The purpose of the study is to evaluate if there is a specific association between the presence of anti Rach antibodies in the CSF and the presence of a cogntive disorder in myasthenic patients. Moreover the investigator wants to study if there is a link between the presence of Anti RACH antibodies in myasthenia and Alzheimers's disease.

For that, the investigator will recruit myasthenic patient with cognitive disorder that has undergo a diagnostic process including lombar punction for memory trouble in Nice memory center as well as Alzheimer's patient having go through the same process.

The study will consist in one additionnal blood draw. Anti RACH antibodies will be analyzed in historical CSF stored in biocollection and serum collected for the study.

LCS of healthy control will also be analyzed.

Inclusion criteria

For patient with myasthenia :

1. adult person,
2. Diagnosis of anti-AChR positive autoimmune myasthenia gravis, confirmed by clinical and biological data, and categorized in class I to IV according to the Myasthenia Gravis Foundation of America (MGFA) classification,
3. Mild or major neurocognitive disorder (DSM-5 criterion) having benefited from a diagnosis at the CMRR with CSF biomarker dosage and agreement to bio-collect residual CSF,
4. Agreeing to sign the free and informed consent,
5. Affiliate or beneficiary of a social security system.

For patient with Alzheimer Disease :

1. adult person,
2. Mild or major neurocognitive disorder (DSM-5 criterion) having benefited from a diagnosis at the CMRR with CSF biomarker dosage and agreement to bio-collect residual CSF,
3. Neurocognitive disorder only linked to Alzheimer's disease (IWG-2 criterion): typical or atypical clinical form with biomarkers of Alzheimer's disease in the CSF;
4. Agreeing to sign the free and informed consent,
5. Affiliate or beneficiary of a social security system.

For healty control :

1. absence of memory complaint,
2. absence of neurocognitive disorder,
3. Having agreed to carry out analyzes as part of research, on these CSF samples stored in the biobank of the Institute of Translational Neurology in Münster (Germany)

Exclusion Criteria:

For patient with myasthenia :

1. Person who does not have sufficient command of the French language to understand, read and write, to take neuropsychological tests;
2. Need to use the routine complementary CSF tube to carry out additional diagnostic explorations as part of routine care,
3. Vulnerable people are defined in articles L1121-5 to -8 ( Pregnant women, parturients and breastfeeding mothers, persons deprived of their liberty by a judicial or administrative decision, persons hospitalized without consent under articles L. 3212-1 and L. 3213-1 who do not fall under the provisions of article L. 1121-8, and persons admitted to a health or social establishment for purposes other than research/Adults who are subject to a legal protection measure or who are unable to express their consent.),

For patient with Alzheimer Disease :

1) vulnerable people are defined in articles L1121-5 to -8 ( Pregnant women, parturients and breastfeeding mothers, persons deprived of their liberty by a judicial or administrative decision, persons hospitalized without consent under articles L. 3212-1 and L. 3213-1 who do not fall under the provisions of article L. 1121-8, and persons admitted to a health or social establishment for purposes other than research/Adults who are subject to a legal protection measure or who are unable to express their consent.)