Progression of myoclonus subtypes in subacute sclerosing panencephalitis

Diagnostic criteria of subacute sclerosing panencephalitis (SSPE) include myoclonus, a well-recognized clinical feature. Here, we studied the electrophysiological features of myoclonus with regards to disease staging in SSPE patients.

We included 10 patients diagnosed with SSPE between 2010 and 2018, along with 21 healthy subjects. All participants had detailed electrophysiological evaluation including polymyographic analysis, blink reflex after trigeminal stimulation, auditory startle response, startle response after somatosensory stimuli, F-waves, and long-loop reflexes. Clinical findings were retrieved from the medical records.

Patients were categorized into Gascon stage 2B (n = 5, 50%), 2A (n = 2, 20%), 3B (n = 2, 20%) and 4A (n = 1, 10%) at the time of electrophysiological evaluation. Two patients had cortical myoclonus, four had possible cortico-subcortical myoclonus, and four had brainstem myoclonus. Patients were categorized into Gascon stages 2a and 2b had possible cortico-subcortical myoclonus (85.7%). However, none of the patients with stage 3b or 4a had possible cortico-subcortical subtype but all had the brainstem subtype.

Association was seen between subtypes of myoclonus and clinical staging in SSPE. This suggests that myoclonus in SSPE may primarily involve the cortex and cortico-subcortical structures such as the thalamus at earlier stages of disease, and then involve more caudal structures as the disease progresses.