Histopathologic lesions closely resemble those characteristic of human patients with ARVC/D [30-32]. Most distinguishing is substantial replacement of RV cardiac myocytes by adipose or fibrous tissue which occur in two patterns: a fatty form in approximately two-thirds and a fibro-fatty form in approximately one-third of cases [15].

The fatty form is characterized by diffusely distributed, multifocal regions of adipose cell replacement within the RV wall and trabeculae, extending from epicardium toward endocardium, in association with tiny patchy fibrosis (Fig. 8.5). The fibro-fatty form consists of focal or diffuse regions of adipose cell replacement associated with extensive areas of replacement fibrosis (Fig. 8.6). Both

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Fig. 8.6 • Histopathology of RV wall from an 8-year-old male boxer dog with sustained ventricular tachycardia (180bpm), syncope,and sudden death. a,Low power photomicrograph. Diffuse myocardial loss associated with adipose tissue and replacement-type fibrosis representative of the fibro-fatty form of ARVC/D was present in the subepicardium and mid-mural wall,(trichrome Heidenhain stain,magnification x3) b, Surviving, atrophic myocytes surrounded by, and interspersed within, fat and patchy fibrous replacement tissue (trichrome Heidenhain stain, magnification x25); c, Myocarditis characterized by patchy, mononuclear cellular inflammatory infiltrate (CD45RO) (magnification x40). d, Brown-staining TUNEL-positive apoptotic myocyte nuclei (magnification x40). From [15] with permission the fatty and fibro-fatty forms are characterized by surviving myocytes embedded within regions of fibrous tissue. In ARVC/D dogs, mean percentage of area of RV fat did not differ significantly between anterolateral (46.7±19.7%) and infundibular (45.2±12.2%) sites, but was lower in the posterior wall (29.2±18.9%) (p<0.008). Replacement of RV myocardium by fat was diffuse (involving >2 regions) in 70% of affected dogs, and segmental in the remaining 30% [15].

LV lesions may be present in up to half of ARVC/D hearts and consisted largely of focal, fibrous tissue replacement with some mild fatty tissue replacement. In left or right atrial walls, approximately one-third of ARVC/D boxer dogs display myocyte loss with fatty or fibro-fatty replacement [15]. Myxomatous degeneration of the mitral valve leaflets can occur as a normal aging change in older dogs.

Myocarditis characterized by focal or multifocal T-lymphocytic infiltrates (CD45, CD45RO, and CD43 positive) and associated with myocyte death, has been identified in the RV of almost two-thirds of ARVC/D boxer dogs. Myocarditis is often present in the LV free wall and in the atria. Myocyte apop-tosis has been identified in 39% of ARVC/D hearts. In boxer dogs with ARVC/D that died suddenly there was a significantly greater percent of dogs with myocarditis (9/9; 100%) vs. those who did not die suddenly (7/14; 50%; p=0.04). The fibrofatty form was also more prominent in dogs with (6/9; 67%) than in those without sudden cardiac death (2/14; 14%) (p=0.02).

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