**1. Introduction**

Fibromyalgia (FM) is defined as a chronic pain syndrome of unknown etiology characterized by diffuse, widespread, and non-inflammatory musculoskeletal pain, which is accompanied by symptoms such as morning stiffness, fatigue, mood disorders, sleep disturbances, and cognitive impairment; it predominantly affects middle-aged adult women, that is, women over 50 years of age [1–3]. One of the most common complaints reported by FM patients is cognitive alterations [4–7]. Specifically, these cognitive complaints have been suggested to affect 50–80% of FM patients and include memory loss that extends to the capacity to recall names, conversations, words and quotations, problems expressing thoughts, difficulty in adequately remembering directions and scheduled activities, and a kind of "fog" that prevents sufferers from perceiving daily events clearly [8–10]. These cognitive problems, together with the experience of pain, limit the daily life activities of FM patients and cause a great deal of discomfort [11,12]. In fact, patients report that they are among the most deleterious symptoms of the disease, due to their negative impact on functional capacity, working life, and quality of life [13–15].

Despite patients' complaints, cognitive dysfunction in FM has received relatively less clinical and empirical support/attention compared with clinical pain symptoms; however, more studies have appeared in recent years, with current evidence indicating the presence of cognitive deficits in several neuropsychological domains [13,16]. Although

**Citation:** Muñoz Ladrón de Guevara, C.; Reyes del Paso, G.A.; Fernández Serrano, M.J.; Montoro, C.I. Fibromyalgia Syndrome and Cognitive Decline: The Role of Body Mass Index and Clinical Symptoms. *J. Clin. Med.* **2022**, *11*, 3404. https:// doi.org/10.3390/jcm11123404

Academic Editor: Giuseppe Lanza

Received: 10 May 2022 Accepted: 11 June 2022 Published: 14 June 2022

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short- and long-term implicit and working memory deficits, as well as slowness to complete complex cognitive tasks (i.e., executive control and emotional recognition tasks) have been confirmed [9,13,16–22], the results remain controversial. While most studies point to general deficits in information processing speed; selective and sustained attention; and visuospatial, verbal, and semantic memory in FM patients compared to healthy participants [5,13,18,23–29], others observed no substantial cognitive deficits in association with FM [27,30–34].

In addition, there is no consensus on the factors influencing the cognitive impairments seen in FM, although the intensity of clinical pain has been proposed as one such factor. Several studies have shown an inverse association between clinical pain levels and cognitive performance in FM patients (e.g., [16,19,28,31,35–40]). However, it is important to note that not all studies demonstrated this association [10,23,41,42].

Other factors proposed to explain the presence of cognitive impairments in FM include medication intake, mood and emotional alterations (i.e., depression and anxiety), and fatigue and sleep problems. However, while some studies have reported a significant relationship between depression and/or anxiety, and cognitive deficits in FM [25,38,42–44], others suggested that cognitive deficits are independent of comorbid depression and/or anxiety disorders [5,6,16,28,29,39]. The situation is similar for fatigue and/or sleep disorders, i.e., some studies have supported the notion that these factors influence cognitive performance [27,28,45], while others have not [19,29,39,40,46,47].

Body mass index (BMI) may also influence cognitive performance, especially when meeting the threshold for obesity (i.e., BMI > 25 kg/m2; [48–51]. Indeed, this has been demonstrated in the general population [52–54]. However, although overweight and obesity are highly prevalent in FM [55,56], to the best of our knowledge, only two studies have attempted to elucidate the relationship between BMI and cognitive impairment in FM. The first one, performed by Soriano-Maldonado et al. [57], revealed strong associations between aerobic fitness, attention, working memory, delayed recall, and verbal learning in FM, while no associations were observed between parameters used to assess overweight (BMI, body fat percentage, fat mass index, and waist circumference) and cognitive ability. Contrarily, Muñoz Ladrón de Guevara et al. [16] found a negative influence of BMI on performance in a cognitive test measuring components of executive function (updating, inhibitory control, switching, decision-making, self-regulation, and planning) in FM patients.

Given that findings regarding the negative association between BMI and cognitive function in FM are limited to executive function components, and taking into account the equivocal results regarding the deficits in basic cognitive processes (attention, memory, and processing speed) seen in FM, the aims of the present study were to: (1) test for deficits in basic cognitive processes in FM, in the domains of attention (selective, sustained, and divided attention), memory (verbal and visuospatial), and information processing speed; and (2) explore the association between BMI and these basic cognitive processes. The role of clinical variables such as pain, anxiety, depression, fatigue, sleep problems (insomnia), and medication use on the cognitive performance of FM patients will be also explored.
