Low blood pressure is generally considered a blood pressure reading lower than 90 millimeters of mercury (mm Hg) for the top number (systolic) or 60 mm Hg for the bottom number (diastolic). What's considered low blood pressure for one person might be OK for someone else. Low blood pressure might cause no noticeable symptoms, or it might cause dizziness and fainting. Sometimes, low blood pressure can be life-threatening. The causes of low blood pressure range from dehydration to serious medical conditions. It's important to find out what's causing low blood pressure so that it can be treated, if necessary.

Types

Types of low blood pressure include:

• Orthostatic hypotension (postural hypotension). This is a sudden drop in blood pressure when standing from a sitting position or after lying down. Causes include dehydration, long-term bed rest, pregnancy, certain medical conditions and some medications. This type of low blood pressure is common in older adults.
• Postprandial hypotension. This drop in blood pressure occurs 1 to 2 hours after eating. It's most likely to affect older adults, especially those with high blood pressure or autonomic nervous system diseases such as Parkinson's disease. Eating small, low-carbohydrate meals, drinking more water, and avoiding alcohol might help reduce symptoms.
• Neurally mediated hypotension. This is a blood pressure drop that happens after standing for long periods. This type of low blood pressure mostly affects young adults and children. It might result from miscommunication between the heart and the brain.
• Multiple system atrophy with orthostatic hypotension. Also called Shy-Drager syndrome, this rare disorder affects the nervous system that controls involuntary functions such as blood pressure, heart rate, breathing and digestion. It's associated with having very high blood pressure while lying down.

Symptoms

Low blood pressure (hypotension) symptoms may include:

• Blurred or fading vision
• Dizziness or lightheadedness
• Fainting
• Fatigue
• Trouble concentrating
• Nausea

For some people, low blood pressure may be a sign of an underlying health condition, especially when it drops suddenly or occurs with symptoms. A sudden fall in blood pressure can be dangerous. A change of just 20 mm Hg — a drop from 110 mm Hg systolic to 90 mm Hg systolic, for example — can cause dizziness and fainting. And big drops, such as those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening. Extreme low blood pressure can lead to a condition known as shock. Symptoms of shock include:

• Confusion, especially in older people
• Cold, clammy skin
• Decrease in skin coloration (pallor)
• Rapid, shallow breathing
• Weak and rapid pulse

When to see a doctor

If you have symptoms of extreme low blood pressure (hypotension) or shock, seek emergency medical help. Most health care providers consider blood pressure to be too low only if it causes symptoms. Occasional minor dizziness or lightheadedness can be caused by many things, such as spending too much time in the sun or in a hot tub. It's important to see a health care provider to get a correct diagnosis. If you have consistently low blood pressure readings but feel fine, your provider may just monitor you during routine health checkups. It can be helpful to keep a record of your symptoms, when they occur and what you're doing at the time.