A hypertensive emergency, unlike the similar sounding hypertensive urgency, is characterized by serious, life-threatening complications. A hypertensive emergency means that the blood pressure is >180 mm Hg or the diastolic pressure is >120 mm Hg, and that end-organ damage is occurring. Signs and symptoms can include shortness of breath, anxiety, chest pain, irregular heart rate, confusion, or fainting. 
Because of the need for emergency care, it is important to recognize the early signs of malignant hypertension. The first giveaway is blood pressure of 180/120. You might have bleeding in the eyes due to rupture of the small blood vessels. Other malignant hypertension symptoms can include chest pain, dizziness, a headache, numbness in your extremities, and confusion.
Health issues may happen; however, when a person's blood pressure suddenly drops and their brain is deprived of an adequate blood supply. The condition may lead to lightheadedness or dizziness. A sudden drop in blood pressure usually happens in a person who has risen from a prone or sitting position to a standing one. When this occurs it is referred to as, 'orthostatic hypotension,' or, 'postural hypotension.' Another type of low blood pressure may happen when a person stands for extended periods of time; it is referred to as, 'neurally mediated hypotension.'
Before measuring your blood pressure, do not smoke, drink caffeinated beverages, or exercise for at least 30 minutes before the test. Rest for at least five minutes before the measurements and sit still with your back straight and supported. Feet should be flat on the floor and not crossed. Your arm should also be supported on a flat surface like a table with the upper arm at heart level.
The authors bring several evidence-based yet progressive concepts into the guidelines, the first of which is that high blood pressure should be treated using a team approach. This makes sense, as science supports more and better patient education around self-monitoring, nutrition, and lifestyle changes, as well as stress management. Telehealth is emphasized as a cost-effective method of ongoing monitoring that is more convenient for patients than frequent office visits.
Hypertension does not usually cause any noticeable symptoms. When it does, you might experience dizziness, shortness of breath, headaches, and nosebleeds, which could indicate that your blood pressure is rising. Complications such as heart disease, stroke, and kidney failure can occur if long-term hypertension is not adequately treated. A hypertensive emergency, which is an uncommon and dangerous event, may cause blurry vision, nausea, chest pain and anxiety.

^ Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents, National Heart, Lung, and Blood Institute (Dec 2011). "Expert panel on integrated guidelines for cardiovascular health and risk reduction in children and adolescents: summary report". Pediatrics. 128 Suppl 5: S213–56. doi:10.1542/peds.2009-2107C. PMC 4536582. PMID 22084329.
The kidney can respond to changes in blood pressure by increasing or decreasing the amount of urine that is produced. Urine is primarily water that is removed from the blood. When the kidney makes more urine, the amount (volume) of blood that fills the arteries and veins decreases, and this lowers blood pressure. If the kidneys make less urine, the amount of blood that fills the arteries and veins increases and this increases blood pressure. Compared with the other mechanisms for adjusting blood pressure, changes in the production of urine affect blood pressure slowly over hours and days. (The other mechanisms are effective in seconds.)
Unlike high blood pressure symptoms, which are poorly defined and often totally absent, low blood pressure symptoms tend to be more upfront and easily recognizable. The development of symptoms is often a warning sign of a potentially serious underlying disorder. Generally speaking, your blood pressure would need to fall pretty dramatically before symptoms develop.