Vegetative versus Minimally Conscious States: A Study Using TMS-EEG, Sensory and Event-Related Potentials

Abstract

Differential diagnoses between vegetative and minimally conscious states (VS and MCS, respectively) are frequently incorrect. Hence, further research is necessary to improve the diagnostic accuracy at the bedside. The main neuropathological feature of VS is the diffuse damage of cortical and subcortical connections. Starting with this premise, we used electroencephalography (EEG) recordings to evaluate the cortical reactivity and effective connectivity during transcranial magnetic stimulation (TMS) in chronic VS or MCS patients. Moreover, the TMS-EEG data were compared with the results from standard somatosensory-evoked potentials (SEPs) and event-related potentials (ERPs). Thirteen patients with chronic consciousness disorders were examined at their bedsides. A group of healthy volunteers served as the control group. The amplitudes (reactivity) and scalp distributions (connectivity) of the cortical potentials evoked by TMS (TEPs) of the primary motor cortex were measured. Short-latency median nerve SEPs and auditory ERPs were also recorded. Reproducible TEPs were present in all control subjects in both the ipsilateral and the contralateral hemispheres relative to the site of the TMS. The amplitudes of the ipsilateral and contralateral TEPs were reduced in four of the five MCS patients, and the TEPs were bilaterally absent in one MCS patient. Among the VS patients, five did not manifest ipsilateral or contralateral TEPs, and three of the patients exhibited only ipsilateral TEPs with reduced amplitudes. The SEPs were altered in five VS and two MCS patients but did not correlate with the clinical diagnosis. The ERPs were impaired in all patients and did not correlate with the clinical diagnosis. These TEP results suggest that cortical reactivity and connectivity are severely impaired in all VS patients, whereas in most MCS patients, the TEPs are preserved but with abnormal features. Therefore, TEPs may add valuable information to the current clinical and neurophysiological assessment of chronic consciousness disorders.

Transcranial Magnetic Stimulation and Electroencephalography

“…These results indicate a clear-cut difference of TMS response between unresponsive and minimally conscious patients, which has also been confi rmed recently (Ragazzoni et al 2013 ). A subset of severe acquired brain-injured patients was also evaluated several times in the acute setting.…”

Functional neuroanatomy of disorders of consciousness

“…This technique allows noninvasive stimulation of a subset of cortical neurons, measuring the effects of the perturbation across the cortex [51]. Stimulation of a superficial region of the cerebral cortex of patients with VS/UWS with TMS has been shown to either induce no response or trigger a simple, local EEG response, indicating a breakdown of effective connectivity [52,53], similar to that observed in deep sleep and anesthesia [54,55]. In contrast, in patients with MCS, TMS triggered complex EEG activations which sequentially involved distant cortical areas, similar to activations recorded in locked-in patients, in healthy awake subjects, and during vivid dreams [53,54,56].…”

Assessing consciousness in coma and related states using transcranial magnetic stimulation combined with electroencephalography

“…Another study recently confirmed the ability of the TMS-EEG to differentiate UWS from MCS patients and showed the superiority of the TMS-EEG compared to traditional neurophysiological methods, such as short-latency somatosensory evoked potential and event-related potentials [54]. In the most recent study, brain responses were compared between conscious patients with mild brain injury and healthy awake subjects [55].…”