Do measles Immunoglobulin G (IgG) levels fluctuate or decline during latent Subacute Sclerosing Panencephalitis (SSPE)?

Measles IgG Levels During Latent SSPE

Measles IgG antibodies do not decline during the latent period of SSPE; instead, they remain persistently elevated in both serum and CSF throughout all stages of the disease, including the asymptomatic latency phase. 1

Understanding the Immunologic Timeline in SSPE

The key to answering this question lies in distinguishing between the true latency period (when the patient is asymptomatic) and the clinical disease phase:

During True Latency (2-10 Years Post-Measles)

• After acute measles resolves, IgM normally disappears within 30-60 days, and the patient enters a true latency period lasting typically 2-10 years (though can be as short as 4 months) during which there is no systemic viremia and theoretically no active immune stimulation. 1
• However, the virus establishes persistent infection in CNS neurons during this time, spreading trans-synaptically with accumulating envelope protein mutations. 1

When SSPE Becomes Clinically Apparent

• Once SSPE symptoms emerge, measles IgG levels are dramatically and persistently elevated in both serum and CSF, and this elevation persists for years or even decades regardless of disease stage. 1
• The CSF/serum measles antibody index is ≥1.5, confirming intrathecal synthesis and indicating local CNS antibody production. 1, 2

The Persistent IgM Phenomenon: A Diagnostic Hallmark

A critical and unusual feature of SSPE that helps answer your question:

• 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—often higher in CSF than serum—indicates ongoing immune stimulation from CNS viral replication and remains elevated for years or decades, regardless of disease stage. 1, 3, 4
• The continuing release of measles antigen from persistent virus in the CNS prevents the normal shut-off of IgM synthesis. 3

Clinical Implications for Diagnosis

The diagnostic combination includes:

• Persistent measles IgM in both serum and CSF (sensitivity 100%, specificity 93.3%) 1
• Markedly elevated measles IgG in both compartments 1
• CSF/serum measles antibody index ≥1.5 1, 2
• Characteristic EEG findings with periodic complexes 2

Important Caveats

The antibody patterns in SSPE show remarkable stability rather than fluctuation:

• In three SSPE patients followed for 3-6 months, measles IgM antibody titers remained constant over the course of disease. 4
• The oligoclonal band patterns of measles-specific IgG eluted from serum and CSF show almost identical profiles, suggesting the same cell clones produce antibodies in both compartments. 5

Do not confuse SSPE with:

• The MRZ reaction in multiple sclerosis, which shows intrathecal synthesis against at least two of three viral agents (measles, rubella, zoster), whereas SSPE shows an isolated, extremely strong measles-only response. 1, 2

Bottom Line

Rather than fluctuating or declining, measles IgG (and IgM) antibodies remain persistently and dramatically elevated throughout SSPE, including during what might be considered "latent" phases. This persistent elevation reflects ongoing viral persistence and immune stimulation in the CNS, making it a reliable diagnostic marker at any stage of disease. 1, 3, 4