The diagnosis of WPW is generally straightforward from the baseline ECG. Occasionally fusion between sinus and ventricular beats falsely gives the appearance of preexcitation (for example, late-coupled ventricular bigeminy can look like preexcitation alternans). Preexcitation can sometime be subtle (see Figs. 4.1a and 4.2), due to variable degrees of fusion between antegrade nodal and antegrade pathway conduction. Subtle preexcitation is more likely when the atrial depolarization reaches the pathway late (i.e. left-sided pathways), when nodal conduction is relatively rapid (i.e. young patient, elevated sympathetic tone), or when pathway conduction velocity is slow (particularly certain types of atypical pathways, discussed elsewhere). Antegrade pathway conduction can be absent or intermittent above certain atrial rates, depending on the effective refractory period of the pathway. Antegrade pathway conduction can also be absent for reasons unrelated to pathway refractoriness (so-called “latent” pathways), including the phenomenon of concealed penetration of the His-Purkinje wavefront retrograde into the pathway (see Fig. 4.3a).

Additional rhythm recording (Holter, event recorder) may be indicated depending on symptoms. Echocardiography is indicated at baseline (to evaluate for comorbid structural heart disease, including Ebstein anomaly of the tricuspid valve, other forms of congenital heart disease, and hypertrophic cardiomyopathy) and perhaps periodically thereafter (to evaluate for ventricular dysfunction as mentioned above). Family screening is not generally recommended, as the recurrence risk in relatives is only modestly elevated (although certainly families with multiple affected members do occur).

Finally, the possibility of risk for sudden death must be considered. The mechanism of sudden death is believed to be that of rapidly conducted atrial fibrillation resulting in rapid irregular ventricular stimulation and eventually ventricular fibrillation. Patients with WPW are at increased risk for atrial fibrillation, even in the absence of structural heart disease. Therefore the risk of sudden death depends primarily on the antegrade conduction characteristics of the pathway. Patients with syncope, documented preexcited atrial fibrillation, multiple preexcited morphologies (since multiple pathways may alternate to allow rapid ventricular stimulation), and SVT (especially antidromic SVT) are at increased risk. Generally, older patients are believed to be at decreased risk, presumably because of survivor bias (i.e. those with rapid antegrade conduction would have presented with syncope or sudden death already).

A consensus statement from PACES (the Pediatric and Congenital EP Society) and HRS (the Heart Rhythm Society) provides guidance for the management of asymptomatic WPW in children and young adults [1]. Although studies differ about the exact magnitude of the risk for sudden death (e.g. between 0.1 and 5 per 1,000 patient-years), the guideline authors provide class IIa recommendations for risk stratification (acknowledging that no patient is at zero risk). The first step is non-invasive risk stratification, including ECG and if necessary exercise stress test, with the goal of identifying “abrupt and complete” loss of preexcitation at faster heart rates (felt to predict long accessory pathway refractory period). It is important to include only abrupt loss (i.e., in a single beat, see Figs. 4.3b and 4.4), and not be misled by gradual fusion or day-to-day variation due to concealed retrograde penetration (see Figs. 4.2 and 4.3a). Only 10–20 % of patients will have convincingly reassuring non-invasive evaluations; invasive testing is recommended for the remainder. Invasive testing consists of diagnostic EP pacing study via either trans-esophageal or trans-venous route, with the goal being to induce sustained atrial fibrillation and measure the SPERRI (Shortest Pre-Excited R-R Interval, see Fig. 4.5). Assessment of SPERRI during atrial fibrillation is a stronger predictor than other pathway characteristics (such as SPERRI during rapid atrial pacing, or accessory pathway ERP, see Fig. 4.6) and should always be sought. Atrial fibrillation can be induced, with concerted effort, in well over 90 % of patients with WPW; prolonged rapid pacing just above atrial refractoriness usually suffices, but ramp pacing protocols and administration of adenosine can be helpful in difficult cases. SPERRI < 250 ms, and especially <220 ms, is a predictor of higher risk for sudden death, and therefore an indication for prophylactic ablation (class IIa). The full algorithm (see Fig. 4.7) and accompanying text [1] capture additional nuance about balancing risks and benefits of ablation vs conservative management in various clinical situations.