Spinal cord injury (SCI) causes motor and sensory deficits below the damaged level, reducing patient quality of life [1]. Depending on the level of injury, SCI can lead to serious complications, including autonomic dysreflexia, spasticity, respiratory system impairment, disturbances of the urinary and gastrointestinal systems, and sexual dysfunction.

The prevalence of SCI worldwide ranges from 10 to 100 cases per million individuals [2]. Approximately 60% of spinal cord-related injuries are cervical spinal cord injuries [3], which often involve severe losses of arm and hand functions and diminish life satisfaction [4]. As upper limb function plays a key role in independent daily activities such as self-management, respiration and sphincter management, and mobility, improving the upper extremity function is an important goal in rehabilitation [5].

The conventional rehabilitation (CR) is known to improve upper limb function [6], but providing sufficient treatment is labor intensive and expensive [7]. In addition, the traditional rehabilitation methods involving simple, repetitive movements may be tedious for the patient, making them less motivated to continue the treatment [8,9]. Therefore, new approaches must be introduced to overcome the shortcomings of the CR methods. One of the potential approaches is rehabilitation using virtual reality (VR) technology.

VR can be classified into non-immersive, semi-immersive, and fully immersive VR according to the user’s immersion level. Non-immersive VR systems have been used for many years in rehabilitation therapy for the purpose of cognition, gait, and balance training, including upper extremity motor function. Many previous studies in stroke patients have reported that VR is useful for rehabilitation [10] and can achieve greater improvements than the conventional therapies [11].

In contrast to non-immersive VR systems, in which users experience both the real world and virtual environment, immersive VR systems integrate users into an environment in which all real-world perception is blocked, so only computer-generated images are seen. Head-mounted display (HMD) devices are typically used to deliver a fully immersive VR experience. HMD devices can block the perception of the external environment to create a more realistic VR environment with a high level of immersion and visual scenes based on the user’s movements. As immersion increases, the emotional impact of VR on the user increases, and the desired physiological response can be driven through the production of the VR experience [12]. VR can be used to motivate patients by offering various realistic sensory experiences and entertainment-based treatments [10,13]. Furthermore, VR can be used as a practical technology to encourage patients to exercise regularly and simulate physical movements involved in everyday life for the purpose of improving patients’ capacity to cope with real-world situations [10,11].

Although many VR systems have been used in stroke patients and have produced promising preliminary results [8,10], few studies have examined their usefulness in patients with SCI. Thus, the purpose of the present study was to determine whether a fully immersive VR intervention combined with CR can improve upper limb function more than CR alone in patients with SCI.

Table 1 shows the participants’ baseline characteristics. The study included 10 EG participants (6 males and 4 females) and 10 CG participants (8 males and 2 females) with motor-incomplete cervical SCI. No significant differences in baseline characteristics were observed between the two groups. Both upper extremities were evaluated separately, expressing the results as mean values, but no statistical differences were found. None of the study participants experienced an adverse event due to interaction with the virtual environment (e.g., nausea, dizziness, and headache) or falls, and none of the participants dropped out of the study.

The goal of this study was to evaluate improvements of arm and hand functions and to assess the usefulness of a VR treatment program for cervical SCI as an adjunctive treatment modality for upper limb rehabilitation. The study was conducted with 16 sessions over 4 weeks, and we compared the results of VR treatment combined with CR in in the EG and those of CR alone in the CG.

This study demonstrated that VR training combined with CR resulted in functional improvement, particularly in grip power and K-SCIM score (Table 2). In addition, elbow extensor, wrist extensor, ASIA-UEMS, grip power, lateral pinch power, and palmar pinch power were all significantly improved as compared with those in the CG. Few studies have examined the VR methods in patients with SCI, and the findings of this study were inconsistent with previous studies. Dimbwadyo-Terrer et al. [14] reported that VR combined with CR showed similar results with CR alone in patients with tetraplegia. Unlike our study, that of Dimbwadyo-Terrer et al. [14] only targeted patients with motor-complete SCI (ASIA-A and ASIA-B), of which ASIA-A accounted for 11 (69%) of the 16 participants in the EG and 10 (67%) of the 15 in the CG. The participants in the EG played an ADL-based VR game to improve performance in basic ADL (eating, combing hair, or washing the face) instead of using controllers but freely moved to manipulate virtual objects. Another study by Prasad et al. [15] in 2018 also reported no significant difference in hand function improvement between the two groups. The participants in the EG used a controller in this study, but the study included both subjects with motor-complete and incomplete SCI, with the sum of ASIA-A and ASIA-B accounting for 7 (58%) of the 12 participants in the EG and 10 participants (70%) in the CG, respectively. In addition, the time from SCI onset to enrollment in the present study was about a month, while those in the studies by Dimbwadyo-Terre et al. [14] and Prasad et al. [15] ranged from 4 to 5 months and from 10 to 15 months, respectively. Previous research has shown that the potential for neurological recovery was greater in incomplete SCI than in complete SCI [16], motor recovery decreases 6 months after injury [17,18], early rehabilitation was effective, and the possibility of motor recovery was higher [19]. The discrepancy of findings between this study and the previous studies might be due to the fact that this study performed early rehabilitation for patients with motor-incomplete SCI who had been injured 1 month before or after rehabilitation.

However, previous studies in stroke patients showed consistent results. In a meta-analysis by Laver et al. [20], training with VR in stroke patients was found to improve ADL, with a moderate effect size. In another systematic review and meta-analysis, Lee et al. [21] reported that VR therapy improved muscle strength and ADL function in chronic stroke patients. Several recent studies have reported that rehabilitation using VR helped stroke patients improve their gross motor function [22]. Furthermore, several previous studies reported that VR training can be more effective and intensive than the existing treatments for improving functions in patients with damaged upper extremities due to stroke [23].

We observed functional improvements in both groups, but the EG participants in the present study may have exhibited improved hand strength because they exercised with controllers during the 30-minute intervention period, consistent with a previous study by Oh et al. [24], who reported improved hand strength after a VR intervention using a controller. Besides, the participant sees the results in the form of a score and receives positive feedback. Participants can develop advanced strategies for earning scores while viewing feedback scores during VR therapy [25]. This not only interests subjects but also triggers motor learning process.

Although improvements in upper limb function have been observed after VR treatment with CR in stroke patients, the evidence that applies to tetraplegia is still limited. Therefore, we believe that the present trial provides new insights into the role of VR applications in improving upper extremity function.

However, the present study involved several limitations that should be considered. First, the study sample was comprised of patients with tetraplegia caused by incomplete SCI only. Many previous studies examined patients with different prognoses in incomplete and complete injuries [26-28]. Therefore, patients with complete SCI must be included in future studies to determine the effects of damage due to SCI on VR treatment. Second, this was a relatively short-term study consisting of 16 rehabilitation sessions over 4 weeks. Long-term studies should be undertaken to gain a deeper understanding of the underlying mechanisms, as a short intervention period could not confirm whether VR is more effective than the traditional treatment for upper extremity rehabilitation. Finally, the patient sample in the present study was relatively small. Future studies with larger samples will be needed to confirm the present findings.

Despite the limitations of the present study, our findings demonstrate that VR training of upper limb function after SCI can provide an acceptable adjunctive rehabilitation method without significant adverse effects. In the future, larger controlled studies will be necessary to further evaluate the tolerability, feasibility, and efficacy of VR training.