Possible sinoatrial node dysfunction in a 6-month-old domestic shorthair cat

Case summary: A 6-month-old male entire domestic shorthair cat was presented to the ophthalmology department for nasolacrimal duct cannulation and flushing, and castration under general anaesthesia. On pre-anaesthetic assessment, the cat had a heart rate of 90 beats/min (bpm). Clinical examination was unremarkable, although the cat appeared stressed. The echocardiogram was within normal limits. An ambulatory electrocardiogram (ECG) monitor was fitted overnight, and analysis of the ECG revealed a sinus rhythm with a lower than normal heart rate. The mean 1 min rate was 98 bpm. There was a slower than normal sinus rhythm and frequent ventricular escape beats. Differential diagnoses included increased vagal tone and sinoatrial node dysfunction (SND). The latter was suspected as the cat demonstrated signs of stress although an atropine response test was not performed. Relevant and novel information: SND is relatively common in dogs but extremely rare in cats. To the authors' knowledge, there is only one affected cat mentioned in the literature. In both dogs and humans, most cases described are in middle-aged and elderly patients. Although the condition is recognised in human infants and fetuses, it has not been reported in dogs under 2 years of age. This case is unusual because SND was suspected in an immature cat. On analysis of the ECG, the heart rate was considerably lower than those previously reported in hospitalised cats; however, findings on physical examination were subtle, highlighting the importance of pre-anaesthetic examination in identifying unexpected abnormalities.