J. Alberto Montoya, DVM, PhD
Internal Medicine, Faculty of Veterinary Medicine, Las Palmas de Gran Canaria University
Las Palmas, Spain
Dilated cardiomyopathy (DCM) is a myocardial disease characterised by a progressive dilatation of one or both ventricles with severe impairment of systolic function in the absence of other cardiovascular disorders.DCM is one of the most common acquired heart diseases in the dog. The dogs diagnosed of DCM suffer from congestive heart failure (CHF) and sudden death is common. The aetiology of the myocardial hypokinesis is seldom known in the individual case of DCM, although several theories concerning genetic, nutritional, metabolic, inflammatory, infectious, or drug- or toxin-induced myocardial disease have been discussed. In dogs, familial DCM has been shown in a number of breeds and in most of these, an autosomal dominant mode of inheritance is suspected. Histopathologic findings in canine DCM reveal two histological different forms of DCM. The classification of canine DCM according to histological findings seems to be superior to classification suggesting breed-specific syndromes. A variety of clinical signs, arrhythmias, abnormalities of cardiac conduction and other echocardiographic alterations are commons findings in dogs with DCM, and they vary in different breeds of dogs. Different clinical forms of canine DCM have been described in different breed dogs. Subclinical or asymptomatic cardiomyopathy (occult DCM) is commonly defined as the condition in which there is echocardiographic evidence of left ventricular dilatation, hypokinesis, and electrocardiographic alterations in the absence of clinical and radiographic evidence of CHF.
DCM carries a poor prognosis in dogs, and a few prognostic indicators have been identified. The electrocardiographic abnormalities can have a very important prognostic value in dogs with DCM.
Canine DCM has been recognised in several medium size and giant-breed dogs. DCM usually affects pure-breed dogs although mixed-breed dogs can be affected. The prevalence of the disease varies among different breeds and even within breeds in different geographic regions. DCM has been reported only rarely in dogs weighing less than 12 kg.
Sex also appears to influence the risk of developing CHF by DCM in some dog's breeds. Male predominance of DCM is observed but not in all affected breeds.
DCM occurs in dogs of all ages, but the risk increases substantially with advancing age. The majority of dogs with DCM are between 4 and 10 years old. However a new form of DCM has recently been described affecting young Portuguese water dogs (< 32-weeks-old). In most breeds, males may be affected at an earlier age, with a faster progressive course.
The disease is usually not difficult to confirm once the patient is symptomatic. Diagnosis is much more problematic in dogs in the preclinical, occult stage of the disease. Occult DCM has been described as a specific clinical entity in the Doberman.
Clinical findings in dogs with DCM may include cough, depression, dyspnoea, weight loss, and syncope. Moreover clinical presentation of DCM commonly includes signs of CHF, pulmonary oedema, pleural effusion and ascites.
Clinical signs exhibited by dogs with DCM are similar in all breeds, but the observed frequency of these signs is significantly different in many of the commonly affected breeds. In some breed dogs with DCM, sexual differences concerning clinical characteristics have been observed.
Dobermans and Boxers generally show signs of acute left-sided heart failure or life-threatening arrhythmia. Sudden death may be the first observed sign in these dogs with DCM. Other large breed dogs have lower possibility to syncope or to die suddenly. Ascites and/or pleural effusion are even more prevalent in other giant breed dogs than Doberman or Boxers.
Weight loss and muscle wasting are common in some dogs with DCM. These findings are more severe in dogs that have been symptomatic for several months.
A regurgitant systolic murmur with a low-pitched protodiastolic gallop sound are frequent and important clinical findings. A gallop sound is evidence of severe ventricular impairment.
Pulmonary oedema is the most common finding on thoracic radiographs of dogs with symptomatic DCM, although signs of right-sided heart failure are reported to be common in giant breeds with DCM. Radiographic changes affecting the cardiac silhouette include left atrial enlargement alone, left atrial and left ventricular enlargement, right-sided enlargement or generalised cardiomegaly.
Electrocardiographic abnormalities are common in dogs with DCM, but the nature and prevalence of different arrhythmias varies in different breeds. Atrial fibrillation is the most commonly diagnosed electrocardiographic abnormality in Irish wolfhound, Dalmatian and Presa canario dogs with DCM, although ventricular premature depolarisations and ventricular tachycardia are reported in a majority of Dobermans. These ventricular arrhythmias may be present up to nine months prior to development of echocardiographic evidence of the disease in this breed. In Dobermans, atrial fibrillation is less common and it is a poor prognosis indicator. In Weimaraners, ventricular arrhythmias are common and may result in syncope or sudden death prior to the development of other clinical signs. Ventricular arrhythmias have also a high prevalent in Boxer, Airedale terrier and Newfoundland dogs with DCM. In Boxers sudden death is less common than in Dobermans. Ventricular premature depolarisations in Boxers have a characteristic appearance; the configuration of the complexes is almost consistent with a right ventricular origin. In Cocker spaniels, ECG often shows tall R waves and sometimes a left bundle branch block. In Presa canario dogs with DCM, abnormalities of cardiac conduction have been diagnosed in 34 % of dogs and changes in wave morphology have been recorded in 62 % of patients. Ventricular arrhythmias are uncommon and they have a very important prognostic value in Presa canario dogs with DCM.
Holter monitoring provides a more representative assessment of the genuine incidence of arrhythmias in patients with DCM. Holter recordings are particularly useful in Doberman and Boxer dogs. That breed tends to manifest frequently occult DCM.
Echocardiographic evaluation of left ventricular systolic performance reveals increased end-systolic and end-diastolic dimensions, dilatation of the left atrium, and decreased fractional shortening, as well as changes in systolic time intervals, in dogs with DCM. Diastolic dysfunction, as evidenced by Doppler examinations derived prolonged relaxation time and left ventricular inflow pattern of increased early diastolic to atrial wave ratio has been correlated to clinical deterioration in dogs with DCM. Echocardiography has been used to screen for DCM. However, currently it is difficult to differentiate between normal and abnormal cases and to assess echocardiographic abnormalities in the absence of the classical findings of DCM, because echocardiographic parameters vary with the breed of the dog as well as with the weight of the animal.
EFFECT OF MEDICAL THERAPY ON SURVIVAL
Medical therapy for DCM is generally unsatisfactory once overt CHF develops.
Neither diuretics nor nitrates have been shown to exert a favourable influence on survival in terms of altering disease progression in dogs with DCM.
Digoxin exerts neither a favourable nor an unfavourable influence on survival in dogs with DCM.
Increased survival times were detected in a group of Dobermans with DCM treated with conventional therapy and pimobendan. However, the same protocol failed to show any effect on survival in a group of Cocker Spaniel dogs with DCM.
ACE inhibitors have been shown to exert a favourable influence on disease progression.
Beta-blocking drugs is known to influence survival in dogs and humans with CHF, but it has been only evaluated in few of studies.
Antiarrhythmic therapy can prolong survival in dogs with DCM. In that short-term (from 2 to 7 months) efficacy is generally good.
Prediction of survival times and identification of factors influencing mortality are of interest for canine patients.
Predictive factors of survival rate include clinical class of CHF, presence of ECG changes, increased left ventricular filling pressure, increased systemic vascular resistance, increased pulmonary capillary wedge pressure, left intraventricular conduction delay, increased hypertrophy-dilatation index, Doppler indices, and Doppler-derived assessment of pulmonary hypertension.
In dogs wit occult DCM, survival times depends, in part, on the degree of myocardial failure. Sudden death is common in Boxer and Doberman dogs and can occur even with normal heart size and contractility. Sudden death is less common in other breeds. In general, dogs with a younger age of onset of echocardiographic abnormalities or clinical evidence of DCM have a more rapidly progressive course.
Survival times are generally short after the onset of overt CHF. Many dogs die without ever being stabilized from their first episode of failure. Dobermans have the worst prognosis.
After the advent of clinical signs of CHF, poor prognostic indicators include the presence of pleural effusion, pulmonary oedema and tachydysrhythmias.
In dogs with overt CHF echocardiographic data and/or radiographic heart size does not predict relative survival time. In Dobermans with overt CHF, both atrial fibrillation and bilateral heart failure exert a negative influence on survival.