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Chapter 6: Disorders of the kidney 253

secondary hyperaldosteronism and hypokalaemic dioxide). RTA is classified into three main types each
alkalosis. There is hypercalciuria and nephro- with differing pathophysiology.
calcinosis. Type1(DistalRTA)causesincludeprimary,idiopathic
Bartter type II syndrome is an autosomal recessive and connective tissue disorders such as Sj¨ ogren’s and

defect in the gene encoding the ROMK channel. rheumatoid arthritis. Impaired H secretion in the
+
Decreased potassium excretion causes hyperkalaemia, distal tubule leads to progressive accumulation of acid
which interferes with the action of the NKCC2 pump. in the body. Even when bicarbonate levels fall to as low
This results in a similar syndrome of sodium loss, de- as 10 mmol/L or below, the urine remains relatively
hydration and hypercalciuria as Bartter type I; how- alkaline (pH ≥ 5.5). If untreated, persistent metabolic
ever, hypokalaemia only occurs after treatment with acidosis leads to increased mobilisation of calcium
sodium supplements. phosphate from bone, and this causes hypercalciuria
Distal convoluted tubule: Thiazide diuretics act from promoting the formation of renal stones. Type 1 RTA
within the lumen of the distal tubule binding to the is treated with bicarbonate replacement, with correc-
chloride site of the NCCT channel reducing sodium and tion of hyperkalaemia.
water reabsorption. Calcium reabsorption is increased Type 2 (Proximal RTA) usually occurs in the con-
leading to hypercalcaemia. text of a general proximal tubule disorder called Fan-
Gittleman syndrome is an autosomal recessive muta- coni syndrome, which may be familial inherited, id-

tion in the gene encoding the NCCT channel. As there iopathic or acquired, e.g. due to drugs, heavy metals
isincreasedsodiumintheurinereachingthecollecting or myeloma. The proximal tubule fails to reabsorb
duct there is exchange of sodium for potassium in the the HCO 3 ,so this passes to the distal tubules. Once
−
collecting duct resulting in a hypokalaemic metabolic plasma bicarbonate levels fall to about 12–16 mmol/L,
alkalosis. the distal tubule is able to reabsorb the amount of fil-
Collecting duct: At the collecting duct there is reabsorp- teredbicarbonate, so severe acidosis does not occur.
tionofsodiuminexchangeforpotassiumexcretion.This The main problems occur due to the loss of other
is under the influence of aldosterone which increases substances such as amino acids and phosphate. Type
sodium (and hence water) reabsorption. Spironolactone 2istreated with bicarbonate, thiazide diuretic and
and amiloride affect this exchange and hence increase potassium bicarbonate or potassium-sparing diuret-
urinary water and sodium loss. In contrast to other ics. Fanconi syndrome is treated with large doses of
diuretics, these cause potassium reabsorption and are vitamin D.
termed potassium-sparing diuretics. The permeability Type 4 is hypoaldosteronism or resistance to the ef-
of the collecting duct is under the influence of anti di- fects of aldosterone such as caused by mild renal
uretic hormone (ADH or vasopressin). failure, interstitial nephritis or potassium-sparing di-
In diabetes insipidus (see also page 445) the action of uretics like spironalactone. It appears that aldosterone

ADH is impaired. This results in excessive water loss deficiency causes hyperkalaemia, which is associated
in the urine. with a mild metabolic acidosis. Urinary pH usually
remains appropriately low (<5.3). Mineralocorticoid
deficiency is corrected using fludrocortisone. Hyper-
Renal tubular acidosis
kalaemia may be life-threatening and the underlying
Definition disorder often shortens life expectancy.
Renaltubularacidosis(RTA)isametabolicacidosisofre-
nal origin, characterised by hyperchloraemic metabolic
acidosis with a normal anion gap. Hyperuricaemic (gouty) nephropathy

Aetiology/pathophysiology Definition
RTAmay be inherited or acquired. Under physiologi- Disorders of uric acid metabolism may cause renal dis-
cal conditions, the kidneys help to maintain acid–base easeduetoachronicnephropathy,anacutenephropathy
balance, together with the lungs (which remove carbon or through the formation of uric acid stones.