Neurocardiogenic (Autonomic, Vasodepressor, Vasovagal) Syncope
The most common syncope in children is neurocardiogenic (vasodepressor or vasovagal) syncope. There is a sudden, brief loss of consciousness because of vasodilatation and decreased peripheral resistance, resulting in decreased arterial pressure, hypotension, bradycardia, and then decreased cerebral blood flow (Bezold–Jarisch reflex). Current theories suggest that the systemic vasodilation and the changes in heart rate and blood pressure are caused by sympathetic withdrawal, rather than increased parasympathetic (vagal) activity.1 Orthostatic hypotension may be a result of volume depletion, anemia, or drugs, but it can also be because of a paradoxic response to the vasodepressor reaction.4,6 A tilt-table test can be performed by a cardiologist to diagnose true neurocardiogenic syncope. A positive head upright tilt-table test response, consisting of an initial increase in heart rate followed by bradycardia and syncope, may warrant drug therapy if frequent episodes occur.1
Environmental factors, such as prolonged standing, heat, fatigue, crowding, or hunger, can trigger syncope. Emotional stress or a recent illness can also play a role. Patients may have symptoms beforehand such as blurred vision, dizziness, nausea, or pallor. This is what is commonly referred to as a “simple faint.” Placing the person in a supine position with the head down usually results in improvement, although the patient may still complain of dizziness.6
The term situational syncope can be used for those patients that have syncope triggered by specific events. The common denominator is that these actions are accompanied by a Valsalva-like maneuver. This includes coughing, micturition, hairgrooming, diving, weight lifting, and sneezing. Another form is carotid sinus syncope, which occurs with head rotation or pressure on the carotid sinus. This can occur with shaving or tight collars.1
Breath-holding spells are another example of reflex syncope. The age of onset of breath-holding spells is 6 to 18 months.5 The two types of breath-holding spells are classified based on the color change:
- The pallid breath-holding spell is a form of reflex syncope. Pallid breath-holding spells are usually provoked by some mild antecedent trauma (usually to the head), pain, or fright. The child may gasp and cry, then become quiet, lose postural tone and consciousness, and become pale. The child may have clonic movements and incontinence in more severe episodes. The child regains consciousness in less than 1 minute, but may be tired after the episode for several hours.5
- A cyanotic breath-holding spell is often precipitated by anger or frustration. The child cries, becomes quiet, and holds the breath in expiration. This apnea is associated with cyanosis and there may be a loss of consciousness, limpness, or opisthotonic posturing, with recovery usually within 1 minute.5
Cardiac syncope is important to exclude, as this is the type that is truly life-threatening. The differential in this subgroup includes arrhythmia, obstruction, cyanosis, and other cardiac etiologies. Arrhythmias that result in a heart rate that is too fast or too slow can cause a decrease in cardiac output and lead to decreased cerebral perfusion. Included in this group are supraventricular tachycardia (SVT), atrial tachycardia, Wolff–Parkinson–White syndrome, atrial flutter, ventricular tachycardia, and ventricular fibrillation. Conduction abnormalities such as AV block, sick sinus syndrome (may occur after cardiac surgery), long QT syndrome, congenital short QT, or Brugada syndrome may be present.
(Fig. 53–2.) Long QT syndrome is a disorder of myocardial repolarization that can lead to polymorphic ventricular tachycardia (torsades de pointes). There are congenital forms; Romano–Ward syndrome (autosomal dominant), and Jervell and Lange–Nielsen syndrome (autosomal recessive). The latter form is associated with sensioneural deafness. There are also acquired forms usually the result of the metabolic disorders such as hypokalemia and hypomagnesemia, some medications (quinidine, procainamide), or a combination of medications (erythromycin or ketoconazole and terfenadine).6 Although prolongation of the corrected QTc is the main requirement, this value changes with the patient's age, and sex. (QTc is >450 ms in males and >460 in females).7 To calculate the QTc, use Bazett's formula QTc = QT/√RR.2 After evaluation by a cardiologist, treatments include β-blockers, implantable defibrillators, and avoidance of medication that prolong the QT interval.8
Congenital Short QT Syndrome
This syndrome has led to sudden cardiac death, syncope, and atrial fibrillation. The QTC is ≤0.30 ms.2
Patients with this inherited autosomal dominant disorder have a characteristic ECG pattern: ST segment elevation (≥2 mm) in leads V1–V3. There is an increased risk of sudden cardiac death resulting from polymorphic ventricular tachyarrhythmias.9 Although it is more likely to occur in young males, with a reported mean age of sudden death at 40 years, children have been detected with this disorder during family screening, and there was no male predominance, and febrile illness was the most important precipitating event.9
Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy
Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD) is characterized by ventricular tachycardia and abnormalities of the right ventricle, caused by myocyte replacement by fibrosis or adipose tissue. Although sudden cardiac death may be the presenting feature (mean age of 30 years), PVCs, syncope, and ventricular tachycardia with left bundle branch block may be forewarning findings. Diagnosis is based on ECG findings (epsilon waves after the QRS) as well as structural, and functional criteria.2,6,8
All these problems should be excluded by evaluation of a 12-lead electrocardiogram and rhythm strip. If the problem is intermittent, 24-hour Holter monitoring and referral to a cardiologist should be included in the evaluation.
Obstructive lesions can impair cardiac output and cerebral blood flow, leading to syncope. These include congenital lesions such as aortic stenosis, pulmonic stenosis, idiopathic hypertrophic subaortic stenosis (IHSS), hypertrophic cardiomyopathy, mitral stenosis, coarctation of the aorta, tetralogy of Fallot, anomalous origin of the left coronary artery, and transposition of the great vessels. Children who have undergone surgical correction of tetraology, transposition, and aortic stenosis are at greater risk of arrhythmias.2 The presence of chest pain and syncope with exercise, as well as a murmur on physical examination, can suggest left ventricular outflow obstruction because of IHSS or aortic stenosis. Cyanosis with or without syncope can result from increased resistance to pulmonary blood flow, causing an increase in the right-to-left shunting of blood. These spells can occur in children with tetralogy of Fallot, tricuspid atresia, and Eisenmenger's syndrome.
Evaluation for these disorders includes an ECG, chest radiograph, echocardiogram, and cardiology consultation.2
This autosomal dominant disorder is characterized by asymmetric hypertrophy of the left ventricle, without dilatation. It is a common cause of sudden death associated with exercise in young patients. Syncope is a major risk factor for sudden death. The mechanisms that cause syncope and sudden death include bradyarrhythmias, ventricular arrhythmias, severe outflow tract obstruction, and decreased blood pressure in response to exercise.2,8 Implantable defibrillators have been used to treat this disorder with some success.8
Acquired lesions include cardiac tumors and conditions secondary to myocarditis, pericarditis, cardiac tamponade, cardiomyopathy, and pulmonary hypertension. Primary pulmonary hypertension can cause dyspnea on exertion but can also cause syncope from inadequate cardiac output. Evaluation includes a 12-lead ECG, chest radiograph, and prompt cardiology referral.2
Noncardiac causes of syncope include neurologic, metabolic, psychologic, and toxicologic problems. Seizures should be considered whenever there is a loss of consciousness, especially if accompanied by increased muscle tone or tonic-clonic movements. If syncope occurs while the child is in a recumbent position, a seizure is a likely diagnosis. The diagnostic workup should proceed based on the most likely etiology and type of seizure.2
Hypoglycemia is the main metabolic disorder that can cause syncope. Prior to a loss of consciousness there is often a period of confusion and weakness. Hypocalcemia and hypomagnesemia can also cause syncope, but this is secondary to the arrhythmias generated by these disorders.
Psychologic causes of syncope include hyperventilation and hysteria. Hyperventilation results in hypocapnia, which causes cerebral vasoconstriction and decreased cerebral blood flow. The patient may complain of shortness of breath, chest tightness, numb fingers before syncope ensues.1 Hysteric syncope occurs when the patient mimics a loss of consciousness and falls to the ground without injury. No abnormalities of heart rate, blood pressure, or skin color are detected, and clues regarding surrounding events may point to the correct diagnosis, such as prolonged recovery after the event, and indifference to syncope.2,3
Drug-induced syncope can be caused by prescription drugs, over-the-counter medications, or drugs of abuse. Drugs of abuse such as cocaine are well known to result in syncope as well as more serious cardiac arrhythmias. Marijuana, alcohol, and opiates can all cause a loss of consciousness.1,2 Inhalant use can result in ventricular tachycardia and death. Antihypertensive agents, phenothiazines, calcium channel blockers, nitrates, and barbiturates can block the increased blood pressure response, and β-blockers and digitalis will block the tachycardia needed to respond to decreased systemic vascular resistance prior to syncope. Some of the newer antihistamines can cause prolonged QT and even torsades de pointes, if given with macrolides or ketoconazole.1