What is mastocytosis?

Mastocytosis is a condition that occurs when mast cells accumulate in skin and/or internal organs such as the liver, spleen, bone marrow, and small intestines. The signs and symptoms vary based on which part(s) of the body are affected.

There are two main forms of mastocytosis: cutaneous and systemic. Cutaneous mastocytosis only affects the skin and is usually diagnosed in children. Systemic mastocytosis affects more than one part of the body and is usually diagnosed in adults. It is usually caused by changes (mutations) in the KIT gene. Most cases are caused by somatic mutations which are not inherited or passed on to the next generation; however, it can rarely affect more than one family member.

Treatment is based on the signs and symptoms present in each person.

What are symptoms of mastocytosis?

The Human Phenotype Ontology (HPO) provides the following list of features that have been reported in people with this condition. Much of the information in the HPO comes from Orphanet, a European rare disease database. If available, the list includes a rough estimate of how common a feature is (its frequency). Frequencies are based on a specific study and may not be representative of all studies.

What is the cause of mastocytosis?

Most cases of mastocytosis are caused by changes (mutations) in the KIT gene. This gene encodes a protein that helps control many important cellular processes such as cell growth and division; survival; and movement. This protein is also important for the development of certain types of cells, including mast cells (immune cells that are important for the inflammatory response). Certain mutations in the KIT gene can leads to an overproduction of mast cells. In mastocytosis, excess mast cells accumulate in the skin and/or internal organs, leading to the many signs and symptoms of the condition.


Most cases of mastocytosis are not inherited. They occur spontaneously in families with no history of the condition and are due to somatic changes (mutations) in the KIT gene. Somatic mutations occur after conception and are only present in certain cells. Because they are not present in the germ cells (egg and sperm), they are not passed on to the next generation.

Mastocytosis can rarely affect more than one family member. In some of these cases, the condition is inherited in an autosomal dominant manner. This means that to be affected, a person only needs a change (mutation) in one copy of the responsible gene in each cell. A person with familial mastocytosis has a 50% chance with each pregnancy of passing along the altered gene to his or her child.

How is mastocytosis diagnosed?

Mastocytosis is often first suspected by a doctor due to the symptoms present. The diagnosis may be confirmed by a skin biopsy. During a skin biopsy, a sample of skin tissue is taken and looked at under a microscope for the presence of dense areas of mast cells. In some cases, a bone marrow biopsy may be performed.

If the diagnosis is uncertain, blood and/or urine tests may be used to measure the levels of specific chemicals or substances related to mast cells. High levels of certain substances support the diagnosis of mastocytosis. Some substances may be elevated in systemic mastocytosis, but not in cutaneous mastocytosis. Other evaluations or tests used to confirm a diagnosis may include a bone scan, gastrointestinal workup, and/or genetic testing.

How is mastocytosis treated?

FDA-approved treatments

The medication(s) listed below have been approved by the Food and Drug Administration (FDA) as orphan products for treatment of this condition.

  • Cromolyn sodium (Brand name: Gastrocrom® (oral)) - Manufactured by Azur Pharma FDA-approved indication: Treatment of mastocytosis.
  • Midostaurin (Brand name: Rydapt®) - Manufactured by Novartis Oncology FDA-approved indication: Treatment of adult patients with aggressive systemic mastocytosis (ASM), systemic mastocytosis with associated hematological neoplasm (SM-AHN), or mast cell leukemia (MCL).

How common is mastocytosis?

Mastocytosis is described as a rare disorder, but to our knowledge, the exact incidence and prevalence are not known. A disorder is considered rare if it affects fewer than 200,000 people in the United States at any given time. An estimate of prevalence from a recent population-based study is approximately 1 case per 10,000 people.

While mastocytosis in general affects males and females in equal ratios, there appears to be a slight male predominance in childhood and a slight female predominance in adulthood. In children, 80% of cases appear during the first year of life, and the majority is limited to the skin. Adults who develop mastocytosis more often have systemic forms of the disease. Cutaneous forms of the disease account for less than 5% of adult cases.

Last reviewed by a Cleveland Clinic medical professional on 11/18/2015.

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