Cholesterol
Dyslipidaemia
Classification:
The first classification of dyslipidaemias was the Fredrickson (WHO) classification, which is based on plasma analysis for various lipoprotein fractions, but takes no account of the underlying aetiology of any of the dyslipidaemias.1 Today it is more common to identify dyslipidaemia by the particular lipoprotein or apolipoprotein that is abnormal. Once a dyslipidaemia has been identified it is important to determine the cause where possible. Dyslipidaemia can be either a primary abnormality or secondary to other disorders.
Primary causes:
The most severe forms of primary dyslipidaemia are caused by genetic disorders of lipoprotein metabolism. In clinical practice, the most easily identifiable of these are familial hypercholesterolaemia (FH), polygenic hypercholesterolaemia and familial combined hypercholesterolaemia. These forms of dyslipidaemia are usually characterised by elevated levels of LDL-C, which increase the risk of developing atherosclerosis and cardiovascular disease. Treating such disorders involves intensive reduction of LDL-C, depending on the overall cardiovascular risk of an individual patient.2 Patients presenting with severe forms of hypercholesterolaemia should also undergo family screening to detect other family members for therapy.2
Associated disorders:
Common causes of secondary dyslipidaemia include diabetes mellitus, nephrotic syndrome, chronic renal failure, hepatobiliary disease (generally of the obstructive variety) and hypothyroidism.2 These conditions can cause some but not all types of dyslipidaemia. For example, diabetes can lead to elevation of triglyceride-rich lipoproteins and reduction of HDL-C, but does not usually increase the level of LDL-C. On the other hand, hepatobiliary disease is associated with an increased level of LDL-C. Some of these associated disorders are also independent cardiovascular risk factors.
References:
1. Yeshurun D, Gotto AM. Southern Med J 1995; 88(4): 379-391.
2. Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. JAMA 2001: 285; 2486-2497.