Where stenosis is severe, abrupt changes in blood pressure can temporarily result in inadequate flow through the stenosis, causing symptoms that are usually very brief. A common cause is orthostatic hypotension which results in a fall in blood pressure when the patient changes posture and may be exacerbated by medicines (particularly antihypertensives), dehydration and heat. In reality, orthostatic hypotension can cause vague symptoms (e.g. dizziness) which are similar to those caused by VBI; VBI should only be considered where the stenosis is severe and/or there are focal neurological symptoms specific to the posterior circulation.