| Summary: | Abstract Background The pursuit for clarity in diagnostic and treatment pathways for the complex, chronic condition of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) continues. This systematic review raises a novel question to explore possible overlapping aetiology in two distinct conditions. Similar neurocognitive symptoms and evidence of d-lactate producing bacteria in ME/CFS raise questions about shared mechanisms with the acute condition of d-lactic acidosis (d-la). Methods d-la case reports published between 1965 and March 2016 were reviewed for episodes describing both neurological symptoms and high d-lactate levels. Fifty-nine d-la episodes were included in the qualitative synthesis comparing d-la symptoms with ME/CFS diagnostic criteria. A narrative review of d-la mechanisms and relevance for ME/CFS was provided. Results The majority of neurological disturbances reported in d-la episodes overlapped with ME/CFS symptoms. Of these, the most frequently reported d-la symptoms were motor disturbances that appear more prominent during severe presentations of ME/CFS. Both patient groups shared a history of gastrointestinal abnormalities and evidence of bacterial dysbiosis, although only preliminary evidence supported the role of lactate-producing bacteria in ME/CFS. Limitations Interpretation of results are constrained by both the breadth of symptoms included in ME/CFS diagnostic criteria and the conservative methodology used for d-la symptom classification. Several pathophysiological mechanisms in ME/CFS were not examined. Conclusions Shared symptomatology and underlying microbiota–gut–brain interactions raise the possibility of a continuum of acute (d-la) versus chronic (ME/CFS) presentations related to d-lactate absorption. Measurement of d-lactate in ME/CFS is needed to effectively evaluate whether subclinical d-lactate levels affect neurological symptoms in this clinical population.
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