Can measles Immunoglobulin G (IgG) in serum be low in latent Subacute Sclerosing Panencephalitis (SSPE)?

Measles IgG in Serum During Latent SSPE

No, measles IgG in serum is characteristically elevated, not low, during latent SSPE—in fact, 100% of SSPE patients maintain persistently elevated measles-specific antibodies in serum, including abnormally persistent IgM that should have disappeared months after acute measles infection. 1

Characteristic Antibody Pattern in SSPE

The antibody profile in SSPE is pathognomonic and opposite to what the question suggests:

• Serum measles IgG remains markedly elevated throughout the latent period and clinical disease, as the same oligoclonal B-cell clones produce measles-specific antibodies both in the CNS and systemically 2
• Measles-specific IgM persists abnormally in 100% of SSPE patients, which is highly unusual since IgM typically disappears 30-60 days after acute measles infection 1, 3
• The oligoclonal measles-specific IgG isolated from serum and CSF show almost identical band patterns with respect to number, intensity, isoelectric point, and light chain class, confirming simultaneous systemic and CNS antibody production 2

Diagnostic Implications

The hallmark laboratory finding is dramatically elevated CSF/serum measles antibody index (CSQrel ≥ 1.5), with typical values ranging from 2.3 to 36.9 in confirmed cases, demonstrating intrathecal antibody synthesis superimposed on already-elevated serum levels 4, 5

Key diagnostic features include:

• CSF measles antibodies are elevated in 90.9-100% of SSPE cases, with sensitivity of 100% and specificity of 93.3% 6
• The elevated CSF/serum ratio indicates local CNS production of antibodies, not simply leakage from serum 1, 5
• In SSPE, most or all oligoclonal IgG proteins in CSF carry measles antibody activity, distinguishing it from other conditions like multiple sclerosis where only partial oligoclonal IgG has measles reactivity 7

Clinical Context: Understanding SSPE Pathophysiology

SSPE develops from persistent mutant measles virus infection specifically in the CNS, occurring years after the initial measles infection when systemic viremia has long resolved 1

The timeline clarifies why antibodies remain elevated:

• Initial measles infection occurs with viremia during acute illness 1
• Years of latency follow with no detectable viremia but persistent CNS infection 1
• SSPE emerges with insidious neurological symptoms while antibody production continues 1
• The latency period is typically 2-10 years, though recent reports show decreasing intervals (as short as 4 months in some cases) 8

Common Pitfall to Avoid

Do not confuse the absence of active viremia during latent SSPE with low antibody levels—these are distinct phenomena. The virus persists in a defective form in the CNS, continuously stimulating antibody production without causing systemic viremia 1, 5

Diagnostic Algorithm When SSPE is Suspected

When white matter lesions are identified on neuroimaging with compatible clinical features 9, 4:

1. Obtain simultaneous serum and CSF samples for measles-specific IgG measurement 5
2. Calculate CSF/serum measles antibody index (CSQrel)—values ≥1.5 confirm intrathecal synthesis 4, 5
3. Expect elevated, not low, serum measles IgG in all confirmed cases 2, 6
4. Look for persistent measles-specific IgM in serum (abnormal finding specific to SSPE) 1
5. Correlate with characteristic EEG findings (periodic complexes with 1:1 relationship to myoclonic jerks) 3