Definition, Etiology, and Symptoms of Diabetes Mellitus
Diabetes mellitus is a group of metabolic diseases characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both, leading to long-term damage of the eyes, kidneys, nerves, heart, and blood vessels. 1
Definition
Diabetes represents a heterogeneous metabolic disorder where the fundamental problem is deficient insulin action on target tissues, causing abnormalities in carbohydrate, fat, and protein metabolism 1. This deficient insulin action stems from:
- Inadequate insulin secretion
- Diminished tissue responses to insulin (insulin resistance)
- Both mechanisms frequently coexisting in the same patient 1
The chronic hyperglycemia is directly associated with organ damage, dysfunction, and failure, particularly affecting the eyes, kidneys, nerves, heart, and blood vessels 1.
Etiology
The pathogenic processes underlying diabetes fall into two major etiopathogenetic categories 1:
Type 1 Diabetes (5-10% of cases)
- Absolute insulin deficiency due to autoimmune destruction of pancreatic β-cells 2
- Can be identified by serological evidence of autoimmune pathology in pancreatic islets and genetic markers 1
- Although traditionally associated with childhood onset, can manifest at any age 3
Type 2 Diabetes (>90% of cases)
- Combination of insulin resistance and inadequate compensatory insulin secretion 1
- Associated with genetic predisposition, epigenetic changes, inflammation, and metabolic stress 4
- Environmental factors include physical inactivity, obesity, drugs, and toxic agents 5
Important clinical note: Impairment of insulin secretion and defects in insulin action frequently coexist in the same patient, making it often unclear which abnormality is the primary cause of hyperglycemia 1. The pathogenic spectrum ranges from autoimmune β-cell destruction to abnormalities causing insulin resistance 1.
Clinical Manifestations
Acute Symptoms of Marked Hyperglycemia
The classic triad plus two includes 1:
- Polyuria (excessive urination)
- Polydipsia (excessive thirst)
- Weight loss (sometimes with polyphagia/increased appetite)
- Blurred vision
- Impairment of growth (in children)
- Increased susceptibility to certain infections 6
Life-Threatening Acute Complications
- Diabetic ketoacidosis (DKA)
- Nonketotic hyperosmolar syndrome 1
Long-Term Complications (Impact on Morbidity and Mortality)
Microvascular complications 2:
- Retinopathy → potential vision loss
- Nephropathy → renal failure
- Peripheral neuropathy → foot ulcers, amputations, Charcot joints
- Autonomic neuropathy → gastrointestinal, genitourinary, cardiovascular symptoms, and sexual dysfunction
Macrovascular complications 1:
- Increased incidence of atherosclerotic cardiovascular disease
- Peripheral arterial disease
- Cerebrovascular disease
- Hypertension and lipoprotein metabolism abnormalities commonly coexist
Critical Clinical Considerations
Asymptomatic period: Type 2 diabetes may be present for a prolonged period without clinical symptoms, during which hyperglycemia is sufficient to cause pathologic changes in target tissues 1. This can be detected by measuring fasting plasma glucose, post-challenge glucose load, or A1C 1.
Disease progression: The degree of hyperglycemia may change over time depending on the extent of the underlying disease process. The same pathogenic process can cause impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) without fulfilling diabetes diagnostic criteria 1.
Common pitfall: Many adults with new-onset type 1 diabetes are misdiagnosed as having type 2 diabetes, with misdiagnosis occurring in approximately 40% of cases 4. Use the AABBCC approach for distinguishing diabetes type: Age, Autoimmunity, Body habitus, Background (family history), Control (glucose control on non-insulin therapies), and Comorbidities 4.