Whether in the first episode or relapse, the first noticeable symptom of the illness in this boy is abnormal movements and autonomic instability. The comparison of the MRI imaging findings of the two phases demonstrated progressive cerebellar atrophy. Although this contrasts with the onset of typical psychiatric symptoms seen in most anti-NMDAR encephalitis cases, neurologic symptoms, such as abnormal movements or seizures, are considered significant onset symptoms in children [
16‐
18]. Besides, oligoclonal IgG bands were identified, which were also observed in some other children's cases, although the identical syndrome of multiple sclerosis was not observed in this case [
19]. Regarding imaging results, the current patient showed evident cerebellar atrophy, which differed from earlier reported hRSV-associated or anti-NMDAR encephalitis cases [
20,
21]. This case's clinical syndromes were atypical to some extent compared to other reported anti-NMDAR encephalitis cases.
Human orthopneumovirus, commonly known as a human respiratory syncytial virus (hRSV), is a member of the genus Orthopneumovirus in the family
Pneumoviridae, a group of enveloped negative-strand RNA viruses [
22]. There are two accepted subtypes, A and B, within hRSVs, followed by some unclassified strains [
23]. The hRSV was primarily identified as an essential agent for respiratory tract infection in bronchiolitis and pneumonia [
24]. Beyond this, this virus was also found to cause extrapulmonary diseases such as myocarditis, hyponatremia, and encephalitis [
21,
25,
26]. Notably, a single hRSV-infected pediatric case with limbic encephalitis was also reported as positive for NMDAR antibodies (Additional file
3). Nevertheless, the virus was solely detected in the nasopharyngeal aspirate while negative in the CSF [
27]. Otherwise, the presence of this virus in the CSF of some cases with hRSV-associated neurological abnormalities has been reported in some studies. Despite the lack of laboratory evidence yet confirming that the hRSV is neuroinvasive, some in vitro investigations have revealed that this virus can infiltrate the central nervous system and infect resident cells, which might explain why the virus can cause neurological disorders in people[
28‐
30]. Although the mechanisms driving neurological complications caused by hRSV infection are obscure, it is proposed that hRSV-associated encephalopathies can be divided into four types, among which the cytokine storm type is involved [
31]. Definitely, there has been sufficient clinical evidence of an altered profile of cytokines, such as IL-6, IL8, CCL2, and CCL4, in the CSF of hRSV-infected cases [
32]. Interestingly, the altered cytokine levels in the CSF of anti-NMDAR encephalitis cases were also proposed as diagnosis markers [
33]. Whether autoimmune encephalitis represents a novel form of hRSV-infection-related encephalopathy or merely a symptom of the brain's disturbed immunologic balance, hRSV is likely to play an important role in the condition in this case. Nonetheless, the etiology of the relapse, whether due to altered immunological homeostasis in the brain or an unknown trigger, requires more exploration. Considering this disease is prone to recurrence, the occurrence of relapsing encephalitis following the hRSV infection was a high possibility.
Taken together, we first identified the existence of hRSV group B in the CSF of a child with anti-NMDAR encephalitis. The study is limited by the dearth of solid immunological evidence (such as IgM from the onset of symptoms), as IgG merely confirms that the patient was infected with the virus. Even if further clinical records and experimental data on pathogenesis and more similar cases are required for validation, this work provides a starting point for future research on hRSV-associated anti-NMDAR encephalitis.