Familial cardiomyopathy in Norwegian Forest cats

Abstract

Norwegian Forest cats (NFCs) are often listed as a breed predisposed to cardiomyopathy, but the characteristics of cardiomyopathy in this breed have not been described. The aim of this preliminary study was to report the features of NFC cardiomyopathy based on prospective echocardiographic screening of affected family groups; necropsy findings; and open-source breed screening databases. Prospective examination of 53 NFCs revealed no murmur or left ventricular (LV) outflow tract obstruction in any screened cat, though mild LV hypertrophy (defined as diastolic LV wall thickness ≥5.5mm) was present in 13/53 cats (25%). Gross pathology results and histopathological sections were analysed in eight NFCs, six of which had died of a cardiac cause. Myocyte hypertrophy, myofibre disarray and interstitial fibrosis typical of hypertrophic cardiomyopathy were present in 7/8 cats, but endomyocardial fibrosis suggestive of restrictive cardiomyopathy was also present in the same cats. Pedigree data analysis from 871 NFCs was supportive of a familial cardiomyopathy in this breed.

Figure 1

Two-dimensional echocardiographic measurements of end-diastolic left ventricular wall thickness in an unaffected cat. (a) Right parasternal long axis outflow (five chamber) view; (b) right parasternal long axis inflow (four chamber) view; (c) short axis view at the papillary muscle level. IVS = interventricular septum; LA = left atrium; Ao = aorta; LV = left ventricle; LVFW = left ventricular free wall

Figure 2

Two-dimensional echocardiographic measurements of left atrial diameter in an unaffected cat. (a) Right parasternal short axis view at the level of the aortic valve. The left atrial to aortic ratio was measured in the first frame after aortic ejection, where the aortic leaflets are visible using an inner-edge to inner-edge method. (b) Right parasternal long axis inflow view. The maximal left atrial diameter was measured at end-systole. LA = left atrium; Ao = aorta; LV = left ventricle; PV = pulmonary vein; RA = right atrium

Figure 3

False tendons (arrows) were described as absent or present, based on right parasternal short and long axis views. Most connected the free wall with the septum or the papillary muscles with the septum

Figure 4

Pedigree tree of the cats that underwent necropsy. Squares denote male cats; circles denote female cats. Solid black symbols denote affected cats; solid grey symbols cats with equivocal results and black open symbols unaffected cats. Grey open symbols indicate the cats for which cardiac health status was unknown. Cat D died suddenly and was diagnosed with myocarditis on histopathology. The index case (A, arrow) has an equivocal sire. She was mated to an unaffected sire, producing affected kittens. An equivocal female was mated to two different males with unknown health status; both litters included affected offspring (B and C). The sibling of this dam was diagnosed with hypertrophic cardiomyopathy in the prospective study

Figure 5

Histological section from the interventricular septum of cat F. Myofibre disarray is present. There is distorted arrangement of the myocytes with adjacent cells arranged at perpendicular and oblique angles. Some myocytes show signs of hypertrophy with nuclear enlargement (haematoxylin and eosin stain)

Figure 6

Histological section from the interventricular septum of cat F, showing diffuse interstitial fibrosis and myofibre disarray. Masson trichrome stain demonstrates bands of fibrous tissue (stained blue) between myocytes