10

important to consider the prevalence of the causes of any condition, because that determines the pretest probability, the order of the investigations, and it can also affect to prognosis. The prevalence of visceral disorders that may trigger pain in the neck-shoulder area is high. It is estimated that the one-year prevalence of gastroesophageal reflux with weekly symptoms is 14% [80], and 15% of Americans have silent gallstones, 10–18% of whom develop biliary pain [81]. Besides, non-alcoholic fatty liver is present in around 30% of the population in western countries [82]. Females are more prone than males to have widespread hyperalgesia from recurrent visceral pain [61], and also NP is more prevalent in females. Further, NP of high intensity/low disability or high disability is strongly related to cardiovascular and digestive disorders [83]. That may suggest, eventually, a visceral origin for the pain, which together with cervical spasms have been observed in animal models [84] and in humans [23] with gastric or esophageal disorders. It is also known that NP is highly associated with obesity [85], LDL cholesterol [86], and metabolic syndrome [87]. For instance, the prevalence of NP in those with metabolic syndrome ranges between 16% for males and 25% for females [87]. This is remarkably important because fatty liver, obesity, and metabolic syndromes entail hepatic suffering; e.g., increased pressure, swelling, and hepatomegaly. Phrenic afferents in the hepatic parenchyma, hepatic veins, Diagnostics 2019, 9, 186 16 of 23 and the inferior vena cava just need light pressure to respond [74,88]. Further, all this can stretch and sensitize the Glisson capsule, which is known to evoke phrenic pain [72]. However, most patients do not relate their NP and the concomitant visceral disorder, or fail to report gastrointestinal or hepatic/biliary symptoms [15], which contributes to the misdiagnosis of NP as mechanical or non-specific. It is interesting to note that experimental research of gastric sensitivity is performed in rats by means of gastric distension, which is very common in obesity, and is related to dyspepsia. This gastric distension triggers an increase in the electromyographic activity of the neck muscles and also affects to the neck posture [89]. The addition of substances which increase the insult to the stomach enhances this visceromotor response [84]. The same mechanisms have been used to experimentally study the gastric hypersensitivity frequently observed in patients with long-standing diabetes [90]. The increase of muscle tone in the area of referred hyperalgesia does not appear only when the stomach is injured, since it has also been demonstrated by artificial ureteric stones [91]. Moreover, the neck muscles’ tone decreased in these models when the viscera was treated by means of electrical stimulation [92,93]. Likewise, manual visceral treatment has also been shown to improve NP and electromyographic recordings of the upper trapezius muscles of subjects suffering from chronic non-specific NP and dyspepsia [94], and has improved neck mobility and NP thresholds in subjects suffering from gastroesophageal reflux disease [95]. On the contrary, the likely participation of patients with NP of visceral origin might contribute to explaining the scarce success of usual treatments for NP, achieving at most moderate effects in the short-term [96]. Therefore, the visceral origin of NP might be more easily diagnosed if it only triggers VRP, because there will be no modification of pain related to activity or posture. However, as previously exposed, mechanical consequences can be also elicited in case of visceral aetiology of NP , hindering the correct diagnosis.

1 2 3 4 5 6 7 8 9 10