Presence of Anti-RACH Antibodies and Neurocognitive Disorder in Myasthenic or Alzheimers's Patients.

This study is investigating whether certain antibodies (anti-AChR antibodies) found in patients with myasthenia gravis are also present in people with Alzheimer's disease, and whether they might play a role in memory and thinking problems. **You may be eligible if...** - You are an adult with a confirmed diagnosis of anti-AChR positive autoimmune myasthenia gravis AND a mild or major cognitive disorder, OR - You are an adult with Alzheimer's disease-related cognitive decline confirmed by spinal fluid (CSF) biomarkers, OR - You are a healthy adult with no memory complaints whose CSF samples are stored in a research biobank - You have consented to spinal fluid sample collection for research purposes **You may NOT be eligible if...** - You do not have sufficient French language skills to complete neuropsychological tests - You are a vulnerable person as defined by French law (e.g., pregnant women, people under legal guardianship, patients hospitalized involuntarily) - Your leftover spinal fluid is needed for routine clinical diagnostic purposes Talk to your doctor to see if this trial is right for you.