Measles IgG Levels During SSPE Latency Period
No, measles IgG antibody levels are not low during the SSPE latency period—they remain persistently elevated in both serum and CSF throughout the latency phase and after clinical onset, which is a defining diagnostic feature of this disease. 1
Understanding the Antibody Pattern in SSPE
The antibody response in SSPE is fundamentally different from typical post-measles immunity:
Measles-specific IgG remains markedly elevated in serum throughout the entire latency period (the years between initial measles infection and SSPE symptom onset), unlike normal measles recovery where antibody levels stabilize at lower protective levels 2, 3
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 infection 1
The persistence of both IgG and IgM reflects ongoing viral replication in the CNS, not a waning immune response 4
Diagnostic Significance of Elevated Antibodies
The hallmark diagnostic finding in SSPE is intrathecal synthesis of measles-specific antibodies:
CSF/serum measles antibody index (CSQrel) ≥1.5 confirms intrathecal antibody production, with SSPE patients typically showing indices ranging from 2.3 to 36.9 (mean: 12.9) 2
Measles-specific IgG levels are higher in CSF (diluted 1:5) than in serum (diluted 1:50) when measured simultaneously, demonstrating local CNS production rather than passive transfer from blood 4
The oligoclonal band patterns of measles-specific IgG in CSF and serum are nearly identical, suggesting the same B-cell clones produce antibodies in both compartments, but with massive overproduction in the CNS 5
Clinical Timeline and Antibody Dynamics
Understanding when antibodies are elevated helps with diagnosis:
During the latency period (typically 6-8 years between measles infection and SSPE onset, though can be as short as 2.5 years or as long as 34 years): measles IgG remains elevated in serum, and intrathecal synthesis is likely already occurring even before symptoms appear 6, 3
At clinical presentation: both serum and CSF show markedly elevated measles antibodies, with the CSF/serum index being the key diagnostic criterion 7, 2
Throughout disease progression: antibody titers remain constant over months to years, reflecting persistent CNS infection 4
Common Diagnostic Pitfalls
Do not confuse SSPE with acute measles infection: In acute measles, IgM appears transiently (detectable for about 1 month after rash onset), whereas in SSPE, IgM persists indefinitely 8, 1
Do not assume normal antibody levels rule out SSPE: The absence of elevated measles antibodies in CSF essentially excludes SSPE as a diagnosis 7, 2
Consider SSPE even without documented measles history: Many SSPE patients had unrecognized measles infection in infancy, but the persistently elevated antibodies will still be present 6, 3
Why Antibodies Remain High
The pathophysiology explains the persistent antibody elevation:
SSPE results from persistent mutant measles virus infection in the CNS, not from resolved infection with waning immunity 1
The virus continues to replicate in neurons throughout the latency period, driving continuous antibody production even though there is no systemic viremia 1
This represents a failure of immune clearance, not immune deficiency—the immune system recognizes the virus but cannot eliminate it from the CNS 7, 1