Tuesday, July 11, 2017

Type 2 RTA pathophysiology, notes and mnemonic

Hello! This post is on type 2 renal tubular acidosis.

What causes Type 2 RTA?
Defect in proximal bicarbonate reabsorption - resulting in a hypokalemic hyperchloremic metabolic acidosis.

The defect in proximal reabsorption of filtered HCO3-  in effect leads to decreased proximal NaCl reabsorption and a tendency for salt wasting. This causes hyperaldosteronism -  leading to increased K secretion by the distal nephrons.

What is the urine pH in proximal RTA?

Why is it variable? (Another version of the same doubt from study group 5: Why is the urine acidic even though there is a defect in "bicarbonate reabsorption"?)

To explain this, here is a key point: You must remember that even though bicarbonate is not getting reabsorbed in the proximal tubule, the distal tubule is still working. The distal tubule can generate NH4 and excrete acid.

Untreated patients have metabolic acidosis but are able to excrete the daily acid load (from the distal tubule). These patients have lost most of their bicrabonate in the urine and the concentration has fallen sufficiently so that all of the filtered bicarbonate can be reabsorbed - urine pH will be low.

In patients treated with an alkali, the increasing serum bicarbonate concentration is enough to produce a filtered bicarbonate load that exceeds the reduced proximal reabsorptive capacity. In this case - urine pH will be high.

So you see why the urine pH is variable in type 2 RTA :)

How is it diagnosed?
Using the same principle - When you give bicarbonate, initially, the pH will be low - and then it will become high.

Which conditions are associated with type 2 RTA?

I group them into 3 categories:

PROximal is associated with PROteins
Multiple myeloma

proxiMal is associated with Metals
Heavy metal poisoning (Lead, mercury, cadmium)
Wilsons disease
Fanconi syndrome

proxiMal is associated with Medicines
Tenofovir (Most common cause)
Tetracyclines (outdated)

Fanconis syndrome
Glycogen storage disease, type 1
Renal transplant rejection
Hypocalcemia and vitamin D deficiency

Treatment: Bicarbonate administration

Extra points:
In contrast to type 1 RTA, nephrocalcinosis and nephrolithiasis does not occur, and this is due to normal levels of urinary citrate in this condition (in contrast to type 2) and the ability to acidify the urine which increases the solubility of calcium phosphate.

That's all!
Here's a nice advice that I read: Always be kind, for everyone you meet is carrying a heavy burden.

Related posts:
Renal tubular acidosis types mnemonic
Type 1 RTA pathophysiology, notes and mnemonic

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