*3.1. Review*

A total of 139 articles reported on epidemiology and prevalence (Table S1), 101 articles on pathophysiology (Table S2), 34 articles on the outcome (Table S3) and 93 articles on therapeutic effects (Table S4).

#### 3.1.1. Epidemiology

#### 3.1.1.1. Dysphagia and Disease Course

In principle, IIM is a chronically progressive disease, but sometimes there are also relapsing– remitting episodes. The situation is similar with dysphagia in IIM. Besides relapsing–remitting episodes of dysphagia, several authors report that the prevalence of dysphagia increases as the disease progresses [17–26]. Nevertheless, dysphagia can also be the initial [17–21,23–32] or even the only symptom [18,29,31,33]. Therefore, dysphagia should not be considered a late symptom in IIM. Indeed, IIM might be the underlying disease in patients with unclear dysphagia, even if other investigations, such as laboratory results and electrophysiology, do not refer to IIM [32].

#### 3.1.1.2. Factors Associated with Dysphagia

Several factors are reported to be associated with dysphagia. Among the subgroups, differences in prevalence are found: Higher prevalence is reported in DM compared to PM [34–37] but also vice versa [38], in IBM compared to other forms of IIM [39] and in overlap syndromes compared to other forms of IIM [39]. In addition, an increased risk of dysphagia is reported in patients with associated malignancy [37,40–45]. A number of antibodies are also linked to an increased risk of dysphagia: NXP2 [46–49], FHL-1 [50], SAE [47,51], HMGCR [47,52], NT5c1A [53], SRP [47,54,55], TIF1y [44,47], OJ [56] and myositis-specific or -associated autoantibodies in general [47]. ANA and MDA5 antibodies are reported to be associated with a reduced risk of dysphagia [47,57].

**Figure 1.** Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram of the reviewed literature.

#### 3.1.2. Pathophysiology

#### Inflammation of Swallowing Muscles

IIM can result in impairment of the oral [29,58–64], pharyngeal [7,23,24,27,29,31–33,44,58–116] and esophageal [24,29,38,59,62,73,81,85,89,90,93,95,98,99,101,103–105,112,114–131] phases of swallowing and pharyngeal dysfunction is associated with aspiration [27,29,31–33,58,60,61,70,76–79,81,83,84,86–89,91,93, 94,96–98,102,105,106,108,111,113]. Results from studies and case-reports with biopsies suggest that inflammatory involvement occurs in the affected swallowing muscles [29,31,32,86,90,91,94–97,100, 101,103,104,108,126,132,133], similarly to the well-known inflammatory reactions in the peripheral skeletal muscles in IIM. Interestingly, such changes also seem to occur in smooth muscle tissue of the esophagus [104,119,126]. Besides muscle biopsy, signs for inflammation can be detected by characteristic MRI findings, e.g., edema in the oropharynx [74,134–136]. However, presumably due to the small volume of the respective muscles, MRI findings are inconclusive and, if normal, cannot rule

out myositis as cause of dysphagia [33]. The study data is conflicting on whether dysphagia is related to the clinical impairment of the peripheral skeletal muscles. Some studies report a correlation of peripheral symptoms with dysphagia [17,39,127], while other studies report the opposite [137,138].
