Introduction

The unresponsive wakefulness syndrome, previously named vegetative state (UWS/VS) [1], and the minimally conscious state (MCS) are one of the worst possible outcomes of acquired brain injury. Patients in UWS/VS show no signs of consciousness [2], whereas MCS patients demonstrate minimal signs of consciousness such as following simple commands, gestural and/or verbal yes/no responses, intelligible verbalization, and purposeful behavior [3]. Complexity of behavior varies between MCS patients; therefore, subcategorization into MCS − (minus) and MCS+ (plus) was proposed. Patients in MCS − only demonstrate non-reflex behavior, whereas MCS+ patients demonstrate command following [4]. Differentiating between UWS/VS and MCS is difficult, as demonstrated by misdiagnosis rates of around 40% [5,6,7,8]. A correct diagnosis of MCS is important for several reasons. First, prognosis is more favorable compared to UWS/VS. A follow-up study showed that improvement beyond a year was absent in UWS/VS patients, whereas 1/3 of MCS patients emerged to consciousness beyond a year [9]. Second, MCS patients might have pain perception capacity, which has consequences for pain management [10]. Third, MCS patients have better outcomes from early intensive neurorehabilitation [11,12,13], recently confirmed in a long-term follow-up study [14]. Fourth, MCS patients may benefit from promising treatment options such as deep brain stimulation [15, 16] and pharmacologic therapies [17,18,19]. Compared to UWS/VS, other ethical dilemmas may arise in MCS patients, e.g., regarding suffering or withdrawing or withholding medical treatment [20].

Currently, an accurate diagnosis of MCS is based on behavioral assessment. Techniques, like neuroimaging, have not been implemented in clinical practice yet. Visual pursuit (VP), which has also been described as visual tracking,Footnote 1 and visual fixation (VF) are considered the first signs of emergence of consciousness [21, 22]. According to the Coma Recovery Scale-Revised (CRS-R), which is the most used assessment scale, VP and VF are clinical signs denoting MCS [23]. According to the CRS-R, VP is present when a moving mirror is followed for 45° without loss of fixation in two of four directions, whereas VF is present when the eyes move from the initial fixation point and re-fixate more than 2 s in two of four trials [24].

Previously, it was demonstrated that failure to detect VP and VF caused misdiagnosis of MCS [7]. This was confirmed in a Dutch prevalence study about UWS/VS [8]: 39% of the reported UWS/VS were misdiagnosed and were at least in MCS. In the 15 MCS patients, VP was seen in 8 of them and VF in one. It remains subject of debate whether or not VP and VF are clearly discernible signs of consciousness. In 1994, the Multi-society Task Force on Persistent Vegetative State (MSTF) reported that VP and VF could be either considered as signs of consciousness or as brief visual orienting reflexes. The MSTF advised to be cautious in diagnosing UWS/VS if VP and/or VF are observed [2]. In 1996, an International Working Party doubted the relation of VP with the conscious state, considering the sole presence of VP not as a reliable sign of consciousness [25, 26]. In 2002, the definition and diagnostic criteria for MCS were published [3]. These criteria were consensus based due to the lack of scientific evidence about diagnosis and prognosis of MCS. VP was incorporated into the criteria as it was considered an example of purposeful behavior based on the following data: VP was associated with late improvement [27], more prevalent in MCS patients [21], and preceded interactive and social behavior later in the recovery course [28]. Regarding the incorporation of VF into the criteria of MCS, no supporting data were reported. Currently, the question whether VP and VF are signs of consciousness still remains debatable. However, in daily practice and in the most recommended assessment scale [23, 24], VP and VF are considered important signs of MCS.

To determine if VP and VF are indicative of consciousness, data about their diagnostic validity are necessary. In 2014, a review about eye movement measurement in the diagnostic assessment in disorders of consciousness (DOC) [29] focused on quantitative techniques to measure eye movements rather than on behavioral assessment. However, this review did not address the question whether VP and/or VF are diagnostic signs of consciousness.

The aim of this study is to review the evidence about definition, operationalization and assessment of VP and VF in relation with the state of consciousness.

Methods

We performed an integrative review, which provides a comprehensive understanding of a particular phenomenon or healthcare problem. This method has the possibility to include a variety of data [30, 31].

Search strategy

The databases of PubMed and EMBASE were searched from May 26, 1994 until October 1, 2016. The publication of the position paper of the Multi-society Task Force on the Persistent Vegetative State was chosen as start date, because they discussed the significance of VP and VF for both UWS/VS and higher levels of consciousness [2]. We searched on the internet for guidelines, reports and for manuals of assessment scales and searched the websites of international taskforces on DOC for relevant papers. The bibliographies of the selected articles were searched for additional relevant papers. Searches were limited to the English, German, French, and Dutch languages.

Two search strategies were used: a broad, general search regarding diagnosis and prognosis in DOC patients and a more specific search related to the use of VP and VF in the diagnosis of DOC.

For the broad general search, we combined patient-related terms like ‘persistent vegetative state’ and ‘minimally conscious state’ with a diagnostic filter and the terms ‘misdiagnosis’, ‘assessment’, and ‘prognosis’. For the specific search, we combined the previously mentioned patient-related terms with terms like ‘visual pursuit’, ‘visual tracking’, ‘visual fixation’, ‘visual perception’, and ‘vision disorders’. Finally, we combined the results of the broad and the specific searches (Supplement 1).

Study selection

Papers were selected if they met one or more of the following inclusion criteria: (1) VP and VF were discussed, either described as DOC in general or described as UWS/VS and/or MCS; (2) the etiology of UWS/VS and MCS was brain injury caused by an acute incident; (3) discussion of the operational definition of VP and/or VF; (4) discussion of different assessment methods; (5) use of an assessment scale testing VP and/or VF; (6) discussion of assessment items of either VP and/or VF; and (7) discussion of influencing factors on visual responses in the assessment of DOC. Papers were excluded if DOC was caused by neurodegenerative diseases and if VP and VF were discussed in patients without DOC.

Data extraction and validation

The first author (BO) reviewed the papers. In case of doubt, a second reviewer (HE) was consulted. After discussion, a decision about inclusion was reached by consensus.

Before reviewing all citations, agreement about the inclusion and exclusion criteria was investigated. Two researchers (BO, HE) independently screened a sample 200 titles and abstracts. After extracting 2 duplicates, 198 papers were checked. Agreement about direct inclusion or papers eligible for further analysis of full text was reached in 168 (85%) of the papers.

Since disagreement existed about a considerable number of papers (n = 30, 15%), we added another search strategy. If based on title and abstract no decision could be made, the full text was electronically screened with the term ‘visual’ to find the terms ‘visual pursuit’, ‘visual fixation’, and ‘visual tracking’. If one of these items was discussed in patients with DOC, the article was eligible for screening of the full text. If not, the paper was excluded. Reanalysis of the 30 papers resulted in disagreement in 2 papers. Thus, adding this method to the search strategy decreased disagreement from 15 to 1%. Disagreement about inclusion was resolved through discussion between both reviewers, which led to consensus.

The selected papers were analyzed by the first author with a data extraction form. This form contained information about: type of article, aim, study subjects, outcome measures, main results, and conclusions.

Results

Included studies

Through the database search, 2351 papers and 13 additional documents were found (Fig. 1). After screening all titles and abstracts, 96 papers and documents were selected for full text analysis. No decision based on title and abstract could be made for 169 papers. Electronic full text screening of these papers yielded 111 eligible for further analysis. In total, full text of 207 papers was analyzed. Eventually, 34 papers could be included. After manual searching the bibliographies of the selected papers, four additional papers were included. The final sample consisted of 38 papers.

Fig. 1
figure 1

Flow diagram

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Definition

Descriptions of VP and VF were found in six papers; however, no uniform definitions of VP and VF were found. The papers provided eight descriptions of VP and 3 of VF [3, 25, 26, 32,33,34] (Table 1). VP was denoted by the terms eye tracking, tracking eye movements, horizontal and vertical tracking and pursuit eye movements [3, 25, 26, 32, 34]. VP was described as following objects or people [25, 26], as localizing to a visual stimulus [32], as the ability to follow in the horizontal and visual fields [32], and as a reaction to a moving stimulus [3]. VF was denoted by eye contact which was further explained as the patient’s gaze during the majority of the assessment session [32], as sustained fixation in response to a salient stimulus [3], and as active looking at or for objects [33].

Table 1 Descriptions of visual pursuit/visual tracking and visual fixation
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Assessment and operationalization of VP and VF

Assessment and operationalization of VP were found in 14 papers in which 9 assessment scales were discussed [23, 24, 28, 32, 34,35,36,37,38,39,40,41,42,43] (Table 2). Another scale, the Sensory Modality Assessment Rehabilitation Technique (SMART) was identified [44], but could not be included, since this scale was not available for evaluation. The assessment scales were developed with different purposes and have different testing procedures and variable operational criteria. Scales with a diagnostic purpose are the CRS-R and the Sensory Tool to Assess Responsiveness (STAR) [23, 24, 43]. In these scales, VP indicates MCS. Scales with purposes of detecting and monitoring signs of consciousness are the Western Neuro Sensory Stimulation Profile (WNSSP) [32], Disorders Of Consciousness Scale (DOCS) [35], Loewenstein Communication Scale (LCS) [36], Comprehensive Assessment Measure for Minimally Responsive Individuals (CAMMRI) [37, 38], Sensory Stimulation Assessment Measure (SSAM) [34], Coma Near Coma Scale (CNC) [39, 40] and the Wessex Head Injury Matrix (WHIM) [28, 41, 42]. VP was tested with different stimuli: objects [28, 32, 34,35,36,37,38, 41, 42], pictures and/or photographs [32, 35, 37, 38], mirror [23, 24, 32, 35, 37, 38, 43], and an individual [28, 32, 34, 36, 39,40,41,42]. In the CRS-R [23, 24], VP was operationalized as following a mirror without loss of fixation in 2/4 trials. In the STAR [43], VP is operationalized slightly different from the CRS-R, the number of trials which is 4 compared to 2 in the CRS-R and the duration of fixation on the mirror is set on 2 s or longer. In the Wessex Head Injury Matrix (WHIM) [28, 41, 42], VP is tested in four reactions, which each have a separate operational definition. A reaction is present if the observed reaction is in accordance with the operational definition of the reaction. The other scales score VP by rating the observed reactions with points [32, 34,35,36,37,38,39,40].

Table 2 Assessment and operationalization of visual pursuit/visual tracking
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VF was assessed and operationalized in 12 papers, which discussed seven assessment scales [23, 24, 28, 32, 35,36,37,38,39,40,41,42] (Table 3). Testing procedures and operationalization varied between the scales. The only scale with a diagnostic purpose is the CRS-R [23, 24]. In this scale, VF indicates MCS. Scales with purposes of detecting and monitoring signs of consciousness are the WNSSP [32], DOCS [35], LCS [36], CAMMRI [37, 38], CNC [39, 40] and the WHIM [28, 41, 42]. VF was tested with different stimuli: an individual [28, 32, 37, 38, 41, 42], pictures of familiar faces [28, 35, 37, 38, 41, 42], brightly colored or illuminated objects [23, 24, 37, 38], a mirror [37, 38], objects [28, 37, 38, 41, 42] and light flashes [39, 40]. The CRS-R operationalizes VF as re-fixation on an object 2 s or longer and indicates MCS [23, 24]. In the WHIM, 8 reactions test VF, each reaction has its own operational definition and VF is considered present if the operational definition is met [28, 41, 42]. The other scales score VF by rating to different observed reactions with points [32, 35,36,37,38,39,40].

Table 3 Assessment and operationalization of visual fixation
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Assessment of visual pursuit

Assessment of VP was discussed in seven papers [45,46,47,48,49,50,51] (Table 4). Results were found about the direction of tracking [45, 47], time of assessment [46], different stimuli [47, 50, 51], quantitative assessment with an eye tracker device [48], and the use of personalized stimuli [49].

Table 4 Assessment of visual tracking/visual pursuit
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Regarding direction of tracking, 48% of 76 head injured adults showed a tracking preference: 28% in the horizontal fields and 20% in the vertical fields [45]. Another study investigated the tracking preference in MCS patients and showed that the MCS- group had a preference of tracking in the horizontal field whereas in MCS + no tracking preference was found [47].

Individual variability of VP within the day was investigated and the highest probabilities for detecting VP were seen at 10.30 AM and at 3.00 PM. The lowest probability for detecting VP was at 2.00 PM, being a post-prandial time point [46].

The use of a mirror was the stimulus with the highest scores in DOC patients. In 1995, it was demonstrated that patients following a mirror had significantly higher mean scores on the visual tracking scale of the WNSSP compared to patients following an individual, picture, or object [45]. These results were confirmed by recent studies. VP was investigated in 51 MCS patients. Thirty-eight (75%) of them showed VP, and 11 (29%) only showed VP when a mirror was used [51]. Another study with 88 MCS patients investigated VP with different objects. VP was detected in 61/88 (69%) of patients, and in 16 (26%) of them VP was exclusively detected by a mirror [47].

VP was also studied in DOC patients quantitatively with an infrared eye tracker [48, 49]. Patients looked to either a moving red circle or a moving parrot, which were presented on a screen. VP was measured by electronically calculating the percentage of fixations on the target. MCS patients followed the target more frequent (32.9%) compared to UWS/VS patients (4.9%). In a second study from the same authors, a moving photo of a relative was added as an extra stimulus [49]. In MCS patients, a significant higher frequency of following the moving photo of a close relative was found (37.3%) compared to the images of the parrot (29.9%) and the circle (30.6%). In UWS/VS and healthy control subjects, no significant differences were seen between the applied stimuli [49].

Assessment of visual fixation

Assessment of VF was discussed in five papers [50, 52,53,54,55] (Table 5). Different stimuli were discussed: objects like a mirror, a ball, a light [52], familiar photographs and a card [53,54,55]. In the WHIM, VF is mainly tested by looking at a person. In one reaction tested by the WHIM an object was used, but was not further specified. Two studies tested VF in combination with the techniques Brain Computer Interface (BCI) and functional Magnetic Resonance Imaging (fMRI), respectively [53, 55].

Table 5 Assessment of visual fixation
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Investigation of VF in MCS patients with different stimuli showed that VF was significantly more seen on the mirror (48%) compared to the ball (28%) and a light (25%) [52]. In an analysis of different items of the WHIM, maintaining gaze or gaze shifting reactions were more prevalent in MCS compared to UWS/VS patients [50].

Three studies discussed the use of visual stimuli with images of familiar persons. First, visual attention to a personal stimulus was compared to a neutral stimulus and patients oriented more frequent to the familiar image than to the neutral stimulus [54]. Second, in a BCI study, VF was investigated in patients with UWS/VS, MCS, locked-in syndrome and healthy controls. It was demonstrated that accuracies of attending to one’s own photo were higher than to unfamiliar photos. However, no differences between UWS/VS and MCS were found [53]. Third, an fMRI study investigated visual perception of different pictures in nine MCS patients and ten healthy controls [55]. In 6/9 MCS patients and all healthy controls looking at family pictures had higher activation in the visual networks compared to looking at other pictures.

Influencing factors

Five influencing factors on visual responses were discussed in eight studies: within-day variability [56], inter-rater reliability (IRR) differences due to profession and/or experience [57, 58], presence of an informal caregiver [59], duration of assessment [60], and influences of visual/oculomotor impairments [6, 61, 62] (Table 6). Most of the results of these studies presented CRS-R visual subscale scores, which were not subdivided in VP and/or VF. First, visual subscale scores on the CRS-R were higher in the morning than in the afternoon, which could be explained by individual changes in visual functioning or by the presence of fragmentary cyclic processes [56]. Second, in two studies, IRR was investigated between different professionals and/or different levels of experience [57, 58]. The IRR on the visual subscale of the CRS-R was good (k = 0.73). The IRR of physicians was slightly higher (k = 0.81) compared to psychologists (k = 0.68) and a group of physiotherapists, speech therapists, and nurses (k = 0.73). Assessors who had > 24 months experience in assessing DOC patients showed a higher IRR (k = 0.81) than assessors with less experience (k = 0.62 for experience < 24 months and k = 0.68 for experience < 12 months) [57]. Another study showed a lower IRR for the visual subscale score of the CRS-R in experienced (k = 0.48) as well as in the less experienced assessors (k = 0.47) [58]. Third, the involvement of an informal caregiver in the assessment resulted in higher visual subscale scores on the CRS-R compared to assessment of a clinician alone [59]. Fourth, the duration of the assessment was investigated in 10 DOC patients. When two repeated assessment with the CRS-R (50–60 min) and 10 SMART assessment (600 min) were compared, this led to differences in the level of consciousness in 4/10 patients. In 2/4 patients, these differences were caused by detecting sustained VF with the SMART and not with the CRS-R [60]. Fifth, influences of visual impairments and/or oculomotor defects on assessment of the level of consciousness were found in 3 studies [6, 61, 62]. In misdiagnosed UWS/VS patients, 65% had visual impairments [6], and in MCS patients, 9/52 (17%) scored no visual responses on the CRS-R [62] and analysis of CRS-R subscale scores showed that visual problems such as optical nerve damage, ptosis, ocular apraxia and visual agnosia could cause improbable CRS-R scores [61].

Table 6 Influencing factors on visual responses
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Discussion

To our knowledge, this is the first review that addresses the question whether or not VP and VF are related to consciousness. We found that literature about the importance of these responses in relation with consciousness still is controversial. No agreed-upon definition of VP and VF was found and the assessment methods vary widely regarding procedures and operational criteria. However, the studies generally agreed that the use of a mirror is the most sensitive method to detect VP and VF.

The lack of an agreed-upon definition has led to international differences in interpretations. In the United States, VP and VF are considered signs of MCS, whereas in the United Kingdom (UK) these signs are atypical but viewed as signs of UWS/VS [63,64,65]. In addition, not operationally defined terms like ‘brief’ and ‘sustained’ VP and/or VF, can cause differences in interpretation with a risk of diagnostic inaccuracy. Furthermore, a recent expert opinion stated that there is no rationale why a brief visual response does not require consciousness and a sustained response does [66]. To conclude, evidence for the use of ‘brief’ and ‘sustained’ VP and VF for distinguishing UWS/VS from MCS is lacking.

A wide variety of assessment methods with variable operational criteria of VP and VF were found. Only the CRS-R and the STAR were developed with a diagnostic purpose. The other scales were mainly developed to monitor neurobehavioral functions. Judging the validity of the different scales is difficult because a golden standard is lacking for diagnosis of the level of consciousness. In 2010, 13 DOC assessment scales were reviewed. The CRS-R is the only scale recommended with ‘minor reservations’ because it has acceptable administration and scoring guidelines and good content validity. Despite the recommendations for clinical use, the authors of this review stated that evaluation of diagnostic validity remains problematic. Diagnostic validity was unproven for all assessment scales and interpretation is difficult because of the lack of a standard criterion measure for the assessment of the level of consciousness [67].

The use of a mirror appeared to be the most sensitive method to detect VP and VF [47, 51, 52]. It has been suggested that the use of patient’s own face can be useful to detect residual self-awareness [68] and that personally relevant stimuli increase the probability of detecting a conscious response in DOC patients [69]. However, recent studies published after our search period indicate that the sensitivity of the mirror cannot be explained by a lower cognitive demand [70], neither the self-referential aspect of the mirror is viewed as a complete explanation [71]. Therefore, the rationale for the sensitivity of the mirror has not been clarified yet.

The absence of visual responses in a considerable part of the DOC patients calls for a nuance to the view that VP and VF are important signs for detecting consciousness. Although it was demonstrated that visual responses were the signs most frequently detected in MCS patients, the absence in about 20% of the MCS population cannot be ignored [62]. Examination of the integrity of the visual tract with techniques like visual evoked potentials and imaging is advisable in patients with DOC who do not show visual responses. A closer look into the neurobiology of VP and VF shows that VP is considered to be under volitional control [72]. For VF, however, it remains questionable if this sign is a conscious response because saccadic eye movements are necessary to shift gaze from one position to another. Saccades can be either voluntary or reflexive [73, 74]. The existence of accurate localization in the visual field without consciously processing visual stimuli, which can be present in patients with blindsight and visual form agnosia, further complicates the understanding of the association of VP and VF with consciousness. Since the association of VP and VF with consciousness remains questionable, further research is needed. Longitudinal studies which follow VP and VF during the recovery phase may give insight into the question if and/or how VP and VF are associated with consciousness.

There are some limitations regarding the literature search and the interpretation. First, the methodological quality of the included papers was not systematically assessed. Because we included theoretical, empirical and expert-opinion papers, a uniform quality assessment was not possible. Second, different descriptions that existed for VF such as ‘focusing on the examiner’ and ‘active looking for objects’, might have led to possible misinterpretation of these reactions as VF. Third, the SMART might be a proper scale for assessment; however, we could not evaluate the properties of this scale, since it requires formal training and it must be purchased. Previously, it has also been reported that the SMART may not be accessible for users outside the UK [67].

In conclusion, the question whether or not VP and VF are signs of MCS cannot be answered uniformly yet. This review demonstrates a lack of consensus regarding definition, operationalization and assessment methods. Although VP and VF are widely recognized as signs of emerging consciousness, the supporting evidence is scarce. Moreover, since VP and VF are included into the diagnostic criteria of MCS, it is not surprising that these signs are more prevalent in MCS patients than in UWS/VS patients. One can speak of a circular argument if based on such a prevalence difference, authors conclude that VP and VF are indicative of consciousness. More research is needed to investigate the validity of these signs to measure the level of consciousness before adopting them as important diagnostic signs of MCS. Therefore, we recommend international consensus development about definitions, operational criteria and assessment procedures of VP and VF. Reaching consensus about these first signs of consciousness is highly important for a proper diagnosis and consequently increases the chance for providing rehabilitation to this population. As recently stated by Fins [75], misdiagnosis of MCS patients as UWS/VS, can deny them access to rehabilitation and thereby marginalizes these patients. Proper identification of MCS can pave the way for rehabilitation and thereby breaching the marginalization of these vulnerable patients.