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Laboratory Evaluation of Hyperlipidemia (Hyperlipoproteinemia)
Laboratory Evaluation of Hyperlipidell1ia (HyperlipoproteineDlia) KENNETH G. BERGE, M.D. RALPH D. ELLEFSON, Ph.D.
Hyperlipidemia-high plasma concentrations of cholesterol, triglycerides, or the related lipoproteins - has emerged clearly as a major factor associated with increased risk of atherosclerotic disease. Alone or in conjunction with other ~ajor risk factors such as hypertension and cigarette smoking, hyperlipidemia clearly is associated with a substantial rise in the likelihood of initial development of coronary heart disease 8 • 11 and probably also in the mortality following myocardial infarction. 3 Data from prospective epidemiologic studies indicate that this significance of hyperlipidemia has an inverse relation with age (the younger the person with hyperlipidemia, the greater the risk of vascular disease) and a direct, stepwise relation with the level of lipid. 11 Because of this latter observation, which supports the findings in population studies comparing coronary heart disease and mean lipid levels in various ethnic or geographic groups, the traditional concepts of normal levels are not applicable in assessing risk factors such as levels of blood fats and of blood pressure. The optimal values may be much lower than the upper range of normal, which is derived arbitrarily from values found among general population groups such as North Americans. And indeed, the risk of coronary heart disease is higher among persons whose blood pressure or lipid concentrations are within the upper part of the defined normal range than among those whose levels are lower in that range.
NEED FOR DETERMINATION OF LIPID CONCENTRATIONS Although the data relating to the effect of correction or amelioration of risk factors 12 are less conclusive, available evidence certainly supports a reasonable clinical effort to reduce elevated levels of plasma lipids, especially in younger adults. For this reason, detection of hyperlipidemia Medical Clinics of North America- Vol. 54, No. 4, July, 1970
1029
1030
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
should not be deferred until the onset of premature, symptomatic ischemic heart disease has called attention to its possibility, any more than the detection of hypertension should await the occurrence of heart failure or stroke. As hyperlipidemia is rarely symptomatic except in fairly extreme forms that produce xanthomas in skin or tendons, it is clear that early detection can be accomplished only by liberal application of screening techniques, either generally or in persons with family history or other evidence suggesting elevated concentration of blood fats. The clinicianalthough his concerns usually are different in perspective and scope from those of the epidemiologist-must adopt some of the screening techniques of the latter if his efforts against cardiovascular disease are to be early, effective, and-one hopes-even preventive. The prevalence and seriousness of cardiovascular disease certainly warrant a major effort in prevention. In view of the early recognition of cholesterol in the atherosclerotic plaque and the early use of cholesterol-feeding to induce experimental atherosclerosis, it is not surprising that hypercholesterolemia has the longest history as a recognized lipid risk factor. No significant data have emerged that would disprove its importance. The significance of elevated plasma glycerides (mainly as triglycerides) in vascular risk has been appreciated more recently2 and confirmed by prospective study.8,1l Currently available evidence indicates that hypertriglyceridemia and hypercholesterolemia are important alone and perhaps even more so when coexisting. 8 Although marked triglyceridemia is frequently associated with turbidity of plasma or serum, the absence of turbidity does not exclude significant triglyceridemia. For these reasons, direct determinations of both cholesterol and triglycerides are needed for proper assessment of lipid factors in vascular risk. Secondary hyperlipidemia, such as that associated with hypothyroidism, liver disease, nephrosis, dysproteinemia, and pregnancy or use of progestational drugs, must be considered and excluded before one estimates cardiovascular risk or- more importantly - plans therapy.
ANALYSIS OF LIPOPROTEINS Chemical or physicochemical combination of lipid molecules (such as cholesterol, glycerides, and fatty acids) with proteins makes possible the transport of fat in the aqueous media of the body. These lipid-protein combinations include relatively small complexes (free fatty acid and albumin); larger complexes (cholesterol, other lipids, and certain globulins); and still larger, triglyceride-rich aggregates that cause turbidity of the plasma (these including so-called endogenous particles and chylomicrons, the latter of which are formed from recently digested fat). The different natures of the lipid and protein components of the various lipoproteins cause them to have different densities, and it is possible to separate the lipoproteins by several means into arbitrary classes on the basis of density: high density (HD), of >1.063 gm/ml;
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1031
low density (LD), of SG 1.006-1.063; and very low density (VLD), of <1.006. Separation by analytical ultracentrifugation has led to classification of the lipoproteins according to flotation rate, expressed in Svedberg flotation units (Sf) and grouped for simplification as Sf 0-20 (HD and LD), Sf 20-400 (VLD) and Sf > 400 (particles). The sophisticated apparatus and techniques required for such analyses have limited their application in clinical medicine, however; and indeed the necessity of such detailed information in clinical practice has never been clearly established. An index of atherogenicity based on lipoprotein fractionation by the analytical ultracentrifuge has not been accepted widely as superior to knowledge of lipid levels based on less complicated and expensive analyses. In 1963, Lees and Hatch described a technique of paper electrophoresis that would separate chylomicrons (origin), endogenously synthesized glycerides (pre-heta band), high-density lipoproteins (HDL) rich in phospholipid (alpha hand), and low-density lipoproteins (LDL) rich in cholesterol (beta band). The addition of preparatory ultracentrifugation to this technique allowed further definition of the bands into those carrying HDL and LDL (~0.006 gm/ml) and those carrying only very low density lipoproteins (VLDL). The system for phenotyping familial hyperlipoproteinemia with a numerical classification reported by Fredrickson and Lees in 1965 and elaborated upon in 1967 (Fig. 1)7 enhanced interest in the classification of clinical hyperlipidemia (Table 1). Previous systems of classification included schemes based only upon plasma concentrations of cholesterol or triglycerides, or on dermatologic manifestations of the basic metabolic disorder, or on separation into familial, primary, and secondary types. The resulting confusion in terminology and lack of ready comparability between series of cases made welcome a new system based on lipoprotein patterns. The numerical system of naming is simpler than the attempts to encompass all of the possible variations of a pattern with.adjectival additions. Although this classification of lipoproteins was based on a study of Table 1.
Plasma Findings in Types of Hyperlipoproteinemia as Classified by Fredrickson and Lees HYPERLIPOPROTEINEMIA, TYPE
FINDINGS IN PLASMA
II
Appearance Cholesterol, excess':' Triglyceride, excess':' Electrophoretic pattern
Cloudy 0-2 1-4 Chylomicrons
°
Clear 1-4 0-1 1-4 excess';' (3-lipoprotein
III
IV
V
Cloudy 1-3 1-3 Broad (3 band
Cloudy 1-3 1-4 Pre-(3 lipoprotein
Cloudy 1-3 1-4 Chylomicrons, pre-(3 lipoprotein
':'Graded on basis of (normal) to 4. The grading in this table reflects experience at the Mayo Clinic, which includes analyses of more than 20,000 specimens from more than 8,000 s.ubjects.
1032
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
PAPER ELECTROPHORESIS CHYlOMICRONS
PRE-/3
Figure 1. Schematic representation of the r.najor portions of the lipoprotein spectrum as defined by paper electrophoresis and by ultracentrifugation. (From Fredrickson, D. S., Levy, R. I., and Lees, R. S.: Fat transport in lipoproteins-an integrated approach to mechanisms and disorders. New Eng J Med 276:34-44 [Jan. 5] 1967. By permission of the Massachusetts Medical Society.)
kindreds with hyperlipidemia, it since has been applied more widely in clinical cases without detailed data on significant numbers of family members as well as in cases of secondary hyperlipidemia. Most of these latter instances are not familial. Furthermore, not all cases of hyperlipidemia secondary to a single cause (such as myxedema) fit into a single lipoprotein type. Such variations as these must be appreciated in order to avoid oversimplification of this complex group of disorders-a hazard inherent not in the classification but in its superficial interpretation or application. Another problem in the interpretation of lipoprotein patterns has been the occasional finding of combinations that do not fit within the five types as defined. These patterns may result from inadequate dietary preparation of the patient, existence of unclassified varieties of the primary or secondary types, or improvement in the techniques of preparation and electrophoresis that allow a more refined separation of lipoproteins. 4 The assessment of any patient's usual lipid status can be done properly only when he has been following his general dietary pattern for several weeks and has fasted adequately (preferably 14 hours or more) prior to the tests. Although blood drawn after meals may provide a reasonable reflection of the fasting cholesterol level, the phase of alimentary lipemia includes a marked elevation of triglycerides due to chylomicronemia. The recent consumption of alcohol in amounts unusual for a given patient also may cause a spurious rise in lipids - especially triglycerides.! If the use of alcohol is long-standing and unchanged, however, chronic hyperlipidemia may be due to this factor alone.
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1033
CLINICAL USEFULNESS OF ELECTROPHORESIS Lipoprotein patterns should be correlated with concentrations of plasma cholesterol and triglycerides. Determination of both cholesterol and triglyceride concentrations provides evidence of hyperlipidemia in more than 95% of cases. 6 If those concentrations are not elevated, determination of lipoproteins by electrophoresis is rarely necessary or helpful. The exceptions would include the rare cases of an absence or deficiency of certain lipoprotein fractions in such disorders as Tangier disease and a-beta-lipoproteinemia. The data from the prospective study in Framingham indicated that risk of coronary heart disease was proportional to the level of the lipids, irrespective of the associated lipoprotein patterns, so the lipoprotein pattern was not of major importance in assessing risk. 8 An exception might be the rare cases of Fredrickson's type I hyperlipoproteinemia that exhibit marked triglyceridemia without premature vascular disease. The major uses of information on the lipoprotein pattern are in the evaluation of the possible cause of the excess of plasma lipids and hence in planning for therapy. In cases of hyperlipidemia that appear to be primary - because of positive family history or absence of known factors associated with secondary hyperlipidemia-elevation of plasrna cholesterol only (triglycerides being definitely within the normal range) is almost always associated with an excess of beta-lipoprotein (type 11 of Fredrickson). Lipoprotein electrophoresis may be used to confirm this. It is in the instances of hyperlipidemia characterized by excess of triglycerides - with or without associated hypercholesterolemia- that the lipoprotein electrophoresis is most helpful in assessing the possible cause of the disorder and in choosing the course of management. The presence of more than a trace of chylomicrons (in the absence of other abnormalities of the electrophoretic pattern) immediately suggests the possibility of improper fasting and should lead to a repeat study of a plasma specimen obtained in the morning after an earlyevening low-fat meal, no alcohol intake, and complete overnight fasting. The rare type I hyperlipoproteinemia of Fredrickson, characterized by chylomicronemia in spite of fasting, is a symptomatic disorder of childhood and should present no differential diagnostic problem. Secondary forms of isolated chylomicronemia are seen in pancreatitis, with or without associated alcoholism. The high triglyceride content of chylomicrons usually is reflected in the plasma lipid concentrations, where the value for triglycerides characteristically is elevated much more than that for cholesterol-sometimes reaching a ratio of 8:1. 7 The plasma invariably is turbid. Chylomicrons are the least dense of the lipid-protein complexes; and as a result of their relatively low density, these particles rise to the surface of the serum or plasma within 24 hours during refrigerated storage. Increased concentrations of plasma cholesterol and triglycerides may also be found in association with VLD lipoprotein that has electro-
1034
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
phoretic mobility similar to that of low-density beta-lipoprotein, sometimes causing a "broad beta band" (type Ill). The clinical features and available genetic data suggest that the associated disorder is different from that which is characterized by an excess of LD beta-lipoprotein only (type 11). The relation of type III hyperlipoproteinemia to that characterized by the presence of the faster-moving pre-beta-lipoprotein (type IV) is less clear. In type IV hyperlipoproteinemia, also characterized by elevations of both cholesterol and triglycerides and by turbidity of plasma or serum after fasting, the distinction between primary and secondary forms is less easily made and must include a host of considerations such as latent or overt diabetes mellitus, alcoholism with or without pancreatitis, hypothyroidism, nephrotic syndrome, dysglobulinemia, pregnancy or use of progestational hormones (as in anovulatory agents), and rare inborn errors of metabolism such as glycogen-storage disease, Gaucher's disease, and Niemann-Pick disease. 7 From a practical clinical standpoint, then, an adequate work-up should include a careful history of medicines being used, of familial disorders (including diabetes mellitus), and of dietery habits (including use of alcohol). The clinical evaluation need not always be extensive or extravagant in order to reasonably exclude nephrosis (proteinuria, hypoalbuminemia), dysproteinemia (excessive sedimentation rate, known disease such as multiple myeloma or lymphoma, abnormal serum proteins on electrophoresis), or pancreatitis. Carbohydrate tolerance may be assessed by determination of blood sugar after fasting or by a test of glucose tolerance. In secondary forms of hyperlipidemia, it is essential to consider and evaluate the disorder properly before adopting a therapeutic plan for the patient. Correction of the underlying cause of secondary hyperlipoproteinemia, if feasible, is obviously preferable to adding treatment for the hyperlipidemia itself. In other conditions, such as pregnancy, no treatment of the hyperlipidemia is indicated. It seems unlikely that treatment is either indicated or likely to be effective in patients having a clinical nephrotic syndrome or having dysproteinemia associated with hematologic disorders. In patients with a history of pancreatitis, vigorous interim treatment for hyperlipidemia is indicated, although the etiologic influence of pancreatitis on excess of lipid (or vice versa) is not clear. Alcoholism, of course, may be causally related to either or both of these problems; but hyperlipidemia seemingly secondary to alcohol abuse may be present without evidence of hepatic dysfunction, pancreatitis, or hemolytic anemia. When hyperlipidemia persists after correction of a possible underlying cause (alcohol abuse, hypothyroidism, acute pancreatitis, use of a progestational drug, etc.), the lipoprotein pattern is of help in selection of the therapeutic regimen6 ,7,10 and in predicting response to treatment. Generally, patients with primary hyper-beta-lipoproteinemia (type 11) are the most resistant to dietary treatment. The mixed hyperlipidemias (types III and IV) are usually very sensitive to dietary restriction of calories (hence weight reduction) and of simple carbohydrates in addition to restriction of cholesterol and saturated fats. If dietary measures
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1035
are not wholly effective, a trial of treatment with drugs may be indicated. As in dietary treatment, the selection of a drug and a prediction as to response frequently can be based on knowledge of the type of hyperlipoproteinemia.
SUMMARY An adequate screening evaluation for hyperlipidemia as a cardiovascular risk factor should include a determination of both plasma cholesterol and plasma triglycerides. If the results of these tests are normal-or optimal-nothing further need be done. In the presence of hyperlipidemia, an evaluation of lipoproteins by electrophoretic methods is of benefit in assessment of possible spurious or secondary hyperlipidemias to be considered, in classification of the disorder, and in the selection of dietary or pharmacologic therapy.
REFERENCES 1. Albrink, Margaret J., and Klatskin, G.: Lactescence of serum following episodes of acute alcoholism and its probable relationship to acute pancreatitis. Amer J Med 23 :26-33 (July) 1957. 2. Albrink, Margaret J., Meigs, J.W., and Man, Evelyn B.: Serum lipids, hypertension and coronary artery disease. Amer J Med 31 :4-23 (July) 1961. 3. Coronary Drug Project Research Group: The coronary drug project: Relationship between coronary risk factors and prognosis for survival after recovery from previous myocardial infarction. (Abstr.) Circulation 40(Suppl 3):62-63 (Oct.) 1969. 4. Ellefson, R. D., Jiminez, Barbara J.,and Smith, R. C.: Lipoproteins of human blood. (Unpublished data.) 5. Fredrickson, D. S., and Lees, R. S.: A system for phenotyping hyperlipoproteinemia. Circulation 31 :321-327 (Mar.) 1965. 6. Fredrickson, D. S., Levy, R. 1., Kwiterovich, P. 0., Jr., and Jover, A.: The Typing of Hyperlipoproteinemia: A Progress Report (1968). In Holmes, W. L., Carlson, L. A., and Paoletti, R.: Drugs Affecting Lipid Metabolism. (Proceedings of the Third International Symposium on Drugs Affecting Lipid Metabolism, Milan Italy, September 9 to 11, 1968.) New York, Plenum Press, 1969, pp. 307-325. 7. Fredrickson, D. S., Levy, R. 1., and Lees, R. S.: Fat transport in lipoproteins - an integrated approach to mechanisms and disorders. New Eng J Med 276:34-44 (Jan. 5); 94-103 (Jan. 12); 148-156 (Jan. 19); 215-225 (Jan. 26); 273-281 (Feb. 2) 1967. 8. Kannel, W. B., Castelli, W. P., and McNamara, Patricia M.: Serum lipid fractions and risk of coronary heart disease: The Framingham study. Minn Med 52:1225-1230 (Aug.) 1969. 9. Lees, R. S., and Hatch, F. T.: Sharper separation of lipoprotein species by paper electrophoresis in albumin-containing buffer. J Lab Clin Med 61 :518-528 (Mar.) 1963. 10. Nunn, S. L., Juergens, J. L., Ellefson, R. D., and Berge, K. G.: A lipid clinic: Preliminary observations. Minn Med 52: 1253-1255 (Aug.) 1969. 11. Stamler, J.: Lectures on Preventive Cardiology. New York, Grune & Stratton, Inc., 1967, pp. 107-162. 12. Stamler, J.: Lectures on Preventive Cardiology. New York, Grune & Stratton, Inc., 1967, pp. 165-241.
Hyperlipidemia-high plasma concentrations of cholesterol, triglycerides, or the related lipoproteins - has emerged clearly as a major factor associated with increased risk of atherosclerotic disease. Alone or in conjunction with other ~ajor risk factors such as hypertension and cigarette smoking, hyperlipidemia clearly is associated with a substantial rise in the likelihood of initial development of coronary heart disease 8 • 11 and probably also in the mortality following myocardial infarction. 3 Data from prospective epidemiologic studies indicate that this significance of hyperlipidemia has an inverse relation with age (the younger the person with hyperlipidemia, the greater the risk of vascular disease) and a direct, stepwise relation with the level of lipid. 11 Because of this latter observation, which supports the findings in population studies comparing coronary heart disease and mean lipid levels in various ethnic or geographic groups, the traditional concepts of normal levels are not applicable in assessing risk factors such as levels of blood fats and of blood pressure. The optimal values may be much lower than the upper range of normal, which is derived arbitrarily from values found among general population groups such as North Americans. And indeed, the risk of coronary heart disease is higher among persons whose blood pressure or lipid concentrations are within the upper part of the defined normal range than among those whose levels are lower in that range.
NEED FOR DETERMINATION OF LIPID CONCENTRATIONS Although the data relating to the effect of correction or amelioration of risk factors 12 are less conclusive, available evidence certainly supports a reasonable clinical effort to reduce elevated levels of plasma lipids, especially in younger adults. For this reason, detection of hyperlipidemia Medical Clinics of North America- Vol. 54, No. 4, July, 1970
1029
1030
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
should not be deferred until the onset of premature, symptomatic ischemic heart disease has called attention to its possibility, any more than the detection of hypertension should await the occurrence of heart failure or stroke. As hyperlipidemia is rarely symptomatic except in fairly extreme forms that produce xanthomas in skin or tendons, it is clear that early detection can be accomplished only by liberal application of screening techniques, either generally or in persons with family history or other evidence suggesting elevated concentration of blood fats. The clinicianalthough his concerns usually are different in perspective and scope from those of the epidemiologist-must adopt some of the screening techniques of the latter if his efforts against cardiovascular disease are to be early, effective, and-one hopes-even preventive. The prevalence and seriousness of cardiovascular disease certainly warrant a major effort in prevention. In view of the early recognition of cholesterol in the atherosclerotic plaque and the early use of cholesterol-feeding to induce experimental atherosclerosis, it is not surprising that hypercholesterolemia has the longest history as a recognized lipid risk factor. No significant data have emerged that would disprove its importance. The significance of elevated plasma glycerides (mainly as triglycerides) in vascular risk has been appreciated more recently2 and confirmed by prospective study.8,1l Currently available evidence indicates that hypertriglyceridemia and hypercholesterolemia are important alone and perhaps even more so when coexisting. 8 Although marked triglyceridemia is frequently associated with turbidity of plasma or serum, the absence of turbidity does not exclude significant triglyceridemia. For these reasons, direct determinations of both cholesterol and triglycerides are needed for proper assessment of lipid factors in vascular risk. Secondary hyperlipidemia, such as that associated with hypothyroidism, liver disease, nephrosis, dysproteinemia, and pregnancy or use of progestational drugs, must be considered and excluded before one estimates cardiovascular risk or- more importantly - plans therapy.
ANALYSIS OF LIPOPROTEINS Chemical or physicochemical combination of lipid molecules (such as cholesterol, glycerides, and fatty acids) with proteins makes possible the transport of fat in the aqueous media of the body. These lipid-protein combinations include relatively small complexes (free fatty acid and albumin); larger complexes (cholesterol, other lipids, and certain globulins); and still larger, triglyceride-rich aggregates that cause turbidity of the plasma (these including so-called endogenous particles and chylomicrons, the latter of which are formed from recently digested fat). The different natures of the lipid and protein components of the various lipoproteins cause them to have different densities, and it is possible to separate the lipoproteins by several means into arbitrary classes on the basis of density: high density (HD), of >1.063 gm/ml;
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1031
low density (LD), of SG 1.006-1.063; and very low density (VLD), of <1.006. Separation by analytical ultracentrifugation has led to classification of the lipoproteins according to flotation rate, expressed in Svedberg flotation units (Sf) and grouped for simplification as Sf 0-20 (HD and LD), Sf 20-400 (VLD) and Sf > 400 (particles). The sophisticated apparatus and techniques required for such analyses have limited their application in clinical medicine, however; and indeed the necessity of such detailed information in clinical practice has never been clearly established. An index of atherogenicity based on lipoprotein fractionation by the analytical ultracentrifuge has not been accepted widely as superior to knowledge of lipid levels based on less complicated and expensive analyses. In 1963, Lees and Hatch described a technique of paper electrophoresis that would separate chylomicrons (origin), endogenously synthesized glycerides (pre-heta band), high-density lipoproteins (HDL) rich in phospholipid (alpha hand), and low-density lipoproteins (LDL) rich in cholesterol (beta band). The addition of preparatory ultracentrifugation to this technique allowed further definition of the bands into those carrying HDL and LDL (~0.006 gm/ml) and those carrying only very low density lipoproteins (VLDL). The system for phenotyping familial hyperlipoproteinemia with a numerical classification reported by Fredrickson and Lees in 1965 and elaborated upon in 1967 (Fig. 1)7 enhanced interest in the classification of clinical hyperlipidemia (Table 1). Previous systems of classification included schemes based only upon plasma concentrations of cholesterol or triglycerides, or on dermatologic manifestations of the basic metabolic disorder, or on separation into familial, primary, and secondary types. The resulting confusion in terminology and lack of ready comparability between series of cases made welcome a new system based on lipoprotein patterns. The numerical system of naming is simpler than the attempts to encompass all of the possible variations of a pattern with.adjectival additions. Although this classification of lipoproteins was based on a study of Table 1.
Plasma Findings in Types of Hyperlipoproteinemia as Classified by Fredrickson and Lees HYPERLIPOPROTEINEMIA, TYPE
FINDINGS IN PLASMA
II
Appearance Cholesterol, excess':' Triglyceride, excess':' Electrophoretic pattern
Cloudy 0-2 1-4 Chylomicrons
°
Clear 1-4 0-1 1-4 excess';' (3-lipoprotein
III
IV
V
Cloudy 1-3 1-3 Broad (3 band
Cloudy 1-3 1-4 Pre-(3 lipoprotein
Cloudy 1-3 1-4 Chylomicrons, pre-(3 lipoprotein
':'Graded on basis of (normal) to 4. The grading in this table reflects experience at the Mayo Clinic, which includes analyses of more than 20,000 specimens from more than 8,000 s.ubjects.
1032
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
PAPER ELECTROPHORESIS CHYlOMICRONS
PRE-/3
Figure 1. Schematic representation of the r.najor portions of the lipoprotein spectrum as defined by paper electrophoresis and by ultracentrifugation. (From Fredrickson, D. S., Levy, R. I., and Lees, R. S.: Fat transport in lipoproteins-an integrated approach to mechanisms and disorders. New Eng J Med 276:34-44 [Jan. 5] 1967. By permission of the Massachusetts Medical Society.)
kindreds with hyperlipidemia, it since has been applied more widely in clinical cases without detailed data on significant numbers of family members as well as in cases of secondary hyperlipidemia. Most of these latter instances are not familial. Furthermore, not all cases of hyperlipidemia secondary to a single cause (such as myxedema) fit into a single lipoprotein type. Such variations as these must be appreciated in order to avoid oversimplification of this complex group of disorders-a hazard inherent not in the classification but in its superficial interpretation or application. Another problem in the interpretation of lipoprotein patterns has been the occasional finding of combinations that do not fit within the five types as defined. These patterns may result from inadequate dietary preparation of the patient, existence of unclassified varieties of the primary or secondary types, or improvement in the techniques of preparation and electrophoresis that allow a more refined separation of lipoproteins. 4 The assessment of any patient's usual lipid status can be done properly only when he has been following his general dietary pattern for several weeks and has fasted adequately (preferably 14 hours or more) prior to the tests. Although blood drawn after meals may provide a reasonable reflection of the fasting cholesterol level, the phase of alimentary lipemia includes a marked elevation of triglycerides due to chylomicronemia. The recent consumption of alcohol in amounts unusual for a given patient also may cause a spurious rise in lipids - especially triglycerides.! If the use of alcohol is long-standing and unchanged, however, chronic hyperlipidemia may be due to this factor alone.
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1033
CLINICAL USEFULNESS OF ELECTROPHORESIS Lipoprotein patterns should be correlated with concentrations of plasma cholesterol and triglycerides. Determination of both cholesterol and triglyceride concentrations provides evidence of hyperlipidemia in more than 95% of cases. 6 If those concentrations are not elevated, determination of lipoproteins by electrophoresis is rarely necessary or helpful. The exceptions would include the rare cases of an absence or deficiency of certain lipoprotein fractions in such disorders as Tangier disease and a-beta-lipoproteinemia. The data from the prospective study in Framingham indicated that risk of coronary heart disease was proportional to the level of the lipids, irrespective of the associated lipoprotein patterns, so the lipoprotein pattern was not of major importance in assessing risk. 8 An exception might be the rare cases of Fredrickson's type I hyperlipoproteinemia that exhibit marked triglyceridemia without premature vascular disease. The major uses of information on the lipoprotein pattern are in the evaluation of the possible cause of the excess of plasma lipids and hence in planning for therapy. In cases of hyperlipidemia that appear to be primary - because of positive family history or absence of known factors associated with secondary hyperlipidemia-elevation of plasrna cholesterol only (triglycerides being definitely within the normal range) is almost always associated with an excess of beta-lipoprotein (type 11 of Fredrickson). Lipoprotein electrophoresis may be used to confirm this. It is in the instances of hyperlipidemia characterized by excess of triglycerides - with or without associated hypercholesterolemia- that the lipoprotein electrophoresis is most helpful in assessing the possible cause of the disorder and in choosing the course of management. The presence of more than a trace of chylomicrons (in the absence of other abnormalities of the electrophoretic pattern) immediately suggests the possibility of improper fasting and should lead to a repeat study of a plasma specimen obtained in the morning after an earlyevening low-fat meal, no alcohol intake, and complete overnight fasting. The rare type I hyperlipoproteinemia of Fredrickson, characterized by chylomicronemia in spite of fasting, is a symptomatic disorder of childhood and should present no differential diagnostic problem. Secondary forms of isolated chylomicronemia are seen in pancreatitis, with or without associated alcoholism. The high triglyceride content of chylomicrons usually is reflected in the plasma lipid concentrations, where the value for triglycerides characteristically is elevated much more than that for cholesterol-sometimes reaching a ratio of 8:1. 7 The plasma invariably is turbid. Chylomicrons are the least dense of the lipid-protein complexes; and as a result of their relatively low density, these particles rise to the surface of the serum or plasma within 24 hours during refrigerated storage. Increased concentrations of plasma cholesterol and triglycerides may also be found in association with VLD lipoprotein that has electro-
1034
KENNETH
G.
BERGE, RALPH
D.
ELLEFSON
phoretic mobility similar to that of low-density beta-lipoprotein, sometimes causing a "broad beta band" (type Ill). The clinical features and available genetic data suggest that the associated disorder is different from that which is characterized by an excess of LD beta-lipoprotein only (type 11). The relation of type III hyperlipoproteinemia to that characterized by the presence of the faster-moving pre-beta-lipoprotein (type IV) is less clear. In type IV hyperlipoproteinemia, also characterized by elevations of both cholesterol and triglycerides and by turbidity of plasma or serum after fasting, the distinction between primary and secondary forms is less easily made and must include a host of considerations such as latent or overt diabetes mellitus, alcoholism with or without pancreatitis, hypothyroidism, nephrotic syndrome, dysglobulinemia, pregnancy or use of progestational hormones (as in anovulatory agents), and rare inborn errors of metabolism such as glycogen-storage disease, Gaucher's disease, and Niemann-Pick disease. 7 From a practical clinical standpoint, then, an adequate work-up should include a careful history of medicines being used, of familial disorders (including diabetes mellitus), and of dietery habits (including use of alcohol). The clinical evaluation need not always be extensive or extravagant in order to reasonably exclude nephrosis (proteinuria, hypoalbuminemia), dysproteinemia (excessive sedimentation rate, known disease such as multiple myeloma or lymphoma, abnormal serum proteins on electrophoresis), or pancreatitis. Carbohydrate tolerance may be assessed by determination of blood sugar after fasting or by a test of glucose tolerance. In secondary forms of hyperlipidemia, it is essential to consider and evaluate the disorder properly before adopting a therapeutic plan for the patient. Correction of the underlying cause of secondary hyperlipoproteinemia, if feasible, is obviously preferable to adding treatment for the hyperlipidemia itself. In other conditions, such as pregnancy, no treatment of the hyperlipidemia is indicated. It seems unlikely that treatment is either indicated or likely to be effective in patients having a clinical nephrotic syndrome or having dysproteinemia associated with hematologic disorders. In patients with a history of pancreatitis, vigorous interim treatment for hyperlipidemia is indicated, although the etiologic influence of pancreatitis on excess of lipid (or vice versa) is not clear. Alcoholism, of course, may be causally related to either or both of these problems; but hyperlipidemia seemingly secondary to alcohol abuse may be present without evidence of hepatic dysfunction, pancreatitis, or hemolytic anemia. When hyperlipidemia persists after correction of a possible underlying cause (alcohol abuse, hypothyroidism, acute pancreatitis, use of a progestational drug, etc.), the lipoprotein pattern is of help in selection of the therapeutic regimen6 ,7,10 and in predicting response to treatment. Generally, patients with primary hyper-beta-lipoproteinemia (type 11) are the most resistant to dietary treatment. The mixed hyperlipidemias (types III and IV) are usually very sensitive to dietary restriction of calories (hence weight reduction) and of simple carbohydrates in addition to restriction of cholesterol and saturated fats. If dietary measures
LABORATORY EVALUATION OF HYPERLIPIDEMIA (HYPERLIPOPROTEINEMIA)
1035
are not wholly effective, a trial of treatment with drugs may be indicated. As in dietary treatment, the selection of a drug and a prediction as to response frequently can be based on knowledge of the type of hyperlipoproteinemia.
SUMMARY An adequate screening evaluation for hyperlipidemia as a cardiovascular risk factor should include a determination of both plasma cholesterol and plasma triglycerides. If the results of these tests are normal-or optimal-nothing further need be done. In the presence of hyperlipidemia, an evaluation of lipoproteins by electrophoretic methods is of benefit in assessment of possible spurious or secondary hyperlipidemias to be considered, in classification of the disorder, and in the selection of dietary or pharmacologic therapy.
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