A. Noncardiac Causes (Autonomic Dysfunction)

• 1.
  

  Orthostatic intolerance encompasses disorders of blood flow, heart rate (HR), and blood pressure (BP) regulation that are most easily demonstrable during orthostatic stress. The recently popularized head-up tilt test has identified three entities: vasovagal syncope, orthostatic hypotension, and postural orthostatic tachycardia syndrome (POTS).

  
  
  
  
  • a.
    

    Vasovagal syncope (also called simple fainting or neurocardiogenic syncope).

    
    
    
    
    • (1)
      

      This is the most common type of syncope seen in otherwise healthy children and adolescents. It is uncommon before ages 10 to 12 but quite prevalent in adolescents, especially girls. It is characterized by a prodrome lasting a few seconds to a minute. The prodrome may include dizziness, nausea, pallor, diaphoresis, palpitation, blurred vision, headache, and/or hyperventilation. The prodrome may be followed by the loss of consciousness and muscle tone with a fall. The unconsciousness does not last more than a minute. The syncope may occur after rising in the morning or in association with prolonged standing, anxiety or fright, pain, blood drawing or the sight of blood, fasting, hot and humid conditions, or crowded places. Typical response of patients with vasovagal syncope to the head-up tilt table test is precipitous drops in both the HR and BP ( Fig. 21.1 ). History is most important in establishing the diagnosis of vasovagal syncope.

      
      

      
      

      
      

      

      
      

      
      

      Schematic drawing of changes in heart rate ( HR) and blood pressure (BP) observed during the head-up tilt test. Thin arrows mark the start of orthostatic stress. Large unfilled arrows indicate appearance of symptoms with changes seen in HR and BP. POTS , postural orthostatic tachycardia syndrome.

      
      

      From Park, M. K., & Salamat, M. (2020). Park’s pediatric cardiology for practitioners (7th ed.). Philadelphia: Mosby.

      
      
      
      
    • (2)
      

      Proposed pathophysiology of vasovagal syncope . The normal responses to assuming an upright posture are a reduced cardiac output, an increase in heart rate, and an unchanged or slightly diminished systolic pressure with about 6% decrease in cerebral blood flow.

    
    

  
  
  
  

  In susceptible individuals, a sudden decrease in venous return to the ventricle produces a large increase in the force of ventricular contraction, which causes activation of the LV mechanoreceptors. A sudden increase in neural traffic to the brainstem somehow mimics the conditions seen in hypertension and thereby produces a paradoxical withdrawal of sympathetic activity. This results in peripheral vasodilatation, hypotension, bradycardia, and subsequent decrease in cerebral perfusion, leading to either presyncope or syncope (with loss of consciousness). Vasovagal syncope always occurs while the patient is in standing position. Hypovolemia (or dehydration) is often a predisposing factor; a dehydrated person is more likely to have syncope.

  
  
  
  
  • b.
    

    Orthostatic hypotension (dysautonomia)

    
    
    
    
    • (1)
      

      The normal response to standing is reflex arterial and venous constriction and a slight increase in HR. In orthostatic hypotension, the normal adrenergic vasoconstriction of the arterioles and veins in the upright position is absent or inadequate, resulting in hypotension without a reflex increase in HR (see Fig. 21.1 ). Unlike the prodrome seen with vasovagal syncope, patients experience only light-headedness in orthostatic hypotension. They do not display the autonomic nervous system signs seen with vasovagal syncope, such as pallor, diaphoresis, and hyperventilation. Prolonged bed rest, prolonged standing, dehydration, medications that interfere with the sympathetic vasomotor response (e.g., calcium channel blockers, antihypertensive drugs, vasodilators, phenothiazines), and diuretics may exacerbate orthostatic hypotension.

      
      
    • (2)
      

      In patients suspected of having orthostatic hypotension, BPs should be measured in the supine and standing positions. In adults, the American Autonomic Society has defined orthostatic hypotension as a persistent fall in systolic/diastolic pressure of more than 20/10 mm Hg within 3 minutes of assuming the upright position without moving their arms or legs, with no increase in the HR but without fainting. Orthostatic hypotension may only be demonstrable in the presence of dehydration. In a well-hydrated state when seen in an office setting, orthostatic hypotension may not occur.

    
    

  
  
  
  
  • c.
    

    Postural orthostatic tachycardia syndrome (POTS)

    
    
    
    
    • (1)
      

      This syndrome, most often observed in young women, is a form of autonomic neuropathy that predominantly affects the lower extremities. Venous pooling associated with assuming a standing position leads to a reduced venous return and a resulting increase in sympathetic discharge with a significant degree of tachycardia. An increased level of adrenomedullin, a potent vasodilator with natriuric and diuretic effects, has been observed in some children with the syndrome. Affected patients often complain of chronic fatigue, exercise intolerance, palpitation, light-headedness, nausea, and recurrent near syncope (and sometimes syncope). These symptoms may be related to chronic fatigue syndrome and may be misdiagnosed as panic attacks or chronic anxiety. Occasional patients develop swelling of the lower extremities with purplish discoloration of the dorsum of the foot and ankle.

      
      
    • (2)
      

      For the diagnosis of POTS, HR and BP are measured in the supine, sitting, and standing positions. POTS is defined as the development of orthostatic symptoms that are associated with at least a 30-beat/min increase in HR (or a HR of ≥120 beats/min) that occurs within the first 10 minutes of standing or upright tilt, with occurrence of symptoms described previously (see Fig. 21.1 ).

    
    

  
  
  
  
• 2.
  

  Exercise-related syncope . Athletic adolescents may experience syncope or presyncope during or after strenuous physical activities. This may signal serious cardiac problems, but in most cases it occurs due to a combination of venous pooling in vasodilated leg muscles, inadequate hydration, and high ambient temperature. To prevent venous pooling, athletes should keep moving after running competitions. Secondary hyperventilation from exercise activities with resulting hypocapnia may also contribute to this form of syncope. Tingling or numbness of extremities may occur with hypocapnia.

  
  
• 3.
  

  Situational syncope .

  
  
  
  
  • a.
    

    Micturition syncope is a rare form of orthostatic hypotension. In this condition, rapid bladder decompression results in decreased total peripheral vascular resistance with splanchnic stasis and reduced venous return to the heart, resulting in postural hypotension.

    
    
  • b.
    

    Cough syncope follows paroxysmal nocturnal coughing in asthmatic children. Paroxysmal coughing produces a marked increase in intrapleural pressure with a reduced venous return and reduced cardiac output, resulting in altered cerebral blood flow and loss of consciousness.

  
  

B. Cardiac Causes of Syncope

Cardiac causes of syncope may include obstructive lesions, myocardial dysfunction, and arrhythmias (see Box 21.1 ).

• 1.
  

  Obstructive lesions . Patients with severe AS, PS, or hypertrophic obstructive cardiomyopathy (HOCM), as well as those with pulmonary hypertension, may have syncope. Exercise often precipitates syncope associated with these conditions. These patients may also complain of chest pain, dyspnea, and palpitation.

  
  
• 2.
  

  Myocardial dysfunction . Although rare, myocardial ischemia or infarction secondary to congenital anomalies of the coronary arteries or acquired disease of the coronary arteries (such as Kawasaki disease, postsurgical, or atherosclerotic heart disease) may cause syncope.

  
  
• 3.
  

  Arrhythmias . Either extreme tachycardia or bradycardia can cause syncope. Commonly encountered rhythm disturbances include SVT, VT, sick sinus syndrome, and complete heart block. Imaging studies may or may not show structural abnormalities.

  
  
  
  
  • a.
    

    No identifiable structural defect is present in long QT syndrome, WPW syndrome, RV dysplasia, and Brugada syndrome.

    
    
  • b.
    

    Identifiable structural heart defects are imaged for the following conditions.

    
    
    
    
    • (1)
      

      Preoperative CHDs (such as Ebstein anomaly, MS, or MR, and L-TGA).

      
      
    • (2)
      

      Postoperative CHDs (such as TOF, TGA, after Fontan operation) may cause sinus node dysfunction, SVT, VT, or complete heart block.

      
      
    • (3)
      

      Dilated cardiomyopathy can cause sinus bradycardia, SVT, or VT.

      
      
    • (4)
      

      Hypertrophic cardiomyopathy is a rare cause of VT and syncope.

    
    

  
  

A. History

Accurate history taking is most important in determining cost-effective diagnostic strategies.

• 1.
  

  About the syncopal event

  
  
  
  
  • a.
    

    The time of day: Syncope occurring after rising in the morning suggests vasovagal syncope. Hypoglycemia is a very rare cause of syncope.

    
    
  • b.
    

    The patient’s position: Syncope while sitting or recumbent suggests arrhythmias or seizures. Syncope after standing for some time suggests vasovagal syncope or other orthostatic intolerance group.

    
    
  • c.
    

    Relationship to exercise

    
    
    
    
    • (1)
      

      Syncope occurring during exercise suggests arrhythmias.

      
      
    • (2)
      

      Syncope occurring immediately after cessation of strenuous physical activities (such as football practice or game) may be due to venous pooling in the leg, and rarely due to arrhythmias. Vigorousness and duration of the activity, relative hydration status, and ambient temperature are important.

    
    

  
  
  
  
  • d.
    

    Associated symptoms

    
    
    
    
    • (1)
      

      Palpitation or racing HR suggests arrhythmia or tachycardia.

      
      
    • (2)
      

      Chest pain suggests possible myocardial ischemia (due to obstructive lesions, cardiomyopathy, carditis, etc.).

      
      
    • (3)
      

      Shortness of breath or tingling or numbness of extremities suggests hyperventilation.

      
      
    • (4)
      

      Nausea, epigastric discomfort, and diaphoresis suggest vasovagal syncope.

      
      
    • (5)
      

      Headache or visual changes also suggest vasovagal syncope.

    
    

  
  
  
  
  • e.
    

    The duration of syncope

    
    
    
    
    • (1)
      

      Syncopal duration less than 1 minute suggests vasovagal syncope, hyperventilation, or syncope due to other orthostatic mechanism.

      
      
    • (2)
      

      A longer duration of syncope suggests convulsive disorders, migraine, or cardiac arrhythmias.

    
    

  
  
  
  
  • f.
    

    The patient’s appearance during and immediately following the episode.

    
    
    
    
    • (1)
      

      Pallor indicates hypotension.

      
      
    • (2)
      

      Abnormal movement or posturing, confusion, focal neurologic signs, amnesia, or muscle soreness suggests the possibility of seizure.

    
    

  
  
  
  
• 2.
  

  Past history of cardiac, endocrine, neurologic, or psychological disorders may suggest a disorder in that system.

  
  
• 3.
  

  Medications, including prescribed, over-the-counter, and recreational drugs, should be checked.

  
  
• 4.
  

  Family history should include the following data:

  
  
  
  
  • a.
    

    Myocardial infarction in family members younger than 30 years of age.

    
    
  • b.
    

    Cardiac arrhythmia, CHD, cardiomyopathies, long QT syndrome, seizures, and metabolic and psychological disorders.

    
    
  • c.
    

    Positive family history of fainting is common in patients with vasovagal syncope.

  
  
  
  
• 5.
  

  Social history is important in assessing whether there is a possibility of substance abuse, pregnancy, or factors leading to a conversion reaction.

B. Physical Examination

Although physical examination is usually normal, it should always be performed, focusing on the cardiac and neurologic systems.

• 1.
  

  Careful auscultation includes heart murmurs or abnormally loud second heart sounds.

  
  
• 2.
  

  If orthostatic intolerance group is suspected, the HR and BP should be measured repeatedly while the patient is supine and after standing without moving for up to 10 minutes.

  
  
• 3.
  

  Neurologic examination should include a fundoscopic examination, test for Romberg sign, gait evaluation, deep tendon reflexes, and cerebellar function.

C. Diagnostic Studies

History and physical examinations guide practitioners in choosing the diagnostic tests that apply to a given syncopal patient.

• 1.
  

  Serum glucose and electrolytes are of limited value, because patients are seen hours or days after the episode.

  
  
• 2.
  

  When an arrhythmia is suspected as the cause of syncope:

  
  
  
  
  • a.
    

    The electrocardiogram (ECG) should be inspected for HR (bradycardia), arrhythmias, WPW preexcitation, heart block, long QTc interval, and abnormalities suggestive of cardiomyopathies and myocarditis.

    
    
  • b.
    

    Ambulatory ECG monitoring (24-hour Holter monitor or event recorder) is usually obtained.

  
  
  
  
• 3.
  

  Echo studies are performed to rule out CHDs, pulmonary hypertension, and cardiomyopathies and to check on the status of postoperative CHDs.

  
  
• 4.
  

  Exercise stress test is indicated if the syncopal event is associated with exercise.

  
  
• 5.
  

  Rarely cardiac catheterization and electrophysiologic study may be indicated in some equivocal cases.

  
  
• 6.
  

  Head-up tilt table test. If patients with positional syncope have autonomic symptoms (such as pallor, diaphoresis, or hyperventilation), tilt table testing is sometimes performed by some centers (see following section).

  
  
• 7.
  

  Neurologic consultation. Patients who faint while sitting or recumbent and those who exhibit prolonged loss of consciousness, seizure activity, and a postictal phase with lethargy or confusion should be referred for neurologic consultation.

D. Head-UP Tilt Table Test

The goal of the test is to provoke the patient’s symptoms during an orthostatic stress while closely monitoring the patient’s cardiac rhythm, HR, and BP responses associated with symptoms. Orthostatic stress is created by a tilting table with the patient placed in an upright position for a certain period of time. Various protocols are available.

Positive responses commonly include light-headedness, dizziness, nausea, visual changes, and frank syncope. Sinus bradycardia, junctional bradycardia, and asystole for as long as 30 seconds are common. Hypotension generally is manifested by systolic BPs of less than 70 mm Hg. Returning the patient to the supine position produces resolution of symptoms rapidly, usually with a reactive tachycardia.

Although several distinct abnormal patterns have been identified during the head-up tilt table tests (as shown in Fig. 21.1 ), there are serious questions about the sensitivity, specificity, diagnostic yield, and day-to-day reproducibility of the tilt test. In adults, the overall reproducibility of syncope by the tilt test is disappointingly low (62%). About 25% of adolescents with no prior fainting history fainted during the tilt test. Moreover, among habitual fainters, 25% to 30% did not faint during the test on a given day.