Really fascinating info on carnitine here: http://www.fodsupport.org/carnitine_therapy.htm

Much more there, but here's an excerpt:
Carnitine deficiency is associated with many symptoms. Since the deficiency is nearly always secondary to another disease process, the symptoms are often those of the primary disease plus additional problems that can be reversed with carnitine replacement therapy. Deficiency of carnitine results in decreased energy available to muscle and muscle weakness and low muscle tone. Growth of muscle, and thus weight gain, also requires energy and the child with carnitine deficiency usually has failure to thrive. Carnitine deficiency can affect the cardiac muscle and result in poor cardiac contractions (cardiomyopathy). Energy is important to brain function and abnormalities of brain function can be seen including convulsions, lethargy, irritability, and even coma. These children are very susceptible to infections and with the frequent infections they often show signs of deterioration of mental and physical status. Liver function may worsen and liver failure may occur. In children with genetic metabolic diseases, carnitine deficiency can be life threatening due to the inability to excrete the unburnt "ashes" left over from incomplete fuel burning. These accumulating ashes are toxic and poison the individual. Without carnitine to take these toxins out, the individual may die or suffer irreversible damage.