Synthesis: A Framework for Comparative Nosology

Synthesis: Three Axes for Classifying the Contested Diagnosis Cluster

The preceding comparisons suggest that the splitting vs. lumping debate is not resolvable by a single criterion — different axes support different groupings.

Axis 1 — Trigger mechanism: Post-infectious (ME/CFS, Long COVID, PTLDS, post-meningitis, QFS, post-infectious IBS) vs. structural/toxic (EDS/hypermobility, skeletal asymmetry, GWI chemical exposure). This axis cleanly separates conditions by etiology and generates testable predictions: post-infectious conditions should cluster on immune/autoimmune markers; structural/toxic conditions should cluster on mechanical/connective tissue/toxin markers.

Axis 2 — Dominant pathophysiology: Metabolic/PEM-dominant (ME/CFS, subsets of Long COVID, subsets of fibromyalgia) vs. pain-dominant (fibromyalgia, CRPS) vs. autonomic-dominant (POTS, orthostatic intolerance) vs. GI-dominant (IBS). This axis maps onto treatment response: metabolic conditions respond to metabolic interventions; pain conditions respond to analgesic/neuromodulatory interventions; autonomic conditions respond to volume expansion/vasoconstrictors; GI conditions respond to dietary/gut-directed interventions.

Axis 3 — PEM status: PEM-positive (ME/CFS, PEM-positive fibromyalgia, PEM-positive Long COVID) vs. PEM-negative (PEM-negative fibromyalgia, POTS, most IBS, most CRPS). This axis is the strongest predictor of exercise response and arguably the most clinically useful single discriminator — it determines whether graded activity is harmful or tolerable, and it separates conditions that share symptom profiles but have opposite physiology on CPET.

The three axes are not mutually exclusive — they represent orthogonal dimensions. A given patient can be classified as post-infectious (Axis 1), PEM-dominant (Axis 2), and PEM-positive (Axis 3). The practical implication: clinical trials and treatment algorithms should use axis scores rather than diagnostic labels for patient stratification. A PEM-positive post-infectious fibromyalgia patient may respond more like a PEM-positive ME/CFS patient than like a PEM-negative fibromyalgia patient — the axis overrides the diagnosis.

Consequence: The three-axis framework replaces “do you have ME/CFS or fibromyalgia or Long COVID?” with “what is your trigger type, your dominant pathophysiology, and your PEM status?” — shifting nosology from categorical assignment to dimensional profiling, and treatment from disease-specific guidelines to mechanism-specific protocols.