Leiomyoma
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This small, painless, sessile submucosal nodule has been  present on the buccal mucosa for almost 6 months.

The benign neoplasm of smooth muscle, the leiomyoma, is rare in the upper aerodigestive tract, being far more common in the genitalia, skin (leiomyoma cutis, arising from pilar arrector muscles) and gastrointestinal tract.  Fewer than 2.7% of human examples occur in the mouth.  Leiomyomas of the head and neck region probably arise from vascular smooth muscle and almost all cases fall into one of two general types, solid leiomyoma (leiomyoma of deep soft tissue) and vascular leiomyoma (angiomyoma, angioleiomyoma). The latter accounts for almost three-fourths of all oral cases, and rare examples of a third type, epithelioid leiomyoma (leiomyoblastoma) have been reported.  As of the year 2000, only 90 oral lesions had been reported.

Leiomyoma of the head and neck region is primarily diagnosed in middle-aged adults but occurs at any age, including infancy (where it is quite rare).  It has no gender predilection. It typically presents as a slowly enlarging, sessile, asymptomatic, firm submucosal mass or nodule, although occasional lesions are tender or painful, especially the vascular leiomyoma. The lesion may continue to slowly enlarge for several years and may reach several centimeters in diameter, although most lesions are quite small at biopsy (Figure 1). The solid leiomyoma routinely appears normal in color but the vascular lesion often exhibits a blue or red discoloration. Tumors usually occur as single lesions of the tongue, lips, palate, and buccal mucosa, but 20% occur on other mucosal sites. Rare examples of intravenous and disseminated leiomyomatosis have been reported, but not from the head and neck regions.  Additionally, a handful of reported oral lesions have been painful or tender, a feature which is much more common (67%) in tumors of the extremities.  The pain may result from ischemia of tumor muscle after contraction of tumor vessels.

All forms of leiomyoma are well encapsulated and show little cellular pleomorphism or mitotic activity. The solid tumor is comprised of interlacing bundles of spindle-shaped or stellate smooth muscle cells with elongated, blunt-ended, pale-staining nuclei (Figure 2). Nuclei may be palisaded and must then be differentiated from neurilemoma, a task usually made easy by the lack of Verocay bodies and wavy, thin, spindled nuclei. Lesions are rather cellular, but collagenic strands often separate the streaming bundles of tumor cells, and occasional leiomyomas have a prominent fibroblastic or myxoid component.  Numerous slit-like vessels are seen in the angiomyoma and polygonal cells are characteristic in the epithelioid variant (Figure 3).

Angiomyomas are sometimes classified into three histologic subtypes: solid, cavernous and venous, with the solid type being most associated with pain and tenderness.  Focal areas of the venous angiomyoma contain numerous thick walled veins with small lumina, surrounding by lesional cells, sometimes oriented in a circumferential fashion (Figure 4).

Older and larger solid tumors may show degenerative changes, even dystrophic calcifications similar to psammoma bodies. The angiomyoma demonstrates numerous tortuous blood vessels with hyperplastic smooth muscle walls and with intertwining bundles of smooth muscle cells between the vessels. The smooth muscle cells often appear to swirl away from the vessel and may demonstrate perinuclear vacuolization. Adipose cells may be intermixed with tumor cells and the lesional vessels typically lack internal and external elastic laminae. These lesions have a remarkable lack of morphologic variation between cases.

The tumor cells of the rare palisaded myofibroblastoma, of course, are derived from myofibroblasts, modified smooth muscle cells, usually those within a lymph node. The cells are typically palisaded and the tumor almost always demonstrates rather unique eosinophilic bands and islands of acellular collagen (amianthoid fibers).

Diagnosis is rarely difficult, but the leiomyoma must be differentiated from nodular fasciitis, myxoma, neurilemoma, fibrous histiocytoma (dermatofibroma), and hamartomatous aggregates of smooth muscle tissues. The smooth muscle nature of the spindle cells can be confirmed by Masson's trichrome stain and myofibrils can be demonstrated by Mallory's phosphotungstic acid (PTAH) stain or van Giesson elastic stain. Lesional cells are immunoreactive for desmin and alpha-smooth muscle actin, although the positivity of these cells with the latter stain is less than that of the surrounding normal smooth muscle fibers.  Lesional cells are nonreactive for S-100 protein.

Conservative surgical excision is the definitive treatment for leiomyoma. Very few recurrences have been reported.  There is no malignant transformation potential.

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		Figure 1:  This small, painless, sessile submucosal nodule has been present on the buccal mucosa for almost 6 months.  [return to text]
		Figure 2:    [return to text]
		Figure 3:  Angiomyoma.  [return to text]
		Figure 4:  Epithelioid leiomyoma.    [return to text]
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