Presumed Neurally-Mediated Bradycardia and Syncope in Boxers with Normal Echocardiograms and Absence of Ventricular Tachycardia

Episodic weakness and syncope are common in Boxers. Reported causes include ventricular tachycardia (VT) and neurally-mediated bradycardia (NMB). The purpose of this study was to describe the features of presumed NMB in Boxers. A total of 27 Boxer dogs were identified. To be included in the study, each dog must have been overtly healthy with a history of syncope or presyncope; had a normal echocardiogram; had absence of ventricular tachycardia (VT) and fewer than 500 ventricular premature complexes on 24 hour Holter recordings; and been alive and clearly healthy for at least six months following the initial evaluation. Heart rhythm were documented at the time of an episode in only eight dogs (30%) and only once on the first Holter recording. Bradycardia was subsequently recorded in five times during 120 hour Holter recordings and during the 5th day of an event recording. Bradycardia was documented during auscultation in two additional dogs. The heart rate was not documented in 19 (70%) dogs in this study. Documentation of the heart rhythm during episodes of collapse was difficult and was carried out in only 30% of dogs and was unlikely to occur during the first Holter recording. In Boxers with suspected NMB, extended ambulatory monitoring, including implantable loop recorders, may be required for heart rate documentation.