Introduction

Intermittent collapse is a common presenting sign in both first-opinion and cardiology referral practice and obtaining a diagnosis can be challenging due to the intermittent nature of clinical signs and also the large and varied differential diagnosis list. This lecture will take a practical approach highlighting features of the history and clinical examination which can help to differentiate seizures from syncope and thereby direct further investigation. Special attention will be given to the diagnosis of arrhythmias that may result in syncope.

Definitions

 Syncope is defined as a sudden yet transient loss of consciousness due to a deprivation of energy substrate (oxygen or glucose), that briefly impairs the cerebral metabolism.

 Seizures are the clinical manifestation of an excessive discharge of hyperexcitable cortical neurones. Usually associated with intense muscle activity but can occasionally be atonic.

History

A detailed history can help to distinguish seizures from syncope but generalisations are difficult as some arrhythmias can result in hypoxic seizures. Videos will be shown during the presentation highlighting some of the classical features of fitting and fainting but also showing the potential overlap particularly as, understandably, owners' recollection of the event may be unreliable due to the stress induced by the episode. Classical features of these three syndromes are shown in Figure 1 but cases sometimes have to be assigned on the basis of 'best fit'.

Figure 1. Classical features of syncope, seizure and falling over.

Differential Diagnosis of Intermittent Collapse

 Cardiovascular disease resulting in reduced cardiac output, tachyarrhythmia, bradyarrhythmia, peripheral or neurogenic dysfunction

 Neurological disease, for example epilepsy, space-occupying lesions

 Endocrine disease, for example hypoadrenocorticism resulting in hyperkalaemia

 Metabolic, for example hypoglycaemia, hypokalaemia, hypovolaemia

 Haematological, for example anaemia

 Musculoskeletal, for example myasthenia gravis

 Iatrogenic, for example over-dosage with alpha-2-agonists drugs or beta-blockers

Physical Examination

Thorough physical examination including auscultation can be useful to detect signs referable to cardiac disease such as a murmur, arrhythmia, pulse deficit or signs of reduced cardiac output such as pale mucous membranes. In view of the large differential diagnosis list it is also important to look for abnormalities associated with other conditions.

Investigation of Suspected Cardiovascular Disease

If there is evidence of cardiovascular disease then further investigation may include:

 An electrocardiogram (ECG) to assess resting heart rate and rhythm

 Echocardiography to assess cardiac dimensions and function

 Full haematology and serum biochemistry to assess whether there is evidence of reduced organ perfusion and/or other systemic disease

 Thoracic radiographs to assess whether there are signs of congestive heart failure

If these tests show that there is underlying structural disease, such as dilated cardiomyopathy, and it is plausible that the episodes of collapse are associated with reduced cardiac output associated with congestive heart failure, then initial therapy is generally aimed at minimising the haemodynamic consequences of this syndrome.

Ambulatory ECGS

If an arrhythmia is detected on the resting ECG and/or an arrhythmia is suspected as the cause of the episodes of collapse then an ambulatory ECG or 'Holter' monitor recording to document the dog's heart rate and rhythm over a longer period may be indicated. Holter monitors used to be large and bulky but modern digital units weight about 150 g and can be attached to a dog using a harness and three adhesive electrodes. These monitors are usually well tolerated and can be worn during normal activities to record heart rate and rhythm continuously for up to 7 days.

Alternative devices for recording heart rate and rhythm during an episode include an event monitor or a Reveal device. The disadvantage of both these approaches is that these monitors generally need to be manually activated by the owner during the episode of collapse and also the Reveal device requires general anaesthesia for implantation and removal.

Arrhythmogenic Collapse

Collapse may be associated with inappropriate tachyarrhythmias, bradyarrhythmias and also an inappropriate heart rate and blood pressure response to a stimulus: vasovagal syncope. Example traces from Holter recordings demonstrating these arrhythmias will be shown during the presentation.

Syncope in Breeds at Risk of Heart Disease Such as Boxers and Dobermanns

Boxers and Dobermanns are predisposed to episodes of collapse associated with ventricular arrhythmias which have the potential to be life threatening, therefore prompt investigation of collapse is strongly indicated. However Boxers are also predisposed to vasovagal syncope and differentiating this relatively benign cause of syncope from arrhythmogenic right ventricular cardiomyopathy (ARVC) can be challenging and is a strong indication for use of an ambulatory ECG.

In the US Holter monitoring is used as a screening test for Boxer cardiomyopathy and this may soon become more widely requested in the UK as the incidence of Boxer cardiomyopathy (which is also known as ARVC) appears to be increasing at present.

Summary

 Collapse is a common presenting sign

 Careful history and physical examination can direct further investigation

 Holter monitors are a non-invasive means of investigating intermittent arrhythmias

 Boxers are predisposed to (ARVC) and also vasovagal syncope