Human Herpesvirus 6A (HHV-6A)

🧠 HHV-6 Encephalitis is a serious complication after stem cell transplants, often presenting with: ⚡ Hyponatraemia | 🧩 Cognitive decline | ⚠️ Seizures | 📉 Memory loss 🧪 CSF & PCR are diagnostic gold standards. MRI (T2, FLAIR, DWI) typically shows hyperintensities in the medial temporal lobes, amygdala, and hippocampus. 💊 Treat with antivirals (foscarnet or ganciclovir). Prompt diagnosis is essential to prevent permanent neurological damage.

🔎 Overview

• HHV-6 = collective term for HHV-6A and HHV-6B.
• HHV-6A belongs to the Herpesviridae family (like HSV & CMV).
• HHV-6B commonly causes 🌸 roseola infantum in children.
• HHV-6A is less understood, often reactivates in immunocompromised patients.
• It is a double-stranded DNA virus, with T-lymphocytes as the main target.

📤 Transmission

• Worldwide, ubiquitous virus.
• HHV-6A: spread via close contact & saliva (exact routes unclear).
• Usually asymptomatic in healthy individuals.
• Reactivation occurs in immunosuppression ➝ drives complications.

🧬 Pathogenesis

• HHV-6A: infects CD4+ T cells, glial cells, macrophages, and epithelial cells. 🛑 Latency in monocytes/macrophages ➝ reactivation when immune defences fail. 🌀 Can disrupt immune balance → chronic inflammation, autoimmunity, or neurological disease.
• HHV-6B: causes roseola infantum (exanthema subitum) with fever + rash. ⚠️ Accounts for ~10–17% of febrile ED attendances in <36-month-olds.

🩺 Clinical Manifestations

• HHV-6A reactivation linked to:
  • Neurological disease: encephalitis, MS associations, cognitive decline, seizures, SIADH.
  • Chronic fatigue syndrome (possible role, under study).
  • Immunosuppression in transplant patients.
  • Worsening HIV progression by depleting CD4+ T cells.
• In healthy people ➝ often asymptomatic or mild flu-like illness.

🧪 Diagnosis

• PCR: most sensitive for viral DNA in blood/CSF.
• Serology: limited in distinguishing HHV-6A vs HHV-6B.
• Viral culture: rarely practical (needs high viral load).
• Tissue biopsy: brain biopsy in severe encephalitis cases.
• MRI: hyperintensity in medial temporal lobes (hippocampus + amygdala).
• EEG: temporal lobe epileptiform discharges may be seen.

🧒 Primary Infection

HHV-6B = roseola infantum → high fever + rash in infants. Complication: febrile seizures (most common). Rarely, acute encephalopathy.

💊 Treatment

• Most primary HHV-6 infections = self-limiting, resolve in 5–7 days.
• Complications: febrile seizures, encephalopathy due to neurotropic effect.
• For severe HHV-6A disease: use antivirals
  • 🧪 Ganciclovir
  • 🧪 Foscarnet
  • 🧪 Cidofovir
• Supportive therapy: seizure control, management of raised ICP, fluid balance in SIADH.

⚠️ Complications

• Encephalitis: transplant recipients most at risk.
• Multiple sclerosis: association proposed, not proven.
• Chronic fatigue syndrome: under research as possible trigger.
• Long-term cognitive decline after encephalitis.

🛡️ Prevention

• No vaccine currently available.
• Strategies: close monitoring in immunocompromised patients (esp. transplant, HIV).
• Pre-emptive screening with PCR in post-transplant settings.
• Rapid initiation of antivirals when reactivation suspected.

📌 Conclusion

• HHV-6A is less understood than HHV-6B but is emerging as a key cause of CNS disease post-transplant.
• Diagnosis requires high suspicion + early PCR/MRI.
• Early antiviral therapy can prevent severe, irreversible neurological injury.

💡 Teaching Pearl: In transplant medicine, always consider HHV-6 encephalitis when you see: 📉 Memory loss + 🌙 confusion + ⚡ seizures + 🔬 hyponatraemia (SIADH). These are red flags for early MRI & PCR testing.