The weakness of legs, dizziness and vomiting may be posterior circulation ischemia. When posterior circulation ischemia tends to cause brainstem ischemia and hypoxia, patients may experience dizziness, nausea, vomiting, and also weakness of both lower limbs, and some patients may even experience weakness of all four limbs, and may also have difficulty swallowing and choking on drinking water. Posterior circulation ischemia is usually caused mainly by hypovolemia, or severe hypotension. Hypovolemia is seen mainly in patients with massive vomiting, diarrhea, or profuse sweating. Severe hypotension may be caused by postural hypotension, where the patient is sitting or recumbent and suddenly stands up to cause a transient drop in blood pressure, which may cause hypoperfusion of the brain and predispose to posterior circulation ischemia. It may also be caused by brainstem hemorrhage or infarction. Patients with brainstem hemorrhage usually start during activity and can suddenly develop the above symptoms. Patients with brainstem infarction usually start in a quiet state and generally have a low-density lesion on cranial CT, while patients with brainstem hemorrhage generally have a high-density lesion on cranial CT.