Postural orthostatic tachycardia syndrome or POTs
This is a very common problem in people with severe CFS. It means the sufferer can only stand for a short time before having to lie down. POTs is said to result from autonomic neuropathy, but my view is that this is the response to falling blood pressure not the cause. Let me explain.
It is much easier for the heart to pump blood on the flat (lying down) than up and down hills (standing up). Indeed we all feel more comfortable lying down or sitting rather than standing because the heart has to work less hard. In severe cases of chronic fatigue syndrome the heart is in a low output state, perhaps sufficiently so that it cannot pump enough blood round the body when standing. So when the sufferer stands, s/he can maintain blood pressure for a certain time, but then the heart muscle becomes fatigued because energy supply to the heart is impaired as a result of mitochondrial failure, so the blood pressure starts to fall. Initially the body tries to compensate by making the heart beat faster. However this too is unsustainable and a combination of weak heart beats which are too fast results in blood pressure falling precipitously. The patient has to lie down quickly to avoid blacking out.
One can test for this by a TILT test - this can be done by Dr Julia Newton at Newcastle but other hospitals within the NHS may well offer the same.
For an explanation of the mitochondrial failure see CFS - The Central Cause: Mitochondrial Failure
As the mitochondrial failure is treated, the symptoms of POTS resolve.