Mast Cell Activation Syndrome (MCAS)

What is MCAS?

Mast Cell Activation Syndrome is a disorder of immune system regulation. In this condition, mast cells activate and release chemical mediators even when there is no identifiable allergen or trigger.

Mast cells normally sit in tissues throughout the body, particularly in the skin, lungs, and gastrointestinal tract. When they detect something harmful, they release substances such as histamine to activate the immune response.

In people with MCAS, this release occurs inappropriately or excessively. The result is repeated episodes of symptoms that resemble severe allergic reactions but occur without a consistent trigger.

Unlike typical allergies, where symptoms occur after exposure to a specific allergen, MCAS episodes may happen unpredictably and can involve multiple organ systems at the same time.

Treatment Options

There is no cure for Mast Cell Activation Syndrome, but symptoms can often be controlled with medications that reduce mast cell activation and block inflammatory mediators such as histamine. Treatment plans are individualized based on symptoms and severity.

Examples include cetirizine, loratadine, famotidine and fexofenadine.

• Reduces itching, hives, and flushing
• Helps control skin and allergy like symptoms
• Helps relieve abdominal pain and nausea
• Reduces stomach acid related irritation
• Often used as first line treatment
• May improve breathing symptoms
• Reduces inflammatory responses
• Helpful for some respiratory symptoms
• Prevents mast cells from releasing inflammatory mediators
• Helps reduce frequency of symptom flares

Symptoms

MCAS symptoms occur in episodes and often affect more than one body system simultaneously. The symptoms resemble allergic reactions because they are caused by the same inflammatory mediators released from mast cells.

Common symptoms include:

• Flushing or warm, red skin
• Itching or hives
• Swelling of the lips, eyes, face, or throat
• Nasal congestion
• Shortness of breath or wheezing
• Low blood pressure
• Rapid heartbeat or lightheadedness
• Abdominal pain
• Diarrhea or constipation
• Nausea or vomiting

Diagnosis

Diagnosing MCAS can be challenging because many of its symptoms overlap with other medical conditions. Evaluation by an allergist or immunology specialist is important to rule out other causes. Diagnosis usually includes:

One of the most commonly used laboratory tests measures serum tryptase, a substance released by mast cells. Blood samples may be collected during a symptom flare and compared with baseline levels when the patient is feeling well.

Additional tests may include urine studies that measure mast cell mediator metabolites such as methylhistamine or prostaglandins.

• A detailed review of symptoms and medical history
• Evaluation of recurrent episodes affecting multiple body systems
• Laboratory testing for markers of mast cell activation
• Assessment of response to mast cell targeted treatments

MCAS and Anaphylaxis

Although not every MCAS episode becomes severe, mast cell activation can sometimes lead to anaphylaxis. Anaphylaxis is a rapid and potentially life threatening reaction that affects breathing, blood pressure, and circulation.

Immediate treatment with epinephrine is required if anaphylaxis occurs. Patients diagnosed with MCAS are often advised to carry an epinephrine auto injector and to have a clear emergency action plan. Early recognition and rapid treatment are critical to preventing complications during severe reactions.

Symptoms of anaphylaxis may include: