**1. Introduction**

There is controversy on the conceptualization of anger [1]. Although there exists a theoretical mismatch about the nature and definition of anger, its complex and dynamic conceptualization is recognized [2–4]. In this sense, anger is considered one of the basic emotions together with fear, disgust, sadness, happiness, and surprise [1,4,5]. Anger may be both, a state and a personality trait [1,6]. Furthermore, anger is mainly expressed in two basic dimensions: anger towards others or outside (also called anger-out), and anger towards oneself or within (which is also known as anger-in, anger turned inward, or unexpressed anger). On purpose, these two basic dimensions have been deeply studied in relation to anger management (i.e., as anger-in (the tendency to repress anger when it

**Citation:** Galvez-Sánchez, C.M.; Reyes del Paso, G.A.; Duschek, S.; Montoro, C.I. The Link between Fibromyalgia Syndrome and Anger: A Systematic Review Revealing Research Gaps. *J. Clin. Med.* **2022**, *11*, 844. https://doi.org/10.3390/ jcm11030844

Academic Editor: Sabrina Ravaglia

Received: 26 December 2021 Accepted: 2 February 2022 Published: 5 February 2022

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is experienced) and anger-out (the leaning to express anger through verbal or physical forms)) [7], and the sensitivity to both, acute and chronic pain [8].

Related to the neurological correlates of anger, several brain structures have been involved in both anger and aggression, including those related with emotion regulation (i.e., the amygdala) [1,5]. For instance, studies focus on human brain imaging have reported greater activity in anterior cingulate cortex and orbitofrontal cortex when individuals are asked to recall past experiences that made them feel angry [5,9]. Historically, studies of anger and aggression have been conducted to explore the involvement of subcortical structures in emotion [1]. Among these structures, the hypothalamus has been one of the earliest and leading associated to anger and aggressive behavior, and based on it a neural circuit for anger and aggression has been frequently proposed [1].

In addition, the neurotransmitter serotonin has also been proposed to play a relevant role in the regulation of anger and aggression [1,9]. In fact, the serotonin deficiency hypothesis (that is, the causal role of diminished serotonin in anger and aggression) is widely supported by the scientific evidence. This evidence shows clearly that aggression is inversely related to serotonergic activity [1,5].

Chronic pain patients usually experience anger [10,11]. Nonetheless, it tends to be underestimated due to the denial and negative social connotation of this emotion [12], especially in women [13]. A possible explanation for anger denial can be related to the social norms and religious values which are assumed to repress its expression [14]. Chronic pain patients, compared to healthy controls, have frequently reported higher levels of anger suppression and/or hostility, which in turn have been related to increased pain and disability [10,11,15]. Kerns et al. [16] reported that the internalization of angry feelings explained a considerable part of the variance in pain intensity, perceived interference and reported frequency of pain behaviors in chronic pain patients. Gaskin et al. [17] revealed that state anger may be a significant predictor of the affective pain. Anger-in has also significantly been associated with depression and pain, whereas anger-out has been linked to greater disability in chronic pain patients [10].

One of the prototypical chronic pain conditions is fibromyalgia syndrome (FMS), which is characterized by generalized and persistent non-inflammatory musculoskeletal pain. Accompanying symptoms frequently comprise depression, anxiety, fatigue, insomnia, morning stiffness, and cognitive impairments (i.e., memory and attention problems, concentration difficulties, mental slowness, etc.) [18,19]. Moreover, a high percentage of FMS patients generally exhibited negative affect, which encompass alexithymia, catastrophizing, neuroticism [20–23] and deficit in health-related quality of life [21,22,24]. Its prevalence is stablished at 2 to 4% in the general population, and seems to be more frequent in women than in men [18]. However, recent studies reveal a possible gender bias which leads healthcare professionals to overestimate FMS prevalence in women simultaneously is underestimated in men [25,26]. Nowadays, there is not a specific treatment for FMS. Therefore, research should contribute to the clinical practice.

Regarding the empirical investigation of the role of anger in FMS, the available literature is scarce. Several authors have pointed out that FMS patients tend to experience higher levels of anger, in comparison with rheumatoid arthritis (RA) patients and healthy controls [27–29]. In the same line, RA patients are considered to be more prone to experience anger (trait anger) in addition to readily express it, while anger is rather internalized and suppressed by FMS patients [30]. Inhibited anger has been linked to greater intense pain than uninhibited anger [31,32], this being, in fact, more common in FMS patients [20,27]. On this detail, physiological arousal has been proposed as a mechanism that underlies the effect of anger in pain intensity due to effortful suppression, and the counterintuitive rise of the accessibility of angry thoughts and feelings after suppression [7,33]. Additionally, both enhanced [27,34] and reduced [35,36] pain has been reported after anger expression (measured by the 'Anger Expression Inventory', experimental emotion-induction tasks, etc.) [34,35]. These mixed findings may be explained by the trait-by-state matching model [37]. This model proposes a reduction of anger and its negative consequences such

as pain by the matching of a general style to express anger with actual anger-expressive behavior [36]. Oppositely, according to the same model, the lack of anger expression in high trait anger-out individuals intensifies anger and pain [36]. Furthermore, higher pain and anger has been linked to poorer health and quality of life in FMS [24].

Due to the fact that social constraints discourage the expression of anger, overall, in women [13], frequent mismatches may arise between the urge to express anger and its actual expression [38]. Furthermore, given that anger is associated with negative clinical outcomes in FMS, it can be argued that its addressing in clinical practice might be beneficial for optimizing treatment and improving health-related quality of life of these patients. However, fewer studies exploring the beneficial effects of its intervention have been conducted. Based on the above reviewed literature, the present systematic review focuses on exploring the connection between anger and FMS and, based on it, delimiting possible gaps in the research, altogether aimed at improving FMS clinical intervention and guiding future research on this topic.
