Equine Hepatic Encephalopathy Study Guide

Overview and Clinical Importance

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome resulting from severe hepatic dysfunction or portosystemic shunting in horses. It represents a critical complication of liver failure and is characterized by abnormal mentation, behavioral changes, and motor dysfunction secondary to the accumulation of neurotoxins, primarily ammonia, that the failing liver cannot adequately metabolize.

HE is a high-yield topic for the NAVLE because it requires integration of hepatic physiology, neurology, toxicology, and clinical medicine. Understanding the pathophysiology, recognizing clinical signs, and implementing appropriate diagnostic and therapeutic strategies are essential competencies for equine practitioners.

Pathophysiology

The pathophysiology of HE is multifactorial, but hyperammonemia is considered the central mechanism. The liver normally converts ammonia, a byproduct of protein metabolism and bacterial urease activity in the GI tract, into urea via the urea cycle. When hepatic function fails or portal blood bypasses the liver through shunts, ammonia accumulates in systemic circulation.