Hyperammonaemia in four cats with renal azotaemia

Abstract

Hyperammonaemia is well reported in animals with advanced hepatic disease and portosystemic shunts, but is unreported in cats with renal disease. This case series describes four cats with severe renal azotaemia in which elevated ammonia levels were detected during the course of treatment. In two cases hyperammonaemia was detected at a time when neurological signs consistent with encephalopathy had developed. This raises the possibility that hyperammonaemia may play a role in the development of encephalopathy in cats with renal azotaemia.

Ammonia is produced by cellular metabolism within the body and detoxified by organs such as the liver, kidneys, muscle, brain and intestines by incorporation into the amino acid glutamine.^1,2 In the intestinal tract, ammonia is formed primarily through bacterial degradation of amines, amino acids and purines, by the action of bacterial urease on urea and, secondarily, by intestinal catabolism of glutamine.^1,2 The ammonia produced by these processes passes through the portal circulation to the liver, where ammonia is detoxified by either conversion to urea in the mitochondrial urea cycle or by consumption in the creation of glutamine.^1,2

Hyperammonaemia develops if the body’s ammonia load is excessive, portal blood from the intestines bypasses the liver or with abnormalities in urea cycle function. ² Hyperammonaemia has been reported in cats in association with portovascular abnormalities^1,3–9 and in association with arginine or cobalamin deficiencies.^10–14 Hyperammonaemia has not been reported in the cat in association with renal disease. In humans, hyperammonaemia has been reported with inherited organic acid disorders and with inherited defects of the urea cycle, fatty acid oxidation and amino acid transporters. It has also been found in humans in association with arginine deficiency, hypovolaemic shock, congestive cardiac failure, portosystemic shunting, liver disease, medication administration, haematological malignancy, urinary infection with urease-producing bacteria, gastrointestinal haemorrhage, glycine irrigation and cachexia with high-protein feeds. ² Human studies have revealed a positive correlation between breath ammonia levels and blood urea and creatinine levels in humans with renal disease undergoing haemodialysis, ¹⁵ but hyperammonaemia was not found in a study of humans with chronic kidney disease (CKD). ¹⁶ Here, we document high blood ammonia levels in four cats with severe renal azotaemia.

Materials and methods

The cats were seen in a Sydney emergency and referral clinic between January and April 2012.

Ammonia levels were obtained from blood samples collected for the monitoring of azotaemia. Once collected, blood was transferred to lithium–heparin gel tubes, which were centrifuged and processed immediately using standard protocol ¹⁷ on an in-house Idexx Vettest chemistry analyser. The maximum urea level measurable by this machine was 46.4 mmol/l (129.97 mg/dl); in cases in which this was exceeded and in which it was considered relevant to clinical outcome, sample dilution with sterile 0.9% sodium chloride was performed. The reference intervals for this machine were as follows: urea 5.7–12.9 mmol/l (14.29–36.13 mg/dl), creatinine 71.0–212.0 μmol/l (0.8–2.4 mg/dl), total calcium 1.95–2.83 mmol/l (7.8–11.32 mg/dl), albumin 23.0–39.0 g/l (2.3–3.9 g/dl), alanine aminotransferase (ALT) 12.0–130.0 U/l and ammonia 0–95 μmol/l. Electrolyte and ionised calcium analysis was performed using a standard protocol ¹⁷ on an Idexx VetStat analyser. The reference interval for potassium on this machine was 3.5–5.8 mmol/l (mEq/l), and the reference interval for ionised calcium was 1.12–1.38 mmol/l (4.48-5.52 mg/dl).

Case 1

A 12-year-old female neutered crossbreed cat was referred for management of CKD. This had been diagnosed previously on the basis of marked azotaemia with inappropriately dilute urine. Diagnostic procedures performed on admission included systolic blood pressure (SBP) measurement (130 mmHg), blood biochemistry and haematology, abdominal ultrasound and urinalysis with urine culture. Results confirmed the diagnosis of International Renal Interest Society (IRIS) stage 4 CKD. ¹⁸ On admission, the cat was dehydrated, and a biochemical profile run at this time revealed a creatinine level of 1140 μmol/l (12.88 mg/dl) and a urea level of >46.4 mmol/l (>129.97 mg/dl). The cat was treated in hospital with intravenous fluids adjusted to physiological requirement, as well as omeprazole 1 mg/kg PO q24h (Losec; Bayer Healthcare), ondansetron 0.5 mg/kg PO q24h (Zofran Zydis; Aspen Pharmacare), an aluminium hydroxide-based phosphate binder (Phosphobind Paste; Hi-Perform Veterinary Products [no longer available]) and nutritional support. The cat was discharged after a week with a creatinine level of 618 μmol/l (6.98 mg/dl) and was managed after this on an outpatient basis. At re-examination 7 days later, the cat’s creatinine level was 933 μmol/l (10.54 mg/dl). Subcutaneous fluid therapy administration was added to the therapeutic regime.At a recheck appointment 8 days later, the creatinine level had increased to 961 μmol/l (10.86 mg/dl). Nine days after this it had reached 1030 μmol/l (11.64 mg/dl). Urea was 42.4 mmol/l (118.76 mg/dl) and plasma potassium was 4.6 mmol/l. The cat was eating, but vomiting occasionally. A fasting blood ammonia level was run at this time, and was 157 μmol/l. The frequency of subcutaneous fluid administration was increased and the cat continued to be managed as an outpatient for 3 months before being euthanased.

Case 2

A 13-year-old female neutered crossbreed cat was referred for management of IRIS stage 4 CKD. ¹⁸ Diagnostic procedures performed on admission included SBP measurement (132 mmHg), biochemistry and haematology, abdominal ultrasound and urinalysis with culture. The results were consistent with the prior diagnosis of CKD with concurrent pancreatitis. On presentation, the creatinine level was 1075 μmol/l (12.15 mg/dl) and urea was 72.5 mmol/l (203.07 mg/dl). The cat was managed in hospital with intravenous fluids adjusted to physiological requirement. Medications were administered at a reduced frequency owing to renal disease and included ranitidine 2 mg/kg IV q24h (Zantac; Aspen Pharmacare), buprenorphine 0.02 mg/kg transmucosally q12h (Temgesic; Reckitt Benckiser), mirtazapine 1 mg/kg PO q3d (APO-Mirtazepine; Apotex) and a phosphate binder (Phosphobind Paste; Hi-Perform Veterinary Products [no longer available]). Over the next 48 h the cat demonstrated clinical deterioration in its neurological status, with mental dullness that progressed to mental obtundation. The cat retained the ability to respond to noxious stimuli. It was recumbent with normal spinal reflexes. Pupillary light reflexes were slow; oculocephalic reflexes remained present. Occasional muscle twitches developed. Neurological signs were not lateralising. SBP readings at the time were recorded as being between 90 and 124 mmHg. Biochemistry revealed a creatinine level at 972 μmol/l (10.98 mg/dl) and urea at 63.3 mmol/l (177.3 mg/dl). Potassium levels were increased at 6.2 mmol/l, and total calcium was mildly increased at 2.89 mmol/l (11.56 mg/dl) with an albumin level of 26 g/l (2.6 g/dl). Fasting blood ammonia was 832 μmol/l. The cat was euthanased on humane grounds.

Case 3

A 6-year-old female neutered domestic shorthair cat was seen with a history of polyuria and polydipsia of 3 days’ duration, followed by weakness and lethargy. On examination, the cat was quiet, but responsive. Oral ulceration and dehydration were present, and the cat was able to stand and ambulate a short distance before lying down. A mild, generalised weakness was present, but no other neurological abnormalities were detected. Biochemistry, urinalysis and abdominal ultrasound were performed, which led to a diagnosis of acute renal failure. The cat’s owner declined further diagnostic testing or treatment, preventing further classification of this disease. The cat’s creatinine level was 833 μmol/l (9.41 mg/dl), urea was >46.4 mmol/l (>129.97 mg/dl), potassium was 5.3 mmol/l, ALT was <10U/l and fasting blood ammonia level was 415 μmol/l.

Case 4

A male neutered domestic shorthair cat of unknown age was examined for a history of inappetence, polyuria and polydipsia of 4 days’ duration. The cat was severely dehydrated and hypotensive on presentation (SBP 80 mmHg). Diagnostic procedures performed on admission to our facility included biochemistry and haematology, abdominal ultrasound and urinalysis with urine culture. The results led to a diagnosis of kidney disease, which was later classified as IRIS stage 4 CKD. ¹⁸ The cat’s initial creatinine was 1202 μmol/l (13.58 mg/dl) and urea was >46.4 mmol/l (>129.97 mg/dl). The cat was managed in hospital with intravenous fluids adjusted to physiological requirement, with the administration of omeprazole 1 mg/kg IV then PO q24h (Omeprazole Sandoz IV [Sandoz] followed by Losec [Bayer Healthcare]), maropitant 1 mg/kg SC q24h (Cerenia; Zoetis) and a phosphate-restricted diet (K/d; Hills Pet Nutrition). The cat was discharged 4 days later with a creatinine of 1170 μmol/l (13.22 mg/dl) and urea of 68.3 mmol/l (191.31 mg/dl). Maropitant was stopped at this time and ondansetron 0.5 mg/kg PO q24h (Zofran Zydis; Aspen Pharmacare) was started. SBP was 138 mmHg. The cat was re-admitted 2 days later for dehydration and anorexia with a creatinine level of 1630 μmol/l (18.42 mg/dl) and a urea level of 62.9 mmol/l (176.18 mg/dl). The resting ammonia level at this time was 82 μmol/l. Intravenous fluid therapy was restarted and an oesophagostomy tube was placed. The cat was discharged on the next day and re-examined 8 days later. At this stage, the cat’s creatinine was 692 μmol/l (7.82 mg/dl), urea was 27.4 mmol/l (76.75 mg/dl) and fasting ammonia was 0 μmol/l. The cat was seen again 4 weeks later for weakness after a change of primary carer at home. On examination, the cat was initially alert, but hypersalivating, weak and unable to stand. Cranial and spinal nerve reflexes were normal. Neurological signs were not lateralising. Blood biochemistry revealed that creatinine was 1632 μmol/l (18.44 mg/dl), urea was 100.3 mmol/l (280.94 mg/dl) and potassium was 5.7 mmol/l. SBP at the time was 170 mmHg. The cat was again managed intensively in hospital for the next 3 days, but demonstrated progressive mental obtundation and weakness. Signs remained non-lateralising, and normal cranial and spinal nerve reflexes remained present. The cat became semi-comatose and required turning from side to side. It was still partly responsive to auditory and noxious stimuli. At this time, the cat’s creatinine had reduced to 877 μmol/l (9.91 mg/dl) and urea had decreased to 84.7 mmol/l (237.24 mg/dl); potassium was 6.7 mmol/l and ionised calcium was 1.19 mmol/l (4.76 mg/dl). The fasting blood ammonia level was 264 μmol/l. The owners consented to the cat being euthanased.

Discussion

Hyperammonaemia has been described previously in cats in association with a portovascular abnormality,^1,3–9 an arginine-deficient diet^10–12 and in cats with a deficiency of cobalamin.^13,14 This study now documents hyperammonaemia in four cats with renal disease.

The exact cause of the hyperammonaemia documented here is unclear. Causes of hyperammonaemia include excessive ammonia load, portal blood from the intestines bypassing the liver or dysfunction of the urea cycle. ² In these reported cases we consider hepatic disease or portovascular abnormality to be excluded as a cause of the hyperammonaemia. All cats had a full biochemical analysis and abdominal ultrasound. Biochemical analysis included ALT, alkaline phosphatase, and total bilirubin and albumin levels. These were within the reference intervals in all cases except in cat 3, in which an ALT level was recorded at <10 U/l (reference interval 12–130 μ/l). Although abdominal ultrasonography can be considered an insensitive test for hepatic and portovascular disease,^1,3,4 it does exclude overt hepatic or portal structural abnormalities. Furthermore, the very high levels of urea present in these cases are suggestive of adequate hepatic function, as urea is produced by hepatic metabolism and is reportedly below the reference interval in the majority of cats and dogs with a portosystemic vascular anomaly. ¹

If hepatic and portovascular disease are excluded as causes of hyperammonaemia in these cats, remaining causes include excessive ammonia load and dysfunction of the urea cycle. In humans, increases in ammoniagenesis are seen with increased protein catabolism, increased renal hydrogen ion excretion, renal tubular acidosis, infection with urea-splitting bacteria, hypokalaemia and after protein feeding or gastrointestinal bleeding.^2,19–21 Cats with kidney disease are predisposed to increases in ammoniagenesis through these mechanisms,^22–24 but, presumably, in the majority of cases the increased ammonia generated can still be metabolised adequately to urea by the liver. Dysfunction of the urea cycle with secondary hyperammonaemia has been documented previously in cats fed an arginine deficient diet or with cobalamin deficiency.^10–14 In cats 2 and 4, blood urea and creatinine levels had reduced from values taken 2 (cat 2) and 3 days (cat 4) earlier, while neurological signs developed and progressed. A possible cause of stable or decreasing urea levels with increasing fasting ammonia levels would be a failure of the urea cycle to detoxify ammonia, as occurs with arginine or cobalamin deficiency. Absolute deficiency in arginine would be unlikely in the cats in this study as they were all fed partly or wholly on a commercial (non-arginine deficient) diet. Furthermore, a study of L-arginine levels in cats with CKD has previously revealed an increased L-arginine level when compared with normal cats. ²⁵ Even anorexia would not be expected to cause arginine deficiency as during protein degradation tissue arginine would be released along with other cellular amino acids. ¹¹ Cobalamin deficiency is a possible cause of these findings, and deficiency secondary to kidney disease is well recognised in humans. ²⁶ A study of 35 cats with CKD found significantly lower levels of serum cobalamin and higher levels of serum methylmalonic acid than a control group of healthy cats. ²⁷ Although levels of both serum cobalamin and methylmalonic acid were found to be within the reference intervals in almost all of the cats in that study, cobalamin deficiency is still a potential cause of urea cycle dysfunction in cats with CKD.

Cats 2 and 4 developed progressive neurological disease at the end of their management. There are many potential causes of neurological disease in cats with renal azotaemia. Cats with CKD often develop abnormal potassium and calcium levels, ²⁸ and CKD is the most commonly recognised cause of systemic hypertension, predisposing to hypertensive encephalopathy. ²⁸ Diseases such as cryptococcosis or lymphosarcoma may also cause concurrent renal and central nervous signs.^29,30 Not enough information was obtained in our cases to rule out some of these disorders definitively. However, both cases developed signs consistent with an encephalopathy at a time when, based on clinical findings, uraemic encephalopathy would be an expected finding.

Ammonia crosses the blood–brain barrier readily and at increased concentrations is toxic to the brain.^9,31–33 Signs described in cats with hepatic encephalopathy include hypersalivation, seizures, lethargy, head-pressing, central blindness, ataxia, behavioural change and tremors.^5–9 While ammonia is not the only neurotoxin involved in hepatic encephalopathy, there is a marked similarity between these described signs and those described in uraemic encephalopathy. ²⁸ The exact pathogenesis of uraemic encephalopathy remains unknown. ²⁸ Proposed mechanisms include alterations in the sodium potassium adenosine triphosphate and calcium pumps, arterial hypertension or imbalance of neurotransmitter amino acids in the brain. ²⁸ Detecting hyperammonaemia at a time of renal azotaemia and concurrent neurological dysfunction raises the possibility that ammonia may be one of the toxins involved in uraemic encephalopathy.

Laboratory error has to be considered as a possible cause for these findings. We consider this unlikely as hyperammonaemia correlated with clinical findings in two of the cats. Elevated ammonia was also not necessarily correlated with the most marked levels of azotaemia, which might be expected if there was some previously undescribed cross-reactivity between the Vettest ammonia assay and a uraemic toxin. Using an alternative method of ammonia assay to confirm these findings would, however, be worthwhile in future.

Conclusions

We report hyperammonaemia in four cats with renal azotaemia. While only four cases are reported, we believe this may be a more common phenomenon than recognised previously. We would welcome correspondence with positive or negative findings of hyperammonaemia in association with end-stage renal disease or uraemic encephalopathy.