Diabetes Insipidus

Question 1

What is Diabetes Insipidus?

Answer 1

Hyposecretion of, or insensitivity to the effects of antidiuretic hormone (ADH)

ADH is synthesised in the hypothalamus and transported in neurosecretory vesicles to the posterior pituitary. There it is released into the circulation, dependant on the plasma osmolality. Its deficiency or failure to act causes an inability to concentrate urine in the distal renal tubules, leading to the passage of lots of dilute urine (about 3 litres of urine with a very low osmolarity). The effective increase in ADH can be due to a decreased release (Neurogenic DI) or can be due to a resistance to the normal ADH levels at the kidney (nephrogenic).

Question 2

What are the causes of Diabetes Insipidus?

Answer 2

Causes:
Neurogenic:
Common – Pituitary/Hypothalamic Tumour, Head Injury (damaging the hypothalamic-pituitary region), Intracranial Surgery
Others - Infection (Meningitis, TB), sarcoidosis (if infiltrating into posterior pituitary), Congenital/Familial, Idiopathic.

Nephrogenic:
Common - Hypercalcaemia, Lithium Toxicity
Other – Familial, Sickle Cell Disease, Amyloidosis, Hypokalaemia, Idiopathic, CKD, Polycystic Kidney Disease

Question 3

What are the symptoms of Diabetes Insipidus?

Answer 3

Symptoms:
Polyuria and polydipsia (>3l/day)
Nocturia
Hypernatremia (Raising ICP causing irritability, lethargy, hyperreflexia, muscle spasm)
Dehydration

Risk Factors:

Specific Questions to ask:
Ask about headaches or vision problems – Indicates a pituitary lesion
Ask about symptoms of other endocrinopathies that may be linked to a pituitary lesion

Differentials:
Diabetes mellitus – Glucose levels and risk factors e.g. lifestyle
Renal failure – History of renal problems
Primary polydipsia, usually psychogenic in origin
The other form of Diabetes insipidus – Be that nephrogenic or neurogenic

Question 4

What are the signs of Diabetes Insipidus?

Answer 4

End of the bed:
Dehydration - Reduced Skin turgidity
Hands:
Dehydration – Tachycardia, Hypo-tension with Postural Drop, Increased Capillary refill time, Cool peripheries
Face:
Visual field defects may indicate a previous or existing pituitary lesion in patients with central DI
Dehydration - Dry Mucous Membranes, Sunken Eyes
Abdomen:
Bladder can be grossly enlarged and palpable
Legs:
Dehydration - Cool Peripheries

Question 5

What investigations will you order in Diabetes Insipidus?

Answer 5

Bedside:
Urine Dipstick – Looking for any signs of renal failure or to exclude diabetes mellitus as the cause of polyuria.
24-hour urinary collection >3l
Blood Glucose

Bloods:
Urine and Serum Osmolarity - A low urine osmolality (<300 mOsm/kg) in conjunction with high serum osmolality or elevated sodium strongly suggests Diabetes insipidus
U&E – Na will be raised in most patients due to dehydration, but may be normal if patient is drinking to meet their losses. Hypokalaemia, Hypokalaemia can be a cause of nephrogenic diabetes insipidus. Urea/Nitrogen will be raised in dehydration
Ca – Hypercalcaemia can be a cause in nephrogenic
Serum Glucose/HbA1c – If indicated by history or capillary blood glucose

Imaging:
MRI head/anterior pituitary function tests if neurogenic DI suspected

Special:
Water Deprivation Test – Diagnostic test. Patient is deprived of fluids for 8 hours while measuring urine osmolarity. Osmolarity should rise with reduced fluid intake (saving water). If it does not this is a positive test

Question 6

What is the treatment of Diabetes Insipidus?

Answer 6

Resuscitation:
Treat any hypernatremia
Treat any dehydration
Lots of fluids

Medical:
Desmopressin - ADH analogue
Optimise treatment of any underlying cause including removal of any tumours