The disease process starts in the subepicardial region and works its way towards the endocardial surface, leading to transmural involvement (possibly accounting for the aneurysmal dilatation of the ventricles). Residual myocardium is confined to the subendocardial region and the trabeculae of the ventricles. These trabeculae may become hypertrophied. Aneurysmal dilatation is seen in 50% of cases at autopsy. It usually occurs in the diaphragmatic, apical, and infundibular regions (known as the triangle of dysplasia). The left ventricle is involved in 50–67% of individuals. If the left ventricle is involved, it is usually late in the course of disease, and confers a poor prognosis. There are two pathological patterns seen in ACM, Fatty infiltration and fibro-fatty infiltration. Fatty infiltration
The first, fatty infiltration, is confined to the right ventricle. This involves a partial or near-complete substitution of myocardium with fatty tissue without wall thinning. It involves predominantly the apical and infundibular regions of the RV. The left ventricle and ventricular septum are usually spared. No inflammatory infiltrates are seen in fatty infiltration. There is evidence of myocyte (myocardial cell) degeneration and death seen in 50% of cases of fatty infiltration. Fibro-fatty infiltration
The second, fibro-fatty infiltration, involves replacement of myocytes with fibrofatty tissue. A patchy myocarditis is involved in up to 2/3 of cases, with inflammatory infiltrates (mostly T cells) seen on microscopy. Myocardial atrophy is due to injury and apoptosis. This leads to thinning of the RV free wall (to < 3 mm thickness) Myocytes are replaced with fibrofatty tissue.
Some recommend biopsy if the lesions persists more than 6 weeks after giving up smokeless tobacco use, or if the lesion undergoes a change in appearance (e.g. ulceration, thickening, color changes, especially to speckled white and red or entirely red). Surgical excision may be carried out if the lesion does not resolve. Prognosis
Usually this lesion is reversible if the tobacco habit is stopped completely, even after many years of use. In one report, 98% of lesions disappeared within 2 weeks of stopping tobacco use. The risk of the lesion developing into oral cancer (generally squamous cell carcinoma and its variant verrucous carcinoma) is relatively low. Indeed, veruccous carcinoma is sometimes term "snuff dippers cancer". In most reported cases, malignant transformation has occurring in individuals with a very long history of chewing tobacco or who use dry snuff.Smokeless tobacco use is also accompanied by increased risk of other oral conditions such as dental caries (tooth decay), periodontitis (gum disease), attrition (tooth wear) and staining. Epidemiology
STK is extremely common among smokeless tobacco users. Given the association with smokeless tobacco use, this condition tends to occur in adults. A national USA survey estimated an overall prevalence of 1.5% of all types of smokeless tobacco lesions, with males affected more commonly than females. See also
Stomatitis nicotina
Smokers melanosis

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