EEG Findings During the Latent Phase of SSPE
The EEG is typically normal during the latent (preclinical) phase of SSPE, though rare cases have shown persistent focal abnormalities years before clinical onset. 1
Understanding SSPE Disease Phases and EEG Evolution
The latent phase of SSPE represents the asymptomatic period between initial measles infection and the emergence of clinical neurological symptoms, which can span 6-8 years or longer. During this phase:
Expected EEG Findings in Latent Phase
The EEG remains normal in the vast majority of patients during the latent phase, as the characteristic periodic complexes only develop after clinical symptoms appear 2
In exceptional cases, preclinical EEG abnormalities may appear years before symptom onset, consisting of focal paroxysmal discharges with shifting characteristics rather than the classic periodic complexes 1
One documented case showed persistent EEG abnormalities for 4 years before clinical SSPE onset, initially manifesting as multifocal paroxysmal discharges, followed by nonrepetitive generalized slow-wave complexes appearing closer to symptom onset 1
When Characteristic EEG Changes Appear
The pathognomonic periodic complexes of SSPE develop only after clinical manifestations begin, not during the latent phase 2, 3:
Classic periodic high-amplitude complexes appear in over 96% of patients once clinical disease is established (found in all but one of 31 patients in one series) 3
These periodic complexes consist of 2-4 high-amplitude delta waves, are usually bisynchronous and symmetrical, and repeat once every 5-7 seconds 3
A one-to-one relationship exists between clinical myoclonic jerks and periodic EEG complexes when both are present 2, 3
The typical SSPE repetitive complexes were observed 1 month after clinical onset in the documented preclinical case 1
Clinical Implications
Diagnostic Approach
Do not rely on EEG to detect SSPE during the latent phase, as it lacks sensitivity for preclinical disease 1
The presence of normal EEG does not exclude future development of SSPE in at-risk patients (those with history of measles infection) 1
If unexplained focal paroxysmal EEG discharges are found in a child with measles history, consider SSPE in the differential and obtain measles antibody titers in serum and CSF 1, 4
Key Pitfall to Avoid
The critical error would be assuming that a normal EEG rules out early or developing SSPE. The multifocal paroxysmal discharges observed in the rare preclinical case likely arose from restricted cortical areas containing persistently infected neurons, indicating that despite relative clinical tolerance, the brain was not completely unresponsive to viral presence 1. However, this represents an exceptional finding rather than the rule.