From OMIM
Primary systemic carnitine deficiency is due to a defect in the high-affinity carnitine transporter expressed in muscle, heart, kidney, lymphoblasts, and fibroblasts. This results in impaired fatty acid oxidation in skeletal and heart muscle. In addition, renal wasting of carnitine results in low serum levels and diminished hepatic uptake of carnitine by passive diffusion, which impairs ketogenesis (Lamhonwah et al., 2002). If diagnosed early, all clinical manifestations of the disorder can be completely reversed by supplementation of carnitine. However, if left untreated, patients will develop lethal heart failure (summary by Shibbani et al., 2014). See also myopathic carnitine deficiency (212160), which is restricted to skeletal muscle.  http://www.omim.org/entry/212140

From MedlinePlus Genetics
Primary carnitine deficiency is a condition that prevents the body from using certain fats for energy, particularly during periods without food (fasting). Carnitine, a natural substance acquired mostly through food, is used by cells to process fats and produce energy.

Signs and symptoms of primary carnitine deficiency typically appear during infancy or early childhood and can include severe brain dysfunction (encephalopathy), a weakened and enlarged heart (cardiomyopathy), vomiting, muscle weakness, and low blood glucose (hypoglycemia). The severity of this condition varies among affected individuals. Some people with primary carnitine deficiency are asymptomatic, which means they do not have any signs or symptoms of the condition. All individuals with this disorder are at risk for sudden death.

Problems related to primary carnitine deficiency can be triggered by periods of fasting or by illnesses such as viral infections. This disorder is sometimes mistaken for Reye syndrome, a severe disorder that may develop in children who appear to be recovering from viral infections such as chickenpox or flu.   https://medlineplus.gov/genetics/condition/primary-carnitine-deficiency