Hepatic encephalopathy (HE) in dogs is a neurological syndrome caused by toxins, primarily ammonia, building up in the bloodstream when the liver can’t filter them properly. These toxins cross into the brain and disrupt normal function, producing symptoms that range from subtle behavioral changes to seizures and coma. It’s not a disease on its own but a consequence of underlying liver dysfunction or abnormal blood flow that bypasses the liver entirely.
How Ammonia Damages the Brain
Under normal circumstances, the liver converts ammonia, a byproduct of protein digestion, into urea through what’s called the urea cycle. Urea is harmless and gets excreted by the kidneys. When the liver is severely damaged or when blood is rerouted around it, ammonia accumulates in the bloodstream instead. Ammonia crosses the barrier between the blood and the brain easily, and once it’s there, specialized brain cells called astrocytes attempt to neutralize it. In doing so, these cells swell and malfunction, disrupting nerve signaling throughout the brain.
Ammonia isn’t the only culprit. HE also involves inflammation, oxidative stress, and changes in blood clotting. But ammonia remains the central driver, and measuring blood ammonia levels is one of the primary ways veterinarians evaluate the condition.
What Causes It
Veterinary medicine classifies three broad types of HE based on the underlying problem. Type A results from sudden, severe liver failure in a dog with no prior liver disease. Type B, the most common form in dogs, results from portosystemic shunts: abnormal blood vessels that divert blood around the liver so it never gets filtered. Type C involves chronic liver diseases like cirrhosis or portal hypertension, where the liver is damaged over time and can no longer keep up.
Portosystemic Shunts
Congenital portosystemic shunts are the leading cause of HE in dogs. These are blood vessel abnormalities present from birth that route blood from the intestines directly into the general circulation, skipping the liver altogether. Most dogs with congenital shunts are diagnosed before age one or two, though some aren’t caught until they’re much older. Certain breeds are predisposed, including Yorkshire Terriers, Maltese, Irish Wolfhounds, and Labrador Retrievers.
Chronic Liver Disease
Dogs with chronic hepatitis, cirrhosis, or other progressive liver conditions can also develop HE, but these dogs tend to be older and often have jaundice (yellowing of the skin and eyes) alongside neurological symptoms. Their liver tissue has deteriorated to the point where it simply can’t process ammonia efficiently, even though blood flow to the liver may be normal.
Signs to Watch For
The earliest signs of HE are often so subtle that owners chalk them up to a dog just being “off.” Grade 1 HE involves mild, fluctuating lethargy and decreased mental alertness. Your dog might seem confused, slow to respond to commands, or mildly disoriented. These episodes can come and go, often worsening after meals (when protein digestion produces more ammonia).
Grade 2 brings more noticeable changes: increasing drowsiness, personality shifts like unusual aggression, mild unsteadiness on the feet, inappropriate behaviors such as house soiling, and a loss of normal social interactions. Some dogs develop anxiety or seem to stare blankly at walls or ceilings.
As HE progresses, the signs become harder to miss. Dogs may press their heads against walls or corners, wander aimlessly, circle compulsively, or develop apparent blindness. Excessive drooling is common. Fine tremors, twitching, and full seizures can occur, particularly with chronic or recurring HE. In the most severe cases, dogs become stuporous or comatose.
One important pattern: symptoms often flare after high-protein meals, gastrointestinal bleeding (which floods the gut with protein from blood), infections, or exposure to sedative medications. Metabolic alkalosis, a shift in the body’s acid-base balance, can also trigger episodes.
How It’s Diagnosed
Diagnosis relies on a combination of neurological signs and lab work pointing to liver dysfunction. Blood ammonia testing is the most direct tool. A recent study established that healthy dogs typically have blood ammonia levels below 30 micrograms per deciliter. Levels above this threshold, especially in a dog showing neurological signs, strongly suggest HE. That said, a normal ammonia reading doesn’t completely rule it out, since ammonia levels can fluctuate.
Blood ammonia can also be elevated by conditions unrelated to the liver, including kidney disease, gastrointestinal bleeding, vitamin B12 deficiency, and urinary tract infections. Male dogs tend to have higher baseline ammonia levels than females. Your veterinarian will consider these possibilities before landing on a diagnosis.
Bile acid testing provides additional evidence of liver dysfunction. This involves measuring bile acids in the blood before and after a meal. Dogs with cirrhosis show median resting bile acid concentrations near 99 micromoles per liter, far above the normal range. Elevated post-meal bile acids are particularly sensitive for detecting liver problems. Imaging studies like ultrasound or CT scans help identify portosystemic shunts or structural liver changes.
Treatment and Management
Treatment targets two goals: reducing the ammonia load reaching the brain and addressing the underlying liver problem when possible.
Reducing Ammonia
Lactulose is the cornerstone medication. It’s a synthetic sugar that works in the gut by making the intestinal environment more acidic, which traps ammonia so it gets excreted in stool rather than absorbed into the bloodstream. It also alters the bacterial population in the colon, reducing the number of ammonia-producing organisms. For dogs in acute crisis, lactulose can be given as a retention enema. Antibiotics are sometimes added to further suppress ammonia-producing bacteria in the intestines.
Dietary Protein Adjustment
Protein management is essential because protein digestion is the primary source of intestinal ammonia. The goal isn’t to eliminate protein, which dogs still need, but to choose the right sources and amounts. Research comparing soy-based diets to meat-based diets in dogs with portosystemic shunts found that soy protein significantly lowered blood ammonia levels and better supported liver function. Dairy and vegetable proteins produce less ammonia during digestion than meat proteins, making them the preferred options for dogs with HE. Your veterinarian or a veterinary nutritionist can design a diet that provides adequate nutrition while minimizing ammonia production.
Surgical Correction of Shunts
For dogs with congenital portosystemic shunts, surgery to gradually close the abnormal vessel is often the best long-term solution. This redirects blood back through the liver, restoring normal ammonia processing. The outcomes are generally positive: in a study of 50 dogs that developed neurological complications after shunt surgery (a known risk), 90% survived beyond six months. Neurological signs resolved by discharge in 48% of these dogs, and among those who still had symptoms at discharge, 67% eventually recovered. Seizures were the most persistent complication, recurring in about half the dogs that experienced them post-surgery.
Dogs without shunts, whose HE stems from chronic liver disease, rely on long-term medical management. Their prognosis depends heavily on the severity and progression of the underlying liver condition.
Triggers That Can Worsen Episodes
Even well-managed dogs can have HE flare-ups when certain triggers arise. High-protein meals are the most obvious one. Gastrointestinal bleeding is particularly dangerous because digested blood delivers a large protein load to the intestines. Infections anywhere in the body increase inflammation, which worsens HE independently of ammonia levels. Sedatives and certain pain medications can amplify the brain-suppressing effects of circulating toxins. Constipation allows more time for ammonia absorption in the gut. Recognizing and avoiding these triggers is a major part of keeping a dog with HE stable between episodes.