SM is a very rare condition, and only 150-250 cases have been reported in the literature. SM affects men and women equally. Most affected individuals are between the ages of 30-70.

SM occurs due to the excessive deposition of mucin in mid and deep layers of the skin. Mucin is a type of acidic protein that stimulates the production of collagen. Collagen is the main structural protein in the skin and provides support for the skin. Mucin and collagen are both produced by cells called fibroblasts. Fibroblast proliferation is another feature of SM. As a result of the increased production of collagen, the skin thickens and becomes hard, a condition known as fibrosis. 

It is unknown what triggers this mucin deposition. There is some data suggesting that some groups of antiviral drugs such as some hepatitis C drugs might be linked with the development of SM. However, evidence regarding this theory is inconclusive.

SM typically presents with skin abnormalities known as papules. Papules are circular, raised areas of the skin that might be red or the same color as the rest of the skin. The papules are often firm and 2-3 mm in diameter. The papules are most commonly found on the face, neck, and upper extremities and might involve large areas of the skin in a linear pattern. The papules particularly tend to group in the forehead and the neck. Over time, the skin may thicken.

Internal organs may also be affected by SM. Symptoms of internal damage include difficulty swallowing food due to an absence of muscular contractions in the esophagus or esophageal aperistalsis. Affected individuals may develop a hoarse voice, experience neurological dysfunctions, eye abnormalities, heart abnormalities, and breathing difficulty. Inflammation of multiple joints (inflammatory polyarthritis) and reduced joint mobility and diseases related to muscular tissue may also be present.

The diagnostic criteria for SM include monoclonal gammopathy, the histopathology triad which is mucin deposition in deep layers of the skin, fibroblast proliferation, and fibrosis, as well as the absence of thyroid disease. Monoclonal gammopathy is a condition where a specific type of antibody or defensive protein is abnormal.

A skin biopsy may be performed to test the skin for the presence of common abnormalities associated with SM. In a skin biopsy, a piece of the skin is excised out and used for various tests. The skin sample is tested for mucin depositions, excessive collagen, abnormalities in fibroblasts, and other associated findings. 

Due to the rarity of this condition, there is no standard treatment for SM. Intravenous immunoglobulin (IVIG) appears to be relatively effective. IVIG is a form of therapy given to individuals with defective antibodies. Immunoglobulins are proteins that make up antibodies. In IVIG, a mixture of antibodies for many healthy donors is administered to affected individuals to treat antibody deficiencies. Thalidomide is a drug that is used to treat a number of cancers and has also been used to treat SM. Additionally, an autologous bone marrow transplant has also been reported as a treatment option. An autologous bone marrow transplant is a procedure in which normal blood-forming cells of the individual are collected and used to replace a defective bone marrow.

SM is an unpredictable disease and the prognosis varies. As a progressive condition, individuals’ symptoms worsen over time. If treatment is not successful, the condition can be debilitating and even fatal due to associated complications.

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