This is now called Systemic Exercise Intolerance Disease (SEID), or SEID/CFS. It’s a mysterious condition in which previously-well healthy people suddenly become so fatigued that they can’t function normally. The cause is unknown (many “theories” have been proved incorrect), physical examination is normal, tests are all normal.
We don’t consider the condition until 6 months have gone by, since many temporary illnesses causing fatigue get better by then. Then we diagnose it if 3 factors are present in addition to fatigue:
- Feeling significantly bad after exercise
- Sleep is not refreshing
- Either a) some sort of memory problem; or b) symptoms occur upon standing, & feel better lying back down
The tests we do are to rule out other diseases that cause fatigue (see our symptom-topic Fatigue). We also rule out Depression or Anxiety as a cause. This is a sore spot, because many people with fatigue resist the possibility of a psychiatric condition. Also, anyone with SEID/CFS may well become depressed. At any rate, if there’s depression for any reason, it should be treated aggressively, so at least the patient feels better in that respect.
We don’t do lots of expensive tests for “chronic Epstein-Barr,” “chronic Lyme,” or other speculative diseases, because there’s no good evidence that such conditions cause fatigue (or even really exist). If any tests are positive, without other reason to suspect a certain illness, they’re far-and-away most likely false-positives (see Why Not to Test People for Unlikely Conditions).
Most people with ongoing fatigue do not have SEID/CFS. Those who do are truly debilitated; they’re absolutely not malingering or “faking” (as friends or family sometimes label them). Unfortunately, treatment is sub-optimal, because we don’t know the cause. If possible, it’s best to be treated by a clinician experienced in managing the condition.