There has been an immense improvement in our understanding of immune-related seizures with the development of NSAbs detection technology in recent years. This study aimed to investigate the early predictors of immune-related seizures based on clinical features and neuroimaging, EEG, and NSAbs.
NSAbs, as biomarkers for the early identification of AEs, are valuable for immune-related seizures . Several studies have shown that the positive incidence of NSAbs in the serum of epilepsy patients without genetic, structural, or metabolic etiologies is 15–20% [14,15,16]. Currently, in the diagnostic criteria of AE , three months is the time limit for subacute onset. However, recent studies have shown that the onset of antibody-mediated seizures is occasionally indeterminate, even weeks or months before the diagnosis of an AE. Therefore, it is difficult to define an operational time definition  clinically. In this observational study, we found NSAbs-positive serum or cerebrospinal fluid prevalence in 22.1% of 154 patients with initial “unknown etiology” seizures with a duration of ≤ 6 months. These results suggest that even in patients with epileptic seizures of “unknown etiology” with a period longer than three months, testing for NSAbs is still clinically significant and might increase the recognition of immune-associated epileptic seizures.
A prospective clinical study found that neuronal antibody positivity in adults with new-onset epilepsy was not supportive of an immune etiology . Here, 12/34 (35.2%) of NSAbs-positive patients had only a single seizure without other clinical features of AE. After treatment with ASM alone, the patients exhibited a good prognosis, and no new AE symptoms were observed during long-term follow-up, which was not consistent with the diagnosis of AE. This result showed that in patients with epileptic seizures with initial “unknown etiology,“ NSAbs positivity could not be used as the only basis for immune etiology and needs to be combined with the clinical characteristics of patients, which was consistent with the results of the Zelano et al. .
FBDS is a characteristic manifestation of LGI1 encephalitis. The literature has reported that FDG-PET in patients with FBDS often indicates metabolic abnormalities in the basal ganglia, indicating that FBDS may be subcortical epilepsy [19, 20]. In this group of LGI1 patients, 5/6 (83.3%) developed FBDS at different stages of the disease. One patient had frequent attacks of FBDS in the early stage of the disease. Combination therapy of the two ASMs was reported to be ineffective. During the ictal and interictal periods, EEG showed no epileptiform discharges, blood and cerebrospinal fluid NSAbs were negative, and the APE2 score was 7. Post clinical diagnosis of antibody-negative AE and simultaneous pulse therapy with methylprednisolone, the seizures gradually reduced and eventually disappeared.The APE2 score made up for the insufficiency of NSAbs detection. However, we also found it sometimes difficult to distinguish viral encephalitis from AEs using the APE2 score. NSAbs test results could be used as a good supplement, but the second-generation sequencing technology for viral etiology is not very popular in China. Therefore, diagnosing antibody-negative immune-related seizures must be done carefully to avoid magnification of the diagnosis.
Among the patients diagnosed with immune-related seizures, each subtype of NSAbs was 42.9% for NMDAR, 23.8% for LGI1, 14.3% for GABABR, 14.3% for CASPR2, and 4.7% for CASPR2 and NMDAR double-antibody positive, consistent with previous studies . Also, the sensitivity of NSAbs detection in serum and cerebrospinal fluid samples of patients with different subtypes was variable [21, 22]. The positivity rate of cerebrospinal fluid in NMDAR subtypes was higher than that in serum, while LGI1 and CASPR2 were the opposite; for patients with combined tumors, antibodies were more easily detected in serum. We found that low serum titers (1:10) of NMDAR were insignificant in patients with seizures alone, and serum antibody titers were significantly higher in patients with tumors than those without tumors. It is suggested that detecting NSAbs in cerebrospinal fluid and serum is the best choice in patients with suspected immune-associated epileptic seizures. However, due to patient cooperation and informed consent, cerebrospinal fluid testing is sometimes difficult to implement.
APE2 is a predictive model established based on clinical assessment and related auxiliary examination results and can be easily operated in clinical practice. Husari et al. proposed that NSAbs should be detected in all epilepsy patients with unknown etiology and APE2 score ≥ 4 . This study showed that the APE2 score ≥ 4 accounted for approximately 73.9% of patients with AE. Currently, the clinical application of domestic antibody detection is limited by time and technology. APE2 could be used as an effective supplement to antibody detection in early clinical evaluation, especially patients with “unknown etiology” epilepsy underwent antibody detection before or without antibody detection conditions. Also, patients with APE2 scores ≥ 4 should be tested for NSAbs in cerebrospinal fluid. Better sensitivity and specificity results also indicated that for patients with APE2 score ≥ 4, NASbs detection in CSF and serum could be performed simultaneously; for patients with APE2 score < 4, serum detection alone could be considered.
In this study, we also found that “immune” and “structural” etiologies could occur simultaneously in a patient. Brain MRI is one of the etiological examinations for initial epilepsy or encephalitis, especially the abnormal signal of T2/FLAIR in one or both medial temporal lobes could indicate AE . Our study showed that 6/23 (26.1%) patients in the AE group had abnormal neuroimaging findings, supporting the findings of previous studies . Although the results of multivariate analysis showed that brain MRI could not be used as an independent indicator for early prediction (P = 0.807), the abnormal brain MRI lesions in a few patients with AE could be reversibly changed with immunotherapy (Fig. 2). In addition, a right temporal lobe space-occupying lesion was found on the brain MRI of one LG1-positive non-AE patient (Fig. 3), and the postoperative pathological diagnosis was ganglioglioma. Therefore, a brain MRI is necessary to evaluate the early etiology and prognosis of epileptic seizures. Epilepsy surgeons should fully consider immune etiologies in the preoperative evaluation of patients with temporal lobe structural disease .
The results of long-term video EEG showed that IEDs accounted for 77.9% of all patients. However, the detection rate of IEDs in patients with immune-associated epileptic seizures was not high, of which about 43.4% (10/23) showed focal slow wave activity or rhythm. However, some studies showed that IEDs might be a risk factor for the recurrence of epilepsy in patients with all types of AE .
Also, multivariate analysis showed that both NSAbs and APE2 were independent factors for the early prediction of immune-related seizures (P < 0.05). This further highlighted the importance of NSAbs and APE2 in the diagnosis and treatment of this disease. Guidelines including “A clinical approach to diagnosis of autoimmune encephalitis”  and “Expert Consensus on Diagnosis and Treatment of Autoimmune Encephalitis in China”  emphasize the importance of NSAbs; however, there needs to be greater awareness regarding the importance of APE2 score. Interestingly, it has been recently proposed that the diagnosis of AE should not be confined to positive antibody, but should be guided by clinical features . Our study supports this conclusion and more attention needs to be paid to the early diagnosis and prognostic assessment of immune-related seizures.
This study also had certain limitations: First, lack of longitudinal follow-up of antibody-positive cases. Second, although the results showed good statistical power, the number of samples was smaller than that of similar studies. In future studies, longitudinal long-term follow-up needs to be conducted based on the observation of a larger sample size, which would result in a comprehensive and in-depth analysis of the significance of NSAbs and APE2 scores in the diagnosis and treatment of immune-related seizures.