Dehydration: This is often the most common cause of new onset presyncope, especially in individuals who have recently experienced vomiting, diarrhea, or have not been drinking enough fluids. Dehydration leads to a decrease in blood volume, which can cause a drop in blood pressure and reduce blood flow to the brain, resulting in presyncope.

Orthostatic Hypotension: This condition occurs when there is a significant drop in blood pressure upon standing, which can lead to presyncope. It is common in older adults, individuals with diabetes, and those taking certain medications.
Anxiety or Panic Attacks: Anxiety and panic attacks can cause symptoms of presyncope, including dizziness and lightheadedness, due to hyperventilation and other physiological responses to stress.
Vasovagal Syncope: Also known as a common faint, this is a reflex of the involuntary nervous system that can cause a feeling of lightheadedness and fainting, often triggered by stress or certain sights.

Cardiac Arrhythmias: Conditions such as atrial fibrillation or other arrhythmias can lead to presyncope by disrupting the normal rhythm of the heart, potentially reducing cardiac output and cerebral blood flow.
Myocardial Infarction or Acute Coronary Syndrome: Although less common, new onset presyncope can be a symptom of a heart attack, especially in individuals with risk factors for coronary artery disease.
Pulmonary Embolism: A blockage in one of the arteries in the lungs can cause sudden onset of presyncope among other symptoms like chest pain and shortness of breath.
Aortic Dissection: A tear in the inner layer of the aorta can lead to presyncope due to the potential for significant blood loss and disruption of blood flow.

Subclavian Steal Syndrome: A rare condition where there is a reversal of blood flow in the subclavian artery, which can lead to decreased blood flow to the brain and cause presyncope.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of high blood pressure, leading to symptoms including presyncope.
Autonomic Dysfunction: Certain conditions affecting the autonomic nervous system, such as multiple system atrophy or pure autonomic failure, can lead to orthostatic intolerance and presyncope.