While most doctors believe that the failure to clear toxins from the blood system in liver disease patients causes HE. There is still controversy over the exact toxins that may be the cause, with many suspecting that ammonia being the most likely toxin. Some studies have indicated that the correlation between serum ammonia levels and HE severity is inconsistent. There is even debate as to which type of ammonia level measurement (i.e., arterial total, venous total, arterial partial or venous partial) might correlate to levels of HE. In 2003, researchers found that venous total ammonia levels correlate with the severity of HE and this correlation could be used to evaluate patients with HE.
The "West Haven Criteria" was developed as a result of the 2003 study and provides the following scale:
Grade 1 - Trivial lack of awareness; euphoria or anxiety; shortened attention span; impaired performance of addition or subtraction.
Grade 2 - Lethargy or apathy; minimal disorientation for time or place; subtle personality change; inappropriate behavior.
Grade 3 - Somnolence to semi-stupor, but responsive to verbal stimuli; confusion; gross disorientation.
Grade 4 - Coma (unresponsive to verbal or noxious stimuli).
The term minimal encephalopathy (MHE) is defined as encephalopathy that does not lead to clinically overt cognitive dysfunction, but can be demonstrated with neuropsychological studies. This is still an important finding, as minimal encephalopathy has been demonstrated to impair quality of life and increase the risk of involvement in road traffic accidents.