1 Introduction

1.1 Background

Pain, a feeling everyone tries to avoid, may come from disease, injury, retrogression, medical treatment and other sources. Despite our dislikes, pain serves as one of the bases for numerous lives on earth. It directs beings, not only humans but also other creatures, to notice danger, to avoid threat and to seek remedy so as to survive this competitive world. Therefore, human beings’ strategy of coping with pain is not to eliminate it completely, which might be possible in the near future, but rather to relieve it and have it under control when necessary.

As history progresses, humans have developed various ways of pain relief, including chemical pharmacy, physical therapies as well as some mysterious yet effective treatments such as hypnosis and acupuncture. However, there has been no ultimate solution in this field till today, for these methods either have negative side effects or are not applicable or effective under some circumstances. In this situation, the explorations for new ways of pain relief have never stopped, among whose outcomes virtual reality is one of the latest inventions.

Virtual reality, usually abbreviated as VR, is a technology that exploits the latest advancements in computer science, in both hardware and software, to fabricate a virtual world for its users in order to create their illusion of presence in that cyberspace. Furthermore, VR has the inherent potential of being interactive because its content is generated in real time, not ahead of time, which makes the experience even more immersive. Many scholars believe that VR has a promising value in pain relief mainly because of its capability, though not yet rigorously verified in a scientific manner, as a distractor. Despite the doubts whether VR actually helps to cure a disease or physical disorder, abundant evidences have been collected, showing its superiority in relieving pain and other kinds of discomfort caused by disease or the treatment procedures, and thus raising the acceptance of unpleasant therapies as well as patients’ quality of lives.

There have been a number of studies concerning the application of VR technology in pain relief, and the interest in it continues to grow. A search for articles published between 1995 and 2015 (October) from the Web of Science databases with the keywords “virtual reality” and “pain” gave a report as shown in Fig. 1. A sharp increase in the number of articles published and cited in recent 20 years shows clearly that more and more attention has been drawn to this field.

Fig. 1.
figure 1

A search report from web of science

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Also, there are quite a few review articles in this field, while this one aims to serve a different purpose from a different perspective. Besides giving a general overview of the existing research outcomes, this article also focuses on some special instruments, methods and results that are worthy of revisiting and further inspection. These articles, though small in ratio, may suggest future research directions and bear great potential value.

1.2 Procedures and Criteria

All the reviewed articles were collected from three important databases, which were Web of Science Core Collection, Medline database and EI Compendex database, via a search for articles published between 2006 and 2015 using key words “virtual reality” and “pain”.

There were a total of 513 hits from 3 databases. The first pass filtering was conducted to remove apparently irrelevant articles by judging from their titles, and 245 remained (111 from Web of Science Core Collection, 99 from Medline Database and 35 from EI Compendex database). Then, a second pass filtering was conducted to remove duplicated ones found in multiple databases and 171 remained. These 171 articles were read carefully and filtered for a third time using the following inclusion criteria:

  • VR technology was used to relieve pain (or other kinds of discomfort), not to evaluate symptoms, assist treatment planning or educate medical practitioners;

  • The article must contain practical data from experiment, instead of merely presenting a protocol or a future research plan;

  • VR technology must play an important role in the experiment, preferably as a subject for comparison, instead of being briefly mentioned or introduced;

  • It should not be another review article. Review articles were used as references but not included in the final list;

  • If more than one articles were written by the same author(s) and based on similar experiments, only the most cited one was kept. If the citation was unknown, the latest one was kept;

  • The full text version must be available.

After filtering 43 articles remained in the final list. A spreadsheet was created to store extracted information in an organized way. In addition to general information such as the title, author, sponsors, source, time and etc., the other important labels of the classified information were as follows:

  • Objective;

  • Demographic information of the subjects;

  • Sample size;

  • Subject’s health status;

  • Kind or source of discomfort;

  • Hardware and software used;

  • Comparative targets;

  • Research methods and procedures;

  • Measures and instruments;

  • Findings and conclusion.

Based on this organized information, we obtained the following review findings.

2 Common Research Patterns and General Conclusions

The application of VR technology in pain management has been investigated for many years. The research patterns used in most research projects are as follow:

2.1 Pattern A: To Evaluate the Efficacy of VR Therapies

The experiments done in this kind of researches were usually temporal. Researchers firstly inspected subjects’ pain, anxiety and other relevant sensory and emotional levels. Then, after the subjects had receive VR therapies, they were evaluated again to check whether VR therapies contributed to the relief of pain and other discomfort. It was used to test the efficacy of VR therapy among patients suffering from complex regional injury [1], fibromyalgia [2], cancer [3] and motor dysfunction and associated neuropathic pain [4]. All these experiments shew positive result, which meant VR therapies helped the patients to improve their mental status.

Because only VR therapies were investigated, usually this pattern didn’t require many experiment cycles and complex environments, making it simple and easy to implement. But the results might be questionable because it was difficult to tell whether VR treatment or self-recovery over time played the most important role. And it was also difficult to answer the question whether VR was a better choice since no other therapies were compared with it.

2.2 Pattern B: To Compare VR Therapies with Other Analgesic Methods

This was the commonest research pattern adopted by many researchers. In this pattern, VR therapies were usually compared with other kinds of chemical and/or physical analgesia to conclude whether (and/or how) VR therapies were superior in relieving pain and unpleasantness caused by a certain reason such as disease [5], burn [6–8], injury [9, 10], medical treatment [11, 12] and so forth. When healthy subjects participated in the experiment, pain was often inflicted artificially with heat stimulus [13], cold pressor [14, 15] or other equipment [16].

In these experiments, standard care without VR usually served as the baseline. In addition to it, the analgesic effect of VR was compared to a lot of other techniques, including TV programs, music, books or even lollipops [12, 17, 18].

As far as experiment design was concerned, there were two different kinds: the within-subject design and the between-subject design. In the former one, the same group of subjects received more than one treatment, randomized in order, and then underwent the same inquiry procedures to conclude which treatment worked better. In the latter one, subjects were randomly assigned to different groups to receive different treatments, and then similar procedures followed. In this research field, more studies used the within-subject design.

Finally, over 80 % of the reviewed articles drew positive conclusions. They supported the opinion that VR therapies might have great potential in pain management, not only because it was proven to be effective among sick and healthy subjects but also because it had very little side-effect and was much safer than other aggressive or offensive therapies.

2.3 Pattern C: To Explore the Factors Affecting the Effectiveness of VR Therapies

This pattern was used not so often, but the results were usually interesting and valuable. In addition to answering the questions whether VR therapies were useful and more effective than other traditional therapies, this research pattern tried to answer the question what might affect the usefulness of VR therapies.

This pattern was obviously valuable because the effectiveness of VR therapies could hardly get improved until the influential factors were clarified. However, this pattern was, from the reviewers’ point of view, the most difficult one by far. Firstly, there might be a great number of influential factors, major or minor, superficial or profound, on users’ side or VR’s side. To figure out the most significant ones was not an easy job. Secondly, some variables on VR’s side must be adjusted so as to assess their influence, which usually meant that those ready-made VR solutions from 3rd-party organizations could not be used. In this case, the researches would have to, if not by themselves, produce VR materials for intended experiments, which might be expensive and time-consuming. Thirdly, this pattern generally required stricter and more accurate variable control techniques to avoid unwanted overlapping and cross effects.

There were a few successful attempts. They will be introduced in Sect. 3.1.

3 Mentionable Research Projects

Among all the articles reviewed this far, some have uncommon features where future research opportunities possibly lie and are thus worth special attention.

3.1 In-Depth Analysis of Influential Factors

As mentioned in Sect. 2.3, pattern C, to explore the VR therapies’ influential factors, was difficult but important. Many unique research projects and interesting findings were carried out and obtained with this pattern.

One of the article compared high tech VR, i.e. high quality VR content presented with more advanced hardware, with low tech VR, and concluded that the high quality one was more effective [19].

Another article used a series of experiments to verify the hypothesis that a cold VR environment might be more effective than a hot one when a subject was suffering from pain caused by heat and vice versa only to find that the difference between them was not significant [20].

There was another research trying to find out whether the same game played with a first-person view, which was believed to be more immersive, brought better analgesic effect than that played with a third-person view, and concluded that the they did not result in significantly different improvements in pain tolerance [21].

Similarly, one of the researches tried to test the hypothesis that increased body movement while steering a VR game led to the diminished experience of pain, but also failed to find significant relationship between body engagement and pain tolerance level [22].

There were also some experiments trying to measure the value of using HMDs (head mounted displays) in VR therapies, but they all concluded that the value of HMDs was questionable [16, 23, 24].

3.2 Unique Perspectives

In one study, 137 cancer patients receiving intravenous chemotherapy participated in 3 experiments to help to explore the influence of age, gender, state anxiety, fatigue and diagnosis on time perception with a VR distraction intervention [25]. PC games were used as the test material and were presented with HMDs. The author concluded that clinicians should not assume that all patients would become distracted and experience altered time perception while using VR and not all patients welcomed distraction during unpleasant medical treatments. Among all the reviewed articles, this was the only one trying to explore the influence of demographic properties.

Another article introduced an innovative pain management system, Epione, which formed a dynamic pain meter by using facial expression analysis, triggering biofeedback and augmented reality based destruction scenarios in an effort to maximize patient’s pain relief [26]. Among all the reviewed articles, this was the only one that introduced the concept of intelligent self-adaptive VR system and community based mutual aid. The result was positive. According to the authors, the holistic approach and modular development of Epione allowed for flexible expansion to many user-scenarios, making it a supporting environment to the whole community, providing a better quality of life.

3.3 Unusual Results

Despite the fact that most research results were positive, there were still negative results denying the efficacy of VR therapies. Although they were small in number (only one found in our case) and might stem from imperfection in experiment design, these negative results might also suggest unnoticed or not yet understood defects in VR therapies. As a result, they well deserve revisiting and further verification.

This study used 86 adults suffering from burn pain to examine whether pre-procedural VR guided relaxation added to patient controlled analgesia with morphine reduced pain severity during awake dressing changes [27]. This study used cross-subject design and subjects were randomly assigned to 2 groups to receive either VR relaxation plus intravenous morphine PCA infusion or intravenous morphine PCA infusion alone. The result was that VR relaxation added to morphine PCA was associated with significantly greater pain intensity during and after awake dressing changes in the patients with burns.

4 Future Research Opportunities

Investigation of the application of VR technology in pain management started more than 10 years ago, and many valuable results have been already obtained. Researchers generally support its effectiveness, however, the underlying principles and mechanisms of VR therapies still remain in a black box. On the technology side, although VR is not a brand new concept, it is thriving and advancing rapidly these years thanks to the development of hardware and software industries. This tendency will also bring more research opportunities to this field.

In the authors’ point of view, future research opportunities are very likely to be found in the following directions.

4.1 Better Understanding of VR Therapies

As introduced in Sect. 3.1, there are already researches trying to find out what are the influential factors of VR therapies. Only the experiment comparing high tech VR and low tech VR gave a conclusion that high quality ones are more effective. Other experiments all failed to establish a relationship between a potential influential factor and the effectiveness of VR therapies. But in common sense it is not reasonable to believe all VR therapies, no matter what their contents are and how their users are designed to experience them, will produce the same analgesic effect. This means more researches are needed to verify these results with larger sample sizes. Other potential influential factors are also to be discovered and estimated.

In addition, the underlying theories of VR therapies still remain a myth. Most researchers believe that its analgesic effects come from its power as a distractor. But what is actually happening on the psychological and neurological level remains to be unveiled. To make this kind of theoretical and profound discoveries is by no means easy, but it will bring about invaluable breakthroughs when successfully accomplished.

4.2 Better Segmented Subjects

Children, especially young kids are often dealt with as a specially group. The most likely reason is that they lack the ability to experience VR and express their feelings as well as adults. Expect for children, though, most research projects didn’t differentiate subjects based on their demographic features and very few researches tried to how these features might affect experiment outcomes.

There ought to be differences between male and female users, well-educated and poorly-educated users, gamers and non-gamers. Adults shall also be divided into young, middle-aged and elderly people and they have different abilities of perception and comprehension. Moreover, Asian, American, European, African residents may also respond differently to the same VR therapy. There are even more criteria to be defined, and better segmented subjects will lead to more accurate and reliable results and conclusions.

4.3 Application of Latest Technologies

As mentioned above, VR technology is undergoing fast development lately. Many manufacturers are inventing new input and output devices to cater for the increasingly demanding requirements from VR users. Although most of the attention was paid to the entertainment field due to commercial motivations, new technologies may still well suit the need of the medical field. The reason is simple: the entertainment industries always try their best to make VR more immersive and attractive, and here lies the same power that distracts attention from pain, the essence of VR’s analgesic effects.

So it’s recommendable that researchers keep an eye on the latest technological advances, and when a new device or technology comes out, be the first to test it in pain management and other relevant fields.