Diabetes Insipidus vs Diabetes Mellitus
Both diabetes mellitus and insipidus are characterized by increased frequency of urination and increased thirst.
Diabetes Mellitus
Diabetes mellitus is a disease associated with elevated blood sugar levels. There are three types of diabetes mellitus. Type 1 diabetes starts in childhood. Beta cells in islets of Langerhan in pancreas fail to synthesize insulin or defective insulin with minimal biological activity is synthesized. It can be due to genetic impairment of insulin receptors, as well. Type 2 diabetes is due to impairment of insulin sensitivity at target cells. Insulin is synthesized at increasingly higher levels till pancreatic cells fail and then, exogenous insulin is required. Pregnancy induced diabetes mellitus is due to the action of hormones of pregnancy. They tend to increase blood sugar levels opposing the action of insulin.
Classical triad of symptoms is increased thirst (polydipsia), increased hunger (polyphagia) and increased frequency of urination (polyuria). In Diabetes mellitus, fasting blood sugar level is above 120mg/dl. Oral glucose tolerance test is the gold standard in diagnosing diabetes mellitus. Blood sugar level 2 hours after ingesting 75g of glucose is above 140mg/dl in diabetes mellitus.
Type 1 diabetics need exogenous insulin injections to control blood sugar. Type 2 diabetics can be managed with oral hypoglycemic drugs such as metformin and tolbutamide. Complications of diabetes are classified into two broad categories. Complications associated with small blood vessels (retinopathy, nephropathy and neuropathy) are known as micro-vascular complications, and those associated with large blood vessels (peripheral vascular disease, stroke and myocardial infarction) are known as macro-vascular complications.
Diabetes Insipidus
Diabetes insipidus is a disease of water and electrolyte retention. There are two types of diabetes insipidus. Central diabetes insipidus is due to impaired synthesis of vasopressin. Vasopressin formation is impaired in diseases of hypothalamus, hypothalamo-hypophysial tract and posterior pituitary. 30% of hypothalamic diseases are neoplastic (malignant or benign); 30% are post-traumatic and 30% are of unknown origin. The rest may be due to infections, infarctions and genetic errors in prepropressophysin gene. Nephrogenic diabetes insipidus is due to impaired action of vasopressin. The action of vasopressin is reduced if vasopressin receptors (V – 2) or water channels (aquaporin – 2) in collecting ducts of the kidney are defective.
In both central and nephrogenic diabetes insipidus, there is excessive water loss leading to the passage of diluted urine and dehydration. Thirst is what keeps them alive. It ensures adequate water intake to counteract the fluid loss from both intracellular and extracellular compartments.
Diabetes Mellitus vs. Diabetes Insipidus
• Diabetes insipidus (DI) is a disease of reduced vasopressin action and diabetes mellitus (DM) is a disease of reduced insulin action.
• DM is a disease of pancreas and target cells while DI is a disease of the brain and kidneys.
• DM causes high blood sugar levels while DI doesn’t.
• DM causes polyphagia while DI doesn’t.
• DM causes polyuria by osmotic diuresis (increased glucose holds and takes water out in urine with it), and DI causes polyuria by reduced water reabsorption at collecting ducts of kidneys.
• DM is treated with oral hypoglycemic drugs and insulin while DI is treated with synthetic vasopressin.