Rare mixed dementia: A case report

BACKGROUND Autoimmune encephalitis (AE) is a rare and recently described neuroinflammatory disease associated with specific autoantibodies. Anti-leucine-rich glioma inactivated 1 (anti-LGI1) encephalitis is a rare but treatable type of AE discovered in recent years. Alzheimer's disease (AD) is a degenerative brain disease and the most common cause of dementia. AD may undergo a series of pathological physiological changes in brain tissue 20 years before the onset of typical symptoms. The stage of mild cognitive impairment (MCI) that occurs during this process, known as MCI due to AD, is the earliest stage with clinical symptoms. MCI is typically categorized into two subtypes: Amnestic MCI (aMCI) and non-aMCI. CASE SUMMARY This report describes a patient with rapid cognitive impairment, diagnosed with anti-LGI1 antibody-mediated AE and aMCI, and treated at Peking University Shenzhen Hospital in March 2023. The patient was hospitalized with acute memory decline for more than 3 months. Both the cerebrospinal fluid and serum were positive for anti-LGI1 antibodies, biomarkers of AD coexisting in the patient's cerebrospinal fluid. Following combination treatment with immunoglobulin therapy and glucocorticoid, plus inhibition of acetylcholinesterase, the patient's cognitive function significantly improved. Throughout the 3-month follow-up period, a sustained improvement in cognitive function was observed. The results of serum anti-LGI1 antibody were negative. CONCLUSION This case has raised awareness of the possible interaction between AE and early AD (including MCI due to AD), and alerted clinicians to the possibility of concurrent rare and common diseases in patients presenting with cognitive impairment.