1. Introduction
Worldwide, stroke is a highly prevalent reason for long-term disability, morbidity, and death [
1]. During the past years, many advances in preventive and therapeutic strategies for stroke have been made, which led to declining mortality rates; however, the overall global burden of stroke appears to increase continuously [
2]. In this respect, the relevance of care for these individuals, especially during rehabilitative measures, is high and a field of increasing scientific interest [
3]. Thereby, the health-related quality of life is an important aim and outcome of care for individuals after stroke [
4]. It is known that quality of life is worse after stroke and positive or negative development is closely related to coping strategies, different personal and environmental factors, and psychological stressors like depression, anxiety, and posttraumatic stress disorder [
4,
5,
6].
As one sub-aspect of the overall health-related quality of life, the construct of oral health-related quality of life (OHRQoL) has been established and widely recognized in the past decades [
7,
8,
9]. Thereby, the OHRQoL reflects the potential influence of oral conditions, including dental, periodontal, and functional diseases, tooth loss, and various other pathologies on quality of life [
8,
10]. The OHRQoL includes different dimensions, whereby both functional and psychosocial sub-scales are available, which are potentially affected by different oral diseases [
11,
12]. Oral diseases are, in turn, common in stroke survivors; this is particularly related to physical, sensory, and cognitive complaints related to stroke, making oral health care challenging for those individuals [
13]. Therefore, stroke survivors often have a higher burden of dental caries, periodontitis, and tooth loss, combined with a lower frequency of dental attendance [
14]. Moreover, periodontitis has been reported to be a potential risk factor for stroke, whereby a risk ratio of 1.88 and 2.27 was found, depending on study design [
15]. Those issues make the oral situation of patients after stroke a field of high scientific and clinical interest [
14].
Taken together, the high morbidity and quality of life issues on the one hand, and the high oral disease burden of stroke survivors on the other hand, support the potential relevance of OHRQoL in these individuals. Previously, the OHRQoL of patients suffering from several general diseases was studied, including rheumatic diseases, renal insufficiency, organ transplantation, and Alzheimer disease [
16,
17,
18,
19]. Thereby, OHRQoL was repeatedly reported to not reflect physical oral health, but to be related to other (disease-related) issues in systemically diseased individuals [
16,
17,
18,
19]. For those reasons, the OHRQoL of patients after stroke appears an interesting and clinically relevant issue.
Accordingly, the aim of this systematic review was to investigate the OHRQoL of patients after stroke. Thereby, the potential associations of OHRQoL with oral health and general-disease related parameters were evaluated to reveal the most important influential factors on OHRQoL of those individuals. Based on the knowledge about patients with other systemic diseases, it was hypothesized that patients after stroke would show a slightly or moderately reduced OHRQoL, which is not primarily associated with their oral status.
4. Discussion
This systematic review identified eight clinical studies, which investigated the OHRQoL of patients after stroke by different measurements. A healthy control group was only recruited by two studies, showing an ambivalent result [
22,
24]. Therefore, it will be necessary to discuss the applied instruments and the potential interpretation of the OHRQoL results first. The OHIP 14 is widely used across different research questions; thereby, it is a valid tool, which is very suitable for clinical studies [
9]. The OHIP was developed in the early nineties, while different versions depending on the number of questions are available [
30]. The OHIP reflects oral-health related issues, which patients perceived during the past month; thereby, questions are to be answered on a five-point Likert scale between 0 (never) and 4 (very often) [
30]. Accordingly, a higher OHIP score indicates worse OHRQoL. While originally seven domains were defined within OHIP [
31], recent research focused on four dimensions of OHRQoL, which are displayed by OHIP; those include oral function, psychosocial impact, oral pain, and orofacial appearance [
32]. In absence of a healthy control group in the studies using the OHIP 14, the values can only be interpreted with regard to the literature. Overall, an OHIP 14 value between 0 and 6 points, depending on dentition (fully dentate up to toothless having full prosthesis), can be seen as “unaffected” OHRQoL [
33]. Therefore, majority of studies indicated a reduced OHRQoL. The OHIP 14 values across the included studies varied between 2.87 and 33.0 points in sum score; this is a wider range as for other patient cohorts, e.g., rheumatoid arthritis, renal insufficiency, Alzheimer disease, or organ transplantation [
16,
17,
18,
19]. This argues for a certain heterogeneity of the included studies, which can be seen in
Table 1 and will be discussed later below.
The second most common OHRQoL assessment was the GOHAI, which was applied in three studies. This index includes 12 questions related to oral conditions and is answered on a Likert scale, whereby answers range between 1 (never) and 5 (always) [
34]. In contrast to OHIP, higher values indicate better OHRQoL. The GOHAI is widely used for elderly individuals or patients with dentures [
35,
36]. As the mean age of most studies was quite high in the included studies, the application of GOHAI appear reasonable. The study using healthy comparison for GOHAI did not find a difference between stroke survivors and healthy individuals [
22]. Altogether, the GOHAI values in the current study are similar than for elderly individuals in literature [
36,
37], and were slightly better and showed smaller range across studies than for patients with rheumatoid arthritis, renal insufficiency, or Alzheimer disease [
16,
18,
19]. Of course, this comparability is limited, because those diseases are different, but it might help to categorize the current reviews findings.
A slightly or moderately reduced OHRQoL of patients after stroke was previously hypothesized. Considering the abovementioned results, this hypothesis appears to be confirmed. Oral diseases, including tooth loss and periodontal diseases, can negatively affect OHRQoL [
38,
39]. It is known that patients after stroke show worse oral conditions, and periodontal diseases could even be related to stroke onset [
14,
15]. Although evidence is limited, a systematic review concluded stroke to be associated with tooth loss [
40]. Four studies reported on tooth loss within the current review, showing remarkable number of missing teeth and high amount of denture wearers [
22,
23,
24,
25]. This might lead to the assumption that this issue would be an explanation for the reduced OHRQoL; however, only one study found associations between missing teeth and OHRQoL [
25]. Considering the rarity of reported associations between oral health and OHRQoL across included studies, oral health situation might not be the main influential factor on OHRQoL in stroke patients. Nevertheless, this is limited by the fact that not all studies assessed oral health parameters and their potential relation to OHRQoL.
Against this background, other factors might influence the OHRQoL within the included studies. It is known that stroke is a life-changing event, potentially leading to disability and psychosocial complaints which affects quality of life of patients [
4,
5,
6]. Severe general diseases can also influence the patients’ experience of their oral conditions; on the one hand, psychosocial complaints also affect this domain of OHRQoL, and on the other hand, general disease burden can mask oral complaints (response shift) [
11,
17]. Two studies found a relationship between OHRQoL and general health-related quality of life [
28,
29], supporting this assumption. Additionally, the disability and related motoric skills in stroke survivors might cause functional complaints related to the oral cavity, resulting in reduced OHRQoL. This is, however, not plausibly supported by the current systematic review, because one study found an association between OHRQoL and disability [
25], while three did not [
22,
24,
28]. Nevertheless, one other issue would argue for such an influence and concomitantly explain the high range of findings to some extent: the differences in time since stroke across included investigations. It can be seen from
Table 1 that there was an enormous heterogeneity regarding this. Within the respective longitudinal studies, a certain positive time effect on OHRQoL is visible, which might indicate a positive effect of rehabilitation time on OHRQoL [
27,
28,
29]. Rehabilitation is essential for patients after stroke to regain their independence and for a certain recovery of motoric functions [
2,
41,
42]. Onward rehabilitation, especially physical activity, positively influences quality of life of individuals after stroke [
41,
42]. Therefore, it is not surprising that time since stroke might also influence OHRQoL of patients. Another factor, which potentially affects the OHRQoL across studies is the different mean age (see
Table 1). Age is related to tooth loss and thus OHRQoL of individuals [
38,
43]. This is another factor that can explain the high range of OHRQoL outcomes in the current systematic review.
Altogether, the hypothesis that patients after stroke would show a slightly or moderately reduced OHRQoL, which is not primarily associated with their oral status, can be seen as partly confirmed. The major factors limiting the ability to support this hypothesis is the heterogeneity of included studies and the missing consideration of the respective parameters (oral health, quality of life, and disease-related parameters) in potential association with OHRQoL. This leads to the general limitations of this systematic review article. The heterogeneity across studies, regarding country, age, gender distribution, and time since stroke, limits the comparability of included studies. Similarly, the different assessment measurements of OHRQoL need to be recognized; although both OHIP and GOHAI are validated and appropriate for measuring OHRQoL, several differences exist between those measurements [
44]. Furthermore, sub-scales of OHRQoL were rarely reported across included studies (see
Table 5), making conclusions on the main reason for OHRQoL impairment (whether it is more functional or more psychosocial) speculative. In this context, the inhomogeneous consideration of general and disease-related parameters, general quality of life, and oral conditions limits the ability to draw conclusions on respective associations. Altogether, further studies are required, applying standardized and valid methodology and comprehensive assessment of OHRQoL and potential influential factors. Based on the included studies, the discussion limits the impact on oral health mostly to oral disease, which are related to oral hygiene and disease history (tooth loss and periodontal disease). In contrast, the most violent impact of stroke on the OHRQoL may be rather related to the instant facial palsy following stroke, possibly impairing motoric control, sensibility of the oral tissues, and chewing efficiency. This has been rarely reported, yet. Another factor with a potential psycho-social impact is the drooling of saliva, given that the lip closure might be affected by the facial palsy. Therefore, immediate functional impairment and a rather long-term effect regarding tooth loss and periodontal disease (which may have been present already before the stroke, and might have even contributed to its occurrence) should be distinguished and examined in subsequent studies. Future research in the field should therefore focus on the stroke-related functional and psycho-social issues and their impact on OHRQoL.
Finally, it must be discussed what might be the practical implication of the current study’s findings. Although included studies and their results were heterogeneous, a certain relevance of oral conditions and perceived oral health appears relevant for patients after stroke. Therefore, oral conditions and their potential impact on quality of life require consideration during rehabilitation and care after stroke. With regard to the association between worse oral conditions, periodontitis, tooth loss, and stroke [
14,
15,
40], oral health should be fostered in this patient group, starting immediately in the rehabilitation of patients. Therefore, medical staff and caregivers should be sensitized for oral health issues and support oral hygiene and dental (prevention oriented) consultations. In dental context, patients appear to need an interdisciplinary approach, addressing all important risks and needs of the patients, e.g., as displayed in the concept of individualized prevention [
45].