Overview

Inherited disorder resulting in ventricular arrhythmias in otherwise healthy young German shepherd dogs. The phenotypic spectrum is wide with some affected dogs having infrequent single premature ventricular complexes while other dogs have frequent and rapid ventricular tachycardia that is associated with sudden death. The pattern of inheritance is complex, depending heavily on background genetics. Siblings of German shepherd dogs that have died suddenly should be tested for this disorder.

Signalment

• Most dogs develop arrhythmias at approximately 12 weeks of age (identified as young as 6 weeks of age). The number and severity of the arrhythmias tend to peak between the ages of 5 and 9 months of age. By approximately 18–24 months of age most dogs have only a few arrhythmias.
• Male and female dogs are afflicted equally.

Signs

• Signs are very rare (e.g., in >500 dogs examined only 1 had syncope) because the dangerous ventricular tachycardia is non-sustained until in some dogs it degenerates into ventricular fibrillation resulting in death (usually between 5 and 9 months of age).
• Arrhythmias often detected during routine examination before neutering.
• Death is associated with sleep, rest after exercise, or excitement after sleep particularly in the early morning.

Causes & Risk Factors

• The genetic mutation(s) responsible for this disorder has not been identified.
• Multiple electrophysiologic abnormalities have been identified-early and delayed afterdepolarizations, heterogeneous and altered action potential duration, ion channel current density, calcium cycling, and sympathetic innervation have been documented.
• Ventricular tachycardia tends to be most frequent with slow heart rates (drug-induced [e.g., phenlyephrine, fentanyl] or during sleep).

Differential Diagnosis

Rule out myocarditis.

CBC/Biochemistry/Urinalysis

Results of routine laboratory tests are within normal ranges.

Other Laboratory Tests

Troponin concentration-to rule out myocarditis. Dogs with inherited arrhythmias have normal troponin levels.

Imaging

• Thoracic radiographs normal.
• Echocardiography-echocardiograms of individual dogs are usually normal.

Diagnostic Procedures

Pathologic Findings

• Routine gross and histopathologic examination is within normal limits.

Drug(s)

• The ventricular arrhythmias usually are easily suppressed with lidocaine at 2 mg/kg IV.
• Control of the arrhythmias with oral medication is more problematic.
• Sotalol alone can be proarrhythmic and should not be used alone.
• Sotalol at 2–3 mg/kg PO q12h combined with mexiletine at 4–8 mg/kg PO q8h suppresses the ventricular arrhythmias, but the response in individual dogs is highly variable.

Contraindications/Possible Interactions

• Avoid drugs that slow the heart rate until the dogs are older than 18–24 months of age.
• Drugs that slow or prolong the action potential duration such as sotalol, phenylephrine, or fentanyl are proarrhythmic.

Patient Monitoring

• Repeat Holter monitoring to assess drug efficacy is highly advised.
• After 18–24 months of age the Holter monitoring is again repeated. If the number of ventricular ectopic complexes is <2,000 singles with no ventricular tachycardia, the dog's risk of death is very low and the medications may be stopped.
• Although an occasional dog will have a dramatic drop in the arrhythmia count and severity during treatment, most do not. Therefore, the absence of arrhythmias on Holter recordings after 18–24 months of age indicates a change in the disorder rather than an antiarrhythmic effect.
• Lifelong treatment is not required.

Expected Course and Prognosis

• Approximately 50% of affected dogs with >10 runs of ventricular tachycardia/24 hours will die suddenly before 1 year of age. If a dog does not have ventricular tachycardia identified by 24-hour electrocardiographic monitoring, the probability of death is very low.
• If a young German shepherd dog with frequent ventricular tachycardia does not die, it will live. Although this statement is ironic at best, even severely affected dogs that have reached the age of 2 years with documented absence of arrhythmia have lived a normal lifespan >12 years.

Age-Related Factors

Because identification of affected dogs depends on the determination of arrhythmias before the age of 1 (ideally 4–9 months) to 2 years (at most), afflicted dogs can easily be missed because the only clinical sign is sudden death with no evidence of a cause found on routine post-mortem examination.