NPTS symptoms

A remarkable increase in heart rate after standing from a prone position Low or normal blood pressure Dizziness and blurred vision Fainting spells Chest pains and shortness of breath Chronic fatigue, anxiety and irritability

In both instances this appears to lead to problems in controlling blood flow in the legs.

Scientists have been trying to pin down the causes of NPTS for decades.

It has been mistaken for low blood sugar, anxiety, anaemia, or even chronic fatigue syndrome.

The US researchers think the problem is caused by blood accumulating in the legs when a person stands.

Blood accumulation

Blood vessel contraction, which depends on the vascular nerves, is supposed to keep blood pressure at the same level as it was when the person was lying down.

But in NPTS patients, vascular nerves are damaged and contraction is ineffective, causing blood to pool in the legs and reducing the amount of blood to the heart.

This produces an effect similar to dehydration or haemorrhaging, in which the heart rate initially increases to compensate for low blood pressure.

If reduced blood return persists, blood pressure can fall and the patient can faint.

Ways to minimise risk

Eat foods high in salt and water Wear socks or stockings that apply pressure to the legs Walk and swim at a gradually increasing rate to build limb muscles, which provide structural support for veins

The researchers found that inadequate amounts of norepinephrine, the chemical responsible for blood vessel contraction, are released in the arms and legs of NPTS patients, even while resting.

NPTS can be treating by prescribing beta-blockers, drugs commonly used to treat hypertension or arrhythmia that decrease the force and rate of heart contractions.

Professor Julian Stewart, of New York Medical College, said the finding represented a significant breakthrough.

He said: "Prior to this study, many people felt this wasn't an illness of any sort, just a group of neurotic people feeling dizzy.

"Finding a consistent abnormality in blood flow control is very important to treating people with NPTS."