Eczema pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:  Edzel Lorraine Co, DMD, MD[2]

## Overview

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The mechanism of disease of eczema involves a complex interplay of abnormalities of skin microbiomes, a dysfunction in the epidermal barrier, and an immune dysregulation. [1] There are two main theories on the existence of atopic dermatitis, also known as atopic eczema \- the inside-out hypothesis, and the outside-in hypothesis.

## Pathophysiology

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### Immunological Dysregulation

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* Affected skin involves the uptake of antigens and allergens by the epidermal dendritic cells, dermal dendritic cells, and Langerhan cells.
* Sensory nerves are activated by the released cytokines interleukins 4,13 and 31 (IL-4, IL-13, and IL-31), which are responsible for the pruritic sensation.
* As the event becomes chronic, an increased expression of keratinocyte and Th-cell-derived cytokines is achieved, with more pruritogens contributing to itch sensation. [1]

### Genetic Inheritance

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* There are 34 genetic loci which are thought to be associated with eczema.
* These regions contain several genes which have important immunological functions, such as T-cell activation, type-2 helper cell differentiation, and innate immunity. [2] [3][4]

### Epithelium Barrier Dysfunction

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* Multiple factors cause the disruption of epithelial barrier. These include:
* FLG mutations
* Constant scratching
* Microbial dysbiosis brought about by Staphylococcus aureus and Malassezia.
* As a result, an interrupted normal skin barrier with elevated pH, high skin permeability, altered lipid composition, and decreased water retention is observed. [5] [6] [7] [8] [9] [10]

### Hypotheses on Development of Eczema

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* There are two main theories on the existence of atopic dermatitis, also known as atopic eczema \- the inside-out hypothesis, and the outside-in hypothesis.[11]
* Inside-out hypothesis
* This hypothesis denotes that some allergic triggers weaken the skin barrier that leads to more introduction of allergens to the area, causing more inflammatory reaction.[11]
* Outside-in hypothesis
* This hypothesis suggests that an impairment of the skin barrier should occur first before the onset of atopic dermatitis.
* This involves the down-regulation of filaggrin genes (FLG) responsible for maintenance of the proper function of skin barrier.[11] [12]

## References

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1. ↑ 1.0 1.1 Feld M, Garcia R, Buddenkotte J, Katayama S, Lewis K, Muirhead G; et al. (2016). "The pruritus- and TH2-associated cytokine IL-31 promotes growth of sensory nerves". J Allergy Clin Immunol. 138 (2): 500–508.e24. doi:10.1016/j.jaci.2016.02.020. PMID 27212086.CS1 maint: Multiple names: authors list (link)
2. ↑ Paternoster L, Standl M, Waage J, Baurecht H, Hotze M, Strachan DP; et al. (2015). "Multi-ancestry genome-wide association study of 21,000 cases and 95,000 controls identifies new risk loci for atopic dermatitis". Nat Genet. 47 (12): 1449–1456. doi:10.1038/ng.3424. PMC 4753676. PMID 26482879.CS1 maint: Multiple names: authors list (link)
3. ↑ Weidinger S, Willis-Owen SA, Kamatani Y, Baurecht H, Morar N, Liang L; et al. (2013). "A genome-wide association study of atopic dermatitis identifies loci with overlapping effects on asthma and psoriasis". Hum Mol Genet. 22 (23): 4841–56. doi:10.1093/hmg/ddt317. PMC 3820131. PMID 23886662.CS1 maint: Multiple names: authors list (link)
4. ↑ Martin MJ, Estravís M, García-Sánchez A, Dávila I, Isidoro-García M, Sanz C (2020). "Genetics and Epigenetics of Atopic Dermatitis: An Updated Systematic Review". Genes (Basel). 11 (4). doi:10.3390/genes11040442. PMC 7231115 Check `|pmc=` value (help). PMID 32325630 Check `|pmid=` value (help).CS1 maint: Multiple names: authors list (link)
5. ↑ Seidenari S, Giusti G (1995). "Objective assessment of the skin of children affected by atopic dermatitis: a study of pH, capacitance and TEWL in eczematous and clinically uninvolved skin". Acta Derm Venereol. 75 (6): 429–33. doi:10.2340/0001555575429433. PMID 8651017.
6. ↑ Jungersted JM, Scheer H, Mempel M, Baurecht H, Cifuentes L, Høgh JK; et al. (2010). "Stratum corneum lipids, skin barrier function and filaggrin mutations in patients with atopic eczema". Allergy. 65 (7): 911–8. doi:10.1111/j.1398-9995.2010.02326.x. PMID 20132155.CS1 maint: Multiple names: authors list (link)
7. ↑ Tsakok T, Woolf R, Smith CH, Weidinger S, Flohr C (2019). "Atopic dermatitis: the skin barrier and beyond". Br J Dermatol. 180 (3): 464–474. doi:10.1111/bjd.16934. PMID 29969827.CS1 maint: Multiple names: authors list (link)
8. ↑ Halling-Overgaard AS, Kezic S, Jakasa I, Engebretsen KA, Maibach H, Thyssen JP (2017). "Skin absorption through atopic dermatitis skin: a systematic review". Br J Dermatol. 177 (1): 84–106. doi:10.1111/bjd.15065. PMID 27639188.CS1 maint: Multiple names: authors list (link)
9. ↑ Howell MD, Kim BE, Gao P, Grant AV, Boguniewicz M, Debenedetto A; et al. (2007). "Cytokine modulation of atopic dermatitis filaggrin skin expression". J Allergy Clin Immunol. 120 (1): 150–5. doi:10.1016/j.jaci.2007.04.031. PMC 2669594. PMID 17512043.CS1 maint: Multiple names: authors list (link)
10. ↑ Baurecht H, Rühlemann MC, Rodríguez E, Thielking F, Harder I, Erkens AS; et al. (2018). "Epidermal lipid composition, barrier integrity, and eczematous inflammation are associated with skin microbiome configuration". J Allergy Clin Immunol. 141 (5): 1668–1676.e16. doi:10.1016/j.jaci.2018.01.019. PMID 29421277.CS1 maint: Multiple names: authors list (link)
11. ↑ 11.0 11.1 11.2 Silverberg NB, Silverberg JI (2015). "Inside out or outside in: does atopic dermatitis disrupt barrier function or does disruption of barrier function trigger atopic dermatitis?". Cutis. 96 (6): 359–61. PMID 26761930.
12. ↑ Leung DY, Guttman-Yassky E (2014). "Deciphering the complexities of atopic dermatitis: shifting paradigms in treatment approaches". J Allergy Clin Immunol. 134 (4): 769–79. doi:10.1016/j.jaci.2014.08.008. PMC 4186710. PMID 25282559.