Diabetes mellitus



Diabetes mellitus


E.R. Pearson


R.J. McCrimmon



Clinical examination of the patient with diabetes



Diabetes can affect every system in the body. In routine clinical practice, examination of the patient with diabetes is focused on image hands, image blood pressure, image and image axillae and neck, image eyes, image insulin injection sites and image feet.






Diabetes mellitus is a clinical syndrome characterised by an increase in plasma blood glucose (hyperglycaemia). Diabetes has many causes (see Box 21.5, p. 807) but is most commonly due to type 1 or type 2 diabetes. Type 1 diabetes is caused by autoimmune destruction of insulin-producing cells (β cells) in the pancreas, resulting in absolute insulin deficiency, whereas type 2 diabetes is characterised by resistance to the action of insulin and an inability to produce sufficient insulin to overcome this ‘insulin resistance’. Hyperglycaemia results in both acute and long-term problems. Acutely, high glucose and lack of insulin can result in marked symptoms, metabolic decompensation and hospitalisation. Chronic hyperglycaemia is responsible for diabetes-specific ‘microvascular’ complications affecting the eyes (retinopathy), kidneys (nephropathy) and feet (neuropathy).


There is a continuous distribution of blood glucose in the population, with no clear division between people with normal and abnormal values. The diagnostic criteria for diabetes (a fasting plasma glucose ≥ 7.0 mmol/L (126 mg/dL) or glucose 2 hours after an oral glucose challenge ≥ 11.1 mmol/L (200 mg/dL); see p. 807) have been selected to identify those who have a degree of hyperglycaemia which, if untreated, carries a significant risk of microvascular disease, and in particular diabetic retinopathy. Less severe hyperglycaemia is called ‘impaired glucose tolerance’. This is not associated with a substantial risk of microvascular disease, but is connected with an increased risk of large vessel disease (e.g. atheroma leading to myocardial infarction) and with a greater risk of developing diabetes in future.


The incidence of diabetes is rising. Globally, it is estimated that 366 million people had diabetes in 2011 (approximately 8.3% of the world population, or 3 new cases every 10 seconds), and this figure is expected to reach 552 million by 2030. This global pandemic principally involves type 2 diabetes, the prevalence of which varies considerably around the world (Fig. 21.1), being associated with differences in genetic as well as environmental factors such as greater longevity, obesity, unsatisfactory diet, sedentary lifestyle, increasing urbanisation and economic development. A pronounced rise in the prevalence of type 2 diabetes occurs in migrant populations to industrialised countries, as in Asian and Afro-Caribbean immigrants to the UK or USA. Type 2 diabetes is now being observed in children and adolescents, particularly in some ethnic groups, such as Hispanics and Afro-Americans.



The incidence of type 1 diabetes is also increasing, such that between 1960 and 1996, 3% more children were diagnosed worldwide each year. Type 1 diabetes is generally more common in countries closer to the polar regions. Finland, for instance, has the highest rate of type 1 diagnosis per year at around 40 per 100 000 of the population, whereas in China the incidence is only 0.1 per 100 000 of the population. Type 1 diabetes is most common in Caucasians and more people are diagnosed in the winter months.


Diabetes is a major burden upon health-care facilities in all countries. Globally, diabetes caused 4.6 million deaths in 2011, and health-care expenditure attributed to diabetes was estimated to be at least US$465 billion, or 11% of total health-care expenditure.



Functional anatomy and physiology


Normal glucose and fat metabolism


Blood glucose is tightly regulated and maintained within a narrow range. This is essential for ensuring a continuous supply of glucose to the central nervous system. The brain has little capacity to store energy in the form of glycogen or triglyceride and the blood–brain barrier is largely impermeable to fatty acids, so the brain depends on the liver for a constant supply of glucose for oxidation and hence generation of adenosine triphosphate (ATP). Glucose homeostasis is achieved through the coordinated actions of multiple organs, but mainly reflects a balance between the entry of glucose into the circulation from the liver, supplemented by intestinal absorption of glucose after meals, and the uptake of glucose by peripheral tissues, particularly skeletal muscle and brain.


After ingestion of a meal containing carbohydrate, normal blood glucose levels are maintained by:



• suppression of hepatic glucose production


• stimulation of hepatic glucose uptake


• stimulation of glucose uptake by peripheral tissues (Fig. 21.2).



Insulin, the primary regulator of glucose metabolism and storage (Box 21.1), is secreted from pancreatic β cells into the portal circulation in response to a rise in blood glucose (Fig. 21.3). A number of other factors released from the gut following food intake can augment insulin release, including amino acids and hormones such as glucagon-like peptide 1 (GLP-1) and gastrointestinal peptide (GIP). As a result, insulin release is greater when glucose is administered by mouth than when the same rise in plasma glucose is achieved by intravenous glucose infusion, a phenomenon termed the ‘incretin’ effect (see Fig. 21.3). The post-prandial rise in portal vein insulin and glucose, together with a fall in portal glucagon concentrations, suppresses hepatic glucose production and results in net hepatic glucose uptake. Depending on the size of the carbohydrate load, around one-quarter to one-third of ingested glucose is taken up in the liver. In addition, insulin stimulates glucose uptake in skeletal muscle and fat, mediated by the glucose transporter, GLUT 4.




When intestinal glucose absorption declines between meals, portal vein insulin and glucose concentrations fall while glucagon levels rise. This leads to increased hepatic glucose output via gluconeogenesis and glycogen breakdown. The liver now resumes net glucose production and glucose homeostasis is maintained. The main substrates for gluconeogenesis are glycerol and amino acids, as shown in Figure 21.2.


Adipocytes (and the liver) synthesise triglyceride from non-esterified (‘free’) fatty acids (FFAs) and glycerol. Insulin is the major regulator not only of glucose metabolism but also of fatty acid metabolism. High insulin levels after meals promote triglyceride accumulation. In contrast, in the fasting state, low insulin levels permit lipolysis and the release into the circulation of FFAs (and glycerol), which can be oxidised by many tissues. Their partial oxidation in the liver provides energy to drive gluconeogenesis and also produces ketone bodies (acetoacetate, which can be reduced to 3-hydroxybutyrate or decarboxylated to acetone), which are generated in hepatocyte mitochondria. Ketone bodies are organic acids which, when formed in small amounts, are oxidised and utilised as metabolic fuel. However, the rate of utilisation of ketone bodies by peripheral tissues is limited, and when the rate of production by the liver exceeds their removal, hyperketonaemia results. This occurs physiologically during starvation, when low insulin levels and high catecholamine levels increase lipolysis and delivery of FFAs to the liver.



Aetiology and pathogenesis of diabetes


In both of the common types of diabetes, environmental factors interact with genetic susceptibility to determine which people develop the clinical syndrome, and the timing of its onset. However, the underlying genes, precipitating environmental factors and pathophysiology differ substantially between type 1 and type 2 diabetes. Type 1 diabetes was previously termed ‘insulin-dependent diabetes mellitus’ (IDDM) and is invariably associated with profound insulin deficiency requiring replacement therapy. Type 2 diabetes was previously termed ‘non-insulin-dependent diabetes mellitus’ (NIDDM) because patients retain the capacity to secrete some insulin but exhibit impaired sensitivity to insulin (insulin resistance) and initially can usually be treated without insulin replacement therapy. However, 20% or more of patients with type 2 diabetes will ultimately develop profound insulin deficiency requiring replacement therapy, so that IDDM and NIDDM were misnomers.



Type 1 diabetes



Pathology

Type 1 diabetes is a T cell-mediated autoimmune disease (p. 86) involving destruction of the insulin-secreting β cells in the pancreatic islets. Progressive loss of β cell function takes place over a prolonged period (months to years), but marked hyperglycaemia, accompanied by the classical symptoms of diabetes, occurs only when 80–90% of the functional capacity of β cells has been lost.


The pathology in the pre-diabetic pancreas is characterised by ‘insulitis’ (Fig. 21.4), with infiltration of the islets by mononuclear cells containing activated macrophages, helper cytotoxic and suppressor T lymphocytes, natural killer cells and B lymphocytes. Initially, these lesions are patchy and, until a very late stage, lobules containing heavily infiltrated islets are seen adjacent to unaffected lobules. The destructive process is β cell-specific, the glucagon and other hormone-secreting cells in the islet remaining intact.



Islet cell antibodies are present before the clinical presentation of type 1 diabetes, and their detection can be useful in confirming a diagnosis of type 1 diabetes, but they are poorly predictive of disease progression and disappear over time (see Fig. 21.4). Type 1 diabetes is associated with other autoimmune disorders (Ch. 4), including thyroid disease (p. 738), coeliac disease (p. 880), Addison’s disease (p. 777), pernicious anaemia (p. 1025) and vitiligo (p. 1295).



Genetic predisposition

Genetic factors account for about one-third of the susceptibility to type 1 diabetes, the inheritance of which is polygenic (Box 21.2). Over 20 different regions of the human genome show some linkage with type 1 diabetes but most interest has focused on the human leucocyte antigen (HLA) region within the major histocompatibility complex on the short arm of chromosome 6; this locus is designated IDDM 1. The HLA haplotypes DR3 and/or DR4 are associated with increased susceptibility to type 1 diabetes in Caucasians and are in ‘linkage disequilibrium’, i.e. they tend to be transmitted together, with the neighbouring alleles of the HLA-DQA1 and DQB1 genes. The latter may be the main determinants of genetic susceptibility, since these HLA class II genes code for proteins on the surface of cells which present foreign and self antigens to T lymphocytes (p. 87). Candidate gene and genome-wide association studies have also implicated other genes in type 1 diabetes, e.g. CD25, PTPN22, IL2RA and IL-10, which are involved in immune recognition of pancreatic islet antigens, T-cell development and immune regulation. The genes associated with type 1 diabetes overlap with those for other autoimmune disorders, such as coeliac disease and thyroid disease, consistent with clustering of these conditions in individuals or families.




Environmental predisposition

Although genetic susceptibility appears to be a prerequisite for type 1 diabetes, the concordance rate between monozygotic twins is less than 40% (see Box 21.2), and wide geographic and seasonal variations in incidence suggest that environmental factors have an important role in precipitating disease.


Although hypotheses abound, the nature of these environmental factors is uncertain. They may trigger type 1 diabetes through direct toxicity to β cells or by stimulating an autoimmune reaction directed against β cells. Potential candidates fall into three main categories: viruses, specific drugs or chemicals, and dietary constituents. Viruses implicated in the aetiology of type 1 diabetes include mumps, Coxsackie B4, retroviruses, rubella (in utero), cytomegalovirus and Epstein–Barr virus. Various dietary nitrosamines (found in smoked and cured meats) and coffee have been proposed as potentially diabetogenic toxins. Bovine serum albumin (BSA), a major constituent of cow’s milk, has been implicated, since children who are given cow’s milk early in infancy are more likely to develop type 1 diabetes than those who are breastfed. BSA may cross the neonatal gut and raise antibodies which cross-react with a heat-shock protein expressed by β cells. It has also been proposed that reduced exposure to microorganisms in early childhood limits maturation of the immune system and increases susceptibility to autoimmune disease (the ‘hygiene hypothesis’).



Metabolic disturbances in type 1 diabetes

Patients with type 1 diabetes present when progressive β-cell destruction has crossed a threshold at which adequate insulin secretion and normal blood glucose levels can no longer be sustained. Above a certain level, high glucose levels may be toxic to the remaining β cells, so that profound insulin deficiency rapidly ensues, causing the metabolic sequelae shown in Figure 21.5. Hyperglycaemia leads to glycosuria and dehydration, causing fatigue, polyuria, nocturia, thirst and polydipsia, susceptibility to urinary and genital tract infections, and later tachycardia and hypotension. Unrestrained lipolysis and proteolysis result in weight loss. Ketoacidosis occurs when generation of ketone bodies exceeds the capacity for their metabolism. Elevated blood H+ ions drive K+ out of the intracellular compartment, while secondary hyperaldosteronism encourages urinary loss of K+. Thus patients usually present with a short history (typically a few weeks) of hyperglycaemic symptoms (thirst, polyuria, nocturia and fatigue), infections and weight loss, and may have developed ketoacidosis (p. 811).




Type 2 diabetes



Pathology

Type 2 diabetes is a diagnosis of exclusion, i.e. it is made when type 1 diabetes and other types of diabetes (see Box 21.5, p. 807) are ruled out, and is highly heterogeneous. The natural history of typical type 2 diabetes is shown in Figure 21.6. Initially, insulin resistance leads to elevated insulin secretion in order to maintain normal blood glucose levels. However, in susceptible individuals, the pancreatic β cells are unable to sustain the increased demand for insulin and a slowly progressive insulin deficiency develops. Some patients develop diabetes at a young age, usually driven by insulin resistance due to obesity and ethnicity; others, particularly the elderly, develop diabetes despite being non-obese and may have more pronounced β-cell failure. The key feature is a ‘relative’ insulin deficiency, such that there is insufficient insulin production to overcome the resistance to insulin action. This contrasts with type 1 diabetes, in which there is rapid loss of insulin production and an absolute deficiency, resulting in ketoacidosis and death if the insulin is not replaced.




Insulin resistance

Type 2 diabetes, or its antecedent, impaired glucose tolerance, is one of a cluster of conditions thought to be caused by resistance to insulin action. Thus, patients with type 2 diabetes often have associated disorders including hypertension, dyslipidaemia (characterised by elevated levels of small dense low-density lipoprotein (LDL) cholesterol and triglycerides, and a low level of high-density lipoprotein (HDL) cholesterol), non-alcoholic fatty liver (p. 959) and, in women, polycystic ovarian syndrome. This cluster has been termed the ‘insulin resistance syndrome’ or ‘metabolic syndrome’, and is much more common in patients who are obese.


The primary cause of insulin resistance remains unclear; it is likely that there are multiple defects in insulin signalling, affecting several tissues. One theory is centred around the adipocyte; this is particularly appealing, as obesity is a major cause of increased insulin resistance. Intra-abdominal ‘central’ adipose tissue is metabolically active, and releases large quantities of FFAs, which may induce insulin resistance because they compete with glucose as a fuel supply for oxidation in peripheral tissues such as muscle. In addition, adipose tissue releases a number of hormones (including a variety of peptides, called ‘adipokines’ because they are structurally similar to immunological ‘cytokines’) which act on specific receptors to influence sensitivity to insulin in other tissues. Because the venous drainage of visceral adipose tissue is into the portal vein, central obesity may have a particularly potent influence on insulin sensitivity in the liver, and thereby adversely affect gluconeogenesis and hepatic lipid metabolism.


Physical activity is another important determinant of insulin sensitivity. Inactivity is associated with down-regulation of insulin-sensitive kinases and may promote accumulation of FFAs within skeletal muscle. Sedentary people are therefore more insulin-resistant than active people with the same degree of obesity. Moreover, physical activity allows non-insulin-dependent glucose uptake into muscle, reducing the ‘demand’ on the pancreatic β cells to produce insulin.


Deposition of fat in the liver is a common association with central obesity and is exacerbated by insulin resistance and/or deficiency. Many patients with type 2 diabetes have evidence of fatty infiltration of the liver (non-alcoholic fatty liver disease (NAFLD)). This condition may improve with effective treatment of the diabetes and dyslipidaemia, but despite this, a few patients progress to non-alcoholic steatohepatitis (NASH, p. 959) and cirrhosis.




Genetic predisposition

Genetic factors are important in type 2 diabetes, as shown by marked differences in susceptibility in different ethnic groups and by studies in monozygotic twins where concordance rates for type 2 diabetes approach 100%. However, many genes are involved and the chance of developing diabetes is also influenced very powerfully by environmental factors (Box 21.3). Genome-wide association studies have identified over 65 genes or gene regions that are associated with type 2 diabetes, each exerting a small effect. The largest effect is seen with variation in TCF7L2; the 10% of the population with two copies of the risk variant for this gene have a nearly twofold increase in risk of developing type 2 diabetes. Most of the genes known to contribute to risk of type 2 diabetes are involved in β-cell function or in regulation of cell cycling and turnover, suggesting that altered regulation of β-cell mass is a key factor.




Environmental and other risk factors


Diet and obesity

Epidemiological studies show that type 2 diabetes is associated with overeating, especially when combined with obesity and underactivity. Middle-aged people with diabetes eat significantly more and are fatter and less active than their non-diabetic siblings. The risk of developing type 2 diabetes increases tenfold in people with a body mass index (BMI) of more than 30 kg/m2 (p. 115). However, although the majority of patients with type 2 diabetes are obese, only a minority of obese people develop diabetes, as the majority of obese patients are able to increase insulin secretion to compensate for the increased demand resulting from obesity and insulin resistance. Those who develop diabetes may have genetically impaired β-cell function, reduced β-cell mass, or a susceptibility of β cells to attack by toxic substances such as FFAs or inflammatory cytokines.




Metabolic disturbances in type 2 diabetes

Patients with type 2 diabetes have a slow onset of ‘relative’ insulin deficiency. Relatively small amounts of insulin are required to suppress lipolysis, and some glucose uptake is maintained in muscle, so that, in contrast with type 1 diabetes, lipolysis and proteolysis are not unrestrained and weight loss and ketoacidosis seldom occur. In type 2 diabetes, hyperglycaemia tends to develop slowly over months or years; because of this insidious onset many cases of type 2 diabetes are discovered coincidentally and a large number are undetected. At diagnosis, patients are often asymptomatic or give a long history (typically many months) of fatigue, with or without ‘osmotic symptoms’ (thirst and polyuria). In some patients with type 2 diabetes, presentation is late and pancreatic β-cell failure has reached an advanced stage of insulin deficiency (see type 1 diabetes, p. 803). These patients may present with weight loss but ketoacidosis is uncommon. However, in some ethnic groups, such as African Americans, half of those whose first presentation is with diabetic ketoacidosis have type 2 diabetes.


Intercurrent illness, e.g. with infections, increases the production of stress hormones which oppose insulin action, such as cortisol, growth hormone and catecholamines. This can precipitate an acute exacerbation of insulin resistance and insulin deficiency, and result in more severe hyperglycaemia and dehydration (see hyperglycaemic hyperosmolar state, p. 814).



Other forms of diabetes


Other causes of diabetes are shown in Box 21.5. In most cases, there is an obvious cause of destruction of pancreatic β cells. Some acquired disorders, notably other endocrine diseases such as acromegaly (p. 792) or Cushing’s syndrome (p. 773), can precipitate type 2 diabetes in susceptible individuals.



A number of unusual genetic diseases are associated with diabetes. In rare families, diabetes is caused by single gene defects with autosomal dominant inheritance. These subtypes constitute less than 5% of all cases of diabetes and typically present as ‘maturity-onset diabetes of the young’ (MODY), i.e. non-insulin-requiring diabetes presenting before the age of 25 years (Box 21.6). Very rarely, diabetes can develop at or soon after birth. This neonatal diabetes is usually genetic in origin, with 50% due to mutations in the KATP channel of the pancreatic β cell causing insulin deficiency and diabetic ketoacidosis. However, sulphonylurea drugs overcome the defect in potassium channel signalling, so that insulin therapy is not necessary in these cases.


Apr 9, 2017 | Posted by in GENERAL SURGERY | Comments Off on Diabetes mellitus
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