A 66-year-old man, who presented with paroxysmal weakness of the left lower limb during activities for half a month, was admitted to a local hospital in December 2021 ( Figure 1 ). The patient reported one to two transient episodes per day with a duration of 2-10 min for each transient episode. The patient had a history of well-managed hypertension and a 1-year history of RA. This patient presented with pain in the major joints of the extremities without minor arthritis of the extremities and extra-articular manifestations, and he was treated with tripterygium glycoside tablets (three times per day, 25 mg a time) and triamcinolone tablets (8 mg/day). He stopped taking these medicines half a month before admission to the hospital because it was not responding to his joint pain. Magnetic resonance imaging (MRI) examination showed atypical enhancement of the pia meningeal, whereas stereoscopic meningeal biopsy only indicated non-specific inflammatory changes ( Figures 1 ,  2A ). The patient was transferred to our hospital in January 2022 owing to a poor diagnosis ( Figure 1 ).
On examination, the patient was in good general condition without headache, psychiatric behavior, cognitive dysfunction, speech dysfunction, seizures, movement disorder, consciousness disorder, and autonomic dysfunction. Neurological examination was rigorously normal except for a positive Babinski sign on the left lower limb. Musculoskeletal examination revealed only slight tenderness of large joints of extremities. Hyperintensity was observed in the leptomeninges of the right frontal and parietal lobes, and subtle hyperintensity was observed in the left frontal and parietal lobes as indicated in the diffusion-weighted image (DWI) ( Figure 3A ), contrast-enhanced T1-weighted sequences ( Figure 3B ), and contrast-enhanced fluid-attenuated inversion recovery (FLAIR) sequences ( Figure 3C ) after examination through brain MRI. No irregularity was observed in intracranial arteries through magnetic resonance angiography (MRA). The patient underwent 18F-FDG PET/CT scan, showing hypermetabolism of bilateral frontal-parietal meninges and adjacent cortex ( Figure 2B ). Electroencephalography (EEG) did not show any changes in brain activity. Cardiac color Doppler ultrasound, ECG, and CT examinations of the whole abdomen and lung were normal.
A lumbar puncture examination was performed on the 3rd day after admission ( Figure 1 ). Cerebrospinal fluid (CSF) analysis showed increased protein content (0.477 g/L), high pressure (260 mm H2O), and elevated cell count (60 cells/mm3, with high levels of lymphocytes,  Table 1 ). Notably, concentrations of lactic acid, glucose, and chloride were normal, whereas viral and fungal markers and culture results using CSF were negative. Analysis showed positive RF (96.3 IU/mL) and ACPA (>800 U/mL). In addition, C-reactive protein (CRP, 13.30 mg/L) was increased, while the erythrocyte sedimentation rate (ESR, 14 mm/h) was normal. Flow cytometry showed that the concentration of serum IL-6 (37.51 pg/mL) was significantly increased ( Table 1 ). Cell-based assay (CBA) showed the presence of NMDAR Abs IgG in CSF (titer 1:1,  Figure 2D ), while the Abs were absent in serum ( Figure 2C ). Moreover, other autoimmune Abs (ANA, ENA, ANCA, anti-thyroid, anti-AMPAR1, anti-AMPAR2, anti-LG1, anti-CASPR2, anti-GABABR, anti-DPPX, anti-IgLON5, anti-GABAAR alpha1, anti-GABAAR beta3, anti-GlyRalpha1, anti-GluR5, anti-D2R, anti-Neurexin3alpha, and anti-GAD65) were not observed. Serum concentrations of C3, C4, IgA, IgG, and IgM were normal. Analysis showed negative results for infectious pathogens, such as herpesviruses, Mycobacterium tuberculosis, HIV, syphilis, and Lyme disease ( Figure 1 ).
The patient underwent a composite evaluation, and a diagnosis of RM with positive NMDAR Abs was proposed. He received IVIg (0.4 g/kg.d) for 5 days from the 7th day after admission. Notably, the left lower limb weakness was alleviated after the second IVIg treatment. The patient received oral prednisone acetate (20 mg/day) and azathioprine (150 mg/day) to prevent the recurrence of RM ( Figure 1 ). Analysis of the lumbar puncture, some biochemical indexes, and MRI examination were conducted after 7 days of IVIg treatment. CSF analysis showed that the protein content (0.185 g/L) was restored to the normal level, and the pressure (185 mmH2O) and the cell count (20 cells/mm3) were decreased compared with pre-treatment values. CRP level (1.0 mg/L) was normal, and IL-6 level (9.62 pg/mL) was significantly lower than that before treatment. Serum RF (57.5 IU/mL) and ACPA (>800 U/mL) were positive ( Table 1 ). The titer of NMDAR Abs in CSF was 1:1, similar to that observed before treatment ( Table 1 ). Only subtle hyperintensity of the bilateral frontal and parietal lobes was observed in DWI ( Figure 3D ), contrast-enhanced T1-weighted sequences ( Figure 3E ), and contrast-enhanced FLAIR sequences ( Figure 3F ). The patient was generally in good condition, and RM did not relapse during the 1-month follow-up. The patient was instructed to reduce the prednisone dose to 5 mg per day by 5 mg every 2 weeks ( Figure 1 ). After 5 months, the patient was hospitalized for follow-up and was in good physical condition ( Figure 1 ). Indicators of abnormal levels included IL-6 in serum (14.79 pg/mL) and CSF (6.16 pg/mL), ACPA in serum (>800 U/mL) and CSF (2.8 U/mL), and NMDAR Abs in CSF (1:1), while other indicators were basically normal ( Table 1 ). Considering that the IL-6 and ACPA levels of the patient were still abnormal, he was suggested to increase the dosage of prednisone acetate tablet to 10 mg per day ( Figure 1 ).