Hepatic encephalopathy (HE) develops in certain liver disorders associated with portosystemic shunting or fulminant hepatic failure. Clinical signs vary but involve disturbed sensorium ranging from mild dullness and an inability to respond to basic commands to overt abnormalities, including propulsive circling, head pressing, aimless wandering, weakness, ataxia, amaurosis (unexplained blindness), ptyalism, dementia, behavior change (eg, aggression), collapse, seizures, and coma. Although the pathophysiologic mechanisms of HE are not completely known, synergistic effects between the failure of the liver to detoxify ammonia and other endogenous substances, increased cerebral inflammatory cytokines, impaired brain perfusion, development of neuronal edema, hypoxia, mitochondrial dysfunction, development of neuroglycopenia, and oxidative injury, are important interdependent mechanisms. Increased production of reactive oxygen and nitrogen oxide species are thought to trigger protein and RNA modifications that deleteriously influence brain function. The integrated concept of HE explains episodic variability and heterogeneous precipitating factors that correlate with diverse clinical scenarios.

Ammonia plays a key role in HE; it is thought to sensitize the brain to numerous other precipitating factors/mediators. However, blood ammonia concentrations and cerebral ammonia concentrations are discordant, disqualifying blood ammonia as a simplistic measure of HE. In healthy animals, most ammonia is removed by hepatocytes, converted into amino acids or urea, and excreted via kidneys in urine. In liver failure or portosystemic shunting, blood ammonia concentrations increase because of inadequate hepatic detoxification. In the circulation, ammonia can also be excreted by the kidneys (tubular secretion) and used for glutamine synthesis in skeletal muscle (temporary ammonia detoxification). This latter mechanism is why maintenance of lean body mass (muscle) is essential in patients with hepatic insufficiency that are susceptible to hyperammonemia and HE. A number of clinical scenarios and mechanisms can augment blood ammonia concentrations and precipitate HE, including dehydration (prerenal/renal azotemia), alkalemia, hypokalemia, hypoglycemia, catabolism, infection, PU/PD, anorexia, constipation, hemolysis, blood transfusion, GI hemorrhage, high dietary protein, and various drugs (eg, benzodiazepines, tetracyclines, antihistamines, methionine, barbiturates, organophosphates, phenothiazines, diuretics [overdosage], metronidazole, and certain anesthetics).

Ammonia can influence multiple neurotransmitter systems directly (chemical influence) and indirectly (altered substrate availability for transmitters). There is substantial evidence that astrocytes play an important role in the pathogenesis of HE. Ammonia and other endogenous products, inflammatory cytokines, and hyponatremia (associated with portal hypertension) induce astrocyte swelling that can lead to brain edema and herniation most common in acute liver failure and acute severe HE.

Treatment of acute HE is aimed at providing supportive therapy and rapidly reducing the neurotoxins produced in the GI tract. Severely encephalopathic animals may be semicomatose or comatose. Benzodiazepines and other sedatives should not be administered. Food should be withheld until neurologic status improves. Fluids (2.5% dextrose and 0.45% saline with potassium chloride and vitamin B complex added) should be administered to correct dehydration, electrolyte, and acid-base imbalances. Lactated Ringer's solution should be avoided because hepatic failure may thwart lactose metabolism and cause lactic acidosis. Cleansing enemas of warm soapy water are followed by retention enemas of either lactulose or lactitol (3 parts lactulose or lactitol to 7 parts water at 20 mL/kg), 10% povidone-iodine solution (20 mL/kg, rinsed well after 10–15 min dwell), neomycin (22 mg/kg mixed with water), or diluted metronidazole (7.5 mg/kg suspended in water at 10–20 mL/kg) given every 8 hr until the animal is neurologically responsive. Retention enemas should be maintained for 15–20 min by use of a Foley catheter. Administration (oral or rectal) of live Lactobacillus and Bifidobaccilus organisms (live yogurt cultures or probiotic products) also can assist in displacing ammonia-producing microbes. Metronidazole, neomycin, and povidone-iodine solutions can directly alter colonic bacterial flora, decreasing populations of ammonia-producing organisms. However, care is warranted in using neomycin with concurrent inflammatory bowel disease because increased systemic uptake can increase potential for renal and otic (cochlear) toxicity. Metronidazole must be restricted to ≤7.5 mg/kg every 8 hr (combined oral and rectal dosing); higher dosages confer risk of iatrogenic neurotoxicity (vestibular signs initially).

Once the animal is stabilized, treatment is aimed at preventing recurrence. Protein-modified restricted diets should be fed (see Hepatic Disease in Small Animals: Nutrition in Hepatic Disease in Small Animals). Oral probiotic yogurt and lactulose (0.1–0.5 mL/kg, PO, bid-tid initial dose) can be used, with initial dose titrated to achieve several soft, pudding-like stools per day. Feeding milk may achieve a similar effect in some animals. The goal of administration of nondigestible carbohydrate is to promote fermentation in the gut. Concentrated probiotic organisms can prevent other bacteria from growing and replicating through substrate competition and pH-related (acid) growth inhibition or mechanical cleansing (catharsis) induced by fermentation products. These effects diminish uptake of ammonia, inflammatory and oxidative substrates, lipopolysaccharide, and other toxic enteric products contributing to HE.

In recalcitrant HE, antibiotic therapy, preferably metronidazole (7.5 mg/kg, PO, bid) or amoxicillin (13–15 mg/kg, PO, bid) rather than neomycin, is recommended. Antibiotic therapy works synergistically to reduce enteric toxins with indigestible carbohydrates.

Clinical signs of HE can be exacerbated by GI bleeding, infection, glucocorticoid use (enhanced catabolism of tissue protein), hypoglycemia, neoplasia, fever, azotemia or dehydration (increased BUN increases enteric ammonia production), constipation (increased generation and absorption of colonic neurotoxins), metabolic alkalosis (favoring both production of ammonia by the kidneys and uptake of ammonia across the blood-brain barrier), and use of diazepam and barbiturates (synergetic neuroinhibitors). Use of H₂-receptor antagonists and sucralfate, control of fever and infection, proper hydration, and minimal (if any) use of anticonvulsant medications can help alleviate HE complications.

Last full review/revision March 2012 by Sharon A. Center, DVM, DACVIM