The preexcitation syndrome is a congenital disease caused by the existence of an accessory conduction pathway between the atria and the ventricles. This abnormality causes the disappearance of the normal delay of conduction in the atrioventricular node because this structure is bypassed as the impulse is conducted through the high speed abnormal cells. Patients are prone to developing ventricular arrhythmia. Several types of pathway with similar electrocardiographic expression leading to a short PR interval with or without a delta wave have been described.
Preexcitation syndrome patients have complaints of varying severity, depending on the underlying arrhythmia. Furthermore, signs and symptoms vary with the age of the affected individual. Newborns and toddlers may exhibit irritability, modified behavior, and indications of heart failure. Feeding is difficult for them, which may lead to failure to thrive if episodes are frequent and prolonged. Symptoms most often occur during a febrile illness.
An older individual will describe rapid palpitations, possibly accompanied by breathing difficulties and chest pain, even if coronary atherosclerosis is not clinically relevant at rest. Palpitations are usually regular (when they are due to a supraventricular orthodromic tachyarrhythmia conducted 1:1 to the ventricles via the accessory pathway), but may also be irregular (when the substrate is atrial fibrillation) and cause diminished physical effort tolerance. Atrial fibrillation is potentially life-threatening in these patients, leading to ventricular fibrillation . Antidromic tachycardia is another possibility that clinically manifests as dizziness or syncope in the context of rapid, regular palpitations. Paroxysmal episodes can be followed by polyuria.
During the arrhythmia, the patient may be pale, exhibit diaphoresis, develop cool extremities, and display symptoms of low blood pressure. The venous pressure may be increased, leading to jugular turgescence. Hepatomegaly caused by stasis and pulmonary congestion manifesting as tachypnea are sometimes observed. The heart rate varies between 200 and 250 beats/minute. The person is hemodynamically stable unless associated abnormalities, such as Ebstein anomaly or hypertrophic cardiomyopathy exist . The first condition is usually accompanied by cyanosis during the arrhythmic event.
The clinical evaluation may describe muscle weakness, suggesting a glycogen-storage illness, mental retardation in Danon disease, or macroglossia and hepatomegaly, indicators of Pompe disease. The blood workup should include the evaluation of electrolytes, such as potassium, calcium and magnesium. Screening for drugs, especially digoxin, is also important in cases where clinical judgement dictates, as they may act as precipitants for the arrhythmia. A thyroid panel can diagnose hyperthyroidism, another tachycardia trigger.
The main tool used to establish the preexcitation syndrome diagnosis is the 12-lead electrocardiogram. The QRS complex may be narrow, wide, or can change morphology in cases where there are multiple accessory pathways. Its aspect can also change if intermittent preexcitation exists. Preexcitation alternans is characterized by the alternation of a delta wave QRS complex with a normal one, whereas in concertina preexcitation the degree of preexcitation varies from one complex to the other. Typically, preexcitation syndromes exhibit a diminished PR interval (below 0.12 sec) and the presence of a delta wave (broad upstroke of the R wave). The QRS duration is increased above 0.12 sec and repolarization abnormalities are present. In Lown-Ganong-Levine syndrome, the QRS complex has no abnormalities, but the PR interval is short. More detailed information is offered by a 252-lead electrocardiogram combined with computed tomography imaging   or electrophysiological studies . Atrial fibrillation in preexcitation syndrome patients may cause hemodynamic instability  and should be suspected if the electrocardiogram shows a rapid succession of bizarre, shape changing complexes in a young individual . The location of the accessory pathway is suggested by the lead that contains the maximally preexcited complexes .
An echocardiogram is needed in order to evaluate the presence of an Ebstein abnormality, hypertrophic cardiomyopathy, coronary sinus diverticula, L-transposition of the great vessels or atrial and ventricular septal defects. Parietal motion may be depressed during the acute arrhythmic episode, but should normalize when the rhythm returns to normal.
In cases where the arrhythmia has a short duration, a variety of recording devices, such as Holter monitors or implantable loop recorders can be used in order to diagnose it.