SSPE Involves Persistent Measles Virus in the CNS
Yes, latent SSPE definitively involves persistent mutant measles virus infection in the central nervous system, not systemic viremia—the virus establishes chronic CNS infection years after the initial measles infection when systemic viremia has long resolved. 1
Pathophysiology: CNS-Specific Viral Persistence
SSPE results from persistent mutant measles virus infection specifically localized to the CNS, occurring after the initial measles infection when systemic viremia is no longer present. 1 The virus establishes true persistent infection in neurons, spreading trans-synaptically, with envelope proteins accumulating mutations that allow immune evasion. 1
Timeline Demonstrates CNS Localization
The clinical timeline clearly demonstrates that SSPE is a CNS-localized persistent infection rather than ongoing systemic disease:
- Initial measles infection occurs with viremia during the acute illness 1
- Years of latency follow (typically 2-10 years, but can be as short as 4 months) with no detectable systemic viremia 1, 2
- SSPE then emerges with insidious onset of neurological symptoms 1
During the true latency period, there is no systemic viremia and no active immune stimulation—only persistent mutant measles virus residing in the CNS. 1
Diagnostic Evidence Confirms CNS Persistence
Intrathecal Antibody Synthesis
The diagnosis of SSPE relies on detection of intrathecal synthesis of measles-specific antibodies in CSF, indicating local CNS production rather than systemic antibody leakage. 1 Key diagnostic features include:
- CSF/serum measles antibody index ≥1.5 confirms intrathecal synthesis 1, 3
- Persistent measles-specific IgM in both serum and CSF, often higher in CSF than serum 1
- This persistent IgM reflects ongoing immune stimulation from continuous CNS viral replication 1
Abnormal Antibody Patterns
100% of SSPE patients maintain detectable measles-specific IgM antibodies in serum, which is highly abnormal since IgM typically disappears 30-60 days after acute measles. 1 This persistent IgM—remaining elevated for years or even decades regardless of disease stage—indicates ongoing immune stimulation from CNS viral replication. 1
The combination of persistent measles IgM in serum and CSF, elevated IgG, and CSF/serum measles antibody index ≥1.5 has 100% sensitivity and 93.3% specificity for SSPE diagnosis. 1
Mechanism of Viral Persistence
Despite presumably functional cell-mediated immunity and high antiviral antibody titers, immunological control of the CNS infection is not achieved in SSPE patients. 4 The virus undergoes mutations within the host that allow it to:
- Evade immune surveillance 5
- Establish chronic CNS infection 5
- Spread trans-synaptically between neurons 1
- Accumulate envelope protein mutations 1
Functional CD4+ and CD8+ T cells together with IFN-gamma are required to overcome the infection, but this immune control fails in SSPE. 4
Clinical Implications
Not Reactivation of Systemic Virus
SSPE develops years after the initial measles infection, during which time there is no systemic viremia—only persistent mutant measles virus in the CNS. 1 This distinguishes SSPE from:
- Acute measles reinfection (which would show high-avidity IgG with IgM positivity) 1
- Acute measles encephalitis (which occurs during active systemic infection) 3
Prevention Through Vaccination
Measles vaccination is the only effective prevention strategy for SSPE, which has essentially eliminated the disease in highly vaccinated populations. 1, 6 The vaccine prevents the initial measles infection that is the prerequisite for subsequent CNS viral persistence. 6
The MMR vaccine does not increase the risk for SSPE—vaccination actually prevents SSPE by preventing the initial measles infection. 3, 6