HE stands for Hepatic Encephalopathy, the “behavioral, psychological, and neurological changes associated with advanced liver disease.” (answers.com)

    Hepatic: “Of, relating to, or resembling the liver. Acting on or occurring in the liver.” (answers.com)Encephalopathy: any disorder or disease of the brain (wordnet.princeton.edu)

    en·ceph·a·lop·a·thy \inˌsefəˈläpə-thē\ (merriam-webster.com)

Damaged livers do not have the capacity to rid the blood of toxins. Those toxins remain in the bloodstream, reaching the brain, and subsequently causing HE.

Petplace.com has a good write-up on HE, which I excerpt here:

    Hepatic encephalopathy is a neuropsychiatric disorder that occurs in animals and people with advanced liver disease. While any severe liver disease can lead to HE, portosystemic shunts in dogs  (where blood is diverted around the liver) are the most likely disorder to produce HE symptoms. About 95% of animals with portosystemic shunts show signs of HE.HE is often expressed in a range of neurologic abnormalities. The first signs are usually behavioral. At first, the changes are subtle. As the disorder progresses, the signs become more obvious. Symptoms may be triggered by a meal. They often come and go, also varying in degree of seriousness.

    A grading system has been modified from human medicine, for use in animals. In this system animals with HE are graded on a scale of 1 to 4.

  • Grade 1. Listlessness, depression, mental dullness, personality changes, excessive urination.
  • Grade 2. Staggering or incoordination, disorientation, compulsive pacing or circling, head pressing, apparent blindness, personality changes, salivation or drooling and excessive urination.
  • Grade 3. Stupor, severe salivation and seizures, although uncommon, are present.
  • Grade 4. Coma
  • Other symptoms may include: collapse or weakness, hyperactivity, head or muscle tremors, and/or  deafness.