Orthostatic Hypotension (OH): This is a common side effect of levodopa and other dopaminergic medications used in Parkinson's disease. The addition of Seroquel, an antipsychotic that can also cause orthostatic hypotension, increases the risk. The mechanism involves decreased sympathetic tone and vasodilation, leading to a significant drop in blood pressure upon standing.

Dehydration: Patients with Parkinson's disease may have autonomic dysfunction, which can lead to impaired thirst sensation and dehydration. Dehydration can exacerbate orthostatic hypotension and contribute to syncope.
Cardiac Arrhythmias: Parkinson's disease patients are at an increased risk of cardiac arrhythmias, which can be exacerbated by medications such as levodopa and Seroquel. Arrhythmias can lead to reduced cardiac output and syncope.
Seizures: Although less common, seizures can occur in patients with Parkinson's disease, especially those on certain medications. Seroquel has a known risk of lowering the seizure threshold.

Pulmonary Embolism (PE): While not directly related to Parkinson's disease or its treatment, PE is a life-threatening condition that can cause syncope. It's crucial to consider PE, especially if there are other symptoms such as chest pain or shortness of breath.
Cardiac Tamponade: This is a life-threatening condition where fluid accumulates in the pericardial sac, compressing the heart and reducing its ability to pump blood. It can cause syncope and is a medical emergency.
Aortic Dissection: A tear in the aorta can lead to syncope among other symptoms. It's a rare but critical condition that requires immediate attention.

Multiple System Atrophy (MSA): A rare neurodegenerative disorder that can present with symptoms similar to Parkinson's disease, including autonomic dysfunction leading to orthostatic hypotension and syncope.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension and orthostatic hypotension, leading to syncope. It's an important diagnosis to consider due to its potential for severe complications if not treated.
Vasovagal Syncope: Although common in the general population, vasovagal syncope might be less likely in a patient with Parkinson's disease on medications that already predispose to orthostatic hypotension. However, it remains a possibility, especially if triggered by specific events such as pain or stress.