IgG Fluctuation in Latent SSPE
IgG levels do not fluctuate during latent SSPE; instead, they remain persistently and dramatically elevated in both serum and CSF throughout all stages of the disease, including early/latent phases. This persistent elevation is a defining diagnostic feature that distinguishes SSPE from other neurological conditions.
Characteristic IgG Pattern in SSPE
All SSPE patients, regardless of disease stage (including latent/early phases), demonstrate persistently high titers of anti-measles antibodies in both serum and CSF, with antibody activity associated with both IgM and IgG immunoglobulin classes 1.
The elevated measles-specific IgG remains stable rather than fluctuating, showing almost identical oligoclonal band patterns between CSF and serum with respect to number, intensity, isoelectric point, and light chain class, suggesting the same cell clones produce antibodies in both compartments 2.
Geometric mean anti-nucleocapsid (NC) antibody titers in SSPE patients are consistently elevated (6.58 ± 0.98 in serum and 4.38 ± 0.74 in CSF) compared to controls, demonstrating the non-fluctuating nature of this response 3.
Diagnostic Implications
The CSF/serum measles antibody index (CSQrel) remains consistently elevated ≥1.5 in SSPE patients (mean: 12.9, range: 2.3-36.9), confirming intrathecal antibody synthesis that persists throughout the disease course 4.
This persistent elevation occurs because continuing release of measles antigen from persistent virus in the CNS prevents the shut-off of immunoglobulin synthesis, maintaining stable high antibody levels rather than causing fluctuation 1.
Important Caveats
While total IgG to measles virus remains elevated, SSPE patients show a selective and persistent reduction in antibody response specifically to the matrix (M) protein of measles virus (geometric mean anti-M titer: 3.35 ± 0.53 in serum, 2.59 ± 0.86 in CSF), though this reduction is stable rather than fluctuating 3, 5.
In 35% of SSPE cases, the specific IgM response may be more pronounced in CSF than serum, suggesting intrathecal IgM production, but this pattern also remains stable rather than fluctuating 1.
The demonstration of increased measles antibody index in simultaneously obtained serum and CSF samples is crucial for SSPE diagnosis, as this elevation is present even in latent/early disease stages 4.