Left ventricular noncompaction (LVNC) is characterized by numerous prominent trabeculations and deep intertrabecular recesses in hypertrophied and hypokinetic segments of the left ventricle (Sasse-Klaassen et al., 2004). The mechanistic basis is thought to be an intrauterine arrest of myocardial development with lack of compaction of the loose myocardial meshwork. LVNC may occur in isolation or in association with congenital heart disease. Distinctive morphologic features can be recognized on 2-dimensional echocardiography (Kurosaki et al., 1999). Noncompaction of the ventricular myocardium is sometimes referred to as spongy myocardium. Stollberger et al. (2002) commented that the term 'isolated LVNC,' meaning LVNC without coexisting cardiac abnormalities, is misleading, because additional cardiac abnormalities are found in nearly all patients with LVNC. Genetic Heterogeneity of Left Ventricular Noncompaction A locus for autosomal dominant left ventricular noncompaction has been identified on chromosome 11p15 (LVNC2; 609470). LVNC3 (see 605906) is caused by mutation in the LDB3 gene (605906) on chromosome 10q23. LVNC4 (see 613424) is caused by mutation in the ACTC1 gene (102540) on chromosome 15q14. LVNC5 (see 613426) is caused by mutation in the MYH7 gene (160760) on chromosome 14q12. LVNC6 (see 601494) is caused by mutation in the TNNT2 gene (191045) on chromosome 1q32. LVNC7 (615092) is caused by mutation in the MIB1 gene (608677) on chromosome 18q11. LVNC8 (615373) is caused by mutation in the PRDM16 gene (605557) on chromosome 1p36. LVNC9 (see 611878) is caused by mutation in the TPM1 gene (191010) on chromosome 15q22. LVNC10 (615396) is caused by mutation in the MYBPC3 gene (600958) on chromosome 11p11. LVNC can also occur as part of an X-linked disorder, Barth syndrome (302060), caused by mutation in the TAZ gene (300394) on chromosome Xq28.

Hypokalemic periodic paralysis (hypoPP) is a condition in which affected individuals may experience paralytic episodes with concomitant hypokalemia (serum potassium <3.5 mmol/L). The paralytic attacks are characterized by decreased muscle tone (flaccidity) more marked proximally than distally with normal to decreased deep tendon reflexes. The episodes develop over minutes to hours and last several minutes to several days with spontaneous recovery. Some individuals have only one episode in a lifetime; more commonly, crises occur repeatedly: daily, weekly, monthly, or less often. The major triggering factors are cessation of effort following strenuous exercise and carbohydrate-rich evening meals. Additional triggers can include cold, stress/excitement/fear, salt intake, prolonged immobility, use of glucosteroids or alcohol, and anesthetic procedures. The age of onset of the first attack ranges from two to 30 years; the duration of paralytic episodes ranges from one to 72 hours with an average of nearly 24 hours. Long-lasting interictal muscle weakness may occur in some affected individuals and in some stages of the disease and in myopathic muscle changes. A myopathy may occur independent of paralytic symptoms and may be the sole manifestation of hypoPP.