Hyperlipidemia
Hyperlipidemia
Overview:
Hyperlipidemias are characterized by high levels of lipids in the blood (hyper = elevated, lipid, emia = blood).
Hyperlipidemia is often asymptomatic, but it significantly increases one's risk for cardiovascular diseases, especially atherosclerosis.
Two commonly used systems of classification:
– The older scheme, which focuses on inherited lipidemias, is called the Fredrickson Classification system.
– The newer system divides lipidemias according to primary (aka, genetic) or secondary (aka, acquired) causes.
– Primary lipidemias can be exacerbated by secondary causes.
Comprise proteins and phospholipids that transport cholesterol and triglycerides in the body.
Outer surface of a lipoprotein:
– Apolipoprotein = protein that binds lipids.
There are several types of apolipoproteins, some of which are implicated in hyperlididemias, as we'll soon see.
– Phospholipids and free cholesterol.
Core:
– Triglycerides (aka, triacylglycerol)
– Cholesterol esters
Chylomicrons deliver dietary triglycerides and cholesterol to the liver and peripheral tissues.
Chylomicron remnants are produced when triglycerides are removed from chylomicrons; thus, they are rich in cholesterol esters.
Very Low Density Lipoproteins (VLDL) are made in the liver, and are rich in triglycerides.
Intermediate Density Lipoproteins (IDL) are produced when triglycerides are removed from VLDL; thus, like the chylomicron remnants, they are rich in cholesterol.
Because they are formed from VLDL, Intermediate Density Lipoproteins are sometimes referred to as VLDL remnants.
Low Density Lipoproteins are produced after even more triglycerides are removed from the Very Low Density Lipoproteins and their remnants; thus, LDL is very rich in cholesterol, which it carries to the peripheral tissues.
LDL is sometimes referred to as "bad" cholesterol, because it distributes cholesterol throughout the body and vessels. In the vessels, the deposited cholesterol contributes to obstructive plaque formation and atherosclerosis.
High Density Lipoproteins (HDL), which are part of the reverse cholesterol transport pathway, carry cholesterol from the peripheral tissues to the liver.
In addition to removing excess cholesterol, High Density Lipoproteins have various other anti-atherogenic properties, so it they are often referred to as "Good" cholesterol.
Hypercholesterolemia is often defined as:
- Total cholesterol > 200 mg/dL
- Low-Density Lipoproteins > 130 mg/dL
- High-Density Lipoproteins < 40 mg/dL
Xanthomas
These are created by lipid deposits in the skin associated with foam cells (macrophages that have ingested lipids).
Tuberous xanthomas form small to large bulges in the skin over the joints, particularly the elbows and knees.
Eruptive xanthomas are erythematous bumps that tend to appear on the buttocks, shoulders, and extensor surfaces.
Plane xanthomas are thin yellow plaques. Xanthelasma is characterized by plaques around the eyelids.
Palmar xanthomas are characterized by yellow plaques that form along the creases of the palm of the hands.
Tendinous xanthomas are bumps that form over the tendons or ligaments
The Achilles tendon at the posterior ankle is a common site for these xanthomas.
Primary hyperlipidemias and their associated Fredrickson Phenotypes
Secondary hyperlipidemia