Clinical presentation

• Incidental finding on blood test.

• Collapse (see Chapter 13).

Definition

Plasma sodium <130 mmol/L.

Approach to the problem

To distinguish between water overload (excessive intake or inadequate output) and sodium depletion (excessive salt loss).

Other features to look for:

• Pigmented scrotum (see Chapter 11) or ambiguous genitalia (see Chapters 8 and 10) suggesting congenital adrenal hyperplasia.

• Plasma potassium measurement.

• Palpable kidney, or mass (renal vein thrombosis, adrenal haemorrhage).

• Family history of renal problems.

• Blood pressure (liable to be raised in renal rather than adrenal disorders).

Differential diagnosis

May be informed by consideration of:

• Postnatal age (low glomerular filtration rate in the first few days of life).

• Gestational age (renal immaturity resulting in salt loss).

Water overload:

• latrogenic: This is the commonest cause, and may start before birth, i.e. secondary to excess maternal intravenous (IV) fluid administration.

• Renal failure.

• Syndrome of inappropriate antidiuretic hormone secretion (SIADH) (intracranial pathology: asphyxia, meningitis; intrathoracic pathology: pneumothorax).

Salt depletion:

• Immaturity (extreme preterm infant usually from the end of first week of life).

• Congenital adrenal hyperplasia (usually days 4–10).

• Congenital adrenal hypoplasia.

• Renal impairment, e.g. polyuric phase of acute tubular necrosis (ATN), congenital nephrotic syndrome, Bartter syndrome (hyperprostaglandin E syndrome, associated with life-threatening hypokalaemia). Congenital renal abnormality (there may be a history of polyhydramnios secondary to excessive urine output in utero).

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