Severe high blood pressure is referred to as malignant hypertension, in which the diastolic ("bottom") blood pressure reading is over 140 mmHg. Malignant hypertension is a medical emergency and can cause stroke. Symptoms can include nausea, severe headache, blurred vision, and dizziness. Some people with malignant hypertension may develop kidney failure. They may experience a decrease in urine output and swelling (edema), most pronounced in the lower legs.

Blood pressure is the force of your blood pushing against the walls of your arteries. Each time your heart beats, it pumps out blood into the arteries. Your blood pressure is highest when your heart beats, pumping the blood. This is called systolic pressure. When your heart is at rest, between beats, your blood pressure falls. This is the diastolic pressure. Your blood pressure reading uses these two numbers. Usually they're written one above or before the other, such as 120/80. If your blood pressure reading is 90/60 or lower, you have low blood pressure.