web_epilepsyThere are several lines of evidence that indicate the strong possibility of an etiological role of HHV-6 in patients with epilepsy. Herpesviruses have been implicated in the development of seizures. Seizures are thought to result from either primary infection or reactivation of latent virus (Leibovitch 2015). HHV-6B, in particular, continues to be associated with epilepsy, in addition to several other CNS diseases.

HHV-6 and Epilepsy

HHV-6 viral DNA has often been detected at higher levels in resected temporal lobe tissues than in non-epileptogenic control tissues, with the frequency ranging between approximately 30% and 70%. In Leibovitch and colleague’s study, using real-time PCR, 60-70% HHV-6B positivity was observed in the mesial temporal lobe epilepsy (MTLE) resections (Leibovitch 2015).

Kawamura and colleagues recently performed a study involving 75 intractable MTLE patients, including 52 MTS patients and 23 non-MTS patients. Resected hippocampus, amygdala, and mixed samples of amygdala and uncut samples were examined by real-time polymerase chain reaction (PCR) and reverse-transcriptase PCR to detect viral DNA and mRNA, respectively. Host gene expressions, including neural markers, were measured using the TaqMan Gene Expression Assay. Kawamura found HHV-6 DNA in approximately 22.7-28.6% of the studied tissues (hippocampus, amygdala, and mixed amygdala/uncus). Interestingly, significantly greater levels of viral DNA were detected in MTS than in non-MTS patient material, and the incidence of febrile seizures was also significantly higher in the MTS group (Kawamura 2015, Leibovitch 2015).

Furthermore, a German group found HHV-6 DNA in 56% of MTLE patients with a history of encephalitis, but no in the patients with a history of complex febrile seizures (Niehusmann 2010). However, for their nested PCR study, this group used formalin-fixed, paraffin embedded (FFPE) tissues, considered by many to be a less sensitive method for the detection of HHV-6. Recent studies have shown that fresh or flash frozen tissues are far more sensitive because the beta herpesvirus DNA is degraded when the FFPE tissues are used and that the DNA also degrades significantly with the passage of time (Wang 2013).

In addition, a study from the Royal Children’s Hospital in Melbourne, Australia found that at least 50% of patients who developed HHV-6 Post-Transplant Acute Limbic Encephalitis (HHV-6 PALE) developed symptomatic generalized epilepsy (SGE) after a silent period of 11-18 months (Howell 2012). Following additional clinical confirmation by a separate group, this condition is characterized by generalized seizures and cognitive regression following HHV-6 PALE (Raspall-Chaure 2013).

Neuroinflammation

Neuroinflammation is increasingly recognized as a key component underlying epileptic disease pathogenesis (Vezzani 2014). Several studies have demonstrated increased inflammatory pathway expression in HHV-6B-positive patients with MTLE:

  • Kawamura observed significantly increased expression of monocyte chemoattractant protein 1 (MCP-1) and glial fibrillary acidic protein (GFP) in the HHV-6-positive versus HHV-6-negative amygdala tissues. Moreover, these expression levels positively correlated with HHV-6 viral load and markers that directly (MCP-1) or indirectly (GFAP) reflect inflammatory or otherwise injurious processes (Kawamura 2015, Leibovitch 2015).
  • Using nested PCR and immunohistochemistry, neurologists from the West China Hospital examined surgical resections from patients with Mesial Temporal Lobe Epilepsy (MTLE) and found HHV-6B in the brain tissue of 28% of 32 resected brain tissues compared to only 8% of controls. The Chinese group also found that an inflammatory marker, NF-κB, was up-regulated in the glial cells of patients positive for HHV-6B. The virus was found in the subset of patients with a history of febrile seizures (Li 2011). This finding confirmed an earlier report by investigators at the NINDS that showed HHV-6B replication in the hippocampal astrocytes in two-thirds of 24 patients with mesial temporal sclerosis, but not in patients with other causes of epilepsy. At the time, this group speculated that HHV-6B might cause seizures by interfering with the astrocytes’ ability to transport glutamate (Fotheringham 2007).
  • Mao and colleagues reported significantly increased interleukin 17A levels in serum samples from epileptic patients during intricate periods, compared with healthy controls. In this cohort, serum interleukin 17A levels correlated with seizure severity and frequency. Observations from these translational studies corroborate findings from animal models of seizure induction, specifically that pro-inflammatory cytokines, such as interleukin 1β, contribute to seizure duration and recurrence and blood-brain barrier damage, which may then perpetuate brain inflammation (Mao 2013, Librizzi 2012, Leibovitch 2015).

How might these observations in MTS/MTLE of increased HHV-6 viral detection and increased markers of neuroinflammation and astrocyte activation be mechanistically associated with epilepsy?

  • Inflammation and HHV-6 infection have each been demonstrated to induce dysregulation of glutamate homeostasis in astrocytes, which is hypothesized to play a central role in the pathogenesis of epilepsy. Excess glutamate may be excitotoxic, and contribute to neuronal depolarization. Furthermore, glutamate receptor antagonists have demonstrated anticonvulsant properties (Tian 2005, Leibovitch 2015)
  • In vitro, HHV-6 infection of primary astrocytes has been shown to down-regulate levels of glutamate transporter expression, which supports the concomitant observation of decreased glutamate uptake in infected versus uninfected astrocytes (Fotheringham 2008, Leibovitch 2015).
  • Inflammatory cytokines, such as interleukin 1β, can also inhibit astrocyte reuptake of glutamate (Vezzani 2014, Leibovitch 2015).
  • Because HHV-6-infected astrocytes have been demonstrated in CNS disorders, including MTLE, and because the virus can induce a metabolic dysregulation that is considered to contribute to epileptogenesis, this mechanism is biologically plausible. Importantly, the role that HHV-6 (or other viruses) may play in the pathogenesis of epilepsy suggests new clinical interventional approaches that target virus infection in the CNS (Leibovitch 2015).
  • Kawamura and colleagues observed a positive correlation between MCP-1 and GFAP expression and HHV-6 viral DNA loads, suggestive of a latent infection (Kawamura 2015). If the virus persistently infects resident glial cells, disrupting astrocyte homeostasis and triggering CNS immune responses toward latent antigens, this may be one mechanism by which the seizure threshold is lowered. Resultant inflammation from seizures may then contribute to the reactivation of HHV-6 from latency in glial or other CNS resident cells (Leibovitch 2015).

HHV-6 and Status Epilepticus

Prolonged febrile seizures, or febrile status epilepticus (FSE), are associated with an increased risk of temporal lobe epilepsy (TLE) (Shinnar 2003, Leibovitch 2015). HHV-6 encephalitis can present as status epilepticus, even in immunocompetent patients, thus HHV-6 encephalitis should be considered among immunocompetent patients presenting with encephalitis and having signs of temporal lobe involvement (Shahani 2014). HHV-6 should be considered in the differential diagnosis of non-convulsive status epilepticus after alloHSCT, especially in patients with hyponatremia. Empirical antiviral therapy targeting HHV-6 should be administered to these patients (de Souza Franceschi 2014).

The FEBSTAT Study

The “Consequences of Prolonged Febrile Seizures in Childhood” (FEBSTAT) study was a prospective, multicenter study. Enrolled were children, aged 1 month to 6 years of age, presenting with a febrile seizure lasting 30 minutes or longer based on ambulance, emergency department, and hospital records, and parental interview. Two other groups of children are included: a “control” group consisting of children with a first febrile seizure ascertained at Columbia University and with almost identical baseline and 1-year follow-up examinations and a pilot cohort of FSE from Duke University (Hesdorffer 2012).

The FEBSTAT cohort consists of 199 children with median age at baseline of 16 months and a median duration of FSE of 70.0 minutes. Seizures were continuous in 57.3% and behaviorally intermittent (without recovery in between) in 31.2%; most were partial (2.0%) or secondary generalized (65.8%), and almost all (98.0%) culminated in a generalized tonic-clonic seizure (Hesdorffer 2012).
Of the 199 children, 86.4% had normal development and 20% had prior febrile seizures. In one third of cases, FSE was unrecognized in the emergency department. The Duke existing cohort consists of 23 children with median age of FSE onset of 18 months and median duration of FSE of 90.0 minutes. The Columbia control cohort consists of 159 children with a first febrile seizure who received almost the same workup as the FEBSTAT cohort at baseline and at 1 year. They were followed by telephone every 4 months for a median of 42 months. Among the control cohort, 64.2% had a first simple FS, 26.4% had a first complex FS that was FSE, and 9.4% had FSE. Among the 15 with FSE, the median age at onset was 14.0 months and the median duration of FSE was 43.0 minutes (Hesdorffer 2012).

Results of the FEBSTAT Study
Of 199 children evaluated, HHV-6 or HHV-7 status could be determined in 169 (84.9%). HHV-6B viremia at baseline was found in 54 children (32.0%), including 38 with primary infection and 16 with reactivated infection. No HHV-6A infections were identified. HHV-7 viremia at baseline was observed in 12 children (7.1%), including either with primary infection and four with reactivated infection. Two subjects had HHV-6/HHV-7 primary coinfections at baseline. There were no differences in age, characteristics of illness or fever, seizure phenomenology or the proportion of acute EEG or imaging abnormalities in children presenting with FSE with or without HHV infection (Epstein 2012).

HHV-6B infection is commonly associated with FSE. HHV-7 is less frequently associated with FSE. Together, they account for one third of FSE, a condition associated with an increased risk of hippocampal injury and subsequent temporal lobe epilepsy (Epstein 2012).

Key Papers: HHV-6 & Epilepsy