VOLUME 10 NUMBER 2 • JUNE 2013
49
SA JOURNAL OF DIABETES & VASCULAR DISEASE
REVIEW
Diabetic dyslipidaemia is characterized by elevated levels of
VLDL and IDL as well as a preponderance of small LDL particles
(see below). Many authors have thus argued that LDL-C alone
does not adequately reflect the atherogenic burden in diabetic
dyslipidaemia and have proposed alternative lipid markers.
The two best known markers are apoB and non-high density
lipoprotein cholesterol (non-HDL-C). ApoB levels reflect the total
number of atherogenic particles while non-HDL-C is a measure of
cholesterol found in all apoB-containing lipoproteins. Non-HDL-C
is calculated by subtracting HDL-C from total cholesterol (TC).
LDL-C currently remains the primary lipid target in most
guidelines; as most of the outcome evidence has been gathered
for LDL-C, it is well established and understood by physicians and
because apoB measurements are not yet universally available.
6
In
this article I will primarily focus on LDL-C target values but indicate
apoB and non-HDL-C target values where appropriate.
Extensive epidemiological evidence indicates that low HDL-C
is associated with an increased risk of ASCVD. However, HDL
has proven to be a difficult therapeutic target and a much more
complex lipoparticle than the apoB-containing lipoparticles.
Although raising HDL-C intuitively makes sense, none of the
trials examining HDL-C raising strategies has thus far been an
unequivocal success.
Dyslipidaemia in diabetes
Lipid phenotypes in patients with T2DM may vary considerably
and are subject to the same genetic and environmental influences
as in non-diabetic patients. However, T2DM is often accompanied
by a characteristic constellation of lipid abnormalities which is
known either as ‘diabetic dyslipidaemia’ or the ‘atherogenic lipid
phenotype (ALP)’.
7
The ALP includes mild tomoderate TG elevations
in the range of 2-5 mmol/l (reflecting increased amounts of VLDL
and the presence of large VLDL that are particularly TG rich +
elevated concentrations of remnant lipoproteins), low HDL-C
(usually in the range of 0.7-1.0 mmol/l) and LDL-C that is usually
within the population range but with a predominance of small
dense LDL particles.
8-10
On superficial inspection the lipogram
associated with the ALP may not appear to be particularly
abnormal, particularly if the focus is only on TC and LDL-C.
The latter two variables are usually not markedly abnormal and
atherosclerotic risk may be underestimated. The combination of
small dense LDL particles, low and often dysfunctional HDL-C,
and elevated levels of other apoB-containing lipoproteins (VLDL
and remnants) is highly atherogenic.
The pathophysiology of the ALP is complex but one of the
major lipoprotein mechanisms involves hepatic overproduction
of large, TG-rich VLDL particles as a result of the elevated levels
of free fatty acids that are characteristic of T2DM. Cholesterol
ester transfer protein (CETP) exchanges cholesterol esters from
HDL (and a lesser extent LDL) into VLDL in exchange for TGs.
The resultant TG-rich HDL and LDL undergoes further metabolic
processing with lipases removing much of the TG content and the
formation of small dense LDL and HDL particles. This process is
particularly active in the presence of TG-rich VLDL.
11
Diabetes is also the most common secondary cause of severe
hypertriglyceridaemia (TG > 10-15 mmol/l). Hypertriglyceridaemia
in T2DM is usually multifactorial in origin. Mechanistically
there is both overproduction of TG-rich lipoproteins together
with impaired clearance. Clinically high dietary TG intake, poor
glycaemic control and certain medications can aggravate the
hypertriglyceridaemia. Severe hypertriglyceridaemia only develops
in genetically and metabolically predisposed patients and the
majority of patients with poorly controlled T2DM do not develop
severe hypertriglyceridaemia.
Clinical Aspects
Dyslipidaemia is asymptomatic in most patients. Dyslipidaemia
management is thus not aimed at alleviating symptoms but risk
reduction is the focus of management. The two major risks that
need to be addressed are atherosclerosis and acute pancreatitis.
Because treating dyslipidaemia does not improve wellbeing in the
short term, and may be associated with side effects, long term
adherence to lipid-lowering therapy remains an ongoing problem
and challenge for the clinician and patient.
Atherosclerosis
As indicated above, patients with T2DM are at very high risk of
developing ASCVD. Lipid treatment guidelines designate T2DM
as ‘coronary risk equivalent’ indicating that the ten year risk of a
cardiovascular event in patients with T2DM is equivalent to that
of a recurrent event in non-diabetic patients who have already
survived an event. This short term risk equivalence probably does
not apply to all populations, especially in younger diabetics; but
as lifetime risk is markedly elevated with T2DM, assigning all type
2 diabetics ‘very high risk’ status remains clinically useful and
appropriate.
12,13
Because all patients with T2DM are considered as
‘very high risk’, Framingham risk scoring is neither appropriate nor
required. The diagnosis of T2DM is the indication for lipid lowering
therapy. Put differently, the question every doctor needs to consider
is not ‘Should this patient with T2DM be on a statin?’ but ‘Is there
any reason why I am not prescribing a statin for this patient?’
Lipid-lowering therapy is initiated based on the high cardiovascular
risk of patients with T2DM, even if their lipid values appear to be
‘normal’.
Atherosclerosis prevention
Lifestyle modification remains the cornerstone of managing T2DM.
Many of the interventions that improve glycaemic control such
as increased exercise, weight reduction and dietary modification
also reduce cardiovascular risk. Smoking cessation and blood
pressure control are especially important in patients with T2DM.
Good glycaemic control can reduce cardiovascular events in the
long term
14
, but attempting very tight glycaemic control in elderly
patients with established cardiovascular disease may increase
mortality.
15
In patients with relatively long standing diabetes, lipid
and blood pressure control are associated with larger reductions
in cardiovascular outcomes than glycaemic control and are also
usually easier.
16
This does not mean that glycaemic control should
be neglected; achieving tight control early after the diagnosis will
benefit patients many years down the line.
14
In addition to lifestyle modifications, patients with T2DM also
require lipid lowering therapy. There is extensive data to support
the safety and efficacy of statin therapy in patients with T2DM.
17,18
Statins are therefore the agents of first choice, except for patients
with severe hypertriglyceridaemia in whom a different treatment
approach is required (see below).
LDL-C is the best validated lipid target and LDL-C control
therefore remains the primary target of lipid lowering therapy,
