignant. Therefore, it is considered an important condition, despite its rarity. Signs and symptoms
The condition most commonly is located at the junction of the hard and soft palate. However, the condition may arise anywhere minor salivary glands are located. It has also been occasionally reported to involve the major salivary glands. It may be present only on one side, or both sides. The lesion typically is 1–4 cm in diameter.Initially, the lesion is a tender, erythematous (red) swelling. Later, in the ulcerated stage, the overlying mucosa breaks down to leave a deep, well-circumscribed ulcer which is yellow-gray in color and has a lobular base.There is usually only minor pain, and the condition is often entirely painless. There may be prodromal symptoms similar to flu before the appearance of the lesion. Causes
The exact cause of the condition is unknown. There is most evidence to support vascular infarction and ischemic necrosis of salivary gland lobules as a mechanism for the condition.
These may develop in common sites (deep vein thrombosis of the leg and resultant pulmonary embolism when these clots break off and enter the lungs), but in PNH blood clots may also form in more unusual sites: the hepatic vein (causing Budd-Chiari syndrome), the portal vein of the liver (causing portal vein thrombosis), the superior or inferior mesenteric vein (causing mesenteric ischemia) and veins of the skin. Cerebral venous thrombosis, an uncommon form of stroke, is more common in those with PNH. Pathophysiology
All cells have proteins attached to their membranes, often serving as a mode of communication or signaling between the cell and the surrounding environment. These signaling proteins are physically attached to the cell membrane in various ways, commonly anchored by glycolipids such as glycosyl phosphatidylinositols (GPI). PNH occurs as a result of a defect in the assembling of these glycolipid-protein structures on the surface of blood cells.The most common defective enzyme in PNH is phosphatidylinositol glycan A (PIGA), one of several enzymes needed to make GPI. The gene that codes for PIGA is located on the X chromosome, which means that only one active copy of the gene for PIGA is present in each cell (initially, females have two copies, but one is silenced through X-inactivation). A mutation in the PIGA gene can lead to the absence of GPI anchors expressed on the cell membrane.