THIS IS THE FIRST OF TWO articles reviewing the identification and management of feline heart disease in first opinion practice. Rather than a comprehensive review, the aim is to discuss the challenges that such disease presents for first opinion practitioners and hopefully provide some useful guidelines.
This article discusses some of the issues and difficulties surrounding identification and diagnosis of feline heart disease in practice – probably posing more questions than answers, which reflects our current understanding.
The second article will aim to provide practical guidelines for first opinion practitioners when presented with feline heart disease, discussing diagnostic approach and management of cats that are asymptomatic, and cats that have developed clinically evident heart disease.
Heart disease in cats
Feline cardiomyopathies are the most common cause of heart disease in domestic cats. Of the primary cardiomyopathies, hypertrophic cardiomyopathy (HCM) is the most common type; hypertrophic obstructive cardiomyopathy (HOCM) is a sub- classification of HCM characterised by partial obstruction of the left ventricular out flow tract.
Although less common, other types of primary cardiomyopathy are also seen, including restrictive cardiomyopathy (RCM), dilated cardiomyopathy (DCM), unclassified cardiomyopathy (UCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC).
The phenotypic expression of cardiomyopathy is highly variable, which can make the precise classification of feline cardiomyopathy problematic. This is perhaps reflected in the literature, where there has been variable reported prevalence of the different forms of cardiomyopathy (Riesen et al, 2007; Payne et al, 2015; Ferasin et al, 2003).
Secondary cardiomyopathies also occur, most commonly with thyrotoxicosis and systemic hypertension. A detailed discussion of disease classification is beyond the scope of this article but is well reviewed elsewhere. In first opinion clinical practice, exact classification of cardiomyopathy for an individual cat is not always helpful; rather diagnostic investigation is usually focused on determining optimal management options based on that individual cat’s disease phenotype and its likely pathological consequences.
Prevalence of HCM/HOCM
Heart disease is surprisingly common in cats, affecting approximately 15% of the population. Three recent studies of healthy cats where cardiomyopathy was identified by echocardiography reported remarkably consistent prevalence of 14.7% in 780 shelter cats (Payne et al, 2015); 15.6% in 199 healthy cats (Wagner et al, 2010) and 15.5% in 103 healthy cats (Paige et al, 2009).
Natural history
Fortunately, death and morbidity as a consequence of cardiomyopathy appear to be much less common than the disease prevalence might suggest. In fact, most cats with HCM never exhibit clinical signs in their lifetime. However, HCM is an extremely heterogeneous disease, both in terms of presentation and outcome.
Whereas the majority of cats may remain asymptomatic, sadly other cats will progress to develop congestive heart failure, thromboembolic complications, collapse signs or die suddenly; so cardiomyopathy cannot be considered a benign disease.
To put the disease in perspective, however, a study which looked at insurance data reported overall cardiac mortality of 30 deaths per 10,000 cat years (Egenvall et al, 2009). Another retrospective study of cats diagnosed with HCM (Payne et al, 2013) reported median survival time of 5.9 years for cardiac mortality. This same study reported 56/107 cats died (or were euthanased) as a result of congestive heart failure and 34 died (or were euthanased) with thromboembolic complications and 17/107 cats died suddenly.
Presentation
The majority of cats with cardiomyopathy are asymptomatic and it is abnormal findings on physical examination, such as heart murmur, that prompt further investigation.
Heart murmurs, gallop sounds and arrhythmias Heart murmurs are very common in cats. Reported prevalence of heart murmurs in apparently healthy cats varies, but is consistently high. A recent study (Payne et al, 2013) reported a prevalence of 40.8% in 780 healthy cats; previous studies have reported prevalence of 21-44% (Wagner et al, 2010; Paige et al, 2009; Cote et al, 2004).
It is important to realise that the presence of a heart murmur does not necessarily mean heart disease is present, as “innocent” murmurs are common (Payne et al, 2015, reported 70.4% of murmurs were functional). Unlike the situation in dogs, where cardiac auscultation is a sensitive test for the detection of the most common heart disease (myxomatous mitral valve disease), cardiac auscultation is not a sensitive tool for the detection of heart disease in cats.
The detection of other auscultatory abnormalities such as gallop sounds, arrhythmia and sometimes tachycardia are more important findings, and clinically significant heart disease is more consistently detected in these circumstances.
Overt clinical signs
First opinion practitioners will be all too familiar with cats presenting with consequences of cardiomyopathy. Overt heart disease will be addressed in more detail in the second article; but to summarise, the most common scenario is dyspnoea secondary to congestive heart failure, which can manifest as pulmonary oedema, or pleural effusion.
A significant number of cats will develop thromboembolic complications of cardiomyopathy, which present typically with hindlimb paralysis but occasionally other arteries can be affected (brachial, cerebral, renal, etc.).
Cats are not small dogs! Cats, unlike dogs, almost never cough with heart disease. Coughing cats typically have airway disease. Ascites is a rare sign of congestive heart failure in cats. Generally, ascitic cats are much more likely to have abdominal disease.
Diagnostic investigation
Echocardiography is unquestionably the most useful tool for the diagnosis of heart disease in cats; however, the facilities to perform cardiac ultrasound are not always available in general practice. Furthermore, echocardiographic examination of a cat is not a simple or easily performed test, full of pitfalls for the inexperienced sonographer.
In fact, the heterogeneous phenotypic expression of feline cardiomyopathies sometimes provides diagnostics challenges for even the most experienced veterinary cardiologists. That said, the use of thoracic ultrasound where it is available in first opinion practice is to be encouraged, as it is an extremely useful and relatively safe tool for the evaluation of acutely dyspnoeic cats. With minimal training, it is possible to reliably identify pleural effusion and answer fundamental questions, such as, “Does this cat have an enlarged left atrium?”
Thoracic radiography
Thoracic radiography is one of the most commonly performed diagnostic tests for clinical evaluation of cats with suspected heart disease in general practice. Although thoracic radiography is readily accessible, it provides limited information about the type of heart disease present.
Thoracic radiography is indicated for dyspnoeic cats; however, thoracic ultrasound is often a safer method for determining if pleural effusion is present. The main advantage for thoracic radiography over other modalities is in the identification of pulmonary venous congestion and pulmonary oedema; however, feline cardiogenic pulmonary oedema has variable radiographic presentation and pulmonary venous congestion is sometimes not present or recognisable (Guglielmini et al, 2015).
Whereas left atrial enlargement can be reliably identified in canine patients, thoracic radiography is not a reliable tool for identifying left atrial enlargement in the cat.
Blood tests
Cardiac biomarkers, such as N-terminal pro-B type natriuretic peptide (NTproBNP) and cardiac troponin-I (cTnI), are useful for the identification of heart disease in cats. NT-proBNP is a marker of abnormal wall strain, cTn-I is a marker of myocardial damage, so they provide different types on information.
Their major advantage is accessibility, particularly now with the advent of point-of-care tests (Borgeat et al, 2015). A significant disadvantage is that it is sometimes not possible to safely obtain a blood sample from an unstable patient.
NT-proBNP reliably discriminates between normal cats and cats with heart disease, also between cats with mild heart disease and those with moderate to severe disease. NTproBNP can also be measured in pleural fluid, obviating the need to obtain blood samples in cats presenting with pleural effusion.
Unfortunately, cardiac biomarkers do not help the clinician determine what form of heart disease a cat may have, and therefore cannot provide reliable guidance to optimal therapy for an individual animal; however, they are extremely useful markers for the presence of disease.
In practice, cardiac biomarkers are of particular value in the following settings:
- discriminating dyspnoea caused by congestive heart failure from other non-cardiac causes of dyspnoea such as asthma
- determining if coincidental findings such as murmur or arrhythmia warrant further investigation
Not to be confused with cardiac biomarkers, genetic testing is available commercially for single point mutations (A31P and R280W) of the MYBPC3 gene; however, the value of genetic testing remains controversial and is probably not helpful for HCM screening in a first opinion practice setting (Haggstrom et al, 2015).
Electrocardiography (ECG)
ECG is essential for the characterisation of arrhythmias, but has limited value in cats with normal heart rhythm. Some cats with HCM will exhibit anterior fascicular block pattern, which although associated with HCM, does not help determine optimal clinical management.
Blood pressure measurement
It is important to measure blood pressure in older cats, or cats with renal and/or thyroid disease, where systemic hypertension may be present. In primary cardiomyopathies, clinically significant heart disease is more commonly associated with low blood pressure.
Blood pressure measurement is strongly recommended in the assessment of cats with heart disease.
Summary
Managing feline heart disease is challenging. Although there has been more research interest in feline heart disease recently, we still have a lot to learn, particularly about treatment.
Our first challenge in practice is identifying disease in apparently healthy cats. Murmurs are common, most are functional and not all cats with cardiomyopathy have heart murmurs, so we do not have a simple method for disease screening.
When presented with an asymptomatic cat, the clinician should consider risk factors associated with a diagnosis of HCM which are: male sex, increasing age, increasing body condition and heart murmur (particularly grade III/VI or louder).
Echocardiography is the gold standard diagnostic test, but for a variety of reasons including availability, skills required and cost, it is not a practical screening tool in first opinion practice. Thoracic radiography, ECG and blood pressure measurement are generally available in practice and all provide useful information when investigating overt heart disease, but have limited use when screening apparently healthy cats.
The use of cardiac biomarkers is encouraged, particularly NT-proBNP, as they provide an accessible, reliable and relatively inexpensive method to identify heart disease. Cardiac biomarkers also can discriminate normal/mild disease from moderate/severe disease, helping to determine which cats may require the services of a cardiologist.
- The full list of references is available on request from editor@veterinary-practice.com. Part 2 will be in the October issue.
