the autonomic or involuntary nervous system, which is involved in the control of body functions such as heart rate and blood pressure, is affected in two thirds of people with guillain–barré syndrome, but the impact is variable. twenty percent may experience severe blood-pressure fluctuations and irregularities in the heart beat, sometimes to the point that the heart beat stops and requiring pacemaker-based treatment. other associated problems are abnormalities in perspiration and changes in the reactivity of the pupils. autonomic nervous system involvement can affect even those who do not have severe muscle weakness.