Hyperlipoproteinemia: Description, Causes and Risk Factors:
Hyperlipoproteinemia is a metabolic disorder characterized by abnormally elevated concentrations of specific lipoprotein particles in the plasma.
Lipoprotein: Any complex or compound containing both lipid and protein. Lipoproteins are important constituents of biological membranes and of myelin (A white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers) . Conjugation with protein facilitates transport of lipids, which are hydrophobic, in the aqueous medium of the plasma. Plasma lipoproteins can be separated by ultracentrifugation, electrophoresis, or immunoelectrophoresis; they migrate electrophoretically with alpha- and beta- globulins, but are usually classified according to their densities (flotation constants). The principal classes by density are chylomicrons, which transport dietary cholesterol and triglycerides from the intestine to the liver and other tissues; very low density lipoproteins (VLDL), which transport triglycerides from intestine and liver to muscle and adipose tissue; low density lipoproteins (LDL), which transport cholesterol to tissues other than the liver; and high density lipoproteins (HDL), which transport cholesterol to the liver for excretion in bile. The properties of these and other plasma lipoproteins are set forth in the accompanying table. The protein moiety of a lipoprotein is called an apolipoprotein (or apoprotein). Besides rendering lipids soluble, some apolipoproteins perform biochemical functions such as enzyme activation. The apolipoproteins of plasma lipoproteins are synthesized by the liver and intestinal mucosal cells and vary in molecular weight from 7000 to 500,000. Protein makes up more than 50% of some HDLs but only 1% of chylomicrons. As the proportion of lipid in a lipoprotein increases, its density decreases. A plasma lipoprotein particle is typically spherical, with a hydrophobic core of triacylglycerol, cholesteryl esters, and apolar amino acid residues surrounded by hydrophilic protein structures and phospholipids.
The concentrations of certain serum lipoproteins correlate closely with the risk of atherosclerosis. An HDL cholesterol level below 35 mg/dL (0.90 mmol/L), an LDL cholesterol level above 160 mg/dL (4.15 mmol/L), and a fasting triglyceride level above 250 mg/dL are all independent risk factors for coronary artery disease (CAD). Although dietary factors are important in some persons, basal levels of lipoprotein, cholesterol, and triglycerides depend chiefly on heredity. Several phenotypes of familial hyperlipoproteinemia associated with risk of premature cardiovascular disease and death have been identified. SEE hyperlipoproteinemia. Medical management of patients with coronary artery disease (myocardial infarction, angina pectoris, history of coronary artery bypass graft or coronary angioplasty) and other atherosclerotic disorders (peripheral arterial disease, abdominal aortic aneurysm, carotid artery disease) includes detection and correction of hypercholesterolemia and hyperlipoproteinemia. Reducing elevated LDL cholesterol diminishes the risk of coronary artery disease; besides halting the progression of atherosclerosis, it may even shrink established atherosclerotic lesions. Of persons with elevated LDL cholesterol, 75% can achieve normal levels with diet, weight reduction, and exercise; the remainder need drug treatment. Factors besides familial hyperlipoproteinemias that can elevate LDL cholesterol include diabetes mellitus, hypothyroidism, nephrotic syndrome, obstructive liver disease, and drugs (progestogens, anabolic steroids, corticosteroids, thiazide diuretics). Dietary saturated fat raises LDL cholesterol more than any other dietary component, cholesterol itself not excepted.
Primary hyperlipoproteinemias that are due to genetically determined defects in lipid or lipoprotein metabolism or are caused by some environmental factors through an unknown mechanism.
All five major types of hyperlipoproteinemia may be familial and probably represent many different mutations. Environmental factors that may cause primary hyperlipoproteinemia include: (1) diet, including alcohol intake; and (2) drugs. Many drugs cause hyperlipoproteinemia, particularly the estrogens as contained in contraceptive medications, and steroid hormones.
Common diseases that are often associated with hyperlipoproteinemia may include hypothyroidism, diabetes, nephrotic syndrome, biliary obstruction, pancreatitis, and dysglobulinemia (including autoimmune hyperlipoproteinemia).
hyperlipoproteinemia very seldom occurs without hyperlipidemia and, consequently, hyperllpidemia may be used to detect hyperlipoproteinemia; a classification based on lipoproteins offers more information than one based on lipids alone; a classification should distinguish between disorders in the metabolism of lipoproteins as well as lipids.
If uncontrolled, higher mortality occurs from cardiovascular and cerebrovascular diseases. Abnormalities of the vascular system can develop, including ischemic cardiac disease. Obtaining appropriate medical evaluation is important, especially in patients younger than 40 years who have a prominent corneal arcus, xanthelasma, or lipemia retinalis.
A high level of lipoproteinin the blood does not have obvious symptoms but can be a risk factorfor other conditions that do have recognizable symptoms, including angina
,heart disease, high blood pressure, stroke and other circulatory ailments. Soft, yellowish skingrowths or lesions called xanthomas may indicate a genetic predisposition to the body's inabilityto process cholesterol and triglycerides normally. Obesity and diabetes may be associated withhigh cholesterol levels. In men, impotence may be due to arteries affected by excessive bloodcholesterol.
A useful clinical appraisal of lipids can usually be made by determining plasma TC, HDL-cholesterol, and triglyceride levels after the patient has fasted for >= 12 h. The specimenshould also be observed for a milky chylomicron layer after it stands overnight in a refrigeratorat 4° C (39.2° F). Colorimetric, gas-liquid chromatographic, enzymatic or otherautomated “direct” methods may determine plasma TC. Enzymatic methods are usually mostaccurate and are standard in virtually all clinical laboratories. Plasma triglyceride is usuallymeasured as glycerol by either colorimetric, enzymatic, or fluorometric methods after alkalineor enzymatic hydrolysis to glycerol and formaldehyde. HDL levels are measured enzymaticallyafter precipitation of VLDL, IDL, and LDL from plasma.
In some cases, performing electrophoresis and ultracentrifugation of whole plasma specimens may be necessary to help establish a diagnosis.
- Drugs are used to lower cholesterol and triglyceride levels.
- Because of the possibility of adverse effects and the question of whether the triglyceride level is an independent risk factor for atherosclerosis, many physicians use drugs to reduce the triglyceride level only when the level exceeds 500 mg/100 mL. Examples of these drugs include lovastatin, pravastatin, and simvastatin.
- Researchers found that the impact on lipoprotein subclass profiles varies between pitavastatin and atorvastatin. Determining the lipoprotein subclass profile and selecting the appropriate statin in patients with diabetes and an additional cardiovascular risk, such as low HDL cholesterol or hypertriglyceridemia may be beneficial.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
- Weight reduction and a diet low in saturated fat and cholesterol are advocated.
- Patients should avoid alcohol and estrogen in certain types of hyperlipoproteinemia.