Why Low blood pressure when standing up for the patients of multiple system Atrophy

http://www.nhs.uk/Conditions/multiple-system-atrophy/Pages/Introduction.aspx

Low blood pressure when standing up 

It's common for someone with multiple system atrophy to feel lightheaded, dizzy and faint when they suddenly sit or stand up. This is because their blood pressure drops when they become upright, which is known as postural hypotension.

When you stand up after lying down, your blood vessels usually narrow quickly and your heart rate increases slightly, to prevent blood pressure dropping and maintain blood flow to the brain. This function is carried out automatically by the autonomic nervous system.

However, because this doesn't work properly in people with multiple system atrophy, this control is lost.

Problems with coordination, balance and speech

A part of the brain called the cerebellum becomes damaged in multiple system atrophy, which can make the person clumsy and unsteady when walking, and can also cause slurred speech.

These problems are collectively known as cerebellar ataxia.

Feeling stiff, and slowness of movement 

A person with multiple system atrophy typically has much slower movements than normal. This is referred to as "bradykinesia" and can make everyday tasks difficult. Movement is hard to initiate, and they often have a distinctive slow, shuffling walk with very small steps. 

Some people may also have stiffness and tension in the muscles, which can make it even more difficult to move around and can result in painful muscle cramps (dystonia).

The above symptoms are typical of Parkinson's disease, but unfortunately the medication used to relieve these symptoms in people with Parkinson's disease (levodopa therapy) is not very effective for people with multiple system atrophy.