What does the presence of both measles Immunoglobulin M (IgM) and Immunoglobulin G (IgG) antibodies in the silent phase of Subacute Sclerosing Panencephalitis (SSPE) indicate?

Measles IgM and IgG Presence in SSPE

Direct Answer

The statement is TRUE—both measles IgM and IgG antibodies are present throughout all phases of SSPE, including the silent (latent) phase, which is a pathognomonic diagnostic feature that distinguishes SSPE from acute measles infection. 1

Understanding the Abnormal Antibody Pattern

The persistent presence of measles-specific IgM in SSPE is highly abnormal and diagnostically significant:

• In acute measles infection: IgM appears 1-2 days after rash onset, peaks at 7 days, and becomes undetectable within 30-60 days 1
• In SSPE: 100% of patients maintain detectable measles-specific IgM antibodies in serum regardless of disease stage, even years after the initial measles infection when systemic viremia has long resolved 1
• This persistent IgM is present in both serum and CSF, with 35% of SSPE cases showing more pronounced IgM response in CSF than serum, suggesting intrathecal IgM production within the central nervous system 2

Diagnostic Significance

The combination of persistent IgM with elevated IgG and a CSF/serum measles antibody index ≥1.5 has 100% sensitivity and 93.3% specificity for SSPE diagnosis. 1

The antibody pattern reflects ongoing antigen release:

• The continuing release of measles antigen from persistent mutant virus in the CNS prevents the normal shut-off of IgM synthesis 2
• Both IgM and IgG antibodies are associated with oligoclonal bands, indicating clonal expansion of specific B-cell populations 2, 3
• The same cell clones appear responsible for synthesis of measles-specific IgG in both the CNS and serum, as demonstrated by identical oligoclonal band patterns 4

Clinical Context: Why This Matters

SSPE develops from persistent CNS infection, not active viremia:

• The disease occurs 6-8 years after initial measles infection, during which time there is no systemic viremia—only persistent mutant measles virus localized to the CNS 1
• The persistent antibody response (both IgM and IgG) reflects continuous antigen stimulation from the defective virus that cannot complete its replication cycle 2

Critical Diagnostic Algorithm

When evaluating for SSPE, look for:

1. Persistent measles-specific IgM in serum (abnormal beyond 30-60 days post-infection) 1
2. Elevated measles-specific IgG in both serum and CSF 1
3. CSF/serum measles antibody index ≥1.5 (confirms intrathecal synthesis) 1
4. Compatible clinical presentation: insidious personality changes, intellectual decline, myoclonic jerks, seizures 5
5. Characteristic EEG findings: periodic complexes with 1:1 relationship to myoclonic jerks 5

Important Caveat

Do not confuse SSPE with the MRZ reaction seen in multiple sclerosis, which shows intrathecal synthesis against at least two of three viral agents (measles, rubella, zoster), whereas SSPE demonstrates an isolated, extremely strong measles-only response 1