Seizure
Toxicologic exposure (stimulants or depressant)
CO poisoning 
Breath-holding spell
Tet spell
Cardiac disease
Pregnancy (especially ectopic)
Hypoglycemia
Cardiovascular-mediated syncope
Dysrhythmias:
WPW (pre-excitation)
Long QT Syndrome/QT prolongation
Brugada Syndrome<unk> 
2nd/3rd AV Block
Afib/aflutter
Vtach/torsades
Sick sinus syndrome
Arrhythmogenic right ventricular dysplasia
Short QT syndrome
Cardiovascular disease
Valvular Disease (AS, MS, tricuspid stenosis)
Aortic Dissection
Myocardial Infarction
CHF
Hypertrophic Cardiomyopathy
PE
Pericardial Tamponade
Myxoma
Pulmonary Hypertension
Pacemaker malfunction
Neurally mediated syncope
Vasovagal:
Fear, pain, emotion, valsalva, breath-holding spell
Situational (associated with):
Coughing, micturition, defecation, vomiting, swallowing, postexercise
Carotid sinus stimulation
Orthostatic hypotension-mediated syncope:
Volume depletion:
Dehydration (vomiting, diarrhea)
Hemorrhage, (Ectopic Pregnancy, Upper GI Bleed/Lower GI Bleeding, Abdominal Aortic Aneurysm)
Sepsis
Autonomic Dysreflexia
Autonomic failure due to meds
Other serious causes
Stroke
SAH
TIA
Vertebrobasilar Insufficiency
Subclavian steal
Heat syncope
Hypoglycemia
Hyperventilation
Asphyxiation
Seizure
Narcolepsy
Psychogenic (anxiety, conversion disorder, somatic symptom disorder)
Toxic (drugs, carbon monoxide, etc.)
Dysrhythmias:
WPW (pre-excitation)
Long QT Syndrome/QT prolongation
Brugada Syndrome<unk> 
2nd/3rd AV Block
Afib/aflutter
Vtach/torsades
Sick sinus syndrome
Arrhythmogenic right ventricular dysplasia
Short QT syndrome
Cardiovascular disease
Valvular Disease (AS, MS, tricuspid stenosis)
Aortic Dissection
Myocardial Infarction
CHF
Hypertrophic Cardiomyopathy
PE
Pericardial Tamponade
Myxoma
Pulmonary Hypertension
Pacemaker malfunction
WPW (pre-excitation)
Long QT Syndrome/QT prolongation
Brugada Syndrome<unk> 
2nd/3rd AV Block
Afib/aflutter
Vtach/torsades
Sick sinus syndrome
Arrhythmogenic right ventricular dysplasia
Short QT syndrome
Valvular Disease (AS, MS, tricuspid stenosis)
Aortic Dissection
Myocardial Infarction
CHF
Hypertrophic Cardiomyopathy
PE
Pericardial Tamponade
Myxoma
Pulmonary Hypertension
Pacemaker malfunction
Vasovagal:
Fear, pain, emotion, valsalva, breath-holding spell
Situational (associated with):
Coughing, micturition, defecation, vomiting, swallowing, postexercise
Carotid sinus stimulation
Fear, pain, emotion, valsalva, breath-holding spell
Coughing, micturition, defecation, vomiting, swallowing, postexercise
Carotid sinus stimulation
Volume depletion:
Dehydration (vomiting, diarrhea)
Hemorrhage, (Ectopic Pregnancy, Upper GI Bleed/Lower GI Bleeding, Abdominal Aortic Aneurysm)
Sepsis
Autonomic Dysreflexia
Autonomic failure due to meds
Dehydration (vomiting, diarrhea)
Hemorrhage, (Ectopic Pregnancy, Upper GI Bleed/Lower GI Bleeding, Abdominal Aortic Aneurysm)
Sepsis
Stroke
SAH
TIA
Vertebrobasilar Insufficiency
Subclavian steal
Heat syncope
Hypoglycemia
Hyperventilation
Asphyxiation
Seizure
Narcolepsy
Psychogenic (anxiety, conversion disorder, somatic symptom disorder)
Toxic (drugs, carbon monoxide, etc.)
Catecholaminergic polymorphic ventricular tachycardia
Seizure
Idiopathic fascicular left ventricular tachycardia
Toxicologic exposure 
Wandering atrial pacemaker
CO poisoning 
Polymorphic ventricular tachycardia
Breath-holding spell