It was also noted that all studies had an unclear or high overall risk of bias and that all evidence was of low or very low quality. Alpha lipioc acid (ALA) with benfothiamine is a proposed pathogenic treatment for painful diabetic neuropathy only. The results of two systematic reviews state that oral ALA produced no clinically significant benefit, intravenous ALA administered over the course of three weeks may improve symptoms and that long term treatment has not been investigated. A 2008 literature review concluded that, "based on principles of evidence-based medicine and evaluations of methodology, there is only a 'possible' association [of celiac disease and peripheral neuropathy], due to lower levels of evidence and conflicting evidence. There is not yet convincing evidence of causality." A 2019 review concluded that "gluten neuropathy is a slowly progressive condition. About 25% of the patients will have evidence of enteropathy on biopsy (CD [celiac disease]) but the presence or absence of an enteropathy does not influence the positive effect of a strict gluten-free diet." Stem-cell therapy is also being looked at as a possible means to repair peripheral nerve damage, however efficacy has not yet been demonstrated.