Nutritional neurobiology and central nervous system sensitisation: missing link in a comprehensive treatment for chronic pain?
2019; Elsevier BV; Volume: 123; Issue: 5 Linguagem: Inglês
10.1016/j.bja.2019.07.016
ISSN1471-6771
AutoresJo Nijs, Ömer Elma, Sevilay Tümkaya Yılmaz, Patrick Mullie, Luc Vanderweeën, Peter Clarys, Tom Deliens, Iris Coppieters, Nathalie Weltens, Lukas Van Oudenhove, Anneleen Malfliet,
Tópico(s)Exercise and Physiological Responses
ResumoIncreasing attention is being paid to dietary and nutritional factors in relation to chronic pain, not only for abdominal (i.e. visceral) pain, but also for other chronic (somatic) pain disorders, such as migraine headache, neuropathic, osteoarthritis, (post)cancer, and low back pain. A recent meta-analysis confirmed that nutritional interventions, especially an altered dietary pattern and an altered specific nutrient intake, can result in significant pain relief in patients having chronic pain.1Brain K. Burrows T.L. Rollo M.E. et al.A systematic review and meta-analysis of nutrition interventions for chronic noncancer pain.J Hum Nutr Diet. 2019; 32: 198-225Crossref PubMed Scopus (44) Google Scholar Even though they may be merely secondary to the mechanical impact of weight loss, such clinical benefits may arise from a potential dietary link to CNS sensitisation.2Holton K.F. Kindler L.L. Jones K.D. Potential dietary links to central sensitization in fibromyalgia: past reports and future directions.Rheum Dis Clin North Am. 2009; 35: 409-420Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar CNS sensitisation, defined as 'an amplification of neural signalling within the central nervous system that elicits pain hypersensitivity',3Woolf C.J. Central sensitization: implications for the diagnosis and treatment of pain.Pain. 2011; 152: S2-S15Abstract Full Text Full Text PDF PubMed Scopus (2661) Google Scholar is a well-established feature in many chronic pain patients, including those with chronic spinal pain, post-cancer pain, fibromyalgia, osteoarthritis, and paediatric pain. How can diet potentially contribute to CNS sensitisation? Poor diet (i.e. low fibre, energy-dense diet, etc.) is associated with oxidative stress, cell necrosis, and tissue damage throughout the body, each of which is a potential endogenous activator of Toll-like receptors (TLRs).4Nicotra L. Loram L.C. Watkins L.R. Hutchinson M.R. Toll-like receptors in chronic pain.Exp Neurol. 2012; 234: 316-329Crossref PubMed Scopus (183) Google Scholar Upon activation, pattern-recognition TLRs trigger pro-inflammatory central immune signalling events,4Nicotra L. Loram L.C. Watkins L.R. Hutchinson M.R. Toll-like receptors in chronic pain.Exp Neurol. 2012; 234: 316-329Crossref PubMed Scopus (183) Google Scholar including glial cell activation, which in turn results in low-grade neuroinflammation5Nijs J. Loggia M.L. Polli A. et al.Sleep disturbances and severe stress as glial activators: key targets for treating central sensitization in chronic pain patients?.Expert Opin Ther Targets. 2017; 21: 817-826Crossref PubMed Scopus (70) Google Scholar (Fig. 1). Aberrant glial activity also causes astrocyte activation in the CNS, which leads to the production of pro-inflammatory substances by hypertrophied and activated astrocytes (astrogliosis).6Raghavendra V. Tanga F.Y. DeLeo J.A. Complete Freunds adjuvant-induced peripheral inflammation evokes glial activation and proinflammatory cytokine expression in the CNS.Eur J Neurosci. 2004; 20: 467-473Crossref PubMed Scopus (460) Google Scholar Increased brain glial activation has been shown in patients with chronic pain, including those with chronic non-specific low back pain7Loggia M.L. Chonde D.B. Akeju O. et al.Evidence for brain glial activation in chronic pain patients.Brain. 2015; 138: 604-615Crossref PubMed Scopus (306) Google Scholar and fibromyalgia.8Albrecht D.S. Forsberg A. Sandstrom A. et al.Brain glial activation in fibromyalgia—a multi-site positron emission tomography investigation.Brain Behav Immun. 2019; 75: 72-83Crossref PubMed Scopus (122) Google Scholar Spinal-cord glial activation was found in patients having chronic lumbar radiculopathy pain.9Albrecht D.S. Ahmed S.U. Kettner N.W. et al.Neuroinflammation of the spinal cord and nerve roots in chronic radicular pain patients.Pain. 2018; 159: 968-977Crossref PubMed Scopus (80) Google Scholar In addition to TLRs (TLR-4 and TLR-2), another important trigger of microglial activation is the fractalkine receptor (CX3R1), which is a rapid and selective trigger of spinal dorsal horn microglia.10Verge G.M. Milligan E.D. Maier S.F. Watkins L.R. Naeve G.S. Foster A.C. Fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) distribution in spinal cord and dorsal root ganglia under basal and neuropathic pain conditions.Eur J Neurosci. 2004; 20: 1150-1160Crossref PubMed Scopus (330) Google Scholar Animal models have shown that fractalkine plays an important role in the early activation of high-fat diet-induced hypothalamic inflammation.11Morari J. Anhe G.F. Nascimento L.F. et al.Fractalkine (CX3CL1) is involved in the early activation of hypothalamic inflammation in experimental obesity.Diabetes. 2014; 63: 3770-3784Crossref PubMed Scopus (97) Google Scholar Aberrant glial activity has the potential to initiate CNS sensitisation through several mechanisms. Activated microglia have been identified as a major source for the synthesis and release of several neurotrophic factors, including brain-derived neurotrophic factor, which is responsible for increasing neuronal excitability by causing disinhibition in dorsal horn neurones in the spinal cord.12Ferrini F. De Koninck Y. Microglia control neuronal network excitability via BDNF signalling.Neural Plast. 2013; 2013: 429815Crossref PubMed Scopus (221) Google Scholar Aberrant glial activity is also accompanied by increased tumour necrosis factor-α, which in turn induces long-term potentiation,13Gao Y.J. Ji R.R. Activation of JNK pathway in persistent pain.Neurosci Lett. 2008; 437: 180-183Crossref PubMed Scopus (119) Google Scholar leading to enhanced synaptic efficacy14Delpech J.C. Madore C. Nadjar A. Joffre C. Wohleb E.S. Laye S. Microglia in neuronal plasticity: influence of stress.Neuropharmacology. 2015; 96: 19-28Crossref PubMed Scopus (109) Google Scholar and, ultimately, pain sensitisation.13Gao Y.J. Ji R.R. Activation of JNK pathway in persistent pain.Neurosci Lett. 2008; 437: 180-183Crossref PubMed Scopus (119) Google Scholar Long-term potentiation and enhanced synaptic efficacy are (partly overlapping) key mechanisms underlying CNS sensitisation15Ji R.R. Kohno T. Moore K.A. Woolf C.J. Central sensitization and LTP: do pain and memory share similar mechanisms?.Trends Neurosci. 2003; 26: 696-705Abstract Full Text Full Text PDF PubMed Scopus (1101) Google Scholar and the formation of (maladaptive) pain memories in patients with chronic pain. The peripheral pro-inflammatory effects of poor nutrition are well established,16Schwingshackl L. Hoffmann G. Mediterranean dietary pattern, inflammation and endothelial function: a systematic review and meta-analysis of intervention trials.Nutr Metab Cardiovasc Dis. 2014; 24: 929-939Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar generating another route through which diet can impact the CNS. Indeed, peripheral pro-inflammatory cytokines can cross the blood–brain barrier and migrate into the CNS. Conversely, human studies suggest that a diet rich in vegetables, fruits, healthy oils, and fibres exerts anti-inflammatory action.17Tick H. Nutrition and pain.Phys Med Rehabil Clin N Am. 2015; 26: 309-320Abstract Full Text Full Text PDF PubMed Scopus (22) Google Scholar In addition, other routes potentially explain how poor diet can trigger CNS sensitisation. High-(saturated) fat or energy-dense diet, and the pro-inflammatory mediators that come along, is sensed via vagal afferent nerves in the gastrointestinal system.18Vaughn A.C. Cooper E.M. DiLorenzo P.M. et al.Energy-dense diet triggers changes in gut microbiota, reorganization of gutbrain vagal communication and increases body fat accumulation.Acta Neurobiol Exp (Wars). 2017; 77: 18-30PubMed Google Scholar More specifically, enteroendocrine cells are the primary 'gut (chemo)sensors' dispersed throughout the gastrointestinal mucosa that secrete hormones and amines in response to the presence of three macronutrient types: carbohydrates, proteins, and lipids.19Dockray G.J. Luminal sensing in the gut: an overview.J Physiol Pharmacol. 2003; 54: 9-17PubMed Google Scholar Vagal afferent neurones express receptors for regulatory peptides and molecules released from the intestinal wall, pancreas, and adipocytes, such as cholecystokinin.20Du Y. Yang M. Lee S. et al.Maternal western diet causes inflammatory milk and TLR2/4-dependent neonatal toxicity.Genes Dev. 2012; 26: 1306-1311Crossref PubMed Scopus (40) Google Scholar This way, vagal afferent neurones inform the brain about dietary intake, which may in turn lead to microglial activation18Vaughn A.C. Cooper E.M. DiLorenzo P.M. et al.Energy-dense diet triggers changes in gut microbiota, reorganization of gutbrain vagal communication and increases body fat accumulation.Acta Neurobiol Exp (Wars). 2017; 77: 18-30PubMed Google Scholar (Fig. 1). An alternative route implies that increased peripheral (local gastrointestinal or systemic) inflammation activates vagal afferents (they have cytokine receptors), which in turn induces neuroinflammation.21Watkins L.R. Maier S.F. Implications of immune-to-brain communication for sickness and pain.Proc Natl Acad Sci U S A. 1999; 96: 7710-7713Crossref PubMed Scopus (211) Google Scholar Moreover, diet-induced changes in gut-microbiota composition can also trigger vagal afferent activation18Vaughn A.C. Cooper E.M. DiLorenzo P.M. et al.Energy-dense diet triggers changes in gut microbiota, reorganization of gutbrain vagal communication and increases body fat accumulation.Acta Neurobiol Exp (Wars). 2017; 77: 18-30PubMed Google Scholar and (low-grade) gut inflammation.22de Lartigue G. de La Serre C.B. Raybould H.E. Vagal afferent neurons in high fat diet-induced obesity; intestinal microflora, gut inflammation and cholecystokinin.Physiol Behav. 2011; 105: 100-105Crossref PubMed Scopus (106) Google Scholar Likewise, under free-fatty-acid-rich obese conditions, astrocytes participate in obesity-induced hypothalamic inflammation by promoting microglial migration and activation.23Kwon Y.H. Kim J. Kim C.S. et al.Hypothalamic lipid-laden astrocytes induce microglia migration and activation.FEBS Lett. 2017; 591: 1742-1751Crossref PubMed Scopus (37) Google Scholar In animals, a caloric restriction diet (i.e. 6 weeks at 60% of the ad libitum food intake of their counterparts) inhibited neuroinflammation, including glial cell activation, and consequently decreased CNS sensitisation and pain behaviour.24Liu Y. Ni Y. Zhang W. et al.Anti-nociceptive effects of caloric restriction on neuropathic pain in rats involves silent information regulator 1.Br J Anaesth. 2018; 120: 807-817Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar The same researchers reported that such a caloric restriction diet in non-obese rats results in anti-nociceptive effects on postoperative pain, possibly mediated by inhibition of inflammation.25Liu Y. Ni Y. Zhang W. Sun Y.E. Ma Z. Gu X. Antinociceptive effects of caloric restriction on post-incisional pain in nonobese rats.Sci Rep. 2017; 7: 1805Crossref PubMed Scopus (16) Google Scholar Taken together, it is plausible that an unhealthy diet characterised mainly by high-calorie intake (e.g. high saturated fat or energy-dense diet) is in part responsible for glial activation as observed in patients with chronic pain. This idea of diet-induced (or diet-maintained) neuroinflammation is strengthened by a series of observations in humans. First, a cross-sectional study showed that markers of neuroinflammation are associated with peripheral glucose concentrations in patients having complex regional pain syndrome,26Jung Y.H. Kim H. Jeon S.Y. et al.Brain metabolites and peripheral biomarkers associated with neuroinflammation in complex regional pain syndrome using [11C]-(R)-PK11195 positron emission tomography and magnetic resonance spectroscopy: a pilot study.Pain Med. 2019; 20: 504-514Google Scholar a severely debilitating subtype of chronic pain. Another cross-sectional study revealed that age-adjusted elevated concentrations of blood glucose and low high-density lipoprotein (HDL) cholesterol are each independently associated with higher chronic pain intensity.27Goodson N.J. Smith B.H. Hocking L.J. et al.Cardiovascular risk factors associated with the metabolic syndrome are more prevalent in people reporting chronic pain: results from a cross-sectional general population study.Pain. 2013; 154: 1595-1602Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar Low HDL cholesterol concentrations might be the result of a low-grade inflammatory process in (some) patients with chronic pain.27Goodson N.J. Smith B.H. Hocking L.J. et al.Cardiovascular risk factors associated with the metabolic syndrome are more prevalent in people reporting chronic pain: results from a cross-sectional general population study.Pain. 2013; 154: 1595-1602Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar Unsaturated fatty acids increase HDL (and in turn decrease blood cholesterol).28Mensink R.P. Zock P.L. Kester A.D. Katan M.B. Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials.Am J Clin Nutr. 2003; 77: 1146-1155Crossref PubMed Scopus (2082) Google Scholar Therefore, it can be hypothesised that unsaturated fatty acids have the potential to reduce chronic pain intensity through the increase in HDL, although further research is needed to verify this. Animal work revealed that high-glucose conditions facilitate CNS sensitisation through increasing expression and activation of high mobility group protein B1, a modulator of the inflammatory response, in dorsal root ganglion neurones.29Bestall S.M. Hulse R.P. Blackley Z. et al.Sensory neuronal sensitisation occurs through HMGB-1–RAGE and TRPV1 in high-glucose conditions.J Cell Sci. 2018; 131: 215939Google Scholar Metformin, a drug for controlling blood sugar concentrations in diabetes mellitus, not only decreases body weight in animals, but also decreases inflammation and CNS sensitisation (i.e. decreased sensitivity to mechanical and thermal allodynia).30Afshari K. Dehdashtian A. Haddadi N.S. et al.Anti-inflammatory effects of metformin improve the neuropathic pain and locomotor activity in spinal cord injured rats: introduction of an alternative therapy.Spinal Cord. 2018; 56: 1032-1041Google Scholar The glucocorticoid system may be a potential mediator for the link between inflammation, glucose, and pain, as mice exposed to interleukin-1β showed increased blood glucose concentrations and pain behaviour via the activation of the glucocorticoid system.31Sim Y.B. Park S.H. Kang Y.J. et al.Interleukin-1beta (IL-1beta) increases pain behavior and the blood glucose level: possible involvement of glucocorticoid system.Cytokine. 2013; 64: 351-356Google Scholar This has obvious relevance to diabetics who may suffer from neuropathic pain, but may also be relevant to chronic pain independent of diabetes mellitus. Indeed, chronic widespread pain in humans, independent of diabetes, is associated with higher cortisol and fasting glucose.32Stehlik R. Ulfberg J. Zou D. Hedner J. Grote L. Morning cortisol and fasting glucose are elevated in women with chronic widespread pain independent of comorbid restless legs syndrome.Scand J Pain. 2018; 18: 187-194Google Scholar The link between glucose, inflammation, and pain is also supported by studies exploring the potential effect of a ketogenic diet in patients having chronic pain. A ketogenic diet is very high in fat, with sufficient protein and restricted carbohydrate intake. It alters cellular (including central neurone) metabolism so that ketones, produced by the liver, are burned instead of glucose: the restricted carbohydrate content of a ketogenic diet minimises glucose metabolism and increases ketolysis (i.e. the use of ketone bodies [acetone, acetoacetate, and β-hydroxybutyrate] as alternate energy sources).33Ruskin D.N. Kawamura M. Masino S.A. Reduced pain and inflammation in juvenile and adult rats fed a ketogenic diet.PLoS One. 2009; 4: e8349Crossref PubMed Scopus (119) Google Scholar Animal work showed that, after 3–4 weeks, such a ketogenic diet resulted in acute peripheral anti-inflammatory and hypoalgesic effects.33Ruskin D.N. Kawamura M. Masino S.A. Reduced pain and inflammation in juvenile and adult rats fed a ketogenic diet.PLoS One. 2009; 4: e8349Crossref PubMed Scopus (119) Google Scholar A similar effect is likely in humans,34Masino S.A. Ruskin D.N. Ketogenic diets and pain.J Child Neurol. 2013; 28: 993-1001Crossref PubMed Scopus (41) Google Scholar but (randomised) interventional studies to confirm this hypothesis are lacking.1Brain K. Burrows T.L. Rollo M.E. et al.A systematic review and meta-analysis of nutrition interventions for chronic noncancer pain.J Hum Nutr Diet. 2019; 32: 198-225Crossref PubMed Scopus (44) Google Scholar When glycolytic enzymes are inhibited, pain thresholds increase (and consequently pain reduced).35Bodnar R.J. Kelly D.D. Glusman M. 2-Deoxy-D-glucose analgesia: influences of opiate and non-opiate factors.Pharmacol Biochem Behav. 1979; 11: 297-301Crossref PubMed Scopus (51) Google Scholar This analgesic effect is mediated centrally,36Bodnar R.J. Merrigan K.P. Wallace M.M. Analgesia following intraventricular administration of 2-deoxy-D-glucose.Pharmacol Biochem Behav. 1981; 14: 579-581Google Scholar and might involve increased brain/spinal-cord inhibition by adenosine.37Zhao Z.Q. Todd J.C. Sato H. Ma X.L. Vinten-Johansen J. Adenosine inhibition of neutrophil damage during reperfusion does not involve K(ATP)-channel activation.Am J Physiol. 1997; 273: H1677-H1687PubMed Google Scholar Inversely, a high blood glucose concentration results in pain hypersensitivity probably by disrupting the functions of cell mitochondria and subsequent generation of reactive oxygen species and oxidative stress,38Feldman E.L. Oxidative stress and diabetic neuropathy: a new understanding of an old problem.J Clin Invest. 2003; 111: 431-433Crossref PubMed Scopus (209) Google Scholar and the activation of microglia.39Wang D. Couture R. Hong Y. Activated microglia in the spinal cord underlies diabetic neuropathic pain.Eur J Pharmacol. 2014; 728: 59-66Crossref PubMed Scopus (98) Google Scholar Therefore, the reduced CNS excitability40Cantello R. Varrasi C. Tarletti R. et al.Ketogenic diet: electrophysiological effects on the normal human cortex.Epilepsia. 2007; 48: 1756-1763Crossref PubMed Scopus (51) Google Scholar in response to a ketogenic diet may be—at least in part—attributed to its anti-inflammatory and antioxidant properties.41Boison D. New insights into the mechanisms of the ketogenic diet.Curr Opin Neurol. 2017; 30: 187-192Crossref PubMed Scopus (147) Google Scholar The reduced CNS excitability can also be triggered directly by ketones or low glucose, fatty acids, or downstream metabolic effects,42Hartman A.L. Gasior M. Vining E.P. Rogawski M.A. The neuropharmacology of the ketogenic diet.Pediatr Neurol. 2007; 36: 281-292Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar but animal work suggests that hypoalgesia does not result from direct actions of elevated ketones or decreased glucose.43Ruskin D.N. Suter T.A. Ross J.L. Masino S.A. Ketogenic diets and thermal pain: dissociation of hypoalgesia, elevated ketones, and lowered glucose in rats.J Pain. 2013; 14: 467-474Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar Alternative explanations include activation of K1 channels, adenosine A1 receptors, or gamma-aminobutyric acid receptors, all causing hypoalgesia.44Sawynok J. Adenosine receptor activation and nociception.Eur J Pharmacol. 1998; 347: 1-11Crossref PubMed Scopus (419) Google Scholar Still, the clinical utility of a ketogenic diet in patients having chronic pain remains to be established via randomised controlled clinical trials that should try to address the problems inherent to this type of nutritional intervention trials (including the difficulties of identifying a good control condition, with blinding, etc.). Another hypothetical route through which diet can sustain or aggravate CNS sensitisation in chronic pain is the capacity of gut microbiota to regulate CNS neurotransmission. This includes the proposed ability of some microbial species to elevate concentrations of tryptophan and subsequently central signalling by serotonin,45O'Mahony S.M. Dinan T.G. Cryan J.F. The gut microbiota as a key regulator of visceral pain.Pain. 2017; 158: S19-S28Crossref PubMed Scopus (55) Google Scholar a neurotransmitter important for brain-orchestrated endogenous analgesia.46Brenchat A. Romero L. Garcia M. et al.5-HT7 receptor activation inhibits mechanical hypersensitivity secondary to capsaicin sensitization in mice.Pain. 2009; 141: 239-247Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar A preclinical study revealed that secretory products from commensal bacteria derived from a healthy human donor contain serine proteases that suppress the excitability of dorsal root ganglion neurones via activation of protease-activated receptor-4.47Sessenwein J.L. Baker C.C. Pradhananga S. et al.Protease-mediated suppression of DRG neuron excitability by commensal bacteria.J Neurosci. 2017; 37: 11758-11768Crossref PubMed Scopus (34) Google Scholar Another animal study showed that commensal intestinal microbiota are necessary for normal excitability of gut sensory neurones.48McVey Neufeld K.A. Mao Y.K. Bienenstock J. Foster J.A. Kunze W.A. The microbiome is essential for normal gut intrinsic primary afferent neuron excitability in the mouse.Neurogastroenterol Motil. 2013; 25: 183-188Crossref PubMed Scopus (207) Google Scholar These findings suggest that therapies that address microbial dysbiosis may also affect the excitability of primary afferent (nociceptive) neurones,47Sessenwein J.L. Baker C.C. Pradhananga S. et al.Protease-mediated suppression of DRG neuron excitability by commensal bacteria.J Neurosci. 2017; 37: 11758-11768Crossref PubMed Scopus (34) Google Scholar possibly reducing CNS sensitisation via a bottom-up route. It has been suggested that probiotics might restore balance of the gut microbiome and introduce beneficial functions to gut microbial communities, resulting in reduced gut inflammation.49Hemarajata P. Versalovic J. Effects of probiotics on gut microbiota: mechanisms of intestinal immunomodulation and neuromodulation.Therap Adv Gastroenterol. 2013; 6: 39-51Crossref PubMed Scopus (534) Google Scholar Data provided by animal studies on visceral pain show that probiotics might exert a direct action through bacterial metabolites on sensitive nerve endings in the gut mucosa, or indirect pathways targeting the intestinal epithelial barrier, mucosal or systemic immune activation, and subsequent central neuronal system sensitisation.50Theodorou V. Ait Belgnaoui A. Agostini S. Eutamene H. Effect of commensals and probiotics on visceral sensitivity and pain in irritable bowel syndrome.Gut Microbe. 2014; 5: 430-436Crossref PubMed Scopus (52) Google Scholar However, even though many claims about causal relationships between gut microbiota and human behaviour are being made, methodologically sound research is often lacking.51Hooks K.B. Konsman J.P. O'Malley M.A. Microbiota-gut-brain research: a critical analysis.Behav Brain Sci. 2019; 42 (e60)https://doi.org/10.1017/S0140525X18002133Crossref Scopus (42) Google Scholar Before claims about consideration of probiotics as a potential new therapeutic target in patients having chronic pain and CNS sensitisation can be made, findings from animal studies require examination in humans having chronic pain. This model of diet-induced neuroinflammation and consequent CNS sensitisation (Fig. 1) provides a rationale for developing innovative treatments for patients with chronic pain, such as dietary interventions and pharmacological treatments. The model first requires further elaboration at the dietary level (i.e. which specific nutritional components play a role in the proposed diet–neuroinflammation–CNS sensitisation pathway?) and experimental testing in humans before causal inferences can be drawn. Study design: JN Review, analysis, and interpretation of literature data; data acquisition: all authors Writing first draft: JN Revising paper and approving final version: all authors The authors declare that they have no conflicts of interest. De Berekuyl Academy Chair, funded by the European College for Lymphatic Therapy, The Netherlands to JN; Ministry of National Education of the Turkish State scholarships to STY and OE; Applied Biomedical Research Program of the Agency for Innovation by Science and Technology and the Research Foundation-Flanders to IC; KU Leuven Special Research Fund to LVO; Research Foundation Flanders to AM.
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