Pathophysiology of corticobasal degeneration: Insights from neurophysiological studies

BACKGROUND: Several studies have applied electrophysiological techniques to physiologically characterize corticobasal degeneration (CBD). METHODS: We performed a systematic literature search of these studies and reviewed all 25 identified articles. RESULTS: Conventional electroencephalography (EEG) is usually normal even in the late stages of disease. Quantitative EEG (qEEG) with spectral analysis revealed mainly lateralized abnormalities, such as an increase of slow wave activity and occasionally the occurrence of sharp waves, and a significant increase of coherence between left parietal-right premotor areas. CBD patients generally have long latency reflexes (LLR) with shorter latencies than in the classic cortical reflex myoclonus observed in progressive myoclonic epilepsy. The somatosensory evoked potentials (SEPs) showed reduced amplitude of the N20-P25 component. These abnormalities may reflect dysfunction of sensory projections to the motor cortex, while the localized parietal cortical damage is thought to be a pivotal factor for the absence of giant SEPs in these patients. Transcranial magnetic stimulation (TMS) revealed asymmetric intracortical disinhibition and asymmetric maps organization; an impaired transcallosal pathways function correlates with the atrophy of the corpus callosum. These findings suggest a pathologic hyperexcitability of the motor cortex, due to a loss of inhibitory input from the sensory cortex. CONCLUSIONS: Neurophysiological techniques, in combination with neuroimaging studies, may shed light on the pathophysiological mechanisms of CBD. A better understanding of the disease processes may help clinicians to make a more accurate and early diagnosis. TMS, SEP, LLR, and co-evaluation of EEG and EMG can aid the in differentiation between CBD and other parkinsonism syndromes.