Hepatic Encephalopathy: Ammonia Neurotoxicity and Astrocyte

Hepatic encephalopathy (HE) results from accumulation of neurotoxic substances — primarily ammonia (NH3) — that the failing liver cannot clear via the urea cycle. In cirrhosis, portal-systemic shunting diverts ammonia-rich portal blood past hepatocytes directly into systemic circulation. Ammonia crosses the blood-brain barrier and is metabolized by astrocytes (the only CNS cells with glutamine synthetase), converting glutamate + NH3 → glutamine. Glutamine is osmotically active, causing astrocyte swelling (cerebral edema) and oxidative stress via mitochondrial dysfunction. This astrocyte swelling disrupts the astrocyte-neuron unit, impairing glutamatergic and GABAergic neurotransmission. Additionally, ammonia activates the neurosteroid-GABA pathway, increasing inhibitory GABAergic tone (explaining the sedation and obtundation). Precipitating factors include GI bleeding (protein load → ammonia production by gut bacteria), infection/sepsis (catabolism), constipation (prolonged colonic ammonia absorption), dehydration, hypokalemia and metabolic alkalosis (promote renal NH3 production and NH3 over NH4+ favoring BBB crossing), sedatives (synergistic CNS depression), and dietary protein excess. West Haven classification grades HE from grade 0 (minimal/subclinical) through grade IV (coma). Treatment targets the gut-liver-brain axis: lactulose (osmotic laxative that acidifies colonic pH, converting NH3 → NH4+ which cannot be absorbed,...