Measles IgG Levels in Latent SSPE
Direct Answer
Yes, measles IgG levels are dramatically elevated in latent SSPE, both in serum and cerebrospinal fluid (CSF), with the hallmark finding being intrathecal synthesis demonstrated by a CSF/serum measles antibody index ≥1.5. 1, 2
Understanding the Immunologic Profile
The antibody pattern in SSPE—even during the latent period—is distinctive and pathognomonic:
Persistently elevated measles-specific IgG is present in both serum and CSF at all stages of disease, regardless of whether the patient is symptomatic or in the latent phase 1, 3
The CSF/serum measles antibody index (CSQrel) ≥1.5 confirms intrathecal synthesis, meaning the CNS is actively producing these antibodies locally, not just receiving them from systemic circulation 1, 2, 4
In confirmed SSPE cases, the CSQrel typically ranges from 2.3 to 36.9 (mean: 12.9), indicating massive intrathecal antibody production 4
The Persistent IgM Phenomenon
A critical and highly unusual finding that distinguishes SSPE from normal measles immunity:
100% of SSPE patients maintain detectable measles-specific IgM antibodies in serum, which is profoundly abnormal since IgM normally disappears completely within 30-60 days after acute measles infection 1, 2
This persistent IgM—present for years or even decades—reflects ongoing immune stimulation from continuous CNS viral replication, not acute infection or reinfection 1, 2, 3
In 35% of SSPE cases, the specific IgM response is more pronounced in CSF than serum, suggesting IgM production within the CNS itself 3
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, 2
Pathophysiologic Mechanism During Latency
The "latent" period is actually characterized by active viral persistence:
SSPE develops from persistent mutant measles virus infection specifically in the CNS, occurring years after the initial measles infection when systemic viremia is no longer present 2
During the typical 2-10 year latency period (though can be as short as 4 months), there is no systemic viremia but the virus establishes true persistent infection in neurons, spreading trans-synaptically 2
The persistently elevated IgG and IgM levels indicate ongoing immune stimulation from CNS viral replication throughout this "latent" phase—it's not truly latent immunologically 1, 2, 3
Oligoclonal Antibody Pattern
The antibody response shows a unique oligoclonal pattern:
Measles-specific IgG isolated from CSF and serum shows almost identical oligoclonal band patterns with respect to number, intensity, isoelectric point, and light chain class 5
In SSPE, most or all oligoclonal IgG proteins in the CSF carry measles antibody activity, creating an isolated, extremely strong measles-only response 6
This oligoclonal pattern indicates that the same cell clones are responsible for synthesis of measles-specific IgG in both the CNS and serum 5
Progressive Nature of Antibody Levels
Serum IgG and IgA levels remain persistently elevated and show progressive rise in later clinical stages, correlating with the progressive nature of the illness 7
However, the measles antibody titer itself does not vary significantly with clinical stage or duration of illness—it remains consistently and dramatically elevated throughout 7
CSF IgG progressively increases with clinical stage, though not always accompanied by a corresponding rise in measles antibody titer, suggesting other antigenic determinants may play a role 7
Critical Diagnostic Distinction
This pattern must be distinguished from other conditions:
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, 2
Acute measles reinfection shows high-avidity IgG with IgM positivity but a normal CSF/serum index, whereas SSPE shows extremely high titers with an elevated CSF/serum index ≥1.5 2
In low-prevalence settings, false-positive IgM results can occur from other conditions (infectious mononucleosis, CMV, parvovirus, rheumatoid factor), requiring confirmatory testing with direct-capture IgM EIA method 2
Clinical Implications
The dramatically elevated measles IgG with intrathecal synthesis is present throughout the disease course, including the so-called "latent" period, because the virus is actively persisting and replicating in the CNS even before clinical symptoms emerge. 1, 2, 3 This makes serologic testing a reliable diagnostic tool even in early or subclinical disease stages.