J-difference GABA-edited MRS reveals altered cerebello-thalamo-cortical metabolism in patients with hepatic encephalopathy
Authors: Zöllner Helge J. , Thiel Thomas A. , Füllenbach Nur-Deniz , Jördens Markus S. , Ahn Sinyeob , Häussinger Dieter , Butz Markus , Wittsack Hans-Jörg , Schnitzler Alfons , Oeltzschner Georg 
Published: September 2022

Hepatic encephalopathy (HE) is a common neurological manifestation of liver cirrhosis. Clinical symptoms range from subtle attention deficits and motor disturbance to stupor and hepatic coma in the most severe cases. HE pathophysiology is characterized by an increase of ammonia in the brain due to impaired clearance in the cirrhotic liver. This results in disturbed glutamate-glutamine homeostasis as ammonia is increasingly metabolized by glutamine synthetase. Ammonia accumulation furthermore causes increased oxidative stress and disrupts neurotransmitter balance, including the GABAergic and glutamatergic systems. Clinical symptoms in the motor domain suggest that the cerebello-thalamo-cortical system plays a key role in HE. The aim of this study is to investigate metabolic abnormalities in the cerebello-thalamo-cortical system of HE patients using GABA-edited MRS. The study also investigates links between metabolite levels, disease severity, critical flicker frequency (CFF), motor performance scores, and blood ammonia levels.

GABA-edited MRS was performed in 35 participants (16 controls, 19 patients (3 minimal HE, 16 HE)) on a clinical 3T MRI system. MRS voxels were placed in the right cerebellum, left thalamus, and left motor cortex. GABA+ levels were estimated from the GABA-edited difference spectra using Gaussian fitting with the Gannet software. Levels of other metabolites of interest (glutamine, glutamate, myo-inositol, glutathione, total choline, total NAA, and total creatine) were assessed using linear-combination modeling in LCModel. Creatine- and water-referenced levels were reported to minimize biases of both reference standards. Group differences in metabolite levels and associations with clinical metrics were tested. Modeling uncertainty estimates of metabolite levels (Cramer-Rao Lower Bounds) were included as statistical weighting factors.

GABA+ levels were significantly increased in the cerebellum of patients with HE. GABA+ levels in the motor cortex were significantly decreased in HE patients, and correlated with the CFF (r = 0.73; p < .05) and motor performance scores (r = −0.65; p < .05). Well-established HE-typical metabolite patterns (increased glutamine, decreased myo-inositol and total choline) were confirmed in all three regions. These alterations were closely linked to clinical metrics. Increased glutathione levels were found in the thalamus and motor cortex. Explorative analysis indicated increased aspartate levels in all three regions and decreased scyllo-inositol levels in the motor cortex.

In summary, our findings provide further evidence for alterations in the GABAergic system in the cerebellum and motor cortex in HE. These changes were accompanied by characteristic patterns of osmolytes and oxidative stress markers in the cerebello-thalamo-cortical system. These metabolic disturbances are a likely contributor to HE motor symptoms in HE.

Graphical Abstract

In patients with hepatic encephalopathy, GABA+ levels in the cerebello-thalamo-cortical loop are significantly increased in the cerebellum and significantly decreased in the motor cortex. GABA+ levels in the motor cortex strongly correlate with critical flicker frequency (CFF) and motor performance score (pegboard test tPEG), but not blood ammonia levels (NH3).

Highlights

Motor deficits in HE may originate from the cerebello-thalamo-cortical system

Altered GABAergic neurotransmission plays a critical role in the pathophysiology of HE

J-difference GABA-edited MRS can be used to study in vivo GABA+ levels

Cerebellar and motor cortical GABA+ levels were significantly altered in HE

GABA+ levels in the motor cortex strongly correlated with clinical metrics