Posterior pituitary: diabetes insipidus
The posterior pituitary gland
secretes two hormones, arginine vasopressin (AVP) and oxytocin.
Diabetes insipidus (DI) is defined
as the inappropriate passage of large volumes of dilute urine (<300mOsm/L).
Due to either deficiency in AVP production (cranial DI) or resistance to its
actions at the kidney (neph-rogenic DI). The most common cause of DI is primary
deficiency of AVP production (i.e. cranial DI). This may be acquired or
inherited in origin
Children present with polydipsia,
polyuria, and nocturia, which must be distinguished from more common causes.
Infants may exhibit failure to thrive, fever, and constipation. Other symptoms
may be related to the underlying cause, e.g. headache, visual acuity/visual
field impairment.
When suspected, assessment of 24hr
urinary volume and osmolality under conditions of ad libitum fluid intake should be undertaken. Serum osmolali-ty,
U&E (Na+), and blood glucose should also be measured.
Blood hypertonicity (serum
osmolality >300mOsm) with inappropriate urine hypotonicity (urine osmolality
<300mOsm) should be demonstrated. Diabetes mellitus and renal failure should
be excluded.
A water deprivation test and
assessment of responses to exogenously administered ADH is required to diagnose
the type of DI. Other tests to determine the underlying cause of DI will also
be needed (e.g. cranial MRI imaging).
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