Description
Diabetes insipidus (DI) is a disorder characterised by the production of large volumes of dilute urine due to deficient activity of antidiuretic hormone (ADH), also known as vasopressin. This disorder can occur due to abnormalities in the production, release, or action of ADH.
Pathogenesis
The pathophysiology of diabetes insipidus involves either insufficient production or release of ADH from the posterior pituitary gland (central DI) or renal resistance to the effects of ADH (nephrogenic DI). ADH is vital in regulating water reabsorption in the kidneys; insufficient ADH action results in excessive water excretion and resultant polyuria and polydipsia.
Subtype
- Central Diabetes Insipidus: Caused by a lack of ADH production or secretion by the posterior pituitary gland. This can be idiopathic or due to damage to the hypothalamus or pituitary gland from a variety of causes, including trauma, surgery, malignancy, or infection.
- Nephrogenic Diabetes Insipidus: Occurs when the kidneys do not respond properly to ADH. This can be due to inherited genetic disorders or acquired causes such as chronic kidney disease or certain medications.
Epidemiology, Risk Factors & Associations
- Central DI is usually idiopathic or secondary to pituitary surgery, head trauma, or intracranial tumours.
- Nephrogenic DI can be inherited (X-linked recessive or autosomal dominant/recessive) or acquired due to chronic renal disease, hypercalcemia, or drugs like lithium.
Clinical Features
Patients typically present with polyuria (excessive urination), polydipsia (excessive thirst), and nocturia. Children may present with failure to thrive, hypernatremia, and fever.
Complications
If untreated, DI can cause dehydration, electrolyte imbalance, and, in severe cases, hypovolemic shock.
Pathological Features
Histopathology
Histopathology is not typically applicable in the diagnosis of DI as it is primarily a clinical and biochemical diagnosis.
Serology
- Water Deprivation Test: Gold standard for diagnosis. Demonstrates inability to concentrate urine despite dehydration.
Biochemistry
- Serum Osmolality and Electrolytes: Increased serum osmolality and hypernatraemia.
- Urine Osmolality: Decreased urine osmolality.
Radiological Features
Imaging studies are primarily useful to identify the cause of central DI.
General Features
- Characteristically demonstrates findings related to the underlying cause, such as pituitary masses or changes consistent with sarcoidosis or histiocytosis.
CT
- Non-contrast: May identify calcifications associated with certain granulomatous diseases.
- Contrast-enhanced: May identify pituitary or hypothalamic masses, infiltrative processes, or post-surgical changes.
MRI
- T1WI: Hypothalamic and pituitary lesions can often be visualised. Absence of the normal posterior pituitary bright spot is a supportive but not definitive sign of central DI.
- T1 C+: Enhancing lesions may be visible in the hypothalamus or pituitary gland.
Grading and Staging
Not applicable as diabetes insipidus is not a malignancy and does not have a staging or grading system.
Diagnosis
Diagnosis is typically based on clinical findings and results of the water deprivation test, with imaging used to identify potential underlying causes.
Differential Diagnosis
- Primary Polydipsia: Characterised by excessive water intake driven by an abnormal increase in thirst. Differentiating features include lower serum osmolality and higher urine osmolality compared to DI, along with a history of psychiatric illness in many cases.
- Diabetes Mellitus: Excessive urination and thirst may also be seen in diabetes mellitus due to glucose-induced osmotic diuresis. However, differentiating features include elevated blood glucose levels and presence of glucose in urine, which are not seen in DI.
- Chronic Kidney Disease: May cause polyuria in early stages, but unlike DI, it is often accompanied by elevated serum creatinine, decreased glomerular filtration rate (GFR), and other signs of kidney dysfunction. As the disease progresses, oliguria becomes more common.
- Hypercalcemia: Excessive calcium in the blood can impair the kidney’s ability to concentrate urine, leading to polyuria. However, hypercalcemia can be differentiated by elevated serum calcium levels and may also present with other symptoms such as fatigue, bone pain, or arrhythmias.
- Hypokalemia: Low blood potassium levels can lead to polyuria by impairing the concentration ability of the kidneys. Differentiating features include low serum potassium levels and associated symptoms such as muscle weakness or cramps, constipation, and palpitations.
Management
Management depends on the type of DI:
- Central DI is treated with desmopressin (a synthetic form of ADH).
- Nephrogenic DI is managed by treating the underlying cause, discontinuing offending medications, and maintaining adequate hydration. Certain medications such as thiazide diuretics or NSAIDs may be used in some cases.