Medically reviewed by Doru Paul, MD

• Indolent systemic mastocytosis (ISM) progresses slowly, and symptoms vary depending on the affected organs.
• Symptoms can include skin changes, digestive problems, severe allergic reactions, neurological changes, and bone pain.
• Schedule a visit with a healthcare provider if you recognize these symptoms, and seek emergency care if you have signs of a severe allergic reaction.

Indolent systemic mastocytosis (ISM) is a condition where there’s an excess of abnormal mast cells—a type of white blood cell—in the body. This condition develops slowly, and signs and symptoms vary depending on which organs are affected. 

Skin Changes

The skin is a commonly affected organ in ISM, and for many people, it’s where the first signs of the condition appear. Skin changes occur when mast cells accumulate in the skin and release substances known as mediators, such as histamine and tryptase, that cause irritation, inflammation, and visible changes.

A common skin symptom in ISM is small spots called urticaria pigmentosa, usually found on the chest, stomach, back, arms, and legs. On lighter skin tones, the spots are reddish-brown. On darker skin tones, the spots may appear darker brown, gray, or purple. The spots may itch, turn red, and swell after being rubbed.

ISM can cause other skin-related symptoms, including:

• Flushing: Sudden redness or warmth, usually on the face, neck, or chest, can be triggered by heat, stress, alcohol, or certain foods. It may be harder to notice on darker skin tones, though the skin may still feel warm or look slightly darker. This can sometimes delay diagnosis or lead to symptoms being mistaken for another condition. 
• Itching: This can come and go or be ongoing. It may feel intense even without a visible rash.
• Hives: Raised, itchy welts may appear suddenly and come and go.

Digestive Symptoms

With ISM, an excess of abnormal mast cells can accumulate in the digestive tract. Things like spicy or fermented foods, alcohol, and stress can trigger these cells. They can then release a rush of mediators, which can irritate the gut and lead to symptoms such as:

• Bloating
• Abdominal cramping or pain
• Nausea and vomiting
• Diarrhea or loose stools
• Gastroesophageal acid reflux (GERD) 

Anaphylaxis

Some people with ISM have a higher risk of anaphylaxis, a sudden and severe allergic reaction that requires immediate medical attention. With ISM, these severe reactions are recurring. They occur when mast cells release a large amount of mediators at once. Insect stings, certain medications, and foods can trigger anaphylactic reactions, though they can also happen without a clear cause. 

Symptoms of anaphylaxis include:

• Flushing 
• Swelling
• Dizziness 
• Fainting
• Difficulty breathing 
• Wheezing
• Rapid or weak pulse
• Heart palpitations
• Nausea, vomiting, or diarrhea
• A sudden drop in blood pressure
• Loss of consciousness in severe cases

If you or someone else develops these symptoms, call 911 or get emergency medical help right away. Many people with ISM carry an epinephrine auto-injector (EpiPen) to use at the first sign of a severe reaction.

Neurological and Cognitive Symptoms

The mediators released by mast cells can affect the nervous system. When this occurs, symptoms may include:

• Headaches
• Difficulty concentrating, or brain fog
• Memory problems
• Fatigue, or low energy that doesn’t improve with rest
• Anxiety and depression
• Mood swings 
• Daytime sleepiness

These symptoms can be mistaken for stress, burnout, or other conditions. They may come and go or flare with other ISM symptoms.

Bone and Joint Symptoms

Because mast cells are made in the bone marrow, they can accumulate in the bones when you have ISM. The mediators released by these cells can disrupt the normal process of breaking down and rebuilding bone.  

With time, this can lead to bone thinning, including osteopenia (loss of calcium and other minerals that weakens the bones) or osteoporosis (more severe bone loss that makes bones easier to break). Since ISM progresses slowly, many people don’t notice these changes until a bone density scan shows thinning or a fracture happens unexpectedly.

Other bone and joint symptoms you may experience with ISM include:

• Bone pain: A dull, heavy ache, often felt in the hips or legs
• Joint pain: Soreness or stiffness that can come and go
• Lower back pain: Sometimes linked to changes in bone density in the spine

Enlarged Organs and Lymph Nodes

Abnormal mast cells can accumulate in organs such as the spleen, liver, or lymph nodes, causing them to enlarge. This is rare with ISM and can occur if the condition progresses to a more aggressive form of systemic mastocytosis. When symptoms occur, they may include: 

• Fullness or discomfort in the upper left abdomen due to an enlarged spleen (splenomegaly), which can affect about 1 in 2 people with ISM
• Swollen lymph nodes (lymphadenopathy), often in the neck, armpits, or groin, which can affect about 1 in 5 to more than half of people with ISM
• Fullness or discomfort in the upper right abdomen due to an enlarged liver (hepatomegaly), which is less common in ISM

Next Steps

ISM progresses slowly and is a milder form of systemic mastocytosis. For most people, symptoms don’t progress to more aggressive forms of the disease. With treatment and some lifestyle adjustments, many people with ISM manage their symptoms well and stay active. 

If you recognize these symptoms in yourself or a loved one, schedule a visit with a healthcare provider. They may refer you to a specialist, such as an allergist or hematologist, for testing and treatment recommendations.

While you’re waiting for an appointment, it may help to:

• Keep a symptom log, and write down what you feel and see, when it happens, and how long it lasts.
• Track possible triggers, such as foods, drinks, stress, or temperature changes. 
• Make a list of your medications and supplements, and bring the list to your appointment.
• Write down questions so you don’t forget what you want to ask.

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Kristina Dawson