Taber's Dict:
hypertensive ¶crisis = Arbitrarily defined as severe elevation in diastolic blood pressure above 120 to 130 mm Hg.
This is considered to be an emergency if there is evidence of rapid or progressive central nervous system, myocardial, hematological, or renal deterioration.
TREATMENT
Blood pressure must be reduced without delay by use of appropriate therapy, usually sodium nitroprusside (SNP), labetalol, and diazoxide. If the ¶crisis is due to eclampsia, SNP may not be indicated because of risk to fetus.

From Harrison Principles of Internal Medicine:
Discretion is warranted in prescribing medication to lower BP that may affect cardiovascular risk adversely in other ways (e.g., glucose control, lipid metabolism, uric acid levels). In the absence of ¶hypertensive ¶crisis, BP should be reduced gradually to avoid end-organ (e.g., cerebral) ischemia.
...Sodium nitroprusside, a direct-acting arterial and venous vasodilator, is the drug of choice for most ¶hypertensive ¶crises. It reduces BP rapidly and is easily titratable, and its action is short-lived when discontinued.
... In ¶hypertensive ¶crisis, sudden visual loss can result from vasospasm of retinal arterioles and consequent retinal ischemia. In addition, acute hypertension may produce visual loss from ischemic swelling of the optic disc.