SN6
SHORT NOTE
5M
Define Renal Tubular Acidosis. Describe its types, clinical features, diagnosis and treatment.
Definition
Renal tubular acidosis (RTA) is a group of disorders in which the kidney fails to acidify the urine appropriately, producing a normal anion gap (hyperchloraemic) metabolic acidosis despite a relatively preserved GFR.
Types
| Type | Defect | Key features |
|---|---|---|
| Type 1 (distal) | Impaired H⁺ secretion | Hypokalaemia, nephrocalcinosis, rickets, failure to thrive; urine pH high |
| Type 2 (proximal) | Bicarbonate wasting | Often part of Fanconi syndrome; hypokalaemia |
| Type 4 | Aldosterone deficiency/resistance | Hyperkalaemia |
Clinical Features
- Failure to thrive/growth failure, polyuria, polydipsia, and dehydration.
- Rickets/osteomalacia and nephrocalcinosis (distal RTA); muscle weakness (hypokalaemia).
Diagnosis & Management
- Blood gas (normal anion gap acidosis), serum electrolytes, and urine pH.
- Alkali therapy — oral sodium bicarbonate or potassium citrate — to correct the acidosis; potassium supplementation as needed; treat the underlying cause.
When to Suspect Rta
- A child with failure to thrive plus a normal anion gap metabolic acidosis (and no diarrhoea).
- Unexplained rickets, nephrocalcinosis, polyuria or persistent hypokalaemia.
Principles of Treatment
Correcting the chronic acidosis with alkali therapy improves growth, bone health and the associated electrolyte disturbances; distal RTA usually needs smaller alkali doses than proximal RTA, and the underlying cause (e.g. Fanconi syndrome) is treated where possible.
Causes
- Primary/hereditary forms; and secondary to systemic disease.
- Distal RTA — nephrocalcinosis, autoimmune disease; proximal RTA — Fanconi syndrome (cystinosis, Wilson disease, drugs).
Diagnostic Approach
- Confirm a normal anion gap metabolic acidosis; then use serum potassium to narrow: low K → type 1/2, high K → type 4.
- Urine pH: inappropriately high (> 5.5) in distal (type 1) RTA despite acidosis.
- Urinary anion gap and a bicarbonate challenge help separate proximal from distal RTA.
All types give normal anion gap hyperchloraemic acidosis.
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