The disease process starts in the subepicardial region and works its way towards the endocardial surface, leading to transmural involvement (possibly accounting for the aneurysmal dilatation of the ventricles). Residual myocardium is confined to the subendocardial region and the trabeculae of the ventricles. These trabeculae may become hypertrophied. Aneurysmal dilatation is seen in 50% of cases at autopsy. It usually occurs in the diaphragmatic, apical, and infundibular regions (known as the triangle of dysplasia). The left ventricle is involved in 50–67% of individuals. If the left ventricle is involved, it is usually late in the course of disease, and confers a poor prognosis. There are two pathological patterns seen in ACM, Fatty infiltration and fibro-fatty infiltration. Fatty infiltration
The first, fatty infiltration, is confined to the right ventricle. This involves a partial or near-complete substitution of myocardium with fatty tissue without wall thinning. It involves predominantly the apical and infundibular regions of the RV. The left ventricle and ventricular septum are usually spared. No inflammatory infiltrates are seen in fatty infiltration. There is evidence of myocyte (myocardial cell) degeneration and death seen in 50% of cases of fatty infiltration. Fibro-fatty infiltration
The second, fibro-fatty infiltration, involves replacement of myocytes with fibrofatty tissue. A patchy myocarditis is involved in up to 2/3 of cases, with inflammatory infiltrates (mostly T cells) seen on microscopy. Myocardial atrophy is due to injury and apoptosis. This leads to thinning of the RV free wall (to < 3 mm thickness) Myocytes are replaced with fibrofatty tissue.
To make a diagnosis of ACM requires either 2 major criteria or 1 major and 2 minor criteria or 4 minor criteria.Major criteria

Right ventricular dysfunction
Severe dilatation and reduction of RV ejection fraction with little or no LV impairment
Localized RV aneurysms
Severe segmental dilatation of the RV
Tissue characterization
Fibrofatty replacement of myocardium on endomyocardial biopsy
Electrocardiographical abnormalities
Epsilon waves in V1 – V3
Localized prolongation (>110 ms) of QRS in V1 – V3
Inverted T waves in V1 -V3 in an individual over 12 years old, in the absence of a right bundle branch block (RBBB)
Ventricular tachycardia with a left bundle branch block (LBBB) morphology, with superior axis
Family history
Familial disease confirmed either clinically or on autopsy or surgeryMinor criteria

Right ventricular dysfunction
Mild global RV dilatation and/or reduced ejection fraction with normal LV. Mild segmental dilatation of the RV
Regional RV hypokinesis
Tissue characterization
Electrocardiographical abnormalities
Late potentials on signal averaged EKG. Ventricular tachycardia with a left bundle branch block (LBBB) morphology, with inferior or unknown axis
Frequent PVCs (> 500 PVCs / 24 hours)

Management
The goal of management of ACM is to decrease the incidence of sudden cardiac death. This raises a clinical dilemma: How to prophylactically treat the asymptomatic patient who was diagnosed during family screening. A certain subgroup of individuals with ACM are considered at high risk for sudden cardiac death. Associated characteristics include:

Young age
Competitive sports activity
Malignant familial history
Extensive RV disease with decreased right ventricular ejection fraction. Left ventricular involvement
Syncope
Episode of ventricular arrhythmiaManagement options include pharmacological, surgical, catheter ablation, and placement of an implantable cardioverter-defibrillator.