NECROTIZING SIALOMETAPLASIA : CLINICAL FEATURES, HISTOPATHOLOGY & TREATMENT
Author: Sanketh DS, MDS
INTRODUCTION & ETIOLOGY
Necrotizing sialometaplasia (NS) is a non-neoplastic, inflammatory process predominantly involving the minor salivary glands. It is a rare lesion classically occurring in the minor salivary glands of the hard palate. One has to be cautious when diagnosing the lesion since it may mimic malignant diseases like oral squamous cell carcinoma (OSCC) and mucoepidermoid carcinoma both clinically and histologically. NS usually does not require treatment since it is self-healing.
This lesion is believed to arise as a result of ischemic necrosis that may be triggered due to an alteration in the local blood supply. A number of predisposing factors have been reported that may be involved in compromising the local blood supply to the salivary glands. Some of them are blunt force trauma, trauma due to local surgical procedures and injection of local anaesthesia, upper respiratory tract infections, alcohol and tobacco abuse. NS has also been reported to be associated with eating disorders like bulimia nervosa where patients have violent or induced vomitting. However, there have been cases where individuals have been affected without any known predisposing factors.
NS usually occurs in adults with a mean age of 47 and manifests twice as frequently in males than females. It begins as a tender swelling with the mucosa appearing erythematous. After a few weeks, the necrotic tissue breaks down and creates a punched out deep seated crater like ulceration. The ulceration may be at 1-3 cm in diameter and has a yellowish-grey base. NS may at times also cause destruction of the underlying palatal bone. This lesion surprisingly is not very painful and patients usually complain of a mild dull pain although patients may also be totally asymptomatic.
It most commonly affects the palatal salivary glands and can be located unilaterally or bilaterally. This lesion has also been reported to involve minor salivary glands in the buccal mucosa, tongue and retromolar pad. NS is almost exclusively a lesion of the minor salivary gland and very rarely affects major salivary glands.
Clinically the punched out crater like ulceration may mimic lesions like squamous cell carcinoma or other malignant salivary gland tumors. Also lesions like syphilitic gummas and fungal infections like mucormycosis may have a similar manifestation and is important to rule out these lesions with a biopsy!
Under the microscope, NS has distinctive features, although care must be taken not to misdiagnose it as OSCC or MEC. The ulceration manifests as an absence of epithelium, histologically seen as fibrin and granulation tissue replacing the missing epithelium. Under the ulceration, the diseased minor salivary gland tissue appears to have acini that have undergone coagulative necrosis.
Remember that coagulative necrosis is a form of necrosis in which the overall architecture of the necrosed tissue is maintained for a few days.
Hence the cytoplasmic borders of the cells of the acini are maintained while the cells themselves appear to be pale, bloated and distended with no nuclei (necrosed). Neutophils and foamy histiocytes may be seen scattered around the necrosed acini, where mucin may have leaked and attracted the inflammatory cells.
Also seen around the necrosed acini are small nests or islands of squamous cells, which are actually salivary gland ducts that have undergone squamous metaplasia.
Metaplasia is a process where a differentiated adult cell type is replaced with another cell type.
Salivary ducts seen histologically are usually striated ducts or excretory ducts. Striated ducts are composed of columnar cells and excretory ducts are lined by pseudo-stratified columnar cells. In the context of NS, the minor salivary gland ducts undergo a squamous metaplasia, where the columnar or pseudo-stratified columnar cell type is replaced by a squamous cell type.
Ducts that have undergone squamous metaplasia may look like malignant squamous cells seen in OSCC or MEC. Causing further dilemma is the overlying mucosal epithelium showing, what is called a pseudo-epitheliomatous hyperplasia, which again mimics a proliferative epithelium seen in a malignant process like OSCC. However, one way to distinguish NS from malignant diseases is by noting that the squamous ductal cells in NS do not show any dysplasia or atypical features. Also MEC does not usually manifest with necrosis and may also have other cells like intermediate and mucous cells proliferating.
TREATMENT AND PROGNOSIS
NS is a benign and a self-limiting lesion that heals within a few weeks. No surgical intervention is required. Patient reassurance and occasional use of analgesics may be enough.
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