Parkinsonism in cirrhosis: pathogenesis and current therapeutic options

Hepatic encephalopathy in alcoholic cirrhosis

Hepatic encephalopathy (HE) is a serious neuropsychiatric complication of cirrhosis in alcoholic patients that is characterized clinically by personality changes, sleep abnormalities, and impaired motor coordination, as well as cognitive dysfunction progressing to stupor and coma. Procedures used for diagnosis and grading of HE include neurologic assessment, electroencephalography, psychometric testing, and use of the critical flicker frequency test. Neuropathologically, HE in cirrhosis is principally a disorder of neuroglia characterized by Alzheimer type II astrocytosis and activation of microglia. However, thalamic and cerebellar neuronal pathologies have been noted as well as lesions to globus pallidus and substantia nigra, leading to a condition known as "parkinsonism in cirrhosis." Multiple mechanisms have been proposed to account for the pathogenesis of HE in cirrhosis, including the neurotoxic actions of ammonia and manganese (normally removed via the hepatobiliary route), impaired brain energy metabolism, central proinflammatory mechanisms, and alterations of both excitatory and inhibitory neurotransmission. Treatment of HE in cirrhosis continues to rely on ammonia-lowering strategies such as lactulose, antibiotics, probiotics and l-ornithine l-aspartate with nutritional management consisting of adequate (but not excessive) dietary protein and vitamin B1 supplements. l-DOPA may improve parkinsonian symptoms. Liver transplantation leads to recovery of central nervous system function in the majority of cases.

The liver-brain axis in liver failure: neuroinflammation and encephalopathy

Systemic inflammation is common in liver failure and its acquisition is a predictor of hepatic encephalopathy severity. New studies provide convincing evidence for a role of neuroinflammation (inflammation of the brain per se) in liver failure; this evidence includes activation of microglia, together with increased synthesis in situ of the proinflammatory cytokines TNF, IL-1β and IL-6. Liver-brain signalling mechanisms in liver failure include: direct effects of systemic proinflammatory molecules, recruitment of monocytes after microglial activation, brain accumulation of ammonia, lactate and manganese, and altered permeability of the blood-brain barrier. Ammonia and cytokines might act synergistically. Existing strategies to reduce ammonia levels (including lactulose, rifaximin and probiotics) have the potential to dampen systemic inflammation, as does albumin dialysis, mild hypothermia and N-acetylcysteine, the latter two agents acting at both peripheral and central sites. Minocycline, an agent with potent central anti-inflammatory properties, reduces neuroinflammation, brain oedema and encephalopathy in liver failure, as does the anti-TNF agent etanercept.

Manganese and the brain

Manganese (Mn) is an essential trace metal that is pivotal for normal cell function and metabolism. Its homeostasis is tightly regulated; however, the mechanisms of Mn homeostasis are poorly characterized. While a number of proteins such as the divalent metal transporter 1, the transferrin/transferrin receptor complex, the ZIP family metal transporters ZIP-8 and ZIP-14, the secretory pathway calcium ATPases SPCA1 and SPCA2, ATP13A2, and ferroportin have been suggested to play a role in Mn transport, the degree that each of them contributes to Mn homeostasis has still to be determined. The recent discovery of SLC30A10 as a crucial Mn transporter in humans has shed further light on our understanding of Mn transport across the cell. Although essential, Mn is toxic at high concentrations. Mn neurotoxicity has been attributed to impaired dopaminergic (DAergic), glutamatergic and GABAergic transmission, mitochondrial dysfunction, oxidative stress, and neuroinflammation. As a result of preferential accumulation of Mn in the DAergic cells of the basal ganglia, particularly the globus pallidus, Mn toxicity causes extrapyramidal motor dysfunction. Firstly described as "manganism" in miners during the nineteenth century, this movement disorder resembles Parkinson's disease characterized by hypokinesia and postural instability. To date, a variety of acquired causes of brain Mn accumulation can be distinguished from an autosomal recessively inherited disorder of Mn metabolism caused by mutations in the SLC30A10 gene. Both, acquired and inherited hypermanganesemia, lead to Mn deposition in the basal ganglia associated with pathognomonic magnetic resonance imaging appearances of hyperintense basal ganglia on T1-weighted images. Current treatment strategies for Mn toxicity combine chelation therapy to reduce the body Mn load and iron (Fe) supplementation to reduce Mn binding to proteins that interact with both Mn and Fe. This chapter summarizes our current understanding of Mn homeostasis and the mechanisms of Mn toxicity and highlights the clinical disorders associated with Mn neurotoxicity.