The syndrome known as malignant hypertension occurs with systolic blood pressures over 200 and diastolic blood pressures typically over 120 mm Hg. The basis for this increase in blood pressure (BP) is not well understood; however the levels of plasma renin are increased, implicating the renin-angiotensin system in the pathogenesis of this condition. Symptoms of this rapid increase in blood pressure vary as the cardiovascular, renal and neurologic systems are affected. Often, the absolute level of the BP is not as important as the rate of increase when it comes to symptoms.
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There are many cardiovascular effects that occur with such high blood pressure; myocardial infarction (also known as a heart attack) is a definite possibility as the increased pressure causes the heart to pump harder and thus increase the requirement for oxygen. When this requirement is not met, a heart attack can occur. Increased blood pressure is also a significant risk factor for the development of aortic dissection, which is when the lumen of the aorta can suffer a tear from the increased pressures; you can imagine the bleeding that is possible in a vessel of that nature. Congestive heart failure is also possible, as the heart can struggle to consistently pump out blood against that high of a blood pressure and blood can end up "backing up" in the heart.
When blood pressure initially starts rising, protein starts appearing in the urine along with occasional blood, and soon thereafter renal failure can begin. Clinically, this will manifest as a severe decrease in the amount of urine produced. This is a medical emergency, as the damage can be irreversible if allowed to continue.
These fall under the term hypertensive encephalopathy, and the symptoms are numerous. Increased blood pressure can cause hemorrhages in the retina, as the smaller arteries in the eye can rupture, along with papilledema (swelling of the optic disc due to increased pressure intracranially). Persistent optic disc swelling can lead to damage of optic disc fibers resulting in permanent visual impairment. Vague neurologic symptoms such as headache and confusion also commonly occur. The increased blood pressure is also a considerable risk factor for stroke, so a variety of focal deficits can become apparent during the hypertensive episode as well.