EDS and MCAS: Why Connective Tissue Disorders and Mast Cell Activation Go Together

The co-occurrence of hypermobile EDS (hEDS) and mast cell activation syndrome (MCAS) is one of the most striking patterns in the dysautonomia community. Studies suggest that 30–70% of hEDS patients meet criteria for MCAS, a rate far above the estimated 1–17% prevalence of MCAS in the general population. This is not coincidence — there are compelling biological reasons why connective tissue disorders and mast cell activation are linked.

Mast Cells in Connective Tissue

Mast cells are resident immune cells that live in connective tissue throughout the body — skin, gut, lungs, blood vessels, and nervous system. They are strategically positioned at the interface between the body and the external environment, acting as sentinels that detect pathogens, allergens, and tissue damage.

In hEDS, the abnormal connective tissue matrix may directly affect mast cell function. Mast cells interact with the extracellular matrix through integrins and other adhesion molecules, and disruption of the normal matrix architecture may alter mast cell activation thresholds. Some researchers have proposed that abnormal collagen in hEDS creates a pro-inflammatory microenvironment that chronically activates mast cells.

Shared Genetic Mechanisms

Several genetic variants have been identified that may predispose to both connective tissue hypermobility and mast cell dysfunction:

TPSAB1 (tryptase gene). Hereditary alpha-tryptasemia (HαT) — caused by extra copies of the TPSAB1 gene — is associated with elevated baseline serum tryptase, mast cell activation symptoms, and connective tissue hypermobility. HαT is found in approximately 5% of the general population but is significantly enriched in patients with MCAS and hEDS.

KIT mutations. Mutations in the KIT gene (which encodes the stem cell factor receptor on mast cells) can increase mast cell numbers and activation, and some KIT variants are associated with connective tissue abnormalities.

Chromosome 16p11.2 duplications. This chromosomal region contains genes involved in both connective tissue and immune function, and duplications in this region have been associated with both hypermobility and mast cell activation.

The POTS-EDS-MCAS Triad

The triad of POTS + hEDS + MCAS is so common that it has been called the "trifecta" in the dysautonomia community. The three conditions reinforce each other:

• MCAS worsens POTS: Mast cell mediators (histamine, prostaglandins) cause vasodilation and reduce venous return, worsening orthostatic intolerance.
• EDS worsens POTS: Connective tissue laxity allows excessive venous pooling in the legs and abdomen on standing.
• POTS activates mast cells: Sympathetic activation and catecholamines can trigger mast cell degranulation.
• EDS activates mast cells: Tissue microtrauma from hypermobile joints may chronically activate tissue-resident mast cells.

Diagnosis

Diagnosing MCAS in hEDS patients follows the same criteria as primary MCAS: episodic symptoms consistent with mast cell mediator release, response to antihistamines or mast cell stabilizers, and (ideally) elevated mast cell mediators during a symptomatic episode.

Hereditary alpha-tryptasemia testing. Genetic testing for TPSAB1 copy number (available through specialized laboratories) can identify HαT, which provides a genetic explanation for elevated baseline tryptase and mast cell activation symptoms.

Baseline serum tryptase. A baseline serum tryptase (drawn when not in an acute reaction) above 11.4 ng/mL suggests systemic mastocytosis or HαT and warrants further evaluation.

Management of the EDS-MCAS Combination

Treating MCAS first. In patients with the POTS-EDS-MCAS triad, treating MCAS often improves POTS symptoms, because reducing mast cell mediator-driven vasodilation improves venous return and orthostatic tolerance. Starting with H1 and H2 antihistamines and mast cell stabilizers before adding POTS medications can simplify management.

Medication sensitivity. hEDS-MCAS patients are often highly sensitive to medications, particularly those with histamine-releasing properties (opioids, NSAIDs, contrast dye, some antibiotics). Starting medications at very low doses and titrating slowly reduces the risk of MCAS reactions.

Low-histamine diet. Reducing dietary histamine load can significantly reduce MCAS symptom burden in hEDS patients. See our low-histamine diet guide for a complete food list.

Physical therapy for joint stability. Improving joint stability through targeted physical therapy reduces tissue microtrauma and may reduce the chronic mast cell activation driven by repeated joint injuries.

ChatDys resources: Track your MCAS symptoms, joint symptoms, and POTS symptoms together in the Health Tracker. Upload your tryptase results and genetic testing to Biomarkers. Review our MCAS treatment guide and our POTS-EDS-MCAS triad article for comprehensive management information.