Measles IgM in Latent SSPE: Present in Both Serum and CNS
In latent SSPE, measles IgM is detectable in both serum and CSF, not exclusively in the CNS—this persistent IgM in both compartments is actually a diagnostic hallmark that distinguishes SSPE from acute measles infection. 1
Diagnostic Antibody Pattern in SSPE
The presence of measles-specific IgM in SSPE is fundamentally different from acute measles infection:
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
In 35% of SSPE cases, the specific IgM response is more pronounced in CSF than in serum, suggesting intrathecal IgM production within the CNS itself. 2
All SSPE patients, regardless of disease stage, have high titers of anti-measles antibodies in both sera and CSF, with antibody activity associated with both IgM and IgG classes. 2
Why This Matters Diagnostically
The persistent IgM reflects ongoing immune stimulation from CNS viral replication, where the virus establishes true persistent infection in neurons:
The continuing release of measles antigen in SSPE prevents the shut-off of IgM synthesis, making it responsible for the specific IgM activity in both compartments. 2
Detection of virus-specific IgM antibodies in CSF of patients with chronic CNS diseases indicates active viral persistence, and in SSPE, IgM remains persistently elevated for years—even decades—regardless of disease stage. 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
Critical Distinction from Acute Measles
This persistent dual-compartment IgM presence is what separates SSPE from acute infection:
In acute measles, IgM becomes detectable 1-2 days after rash onset, peaks at 7 days, and becomes undetectable within 30-60 days. 1, 3
The presence of measles-specific IgM years after potential measles exposure strongly suggests SSPE, not acute infection. 1
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
Clinical Caveat
Do not confuse SSPE with subacute measles encephalitis (SME) in immunocompromised patients, where neither serum nor CSF antibody titers against measles are typically high, requiring brain biopsy for definitive diagnosis. 4