[1] However, the majority of those with LGL in whom electrophysiological studies have been performed do not have any evidence of an accessory pathway or structural abnormality.

Whilst in a minority of cases some form of intranodal or paranodal fibers that bypass all or part of the atrioventricular node can be found with subsequent conduction down the normal His-Purkinje system, in most cases the short PR interval is caused by accelerated conduction through the atrioventricular node.

[citation needed] LGL syndrome is diagnosed in a person who has experienced episodes of abnormal heart racing (arrhythmias) who has a PR interval less than or equal to 0.12 second (120 ms) with normal QRS complex configuration and duration on their resting ECG.

The QRS complex is often normal but can also be narrow in LGL syndrome, as opposed to WPW, because ventricular conduction is via the His-Purkinje system.

[citation needed] Individuals with LGL syndrome do not carry an increased risk of sudden death.