Case Studies
 

Distal Renal Tubular Acidosis in a Terrier Presented for Dehydration, Lethargy and Hepatopathy.

Distal Renal Tubular Acidosis: Border Terrier

VETERINARIAN SPECIALTY CASE STUDY

Renal tubular acidosis (RTA) is a group of rare disorders that lead to metabolic acidosis with normal anion gap.  In dogs, cases are often “idiopathic,” whereas in cats, dRTA has been associated with pyelonephritis and hepatic lipidosis.

By Mickila Collins, DVM
Diplomate American College of Veterinary Internal Medicine

1. Referral

Ostin, a 17 month old male neutered terrier presented to the Internal Medicine service for dehydration, severe lethargy and hepatopathy. While his owner were out of town he developed diarrhea, extreme PUPD, lethargy and inappropriate urination in the house. Earlier in the year Ostin had been evaluated for a shifting leg lameness, elevated CK, AST and ALT and mild hypoglycemia. While a definitive diagnosis had not been obtained, the lameness had recently been under control. Prior to referral a UA and UMIC was performed by the primary care veterinarian which revealed a USG of 1.007 and 2 + proteinuria. Complete bloodworm revealed marked electrolyte abnormalities K 2.6 m Eq/L, Chloride 122mEq/L, Mg 2.5mg/dL and elevated liver enzymes ALT 549 U/L, AST (596 U/L).

2. Examination

Austin’s general physical examination was largely unremarkable aside from a mildly elevated temperature (102.1), mild dehydration, generalized weakness and quiet mentation

3. Diagnostics

An in house blood gas analysis was performed on admission and revealed a marked metabolic acidosis (pH 7.017; HCO3 8.0, PCO2 31.9, BE -22, PCV 65 and TS 11). The following day an abdominal ultrasound was performed and revealed some subtle changes to both kidneys including reduced corticomedullary distinction and renal pelvis mineralization. Serial blood gas and electrolyte monitoring revealed a persistent acidemia and worsening hypokalemia (pH 7.157, K 2.18). Leptospirosis serology was submitted and found to be negative. The urinalysis revealed an inappropriate pH of 7.0 and the urine culture returned negative.

4. Diagnosis

After ruling out infectious causes (Leptospirosis, pyelonephritis), toxic causes (based on owner’s history) and neoplasia a clinical diagnosis of Idiopathic Distal Renal Tubular Acidosis was made.

5. Treatment

Ostin was started or oral potassium citrate, sodium bicarbonate and benazepril. His dose was titrated until his blood pH and potassium levels were in normal range and then he was discharged from the hospital.

6. Discussion

Renal tubular acidosis (RTA) is a group of rare disorders that lead to metabolic acidosis with normal anion gap. There are 3 types. Distal RTA (dRTA) or type 1 is the most common and is characterized by reduced hydrogen ion secretion in the distal renal tubules.

Proximal RTA or type 2 is characterized by reduced bicarbonate reabsorption from the proximal tubules. pRTA is observed as part of Fanconi syndrome. Type IV RTA is a type of dRTA associated with hypoaldosteronism or aldosterone resistance that results in hyperkalemia.

The exact pathophysiology of dRTA is not clearly understood. Urine cannot be maximally acidified because of impaired H+ secretion (and thus HCO3- generation) in collecting ducts, and urine pH is typically > 6.0 despite moderately to markedly decreased plasma HCO3- concentration. Urinary tract infection by urease-producing bacteria must be ruled out before the diagnosis of dRTA can be made.

In dogs, cases are often “idiopathic”, whereas in cats, dRTA has been associated with pyelonephritis and hepatic lipidosis. In people, cases are inherited or associated with a variety of disorders such as nephrocalcinosis, multiple myeloma, systemic lupus erythematous, toxins, tubulointerstitial nephropathies, autoimmune disorders, and certain medications..

Signs are non-specific and may relate to any associated disease. Diagnosis is based on the presence of hyperchloremic metabolic acidosis (normal anion gap) with a urinary pH that is inappropriately high (pH > 6.0) for the degree of systemic acidosis in the absence of bacterial urease modification of urine. Hypokalemia may be present because of increased renal excretion in some affected animals. dRTA has been associated with demineralization of the skeleton (due to buffering of excess hydrogen ions) and nephrolithiasis (due to hypercalciuria from bone resorption) in dogs.

Diagnosis may be difficult if a blood gas analysis and/ or urinalysis is not performed. The clinical signs and presenting complaints can be vague so it is important to keep an open differential list and complete a thorough diagnostic evaluation in all patients.

7. Recovery

Ostin is currently doing very well on his continued medications. Interestingly, the intermittent shifting leg lameness and elevated liver and muscle enzymes have completely resolved. In retrospect, we felt that some of his long standing issues could have been related to his undiagnosed distal renal tubular acidosis. In people, there are reports of severe muscle and bone pain and muscle cramping. His blood gas and electrolytes are monitored every 3 months. Complete bloodworm and blood pressure analysis is also performed every 6 months.


Photo of Border Terrier is not of Ostin.

Animal Specialty Group

DVM, Diplomate American College of Veterinary Internal Medicine

Dr. Mickila Collins received her Doctor of Veterinary Medicine degree from Ross University in 2005, completing her clinical year at Louisiana State University. After graduation, she completed a one-year rotating internship, followed by an internal medicine internship, both at ASG.