Isolated Leptomeningeal Enhancement in Anti-N-methyl-D-aspartate Receptor Encephalitis: A case report

Background Magnetic resonance imaging findings have been reported to lack specificity. The hippocampus, frontal lobe, white matter, basal ganglia, and even spinal cord can be involved. However, isolated leptomeningeal enhancement is rare in anti-NMDA receptor encephalitis. Case We report a 17-year-old boy who presented with a 2-week history of mild fever and progressive encephalopathy with behavioral and psychological abnormalities. Finally, a diagnosis of anti-NMDA receptor encephalitis was made. Initial magnetic resonance imaging results revealed diffuse contrast enhancement of the leptomeninges without parenchymal lesions. After treatment with high-dose intravenous methylprednisolone, plasma exchange, and intravenous immunoglobulin, the enhancement showed substantially decrease at the 3-week follow-up.

2 Abstract Background Magnetic resonance imaging findings of anti-N-methyl-D-aspartate (anti-NMDA) receptor encephalitis have been reported to lack specificity. The hippocampus, frontal lobe, white matter, basal ganglia, and even spinal cord can be involved. However, isolated leptomeningeal enhancement is rare in anti-NMDA receptor encephalitis.

Case presentation
We report a 17-year-old boy who presented with a 2-week history of mild fever and progressive encephalopathy with behavioral and psychological abnormalities. Finally, a diagnosis of anti-NMDA receptor encephalitis was made. Initial magnetic resonance imaging results revealed diffuse contrast enhancement of the leptomeninges without parenchymal lesions. After treatment with high-dose intravenous methylprednisolone, plasma exchange, and intravenous immunoglobulin, the enhancement showed substantially decrease at the 3-week follow-up.

Conclusions
This case is unusual because isolated leptomeningeal enhancement is rare in anti-NMDA receptor encephalitis. The present case suggests that isolated diffuse leptomeningeal enhancement could be one of the imaging findings in patients with anti-NMDA receptor encephalitis.

Background
Magnetic resonance imaging findings of anti-N-methyl-D-aspartate (anti-NMDA) receptor encephalitis have been reported to lack specificity. The hippocampus, frontal lobe, white matter, basal ganglia, and even spinal cord can be involved [1]. Herein, we report an unusual magnetic resonance imaging finding in a 17-year-old patient with anti-NMDA receptor encephalitis. This case is unusual because isolated leptomeningeal enhancement is rare in anti-NMDA receptor encephalitis [1]. The present case suggests that isolated leptomeningeal enhancement could be one of the imaging findings in patients with anti-NMDA receptor encephalitis.  Figure 1B). An additional movie file shows this in more detail (see Additional file 2).

Discussion
A previous study reported that 5 of 106 patients with anti-NMDA receptor encephalitis had meninges enhancement [2]; however, the authors did not report whether these 5 patients had concomitant parenchymal lesions. Furthermore, the pattern of leptomeningeal enhancement was not reported. The present case suggests that isolated diffuse leptomeningeal enhancement could be one of the imaging findings in patients with anti-NMDA receptor encephalitis.
Leptomeningeal enhancement has received renewed attention in other types of immune-mediated diseases of the central nervous system such as multiple sclerosis [3][4][5]. Histopathologic analysis has shown clustered inflammatory cells in the subarachnoid space in the area of in vivo leptomeningealcompartment enhancement [3]. These cells are sometimes organized into ectopic follicle-like structures containing B and T cells, and they are often found around meningeal vessels; however, this 4 produces non-diffuse enhancement of leptomeninges in multiple sclerosis, unlike the present case.
In studies on meningitis (including viral etiology), leptomeningeal enhancement has also been reported because of the disruption of the blood-brain barrier in the inflamed meninges [6]. Although no evidence of infection was found in the present case after a thorough test of cerebrospinal fluid, concurrent viral meningitis cannot be completely ruled out. However, the meningeal enhancement in the present case was not consistent with viral etiology because the grade of enhancement should be low and subtle in viral meningitis, particularly on contrast-enhanced images based on T1-weighted [6,7].
Leptomeningeal enhancement based on T1-weighted imaging has various limitations, including excessive vascular enhancement, flat images, and inflow effect, which suggest that the observed enhancement might only be adjacent enhanced vessels rather than enhanced meninges [8]. However, repeat contrast-enhanced T1-weighted imaging using the same scan protocol suggested definite leptomeningeal enhancement in the present case. with written informed consent from patient's legal representatives.

Consent for publication
The legal representatives have consented to submission of this case report to the journal, and we 5 have obtained a written informed consent.

Availability of data and materials
All data related to this case report are contained within the manuscript.

Competing interests
The authors report no conflict of interest.