Disorders of consciousness (DoC) are neurological disorders caused by traumatic brain injury (TBI), such as car accidents or falls, and non-traumatic brain injury (non-TBI), such as stroke, etc., and are characterized by decreased awareness and responsiveness. Patients with DoC may exist in states ranging from the unresponsiveness of a vegetative state to minimal yet discernible signs of consciousness found in minimally conscious states (MCSs) (1). These conditions are particularly challenging for clinicians because of the patient’s limited ability to interact with their environment, complicating the assessment of cognitive and sensory functions (2).

Traditional clinical evaluations rely on behavioral scales, like the Coma Recovery Scale-Revised (CRS-R), to provide a standardized method for evaluating the level of consciousness (3, 4). In 2004, the Coma Recovery Scale-Revised (CRS-R) was published, comprising six subscales that assess auditory, verbal, visual, communicative, motor, and arousal domains. The CRS-R includes 23 hierarchically structured items, with scores ranging from 0 to 23, with higher scores signifying greater levels of awareness and functional capacity (5, 6). However, subtle and sporadic responses in DoC can lead to diagnostic ambiguities and difficulties in predicting recovery. The need for more objective and sensitive measures has spurred the exploration of advanced neuroimaging techniques that offer insights into the functional architecture of the brain beyond the confines of behavioral observation (7).

In China, some medical units have prioritized research into wakefulness treatments of patients with DoC. SCS represents a neuromodulation therapy for patients diagnosed with DoC. The stimulation electrode is implanted at the C2–C4 level of the high cervical vertebrae through the epidural cavity. Sending electrical stimulation pulses increases the nerve impulses of the upward reticular activating system (8, 9). The neural conduction is improved, the electrical activity of the nerve cells is enhanced, and neurotransmitters are released as a result of the stimulation. This leads to an increase in blood flow and brain metabolism, promoting wakefulness. The stimulation frequency is 50–100 Hz, the pulse width is between 200–500 microseconds, the amplitude is adjusted to the threshold tolerated by each patient to optimize the therapeutic effect without causing discomfort, and the duration of the stimulation is based on the patient’s clinical response as assessed periodically (10, 11). Resting-state functional MRI (rs-fMRI) has recently become a valuable tool for studying residual brain function in patients with DoC. Since these patients often cannot participate in active tasks in a nuclear magnetic scanner, rs-fMRI is an ideal method to study their brain activity (12). Information regarding the structural and functional integrity of the patient’s brain is obtained through direct functional imaging (13, 14). The amplitude of low-frequency fluctuation (ALFF) indirectly reflects spontaneous brain activation through changes in the BOLD signal, indicating brain abnormalities in the resting state (15–18). An increase in the ALFF indicates heightened spontaneous neuronal activity in the corresponding brain area, suggesting that the activation of this functional area may play a compensatory role in the loss of certain cognitive functions. Conversely, reduced ALFF values suggest a diminished intensity of spontaneous neuronal activity in the corresponding brain regions. This implies that the decreased neural activity in these areas may be associated with a decline in related cognitive functions. Consequently, ALFF can be a valuable tool for investigating fundamental brain activity in various diseases and objectively assessing overall brain function. It helps quantify consciousness levels in patients with DoC, detect residual consciousness, and evaluate the outcome of consciousness in the early stage (19–22).

Despite these promising findings, the field is still developing, and larger, more comprehensive studies are needed to validate these preliminary results (23). There is also an urgent need for nuanced biomarkers to precisely capture the complexity of consciousness (24). Therefore, this study aims to explore the predictive value of ALFF in rs-fMRI for assessing the efficacy of SCS in patients with DoC. By examining the relationship between pretreatment ALFF values and subsequent changes in consciousness levels after SCS, this study seeks to enhance our understanding of the neurobiological substrates of consciousness and support the development of targeted therapeutic interventions (25), potentially improving recovery prospects and quality of life.

Between September 2019 and November 2022, we recruited 58 patients with DoC at the Department of Neurosurgery at Peking University International Hospital. Patients’ consciousness levels were assessed preoperatively and 3 months postoperatively using the CRS-R score. Inclusion criteria were: being over 18 years old with no gender restriction; a diagnosis of DoC such as vegetative state (VS) or unresponsive wakefulness syndrome or MCS; no contraindications to MRI like metal implants or pacemakers; no sedative medication used within 24 h before the operation; and written informed consent provided by the legal guardian. Exclusion criteria included poor image quality due to excessive head movement (translation ˃3 mm or rotation ˃3°) during MRI, a small sample size, inability to complete follow-up or incomplete follow-up data, a history of psychiatric illness or severe cognitive dysfunction before the onset of the disease, and refusal of participation by legal guardians. Ultimately, 53 patients met the requirements based on the nadir criteria, and to explore whether there is a potential commonality between patients with DoC due to different aetiologies for SCS wake-up promoting treatment, they were divided into traumatic brain injury group (31 patients) and non-traumatic brain injury group (22 patients) according to the etiology. The general data for both groups are shown in Figure 1 and Table 1.

After SCS, all patients underwent resuscitation and rehabilitation at our hospital and were followed up at 3 months. Follow-up was conducted via telephone for those discharged or transferred to other facilities. All patients were assessed using the CRS-R by a senior neurosurgeon specifically trained and qualified in its administration. (Items of CRS-R scale: patients in VS state show very low neurobehavioral responses on the CRS-R assessment, with scores below thresholds on all subscales: auditory ≤2, visual ≤1, motor ≤2, oral motor/verbal ≤2, and communication 0, reflecting a lack of awareness and responsiveness to the environment). Patients in the MCS state, show much higher neurobehavioral responses, with scores above the thresholds for VS: auditory 3–4, visual 2–5, motor 3–6, oro-motor/verbal 3, and communication 1–3, indicating some ability to perceive and respond to the environment, as well as more complex motor and communication behaviors. (Chart of CRS-R in the Supplementary Table S1 shows white for VS, light blue for MCS, and dark blue for EMCS).

No statistically significant difference was observed in the general demographic characteristics between the two groups of patients, as shown in Tables 1, 2.

The ALFF method in rs-fMRI was used to assess residual brain activity in patients with DoC with two etiologies and evaluate it in conjunction with the CRS-R scores and consciousness outcomes at 3 months post-SCS surgery. This approach aimed to provide insights into predicting the improvement of consciousness in patients with DoC after wakefulness-promoting treatments.

In this study, elevated ALFF values were observed in the thalamus and anterior cingulate cortex in the TBI group. The thalamus, an important brain structure linked to arousal and consciousness (26–29), is often referred to as the “on/off switch” for consciousness (30, 31). Thalamic activity levels are associated with a patient’s level of arousal (32), and decreased functional connectivity between the thalamus and cortex occurs during unconsciousness (28, 33). Damage to the thalamus and brainstem, as well as diffuse axonal injury, are the most common manifestations of DoC in autopsy and MRI studies (34–37). An increased ALFF in the thalamus may reflect the upregulation of neuronal activity in this brain region, which is a compensatory mechanism for the disruption of thalamocortical circuits (30). An increased ALFF can facilitate the reestablishment of cortical activation patterns necessary for consciousness (31), which may also be essential for the facilitation of arousal in patients with DoC. The cingulate gyrus is critical for managing cognitive and emotional functioning, overseeing conflict resolution, and processing social interactions (38–40). Increased ALFF in the cingulate gyrus may be associated with improved cognitive functioning in patients.

Interestingly, when comparing both groups of patients with DoC with those who showed improved consciousness, both groups of patients with unchanged consciousness had decreased ALFF values in the precuneus and cingulate gyrus. Conversely, both groups of patients with improved consciousness had increased ALFF in these brain regions. The precuneus, located on the medial side of the superior parietal lobule and hidden by the medial longitudinal fissure, plays a crucial role in restoring consciousness. Increased ALFF in the precuneus is associated with improved consciousness. The precuneus’s degree of centrality positively correlates with consciousness recovery in DoC patients, especially when comparing TBI and non-TBI cases, supporting this study’s findings (41). Our study aligns with existing research highlighting the precuneus’s key role in higher cognitive functions. Ionta’s et al. (42) work shows its extensive connectivity to the self-awareness network, especially between the right temporal-parietal junction and the insula, which are highly sensitive to self-positioning and first-person perspective changes. Cavanna and Trimble’s (43) anatomical and MRI studies confirmed the precuneus lobe’s activation during self-related tasks, especially in visuospatial activities and action attribution. These findings highlight the precuneus’s essential role in integrating sensory information and self-awareness, providing a crucial neurobiological basis for cognitive neuroscience and neuropsychiatric disorder treatment. The precuneus plays a central role in consciousness-related functional network aspects. It is a part of the default mode network (DMN) (41), which is associated with consciousness (43). Deficits in consciousness are closely related to the functional dissociation of the DMN, which plays an important role in the origin of consciousness (36, 44, 45). Previous studies have found that comatose patients retain spontaneously synchronized DMN activity (46, 47), suggesting that the different brain regions involved in the DMN may be correlated with the degree of improvement in consciousness. Elevated ALFF values in the precuneus may imply that DMN-connected brain regions are reactivated, which is crucial for self-awareness recovery and environmental interaction and may be a critical point for consciousness recovery (44). Notably, the precuneus is also among the earliest brain regions to show signs of recovery in patients with DoC with improved consciousness (45). A moderate positive correlation was identified between ALFF values and CRS-R scores in the precuneus. This finding suggests that the intensity of local neural activity in the precuneus may be directly correlated with the level of consciousness in patients with DoC. Increased ALFF values, which reflect the intensity of local brain activity, may indicate enhanced neural activity in this region. In the context of impaired consciousness, this increased neural activity could be associated with improvements in the patient’s level of consciousness. A significant decrease in the functional activity of the precuneus indicates an increased degree of impaired consciousness (1). This study found that patients with DoC from both etiological groups who showed no improvement in consciousness after 3 months of SCS treatment had significantly lower ALFF values in both the precuneus and cingulate gyrus. This may account for the poor improvement in consciousness observed. One possibility is that ALFF acts as a proxy for the intrinsic functional connectivity of the brain, which is often disrupted in the brain networks of patients with DoC (1). Improved functional connectivity might facilitate the reintegration of neural networks required for consciousness, as suggested by increased ALFF (48). Brain plasticity may also play an important role in this process. Neuroplasticity allows for the reorganization of brain circuits following injury and increased ALFF in key regions may indicate that these regions are undergoing adaptive changes, potentially facilitating the recovery of consciousness. The biological relevance of ALFF lies in its capacity to elucidate the intensity of spontaneous neural activity in specific brain regions. In patients with DoC, an increase in ALFF may indicate enhanced neural activity in specific brain regions, potentially representing a compensatory mechanism for loss of cognitive function. For example, in the present study, patients with DoC associated with TBI exhibited increased ALFF in the thalamus and anterior cingulate cortex, which are areas strongly associated with arousal and consciousness. An increase in ALFF may reflect enhanced neural activity in these regions, contributing to the re-establishment of cortical activation patterns, which is essential for facilitating arousal in patients. Conversely, a reduction in ALFF may indicate a decline in neural activity in the relevant brain regions, which is associated with a deterioration in cognitive function. In this study, decreases in ALFF were observed in the occipital and temporal lobes of patients with DoC. These regions are typically associated with the processing of sensory input and language functions (49, 50). The diminished activity of these regions may be associated with the sensory and cognitive impairments observed in patients. The potential of ALFF as a biomarker is based on its to predict the response to SCS therapy in patients with DoC. Notably, the present results indicate that patients whose level of consciousness improved following treatment exhibited disparate ALFF patterns in the posterior cingulate cortex and occipital lobe regions compared to those whose conditions remained unchanged. This result indicates that ALFF not only reflects the current state of neural activity but may also predict the patient’s response to treatment and the potential trajectory of consciousness recovery.

Despite these insights, our study was limited by its relatively small sample size and short follow-up period, which may affect the generalizability of our findings. Future studies with larger cohorts and long-term follow-ups are required to confirm these preliminary results. Moreover, while ALFF measures regional neural activity, it does not directly assess cognitive function or connectivity. Combining ALFF with other neuroimaging techniques, such as functional connectivity analysis or diffusion tensor imaging, may provide a more comprehensive understanding of the neural substrates of consciousness and cognition in patients with DoC. Additionally, The absence of a control group in this study limits the ability to definitively that the observed improvement in consciousness was solely attributable to SCS therapy, rather than the natural recovery of the patients or other factors. For patients with impaired consciousness who did not receive SCS therapy, obtaining clinical information, such as MRI and CRS-R scores, was challenging as they were typically managed at home. To enhance the precision of the findings, future studies should include MRI data from the same patients with impaired consciousness before and after SCS therapy (removal of SCS electrodes) to account for potential confounding factors.

In conclusion, our study suggests that ALFF measured using rs-fMRI is a promising biomarker for assessing cognitive function and levels of consciousness in patients with DoC. The observed correlation between the ALFF and consciousness highlights the potential of neuroimaging techniques in guiding therapeutic interventions and predicting outcomes for these patients. Further studies are needed to elucidate the mechanisms underlying the association between ALFF and cognitive recovery and to identify indicators of neural activity that predict the key brain regions closely associated with the recovery of consciousness in patients with DoC.