Post-transplant acute limbic encephalitis (PALE) has been reported, and human herpesvirus 6 (HHV-6) is one of the causes. Reports of PALE in children are still limited. In the present study, the detailed clinico-radiological findings of PALE and the neurological outcomes of four children are reported. Four patients aged three to twelve years of 142 patients who underwent allogeneic hematopoietic stem cell transplantation developed PALE. Underlying disorders were neuroblastoma in three patients and acute lymphoblastic leukemia in one. All four patients showed disorientation, behavioral changes, emotional dysregulation, anterograde amnesia, or decreased level of consciousness between 12 and 22 days after transplantation. None had clinical seizures. In all patients, MRI showed hyperintense signals in unilateral or bilateral hippocampi, especially in the CA1 area, on diffusion-weighted imaging (DWI), with a decreased apparent diffusion coefficient. Though the hippocampi were also hyperintense on T2-weighted and fluid-attenuated inversion recovery imaging, signal intensity was higher on DWI by visual inspection. Electroencephalograms showed poorly organized posterior dominant rhythm in all patients and left occipito-centro-parietal slow waves in one without epileptiform discharges. HHV-6 DNA was positive in both cerebrospinal fluid and whole blood by polymerase chain reaction in three of the four patients. Two patients had pharmaco-resistant epilepsy and cognitive impairment three years after PALE, but the other two recovered neurologically from PALE. A DWI abnormality of the hippocampal CA1 area is a good biomarker for the diagnosis of HHV-6-positive or -negative PALE in children. Glutamatergic dysregulation and hippocampal cytotoxic edema may be involved in PALE.

Post-transplant human herpes virus 6 (HHV6) encephalitis can be followed by refractory epilepsy accompanied by intellectual decline after several months. However, such cases are extremely rare, and the disease mechanism remains elusive. We present the case of an eight-year-old boy who presented with epileptic encephalopathy 11 months after developing post-transplant acute limbic encephalitis (PALE) caused by HHV6. The patient developed multiple types of seizures, primarily characterized by epileptic spasms. Significant electroencephalographic (EEG) abnormalities were noted during the interictal period, along with regression of cognitive and language functions and progressive atrophy of the entire brain, including the hippocampus. He was managed with multiple antiepileptic drugs, although his seizures remained uncontrolled for one year after epilepsy onset. Herein, we summarized and analyzed the clinical features of the previously reported cases and the present case. The median time from the onset of HHV6 PALE to epilepsy was 11.5 months. Developmental regression or cognitive decline, multiple seizure types including tonic seizures, generalized slow waves, multifocal spike-wave activity on interictal EEG, brain changes such as hippocampal sclerosis, and poor seizure prognosis are common features. The disease was classified as epileptic encephalopathy following HHV6-related PALE (EE-PALE). This case not only provides additional evidence that EE-PALE is a distinct disease with consistent clinical features but is also expected to contribute to the identification of its pathogenesis and effective treatment.

Long-term neurocognitive deficits after human herpesvirus-6 (HHV-6) infection are common in stem-cell transplant recipients, but SIADH (Syndrome of inappropriate antidiuretic hormone secretion) with persistent hyponatremia is rare. A 51-year-old woman presented with somnolence, hyponatremia (121 mmol/L) and HHV-6 viremia (80,330 copies/ml) on day +22 post umbilical cord blood transplant (UCBT). With waterrestriction, tolvaptan and combination of foscarnet and ganciclovir, patient's hyponatremia and HHV-6 viremia improved. On day +94 UCBT, hyponatremia and HHV-6 viremia recurred. Foscarnet was restarted and continued until day +269 UCBT due to multiple HHV-6 recurrences with persistent hyponatremia. At day +712, patient remains on water-restriction, tolvaptan for continuous hyponatremia from SIADH.

Human herpesvirus 6 (HHV-6) can reactivate with immunosuppression and cause central nervous system (CNS) dysfunction. Much of the literature describes cases after hematopoietic stem cell transplantation (HSCT), ranging from encephalitis to a post-transplant acute limbic encephalitis syndrome (PALE). Outside of HSCT, studies of HHV-6 encephalitis are limited to case reports. This study was designed to review HHV-6 CNS infection, and evaluate all patients admitted to MD Anderson Cancer Center between March 2016 and December 2018 with detectable HHV-6 DNA in the cerebrospinal fluid (CSF). Patients with HHV-6 DNA detected in the CSF using the Viracor or Biofire® Meningitis Encephalitis Panel platforms and no other identified etiology were identified and demographic features, known risk factors, imaging findings, CSF analysis, treatments and patient outcomes were extracted from medical records. 725 patients underwent HHV-6 testing during the study timeframe, with 19 cases (2.6 %) of HHV-6 mediated CNS disease identified. Most patients, 13/19 (68 %), had undergone HSCT with median time to presentation of 31 days after transplant. Survival at 240 days after transplant was 62 %. CSF had lymphocyte predominance and nearly all patients had peripheral lymphopenia. Other at risk populations identified included patients who received chimeric antigen receptor (CAR) T-cell therapy and biologic immunotherapy. Notable discordance among testing platforms was found in 5/9 (55 %) instances. In addition to HSCT patients, HHV-6 reactivation leading to CNS disease also occurs in settings such as following adoptive T cell therapy or biologic immunotherapy. Significant diagnostic discordance exists between testing platforms.

Hematopoietic cell transplantation (HCT) is widely performed for neoplastic and non-neoplastic diseases. HCT involves intravenous infusion of hematopoietic progenitor cells from human leukocyte antigen (HLA)-matched donor (allogeneic) or from the patient (autologous). Before HCT, the patient is prepared with high dose chemotherapy and/or radiotherapy to destroy residual malignant cells and to reduce immunologic resistance. After HCT, chemotherapy is used to prevent graft rejection and graft versus host disease (GvHD). Neurological complications are related to the type of HCT, underlying disease, toxicity of the conditioning regimens, immunosuppression caused by conditioning regimens, vascular complications generated by thrombocytopenia and/or coagulopathy, GvHD and inappropriate immune response. In this review, neurological complications are presented according to time of onset after HCT: (1) early complications (in the first month) - related to harvesting of stem cells, during conditioning (drug toxicity, posterior reversible encephalopathy syndrome), related to pancytopenia, (2) intermediate phase complications (second to sixth month) - central nervous system infections caused by prolonged neutropenia and progressive multifocal leukoencephalopathy due to JC virus, (3) late phase complications (after sixth month) - neurological complications of GvHD, second neoplasms and relapses of the original disease.