Doctors have run this test for years in cardiac patients and it's useful in ME to look at changes in eyes closed in a 'resting
state' laying flat alone in the room (relaxed as possible with as calm pulse as possible) and then eyes open standing upright alert.
In ME the sympathetic nervous system is aroused at rest more than usual and sharply increases standing up. Conversely, the parasympathetic is largely depressed or absent (as in Autism). I had my HRV test done a long time ago in a private hospital by an 'unconventional' doctor who even today would be scoffed at for 'believing' in the ME patient (sad reflection of the times). The good news is we can get these HRV tests as patients
now, it's just a matter of
finding a private doctor who has the equipment: ECG, Laptop, Software and press 'print' is all it needs.
The benefit for us , is it's a painless and totally safe (non invasive) method to provide evidence of abnormal central nervous system arousal or suppression. This demonstrates we have evidence of a
neurological problem. Ideally you also want a 'TILT' test. I'll get onto that next.
Another test, usually run in a large hospital (or by a specialist in autonomic/cardiology) is a Baroreflex test. This is similar in terms of what it shows. In ME during a TILT test procedure (an autonomic test it will show when you're upright your nerve impulses going crazy, and thus this is evidence your brain and heart are interacting, simply by being upright. This equipment is specialist and costly, it won't be in 'most' hospitals so you'd have to track it down.
What you need to show your CNS getting excited when upright in a hospital setting regarding TILT test is:
TILT table
ECG
Beat to beat blood pressure monitoring (BP taken every second on finger)
Ear clip and PC for Baroreflex response then shown on the PC attached to all the wires.
The benefit of this complex specialist test, is it not only shows you have a physical problem beyond your control, it helps get you 'under' a neurologist or a cardiologist at the hospital (hopefully), who otherwise may have dismissed you for having 'CFS'.
For people worried who are more active, but are still plague by Orthostatic Intolerance (O.I) I'd also add that in myself it made no difference to the test results when I was mobile, or bedridden. Always the same result. The benefit to that finding (at least in myself) is it disproves deconditioning theory of ME CFS, and also disproves de conditioning POTS (autonomic dysfunction) in ME CFS also, because the 'problem' you demonstrate is always there, regardless of your physical fitness level. Thus you must have a chronic neurological disorder. Thus CBT GET won't work, and thus you need better care. If you're lucky enough to have private health insurance, then better avenues of health management open up to you.
So I'd say the first cheap and easy test to try out (if you can't get Hospital based Baroreflex assessment) is the HRV test., which to recap is a painless ECG, a laptop and a print out they hand you to take home and keep showing sympathetic 'tone' vs parasympathetic tone. I'd say, with this, you are then more likely to get a TILT test too, if you're being denied one,or cannot get a doctors referral, for Orthostatic Intolerance or other CNS symptoms that worsen when upright or 'awake' and conversely, also fire off in your sleep.
If you have autonomic problems (common in ME), you'll get lots of weird and wonderful things happening to you in your
sleep too including episodes of:
Waking up with vertigo/dizzyness/fainting when laying flat
Cardiac arrhythmias and shortness of breath with or without each other
Breathing rhythm changes
Cold/hot sweats
Confusion
Hot flashes
Itching
Nocturnal supine polyuria (peeing too much, laying flat, at night) - ruins sleep if you pee multiple times at night.
Hence in total, finding out if you have an autonomic dysfunction via an HRV and then TILT + more (don't forget the valsalva manoeuvre test) also is helpful in getting you a
neurological diagnosis, a diagnosis far removed from the 'fatigue' of Fukuda Criteria CFS. This then bypasses disbelief ME is neurological, as you don't need to go with an ME diagnosis at all to have autonomic problems diagnosed and managed, (you can go with a dysautonomia diagnosis) of which POTS is just one form.