Distal renal tubular acidosis (RTA) was diagnosed on the basis of the patient's presentation of profound weakness, history of renal colic, ECG findings consistent with severe hypokalemia, renal ultrasound showing nephrocalcinosis, and arterial blood gas analysis showing profound metabolic acidosis.
Initial laboratory testing revealed a sodium level of 138 mEq/L (reference range, 135-145 mEq/L), potassium level of 1.6 mEq/L (reference range, 3.6-5 mEq/L), chloride level of 116 mEq/L (reference range, 98-109 mEq/L), bicarbonate level of 10 mEq/L (reference range, 22-31 mEq/L), anion gap of 12 mEq/L (reference range, 6-16 mEq/L), blood urea nitrogen level of 12 mg/dL (reference range, 7-21 mg/dL), and a creatinine level of 0.83 mg/dL (reference range, 0.6-1.2 mg/dL). These values were consistent with a nonanion gap metabolic acidosis, with associated hypokalemia and no evidence of chronic kidney disease. Her pregnancy-induced hyperemesis gravidarum was undoubtedly the inciting factor for the progression of hypokalemia to such a critical level.
The ECG (Figure 1) from the patient showed very prominent U waves, which are classically associated with hypokalemia; in cases where the U wave is taller than the T wave, severe hypokalemia is usually present.
The chest radiograph (not shown) showed subtle diffuse osteopenia. The patient's history of renal colic would be consistent with a history of urinary stones that often occurs in patients with RTA resulting from alkaline urine and hypercalciuria. The renal ultrasound (Figure 2) showed hyperechoic regions in the renal medulla consistent with nephrocalcinosis.
The arterial blood gas analysis demonstrated a profound metabolic acidosis with a pH of 7.25, further confirming the diagnosis. The type of RTA was characterized by looking at the urine pH, which in this patient was 7. This is consistent with type 1 RTA, also known as "distal RTA."
Distal RTA is a rare renal disorder characterized by a nonanion gap hyperchloremic acidosis and hypokalemia. In this condition, the alpha intercalated cells of the cortical collecting duct of the distal nephron fail to secrete acid into the urine. This failure of acid secretion leads to an inability to acidify the urine to a pH < 5.5.
Because renal excretion is the primary means of eliminating acid from the body, a tendency toward systemic acidemia is noted. This leads to the clinical features of type 1 RTA, which include:
Normal anion gap hyperchloremic metabolic acidosis
Hypokalemia (from multiple mechanisms, but often severe during periods of stress)
Nephrolithiasis (related to an inability to acidify urine, hypercalciuria, and low urinary citrate)
Loss of calcium from bones (which can cause rickets in children and osteomalacia in adults)
During periods of stress caused by illness, or in this particular case the vomiting of hyperemesis gravidum, patients can have episodes of profound hypokalemia, resulting in flaccid paralysis, rhabdomyolysis, cardiac arrest, and even death.
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