Introduction

Psoriasis is a chronic inflammatory skin disease [1]. Norway has the highest prevalence of psoriasis (8.5%) compared with other countries [2]. Psoriatic arthritis (PsA) is a chronic inflammatory arthritis disease associated with psoriasis, and has a prevalence in the range of 0.1–0.2% [3]. Both psoriasis and PsA are associated with a high risk of cardiovascular diseases, metabolic syndrome, and gastrointestinal disorders [4–6]. Living with psoriasis and PsA has psychosocial consequences, and depression and anxiety are frequent comorbid conditions [5, 7, 8].

A diagnosis of a chronic disease, such as psoriasis or PsA, requires a person to make adjustments in both the physical and psychological aspects of their life [9]. People’s cognitive beliefs about their illness, often referred to as illness perceptions, are important determinants and predictors of different outcomes. Negative illness perceptions have been reported as associated with poorer functioning and well-being, depression, and more physical symptoms [10–13]. Hence, people’s illness beliefs play an important role in influencing their responses to chronic diseases.

Subjective health complaints (SHCs), such as musculoskeletal pain, gastrointestinal complaints, fatigue, and mood changes, are frequent in the Norwegian population [14–16]. There are no obvious limits distinguishing “normal” endurable health complaints from complaints requiring professional help [14]. SHCs are among the most common reasons for the use of health services, sickness absence, and disability [17]. People with chronic diseases report more subjective health complaints and a higher severity of such complaints compared with the general population [18–23].

Although it is well-documented that psoriasis and PsA are associated with high comorbidity [4–8], there is scarce knowledge of SHCs among individuals with these diseases. Living with a chronic disease can be demanding and patients in the two aforementioned patient groups have reported an impaired quality of life [24, 25]. The fact that SHC can place an additional burden on such individuals, potentially impairing their quality of life even further, together with the fact that other chronically ill individuals have reported high numbers of complaints [18–23], demonstrates the importance of elucidating to what extent people with psoriasis and PsA experience SHCs. Furthermore, investigating associations between SHCs and specific disease characteristics, such as skin symptoms and illness perceptions, could be important to ensure tailored interventions for individuals with psoriasis and PsA.

The study aimed to describe the prevalence of SHCs among individuals with psoriasis and PsA in Norway, and to investigate whether the severity of their skin condition and their illness perceptions were associated with the number and severity of health complaints.

Methods

Study Design and Recruitment Procedure

We conducted a cross-sectional study. A self-administered questionnaire was distributed to 5255 members of the Psoriasis and Eczema Association of Norway, as an attachment to the membership magazine no. 3/2013, during autumn 2013. The criteria for inclusion were in the age 18–70 years and having either psoriasis or PsA. Data collection ended in January 2014.

Questionnaire and Standardized Measurement Tools

The questionnaire included questions covering demographic information, subjective health complaints, severity of the skin condition, and illness perceptions.

Dependent Variables

Subjective health complaints were measured by the Subjective Health Complaints Inventory, in which 29 common health complaints are graded on a four-point scale (0 = not at all, 1 = a little, 2 = some, 3 = severe) as experienced last month [26]. The complaints are divided into five factors [26, 27]: musculoskeletal complaints (headache, migraine, neck pain, shoulder pain, pain in arms, pain in upper back, low back pain, and leg pain) (Cronbach’s α = 0.74), psychological or pseudoneurological complaints (extra heartbeats, heat flushes, sleep problems, tiredness, dizziness, anxiety, and depression) (α = 0.73), gastrointestinal complaints (heartburn, stomach discomfort, ulcer/non-ulcer dyspepsia, stomach pain, gas discomfort, diarrhoea, and constipation) (α = 0.62), allergic complaints (asthma, breathing difficulties, eczema, allergies, and chest pain) (α = 0.58), and flu (cold, flu, and coughs) (α = 0.67). The prevalence of single complaints and complaints divided into subscales, a sum score of the severity of all complaints and of the subscales, and a score for the number of complaints (0–29) are all reported [16, 27].

Independent Variables

Two questions eliciting participants’ self-reported severity of their skin condition were obtained from a psoriasis-specific questionnaire that has been used in the Nord-Trøndelag Health Study [28]. We measured the severity of the skin condition as having a current rash when participating in this study (yes/no) and as the number of places formerly affected with a rash (on a scale of 0–10). From a list of 10 selected body parts, the participants were asked to tick the places where they had ever been affected. We calculated the mean number of places with a rash.

We used the revised version of the Brief Illness Perception Questionnaire (BIPQ-R) to measure illness perceptions [29]. The measurement tool contains nine items: eight items are rated using a 0–10 response scale and one item has an open-ended response. Only the results from the response scale items were used. Five items measure cognitive illness representations: consequences (degree of impact on life), timeline (perceived timeline of illness), personal control (perceived control of illness), treatment control (perceived benefit from treatment), and identity (grade of symptoms associated with the illness). Two items assess emotional illness representations: concern (grade of concern regarding the illness) and emotional affect (grade of affect). The last item assesses comprehensibility/coherence of illness [13, 29]. The mean scores for each item on the response scales were calculated [13].

Statistical Analyses

SPSS Statistics version 23 was used for all analyses, and p values less than 0.05 were considered statistically significant.

The prevalence of single complaints and complaints on the subscales were computed separately for participants with psoriasis and PsA. If there were less than 50% missing values within the subscales for SHCs, the mean values were calculated; otherwise, the scale was regarded as missing [26]. Differences among participants with psoriasis and PsA in categorical variables were examined using the chi-square test for independence. Yates’ correction for continuity was used for 2 × 2 contingency tables. Differences in means were examined using independent samples t tests. To investigate associations between SHCs, severity of the skin condition, and illness perceptions, we used multiple linear regression models. Dependent variables were the total number of SHCs and severity of complaints. A rash during study participation (dichotomous) and number of places with a rash (continuous), both of which represented the severity of the skin condition, were included as independent variables. To determine which of the individual BIPQ-R items representing illness perceptions should be included as independent variables, bivariate correlation analyses with Pearson correlation were performed for all items against each dependent variable. Only items that significantly correlated with the number and severity of SHCs were included in the regression models, resulting in following seven BIPQ-R items as independent variables: consequences, personal control, treatment control, identity, concern, emotional affect, and comprehensibility/coherence (all continuous). Multicollinearity tests were conducted to test intercorrelations among independent variables. Model fit was evaluated by R 2. Regression analyses were conducted separately for each dependent variable and for participants with psoriasis and participants with PsA, resulting in four regression models. We adjusted for age, gender, and education in the first step (Block 1), and then added all independent variables (Block 2).

Ethics

The project was approved by the Regional Committees for Medical and Health Research Ethics (REC) in Norway.

Results

Of the 5255 questionnaires sent to the members of the Psoriasis and Eczema Association of Norway, we received 1135 completed questionnaires, corresponding to a response rate of 21.6%. We excluded 193 persons: 166 who did not fulfil the criteria for age and 27 who did not have psoriasis or PsA. Thus, 942 participants were included in the study, of which 573 (60.8%) had psoriasis and 369 (39.2%) had PsA (Table 1). Participants with PsA were older and fewer reported having a current rash compared with participants with psoriasis (Table 1).

Table 1 Demographic information, severity of the skin condition, and illness perception scores for all participants and for participants with psoriasis and PsA separately
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Participants with PsA perceived their illness more negatively compared with participants with psoriasis. The results showed significantly higher scores on the following subscales: consequences, timeline, identity, concern, and emotions (Table 1). However, participants with PsA reported significantly better comprehensibility/coherence of their illness compared with participants with psoriasis (Table 1).

The prevalence of SHCs in participants with psoriasis and PsA is shown in Fig. 1. The most prevalent single complaints reported by participants with psoriasis were tiredness, low back pain, arm pain, shoulder pain, and neck pain, while for PsA, they were tiredness, arm pain, low back pain, pain in feet, and shoulder pain (Fig. 1). Participants with PsA reported a significantly higher prevalence of 16 out of 29 complaints compared with participants with psoriasis. Participants with PsA also showed a significantly higher prevalence of musculoskeletal and pseudoneurological complaints (Table 2).

Fig. 1
figure 1

Prevalence of SHCs (%) distributed by single items for (a) participants with psoriasis (n = 573) and (b) psoriatic arthritis (n = 369)

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Table 2 Prevalence (%) and mean severity of SHCs on subscales, and mean number of complaints for all participants and for participants with psoriasis and PsA separately
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Among all participants, the mean total severity of complaints was 16.86 (Table 2). Participants with PsA reported significantly a higher mean total severity compared with participants with psoriasis. They also reported a higher severity on all subscales, with significant differences for musculoskeletal, pseudoneurological, and gastrointestinal complaints (Table 2). Out of 29 complaints, participants with psoriasis reported the fewest complaints, whereas participants with PsA reported significantly more complaints (Table 2).

The results of the multiple linear regression analyses showed that the severity of the skin condition was a factor for reporting SHCs. Having a rash during study participation was associated with a number of health complaints among participants with psoriasis only. However, increased numbers of places with a rash were significantly associated with the number and severity of complaints in both groups (Table 3). Items representing cognitive and emotional illness representations were associated with SHCs in participants with psoriasis. A higher perceived illness impact (consequences) was associated with the number and severity of complaints, and a higher perceived emotional affect was associated with the number of complaints. Only items representing cognitive illness representations were associated with SHCs in participants with PsA. In this group, a higher perceived illness impact (consequences) and a higher grade of symptoms perceived as belonging to the PsA (identity) were associated with both the number and severity of complaints (Table 3). The models explained 27.5% of the variance (R 2) in the number of SHCs in persons with psoriasis, and 29.0% in PsA. For severity, the model explained 25.4 and 34.1% of the variance in persons with psoriasis and PsA, respectively.

Table 3 Relationship between severity of the skin condition and illness perceptions with the number and severity of SHCs in participants with psoriasis and psoriatic arthritis
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Discussion

We found a high prevalence of SHCs among individuals with psoriasis and PsA. Participants with PsA reported more health complaints and a higher severity of complaints compared with participants with psoriasis. The number of places with rash was significantly associated with the number and severity of health complaints in both groups. Cognitive illness perceptions (consequences) and emotional illness perceptions (emotional affect) were associated with SHCs in participants with psoriasis, whereas only cognitive illness perceptions (consequences and identity) were associated with SHCs in participants with PsA.

With the exception of flu-like complaints, the prevalence and severity of SHCs were higher among persons with psoriasis and PsA compared with what has been previously reported for the general population [14–16, 30]. Studies of the Norwegian general population have reported prevalences in the range of 75–80% for the musculoskeletal subscale and 59–65% for the pseudoneurological subscale [14–16]. By comparison, 91% of the participants in our study reported complaints for the musculoskeletal subscale and 86% for the pseudoneurological subscale, respectively. Several studies have shown that people with other chronic diseases, such as sciatica, low back pain, whiplash associated disorder, Ehlers-Danlos syndrome, and irritable bowel syndrome, have a higher prevalence of health complaints compared with the general population [18–23]. Parallel to our findings, a tendency for a higher severity of complaints among other chronically ill individuals has also been reported [19]. Psoriasis and PsA are associated with high comorbidity of other diagnoses, such as depression, anxiety, and irritable bowel syndrome [4–8]. Although we had no information on other possible diagnoses, it seems reasonable to assume that such conditions could cause symptoms and complaints that would partially explain our findings.

Somatic and cognitive sensitizations have been proposed as underlying mechanisms to explain why some individuals develop multiple complaints or generalized pain [31, 32]. Somatic sensitization occurs when the nervous system reacts with increased response to stimuli, such as when repeated pain stimuli cause more pain [31]. By contrast, cognitive sensitization occurs through a cognitive bias (e.g., increased focus, attention, and worrying regarding complaints and individual experiences). Such cognitive bias has been reported for several patient groups, including those with psoriasis [33, 34].

We found that the number and severity of SHCs were associated with an increasing numbers of places with a rash in both groups. This could indicate that somatic sensitization is at least partially involved as a mechanism that explains high levels of SHCs. Both psoriasis and PsA involve stimuli such as itching, pain, and discomfort from the skin and joints [1, 3, 35]. Constant, repetitive stimuli, such as rashes, possibly induce somatic sensitization through increased responses in the limbic structures, leading to more complaints and a higher severity of the complaints in individuals with psoriasis and PsA [31, 32]. The associations found between SHCs and higher perceived illness impacts (consequences) and emotional affect may be related to an ongoing continuous cognitive sensitization among these individuals. Both higher perceived consequences and emotional affect could reflect an increased disease impact of complaints already experienced due to having a chronic disease. How already existing complaints are perceived may change and cause reinforcement [36, 37], resulting in a high prevalence and severity of SHCs among individuals with psoriasis and PsA.

As expected, participants with PsA reported a higher prevalence and severity of musculoskeletal complaints. However, they also reported a significantly higher severity of pseudoneurological and gastrointestinal complaints compared with participants with psoriasis. There could be several possible explanations for this finding. First, potential differences in comorbidity between the patient groups could explain the high numbers of SHCs in participants with PsA. However, measures of comorbidity in a lacking point in this study, and it was not possible to reveal any such differences. Second, PsA involves both skin and joint symptoms [3]. It is likely that individuals with PsA experience more constant, repetitive stimuli, such as rashes and pain, than persons with psoriasis, and are thus more prone to experiencing somatic sensitization. However, one weakness of the study was that we did not measure joint symptoms. Third, and finally, participants with PsA perceived their illness more negatively on several items. Hence, cognitive sensitization may cause a greater reinforcement and significantly more complaints in this group.

There were some interesting differences in illness perceptions between the patient groups. Participants with PsA perceived significantly more symptoms (identity), serious impact (consequences), and chronic timeline (timeline) of the illness. These three illness beliefs are associated with more use of health care among primary care patients [38]. Whether such beliefs are critical for predicting the use of health care in PsA, and contribute to differences in health care use between the patient groups, should be explored further. Interestingly, we found that individuals with PsA reported better coherence of their illness, even though they perceived their illness more negatively compared with participants with psoriasis. Longer duration of the chronic disease is associated with better coherence of illness in psoriasis [39]. Since the majority of patients with PsA have been diagnosed with psoriasis almost 15 years prior to their PsA diagnosis [40], we suggest that the difference in perceived coherence of illness could be related to the duration of the disease. In our study, individuals with PsA might have lived longer with their disease compared with individuals with psoriasis, which may also reflect why those with PsA were significantly older.

The study had several limitations. A major weakness was the low response rate. Participation in surveys and epidemiological studies has decreased, and achieving high response rates is an increasing problem [41], which may explain our low response rate. Furthermore, the calculation of the response rate was based on the number of questionnaires sent out to all registered members of the Psoriasis and Eczema Association of Norway. There were weaknesses in this procedure. The membership registry had not been updated and included members not eligible for the participation in the study. Additionally, some questionnaires went to e.g., health care centres rather than to individual members, and therefore, we were not able to ascertain the exact number of members with psoriasis or PsA. We assume that the response rate would have been higher if calculations had been carried out with eligible members only. Since we had no information on which members of the association that was reached, we had no information on the non-responders. It is likely that the members who participated in the study were slightly different from the general population of members with psoriasis and PsA, and therefore, we cannot rule out selection bias. The estimated prevalence values might not represent the general psoriasis and PsA populations in Norway. An overrepresentation of healthy subjects in health surveys has been reported [42], but it is also likely that the most affected members were more motivated to return their questionnaire. Both scenarios could have led to a biased sample. However, the problem of selection bias is less severe for the association studies, as long as the groups are comparable [43]. The number of participants was relatively high, and the length of the confidence intervals for the effect measures were relatively small, thus, strengthening the associations found between groups and variables. Finally, our cross-sectional design did not allow for the determination of the nature of the direction of associations between SHCs, the severity of the skin condition, and illness perceptions, which limits the interpretation of our findings. Contrary to our presumption, the direction of associations could be reverse; for example, experiencing more health complaints could lead to more negative illness perceptions.

Conclusions

In addition to coping with a chronic disease, individuals living with psoriasis and PsA have to manage high levels of SHCs. The observed prevalence and severity of complaints was apparently associated with the severity of the skin condition and negative illness perceptions, and hence, might have been due to somatic and cognitive sensitization. Our findings suggest that holistic approaches in health care, taking into account both the somatic and psychological aspects of having a chronic disease, are essential when managing these patient groups. This in turn has several implications for clinical practice. Carefully examining patients’ experienced SHCs is important in order to rule out manifestations of pathology, as the risk of comorbidity is high in psoriasis and PsA. After excluding pathological explanations, tailored medical treatment to reduce skin symptoms, complemented with psychosocial interventions to achieve less negative illness perceptions, could be important to avoid subjective health complaints.