Page 179 - Small Animal Internal Medicine, 6th Edition

CHAPTER 7 Myocardial Diseases of the Dog 151

and biologic toxins (wasp or scorpion stings, snake venom, trial of supplemental taurine for 3 to 4 months is recom-
spider bites). mended (see p. 73 for supplementation guidelines). However,
VetBooks.ir METABOLIC AND NUTRITIONAL the effect of taurine supplementation on disease progression
is unclear. Taurine-deficient dogs may show echocardio-
DEFICIENCY
L-carnitine graphic improvement after supplementation; however, some
degree of cardiac abnormality often persists, and there is
L-carnitine is an essential component of the mitochon- questionable effect on survival time.
drial membrane transport system for fatty acids, which are Other factors
the heart’s most important energy source. It also transports Myocardial injury induced by free radicals may play a role
potentially toxic metabolites out of the mitochondria in the in a number of diseases. Evidence for increased oxidative
form of carnitine esters. L-carnitine-linked defects in myo- stress has been found in dogs with CHF and myocardial
cardial metabolism have been found in some dogs with failure, but the clinical ramifications of this are unclear. Dis-
DCM. Rather than simple L-carnitine deficiency, one or eases such as hypothyroidism, pheochromocytoma, and dia-
more underlying genetic or acquired metabolic defects are betes mellitus have been associated with reduced myocardial
suspected. There may be an association between DCM and function, but clinical CHF is unusual in dogs secondary to
carnitine deficiency in some families of Boxers, Doberman these conditions alone. Excessive sympathetic stimulation
Pinschers, Great Danes, Irish Wolfhounds, Newfoundlands, stemming from brain or spinal cord injury results in myo-
and Cocker Spaniels. L-carnitine is mainly present in foods cardial hemorrhage, necrosis, and arrhythmias (brain-heart
of animal origin. DCM has developed in some dogs fed strict syndrome). Muscular dystrophy of the fasciohumoral type
vegetarian diets. (reported in English Springer Spaniels) can result in atrial
Plasma carnitine concentration is not a sensitive indicator standstill and heart failure. Canine X-linked (Duchenne)
of myocardial carnitine deficiency. Most dogs with myocar- muscular dystrophy in Golden Retrievers and other breeds
dial carnitine deficiency, diagnosed via endomyocardial also has been associated with myocardial fibrosis and min-
biopsy, have had normal or high plasma carnitine concentra- eralization, with subsequent LV systolic dysfunction and
tions. Furthermore, the response to oral carnitine supple- CHF. Rarely, nonneoplastic (e.g., glycogen storage disease)
mentation is inconsistent. Subjective improvement may and neoplastic (metastatic and primary) infiltrates interfere
occur, but few dogs have echocardiographic evidence of with normal myocardial function. Immunologic mecha-
improved function. Dogs that do respond show clinical nisms also could play an important role in the pathogenesis
improvement within the first month of supplementation; of myocardial dysfunction in some dogs with myocarditis.
there may be some degree of improvement in echo param-
eters after 2 to 3 months. L-carnitine supplementation does ISCHEMIC MYOCARDIAL DISEASE
not suppress preexisting arrhythmias or prevent sudden Acute myocardial infarction resulting from coronary embo-
death. See p. 73 for supplementation guidelines. lization is uncommon. An underlying disease associated
Taurine with increased risk for thromboembolism, such as bacterial
Although most dogs with DCM are not taurine deficient, endocarditis, neoplasia, protein-losing nephropathy or
low plasma taurine concentration has been documented in enteropathy, immune-mediated hemolytic anemia, acute
some and appears particularly common in certain breeds. pancreatitis, disseminated intravascular coagulopathy, and/
Low taurine, and sometimes carnitine, concentrations occur or corticosteroid use, underlies most cases. Sporadic reports
in Cocker Spaniels with DCM. Oral supplementation of of myocardial infarction have been associated with con-
these amino acids can improve LV size and function, as well genital ventricular outflow obstruction, patent ductus arte-
as reduce the need for heart failure medications in this breed. riosus, hypertrophic cardiomyopathy, and mitral insufficiency.
Low taurine concentrations have also been found in some Atherosclerosis of the major coronary arteries, which can
Newfoundlands, Golden Retrievers, Labrador Retrievers, accompany severe hypothyroidism in dogs, rarely leads to
Saint Bernards, and other dogs with DCM. Some such dogs acute myocardial infarction. Clinical signs of acute major
have been fed low-protein lamb meal and rice, or vegetarian coronary artery obstruction are likely to include arrhyth-
diets; others have been consuming diets with adequate mias, pulmonary edema, marked ST segment change on
taurine content. In particular, DCM has been identified in ECG, and evidence of regional or global myocardial contrac-
Dalmatians fed protein-restricted diets for prevention of tile dysfunction on echocardiogram. High circulating cardiac
breed-related urate urolithiasis. troponin concentrations and possibly creatine kinase activity
Assessment of plasma or whole blood taurine concentra- occur after myocardial injury and necrosis.
tion should be considered in “atypical” breeds affected with Disease of small coronary vessels is recognized as well.
DCM or any dog with concern for nutritional deficiency Nonatherosclerotic narrowing of small coronary arteries
based on diet history. Plasma taurine concentrations less could be more clinically important than previously assumed.
than 40 nmol/mL and blood taurine concentrations less than Hyalinization of small coronary vessels and intramural myo-
150 nmol/mL are generally considered deficient. Specific col- cardial infarctions have been described in dogs with chronic
lection and submission guidelines should be obtained from degenerative mitral valve disease; however, they can occur
the laboratory used. If taurine deficiency is documented, a in older dogs without valve disease as well. Fibromuscular