Neurally Mediated Syncope

This category encompasses the terms vasovagal syncope, vasodepressor syncope, neurocardiogenic syncope, reflex syncope, and simple faint. NMS is the most common cause of syncope in children3 and usually is preceded by a sensation of warmth, nausea, light-headedness, and a visual grayout or tunneling of vision. 15 This type of syncope frequently lasts less than 1 min.4 Common precipitating factors include prolonged recumbence just prior to standing or prolonged standing, sight of blood or disfiguring injury (for example, fractures or soft tissue injuries), emotional upset, mild physical trauma or pain, physical exertion, and hot or crowded conditions. Other contributing factors that are less common include hypovolemia, anemia, dehydration, and pregnancy. NMS can also occur with swallowing, urination, defecation, and coughing; breath-holding spells are a variant of this form of syncope. Medications that alter vascular tone or heart rate may contribute to the development of syncope, including b blockers, calcium-channel blockers, and diuretics. Surreptitious use of diuretics is common among athletes, such as wrestlers, who must maintain weight restrictions.

Identifying NMS as the cause of syncope can be difficult in an emergency department. Because NMS is physiologically based on inadequate compensatory mechanisms to maintain blood pressure and cardiac output in a wide range of clinical states, the distinction between syncope from NMS and other causes of orthostatic syncope is often blurred. However, NMS and other orthostatic causes of syncope are generally considered to be benign illnesses.

Three clinical patterns of NMS occur: vasodepressor syncope, cardioinhibitory syncope, and mixed syncope. Any disorder that causes vasodilatation, vagal stimulation, or both can result in syncope.

ORTHOSTATIC SYNCOPE Patients will typically complain of light-headedness and weakness after standing for a period of time ranging from seconds to minutes. Factors that predispose children to orthostatic syncope include anemia, dehydration, and medications, especially calcium-channel blockers and angiotensin-converting enzyme inhibitors.3 A drop of greater than 20 mmHg in blood pressure with an increase in heart rate of more than 20 beats per minute while checking vital signs with the child in the supine to standing position is often considered diagnostic of orthostatic hypotension.

SITUATIONAL SYNCOPE Urination, defecation, coughing, and swallowing have all been described as causing syncope. The pathophysiology is thought to be related to an exaggerated Valsalva response causing cardioinhibitory syncope. Stretching, neck extension, external neck pressure, and hair grooming have also been described as causing syncope, presumably due to carotid sinus hypersensitivity or abnormal Valsalva responses. 14

FAMILIAL DYSAUTONOMIA Abnormalities in heart rate and blood pressure control can be inherited as a primary disorder, such as the Riley-Day syndrome. This disorder results from abnormal development of the sensory and autonomic ganglia, perhaps due to a lack of nerve growth factor during embryogenesis. Manifestations include failure to thrive, developmental delay, temperature instability, abnormal sweating, absent lacrimation, breath-holding spells, and seizures. 14

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