Case of the Month #55 Sudden Onset Generalised Weakness

Renal tubular acidosis (RTA) is a disorder where the kidneys fail to properly acidify urine, leading to a buildup of acid in the blood (metabolic acidosis). There are several types of RTA, classified based on the specific defect in the renal tubules. These types include: 

1. Type 1 RTA (Distal RTA) 

Defect: Impaired acid secretion in the distal tubule, particularly in the alpha-intercalated cells, which normally excrete hydrogen ions (H⁺) into the urine. 

Mechanism: The kidneys can't excrete enough acid, leading to metabolic acidosis with a normal anion gap. 

Key Features: 

Hypokalaemia (low potassium levels due to decreased potassium reabsorption). 

Urine pH typically remains high (above 5.5) even in the presence of metabolic acidosis, which is abnormal. 

Bone demineralization (due to chronic acidosis and hypercalcuria). 

Aetiologies: Can be primary (idiopathic) or secondary to conditions like autoimmune diseases (e.g., Sjögren's syndrome) or medications (e.g., amphotericin B). 

 

2. Type 2 RTA (Proximal RTA) 

Defect: Impaired bicarbonate reabsorption in the proximal tubule, leading to a loss of bicarbonate (HCO₃⁻) in the urine. 

Mechanism: The kidneys can't reabsorb bicarbonate effectively, resulting in a non-anion gap metabolic acidosis. 

Key Features: 

Hypokalaemia (low potassium) due to increased distal tubular potassium secretion as compensation. 

Urine pH is typically low (acidic), especially in the early stages. 

Fanconi syndrome (a complex of proximal tubular dysfunction) may occur, leading to phosphate wasting, glucosuria, aminoaciduria, and low serum phosphate. 

Often associated with bone disease and growth failure in children. 

Aetiologies: Can be primary or secondary to diseases such as multiple myeloma, Wilson’s disease, cystinosis, and certain medications (e.g., tenofovir). 

 

3. Type 3 RTA 

This term is used less frequently now, as it refers to a combination of both distal and proximal tubular dysfunction. It's considered more of a hybrid form of RTAs (a defect in both bicarbonate reabsorption and acid secretion). 

Mechanism: Results in both impaired acidification of urine and impaired reabsorption of bicarbonate. 

Key Features: Presents with features of both Type 1 and Type 2 RTA, but it's rare and not commonly recognized as a distinct clinical entity anymore. 

 

4. Type 4 RTA (Hyperkalaemic RTA) 

Defect: Impaired aldosterone secretion or action in the distal tubule, leading to defective potassium and hydrogen ion excretion. 

Mechanism: Due to low aldosterone (hypoaldosteronism), potassium and hydrogen ions aren't properly excreted, leading to hyperkalaemia (elevated potassium) and acidosis. 

Key Features: 

Hyperkalaemia (elevated potassium). 

Urine pH is typically low (acidic). 

Often associated with renal insufficiency, diabetes, ACE inhibitors, or aldosterone antagonists (e.g., spironolactone). 

Aetiologies: Conditions that lead to reduced aldosterone, such as Addison's disease, diabetic nephropathy, chronic kidney disease, or the use of certain medications.