This may occur from either a triggering event such as exposure to blood, pain, strong feelings, or a specific activity such as urination, vomiting, or coughing. Vasovagal syncope
Vasovagal (situational) syncope is one of the most common types which may occur in response to any of a variety of triggers, such as scary, embarrassing or uneasy situations, during blood drawing, or moments of sudden unusually high stress. There are many different syncope syndromes which all fall under the umbrella of vasovagal syncope related by the same central mechanism. First, the person is usually predisposed to decreased blood pressure by various environmental factors. A lower than expected blood volume, for instance, from taking a low-salt diet in the absence of any salt-retaining tendency. Or heat causing vaso-dilation and worsening the effect of the relatively insufficient blood volume. The next stage is the adrenergic response. If there is underlying fear or anxiety (e.g., social circumstances), or acute fear (e.g., acute threat, needle phobia), the vaso-motor centre demands an increased pumping action by the heart (flight or fight response). This is set in motion via the adrenergic (sympathetic) outflow from the brain, but the heart is unable to meet requirements because of the low blood volume, or decreased return. A feedback response to the medulla is triggered via the afferent vagus nerve. The high (ineffective) sympathetic activity is thereby modulated by vagal (parasympathetic) outflow leading to excessive slowing of heart rate. The abnormality lies in this excessive vagal response causing loss of blood flow to the brain.
Vertebro-basilar arterial disease
Arterial disease in the upper spinal cord, or lower brain that causes syncope if there is a reduction in blood supply. This may occur with extending the neck or with use of medications to lower blood pressure. Other causes
There are other conditions which may cause or resemble syncope. Seizures and syncope can be difficult to differentiate. Both often present as sudden loss of consciousness and convulsive movements may be present or absent in either. Movements in syncope are typically brief and more irregular than seizures. Akinetic seizures can present with sudden loss of postural tone without associated tonic-clonic movements. Absence of a long post-ictal state is indicative of syncope rather than an akinetic seizure. Subarachnoid hemorrhage may result in syncope. Often this is in combination with sudden, severe headache. It may occur as a result of a ruptured aneurysm or head trauma.Heat syncope occurs when heat exposure causes decreased blood volume and peripheral vasodilatation. Position changes, especially during vigorous exercise in the heat, may lead to decreased blood flow to the brain. Closely related to other causes of syncope related to hypotension (low blood pressure) such as orthostatic syncope. Lactose intolerance can cause "a release of histamine, resulting in an extreme dilatation of the bloodvessels, resulting in a drop of blood pressure so that not enough blood reaches the brains, leading to dizziness, fainting, syncope, itching, hives, tingling or swelling of the lips, tongue, or throat; chest tightness, shortness of breath, or difficulty breathing, wheezing." (More in the article on 
 Lactose intolerance).