Maerz, Wilkie L, et al. Familial cardiomyopathy in Norwegian Forest Cats. J Feline Med Surg. 2015 Aug; 17 (8):681-691. (Winn-funded study)
Norwegian Forest Cats (NFCs) are one breed of cats considered predisposed to cardiomyopathy. What is known about the characteristics of cardiomyopathy in NFCs has not been well defined. Hypertrophic cardiomyopathy (HCM) is the most common type of cardiomyopathy though there are other forms described: restrictive cardiomyopathy (RCM), dilated cardiomyopathy (DCM) and arrhythmogenic right ventricular cardiomyopathy (ARVC).
The definition of HCM is abnormal hypertrophy of the non-dilated left ventricle in the absence of another understandable cause. What is noted on histopathology with HCM is a myofibre disarray where the groups of cardiac muscle cells are aligned obliquely or perpendicular to each other. With RCM, there is an increase in cardiac mass along with enlargement of both atria. It is suggested that HCM and RCM are two different phenotypic expressions of the same disease.
Breeders of NFCs in North America and Europe have been concerned about the potential familial nature of the problem in the breed. HCM is an inherited cardiac disorder in humans and mutations affecting myosin binding protein C gene causing HCM in Maine Coons and Ragdoll cats have been reported. HCM is inherited as an autosomal dominant trait in Maine Coons and noted as a progressive disease usually not found in the first few years of life. In Ragdolls, the mode of inheritance is not yet determined though homozygous carriers have a reduced lifespan.
Because the phenotypic characteristics and heritability of cardiomyopathy is not reported in NFCs, the authors did a prospective screening of 53 Norwegian Forest Cats (70% female, 30% male). The aims of the study were 1) to screen NFCs in the UK prospectively for cardiomyopathy using auscultation and echocardiography, 2) to obtain pathological and histopathological data from cases where a NFC was suspected of being affected with cardiomyopathy; 3) to analyze NFC pedigrees using a combination of screening data and necropsy data to assess the inheritance pattern in this breed.
Some of the primary results noted were that murmurs and dynamic left ventricular (LV) outflow tract obstruction were uncommon. The low prevalence of murmurs on auscultation is of important concern since the finding of a murmur could lead to a diagnosis before other clinical signs develop.
A second finding was left ventricular (LV) hypertrophy was modest. The investigators in this study used a low cut-off value (5.5 mm) for maximum LV wall thickness as the criterion for LV hypertrophy. LV hypertrophy was noted as relatively mild in cats examined at necropsy. This included 5/6 cats dying of their heart disease. The authors concluded a lower cut-off than 6.0 mm (maximum) may be justified when screening cases or at least in NFCs and Maine Coons. In 7/8 cats examined by gross pathology and histopathology, myocyte hypertrophy, myofibre disarray and interstitial fibrosis typical of hypertrophic cardiomyopathy (HCM) was present but endomyocardial fibrosis more suggestive of RCM was also present in these same cats.
Additionally, the pedigree analysis was suggestive of an autosomal dominant inheritance pattern with incomplete penetrance in this breed. Other modes of inheritance could not be excluded. The results suggest cardiomyopathy in NFCs has characteristics of HCM and RCM. (VT)
Payne JR, Borgeat K, Connolly DJ, et al. Prognostic indicators in cats with hypertrophic cardiomyopathy. J Vet Intern Med. 2013 Nov-Dec:27(6):1427-36.