A recent case report published in the journal JAAD Case Reports highlights that subtle skin changes — especially widespread red-brown spots — can be an early warning sign of systemic mastocytosis (SM), and should prompt a thorough checkup including a bone marrow biopsyBone marrow biopsy A procedure to collect a sample of bone marrow using a needle. Often used to examine the characteristics of mast cells and diagnose SM..
The case report describes a 52-year-old man who lived with itchy, reddish-brown spots on his skin for over a decade before doctors connected them to SM. The spots were wrongly blamed on environmental causes. He also experienced stomach pain, near-fainting spells, brain fog and joint aches, all signs of mast cellMast cell A type of white blood cell produced in the bone marrow. They help defend against infections and play a key role in allergic reactions. In SM, mast cells become overactive and build up throughout the body. mediator release.
A dermatologist finally recognized the spots as “telangiectatic macularis eruptiva perstans” (TMEP) — a type of mast cell diseaseMast cell disease A group of conditions in which mast cells behave abnormally by building up in excess, releasing chemicals too easily or both. Includes mastocytosis, mast cell activation syndrome and hereditary alpha-tryptasemia. that shows up on the skin. A skin biopsy confirmed the diagnosis, and a blood test showed his tryptaseTryptase A protein enzyme that is primarily produced by mast cells and stored in small pockets within the cells, known as granules. High levels of tryptase are a key indicator of SM. level (a marker of mast cell activity) was four times higher than normal. A bone marrow biopsy confirmed he had indolent SMIndolent SM A subtype of nonadvanced SM caused by the abnormal accumulation of mast cells in the bone marrow and other organs. Indolent SM accounts for around 90% of SM cases., a milder form of the disease.
For years, his condition stayed relatively stable. Then, in 2020, back-to-back infections — Rocky Mountain Spotted Fever (a tick-borne disease) and COVID-19 — triggered major flare-ups. In 2025, after he felt increasingly unwell, a bone marrow biopsy showed that he had developed mast cell leukemiaMast cell leukemia The most severe subtype of SM, caused by the rapid buildup of immature mast cells in the bone marrow and blood. Mast cell leukemia accounts for less than 5% of SM cases and has a poor prognosis., the most dangerous form of SM.
Read more about SM testing and diagnosis
Doctors first tried a cancer drug called imatinib, but the patient’s side effects were too severe to continue. They then started him on a very low dose of avapritinib, which works by blocking the signal that tells the abnormal mast cells to keep growing. Within four months, his mast cell and tryptase levels dropped significantly and the spots on his skin improved significantly.
The patient had initially tested negative for the KIT mutation that typically shows up in SM patients. The case report authors note that “negative initial molecular testing does not rule out systemic mastocytosisMastocytosis Rare disease caused by the buildup of mast cells. Cutaneous mastocytosis primarily affects the skin and is more common in children, while systemic mastocytosis affects internal organs and is more common in adults..”
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