diagnosis and mechanism of Types I and V hyperlipidaemia

Last reviewed 01/2018

• serum triglyceride > 10 mmol/litre (fasting/non-fasting) with hyperchylomicronaemia
• chylomicrons and VLDL are cleared from the circulation via the same mechanism (lipoprotein lipase) and thus, in general, the lipoprotein phenotype is a type V hyperlipidaemia (increased chylomicrons and increased VLDL)
• severe hypertriglyceridaemia occurs when there is an increased VLDL production is associated with a decreased triglyceride clearance:
  • increased hepatic VLDL production - genetic or secondary (e.g. diabetes, alcohol abuse, obesity)
  • decreased triglyceride clearance - genetic or secondary (e.g. diabetes, hypothyroidism, beta-blockers)
    • with the clearance mechanism overloaded with increased levels of VLDL the chylomicrons entering the circulation following a fatty meal may be present in the circulation for days rather than hours (triglyceride levels may be greater than 100 mmol/L)
    • a patient with a normal fasting triglyceride level of say 5 mmol/L may develop a severe hypertriglyceridaemia if there is a secondary concomitant factor such as diabetes or alcohol abuse
• familial lipoprotein lipase deficiency is generally due to mutation of the lipoprotein lipase gene (although may occasionally be due to a deficiency in apo CII - the activator for lipoprotein lipase)
  • in childhood this condition generally produces a type I hyperlipidaemia where only chylomicrons are raised. It is probable that hepatic lipase is able to catabolize the levels of VLDL produced in childhood (although being not able to catabolize the increased levels of chylomicrons sufficiently to prevent a hyperchylomicronaemia). As the child progresses to adulthood however, patients with this condition develop a type V hyperlipidaemia as VLDL production increases beyond the capacity of hepatic lipase