Measles IgG Levels in Latent SSPE
Yes, measles IgG levels are dramatically elevated in both serum and CSF during the latent period of SSPE, with the critical diagnostic finding being intrathecal synthesis demonstrated by a CSF/serum measles antibody index ≥1.5. 1
Understanding the Immunologic Timeline and "Latency"
The term "latent SSPE" requires clarification, as true immunologic latency does not exist in SSPE:
True latency period (2-10 years post-measles): During this asymptomatic interval after acute measles infection, there is no systemic viremia and theoretically no active immune stimulation—IgM from the original measles infection has long disappeared (within 30-60 days). 1 However, once SSPE becomes detectable through antibody testing, the patient is no longer in true latency.
Once antibodies are elevated, viral replication is ongoing: The presence of persistently elevated measles IgG and the appearance of measles-specific IgM indicate continuous CNS viral replication and ongoing immune stimulation, not true latency. 1, 2
Diagnostic Antibody Pattern in SSPE
Measles IgG Characteristics
Measles-specific IgG is dramatically elevated in 100% of SSPE patients, regardless of disease stage: 1, 3
- Serum IgG: Persistently elevated at very high titers throughout the disease course 3
- CSF IgG: Also markedly elevated, with progressive increases correlating with disease progression 3
- CSF/serum measles antibody index ≥1.5: This confirms intrathecal synthesis (local CNS antibody production) and has 100% sensitivity and 93.3% specificity for SSPE diagnosis 1, 4
The Critical IgM Finding
All SSPE patients (100%) maintain detectable measles-specific IgM in serum, which is pathognomonic for the disease: 1
- In acute measles, IgM appears 1-2 days after rash onset, peaks at 7-10 days, and becomes completely undetectable within 30-60 days 1
- The persistence of IgM years after measles infection is highly abnormal and indicates ongoing CNS viral replication 1, 2
- In 35% of SSPE cases, measles-specific IgM is higher in CSF than serum, suggesting intrathecal IgM production 2
Clinical Implications for Testing
Testing should be performed when clinical features suggest SSPE, not during true asymptomatic latency: 1
- Progressive neurological deterioration with history of measles exposure 1
- Behavior changes followed by myoclonic jerks 1
- Characteristic EEG findings showing periodic complexes with 1:1 relationship to myoclonic jerks 5, 6
- White matter lesions or discrete hippocampal high signal on MRI (present in ~60% of cases) 1
Diagnostic Algorithm
Obtain simultaneous serum and CSF samples for: 1
- Measles-specific IgG measurement in both compartments to calculate CSF/serum antibody index
- Measles-specific IgM testing in both serum and CSF
- Total IgG and albumin levels to calculate the antibody index properly 4
Diagnostic criteria (combined sensitivity 100%, specificity 93.3%): 1
- Persistent measles IgM in serum and/or CSF
- Elevated measles-specific IgG in both compartments
- CSF/serum measles antibody index ≥1.5
Important Differential Diagnoses
Multiple sclerosis with MRZ reaction: Shows intrathecal synthesis against at least 2 of 3 viral agents (measles, rubella, zoster), whereas SSPE shows an isolated, extremely strong measles-only response 1, 6
Acute measles reinfection: Shows high-avidity IgG with IgM positivity but a normal CSF/serum index, whereas SSPE shows extremely high titers with elevated CSF/serum index ≥1.5 1
False-positive IgM in low-prevalence settings: Confirmatory testing using direct-capture IgM EIA method is recommended when IgM is detected without epidemiologic linkage to confirmed measles 1
Critical Caveat
Once measles antibodies are detectably elevated (especially with persistent IgM), the patient is no longer in true immunologic latency—ongoing CNS viral replication is occurring. 1, 2 The "preclinical" phase where antibodies are already elevated but symptoms are minimal or absent is better termed "early SSPE" rather than "latent SSPE."