Environmental hyperresponsiveness is this network’s working research hypothesis for what MCS and its overlapping conditions may have in common: an acquired, persistent high-gain state in the systems that detect, process and respond to environmental input. It is a hypothesis, not a diagnosis, and this page says so in its first paragraph on purpose.
The model runs: susceptibility plus an initiating event or exposure produces an altered high-gain environmental responsiveness, after which previously tolerated inputs produce disproportionate physiological responses. Different initiators — pesticide, solvent, mold, infection, injury — may lead into the same state; different inputs — chemicals, and possibly others — may then trigger it.
The systems that may interact: sensory nerves and TRPA1/TRPV1; central sensory processing; autonomic regulation; small-fiber biology; immune, inflammatory and mast-cell signaling; oxidative and metabolic regulation; genetics and epigenetics; and, after illness exists, learned or conditioned amplification. The order is unknown. Conditioning is not placed at the start of the chain unless evidence puts it there.
The hypothesis is built from established pieces: TRP sensitization measured by capsaicin challenge; altered central processing on imaging; autonomic and small-fiber findings in subgroups; inflammatory signatures; the co-occurrence of MCS with EHS, MCAS, POTS, ME/CFS, fibromyalgia and migraine far above chance. Each piece has evidence. The claim that they form one high-gain state is what has to be tested.
The hypothesis is also deliberately falsifiable. If MCS-only, EHS-only, combined and control groups do not share measurable sensory and autonomic traits, the shared-phenotype idea fails and the overlaps are separate conditions co-occurring. The registry is designed to be able to return that answer.
Whether people with MCS, EHS, MCAS, POTS, ME/CFS and related conditions share a measurable high-gain profile; which initiators lead into it; and whether the same profile predicts who recovers.
Environmental hyperresponsiveness is our best working description of what may sit under MCS and its neighbors. It is the question we are asking, stated clearly enough to be proved wrong.