Key Points
Disease summary:
Metabolic syndrome (MetS) is a complex genetic disorder, involving the interaction of multiple genetic loci with environmental factors.
MetS consists of three of the five following criteria: increased waist circumference (Table 12-1); triglyceride (TG) level greater than or equal to 150 mg/dL (1.7 mmol/L) or drug treatment for elevated TG; high-density lipoprotein-cholesterol (HDL-C) less than 40 mg/dL (1 mmol/L) in males or less than 50 mg/dL (1.3 mmol/L) in females or drug treatment for reduced HDL-C level; systolic blood pressure (SBP) greater than or equal to 130 mm Hg and/or diastolic blood pressure (DBP) greater than or equal to 85 mm Hg or antihypertensive drug treatment in a patient with a history of elevated blood pressure (BP); fasting glucose greater than or equal to 100 mg/dL (5.6 mmol/L) or drug treatment for elevated glucose.
Individuals with MetS have a twofold increased risk for cardiovascular disease (CVD) and fivefold increased risk for type 2 diabetes (DM2) compared to individuals without MetS.
Differential diagnosis:
MetS is polygenic in nature and may be often evident in individuals with coexistent DM2, HIV lipodystrophy, or polycystic ovary syndrome. Rare monogenic forms of lipodystrophy such as familial partial lipodystrophy (FPLD), congenital generalized lipodystrophy (CGL), and acquired partial lipodystrophy (APL) are often considered extreme forms of MetS.
Monogenic forms:
Extreme forms of MetS may be evident in monogenic forms of lipodystrophy such as FPLD, CGL, and APL.
Family history:
There may be a family history of one or more of the components of MetS. In the genetic lipodystrophies with MetS phenotype, FPLD is inherited in an autosomal dominant fashion, while CGL is inherited as an autosomal recessive trait.
Environmental factors:
The components of MetS may be exacerbated by environmental factors. For example, hypertension can be aggravated by excessive salt. Dietary components such as intake of saturated fat or simple carbohydrates can influence the onset and severity of dysglycemia and dyslipidemia. Overall, obesity is a significant moderator of all components of MetS.
Genome-wide association studies:
Many associations have been reported for common MetS, especially its individual components. There is no evidence at present for any added diagnostic, prognostic, or therapeutic value of genetic evaluation for common single-nucleotide polymorphism (SNP) genotypes or risk allele scores for the component quantitative traits in common MetS presentation.
Pharmacogenomics:
There is no demonstration yet of a potential role for pharmacogenetic evaluation in common MetS. FPLD2 (due to mutant LMNA) appears to be more responsive to therapy with thiazolidinediones than is FPLD3 (due to mutant PPARG).
Diagnostic Criteria and Clinical Characteristics
Although several different definitions for MetS have been proposed, six different organizations have recently issued a joint statement in an attempt to harmonize the definition of MetS. According to their report, three of the following five criteria are required to make a diagnosis of MetS:
Elevated waist circumference based on population- and country-specific definitions. Although consensus regarding worldwide waist circumference criteria is still pending, the joint statement recommends using International Diabetes Federation (IDF) thresholds for non-Europeans and IDF or American Health Association/National Heart, Lung, and Blood Institute (AHA/NHLBI) thresholds for those of European origin (Table 12-1).
TG level greater than or equal to 150 mg/dL (1.7 mmol/L) or drug treatment for elevated TGs
HDL-C less than 40 mg/dL (1 mmol/L) in males or less than 50 mg/dL (1.3 mmol/L) in females or drug treatment for reduced HDL-C level
SBP greater than or equal to 130 mm Hg and/or DBP greater than or equal to 85 mm Hg or antihypertensive drug treatment in a patient with a history of elevated BP
Fasting glucose greater than or equal to 100 mg/dL (5.6 mmol/L) or drug treatment for elevated glucose
| Waist Circumference Cutoffs (cm) | |||
|---|---|---|---|
| Population | Population | Males | Females |
| Caucasian | ≥102 | ≥88 | |
| Asian | ≥90 | ≥80 | |
| Middle East, Mediterranean, Sub-Saharan Africa | ≥94 | ≥80 | |
| Ethnic Central and South American | ≥90 | ≥80 |
MetS represents a clustering of risk factors for the development of CVD and DM2.
These risk factors include central adiposity, hypertension, hypertriglyceridemia, hypoalphalipoproteinemia, and dysglycemia.
Insulin resistance is often present, and may be clinically manifest as acanthosis nigricans.
Atherogenic dyslipidemia characterized by elevated serum TG, apolipoprotein B (ApoB), small low-density lipoprotein (LDL) particles, and reduced HDL-C level is often demonstrated in these patients.
Individuals with MetS have a twofold increased risk for CVD and fivefold increased risk for DM2 compared to individuals without MetS.
Most individuals with MetS will also demonstrate increased hepatic and visceral adiposity.
Polycystic ovaries and hirsutism may be evident in women.
In most individuals, MetS is polygenic in nature and is often evident in individuals with coexistent DM2, HIV lipodystrophy, or polycystic ovary syndrome.
Less commonly, the cause of MetS can be monogenic in the form of certain lipodystrophies such as FPLD, CGL, and APL.
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