Introduction
Psoriasis vulgaris is a systemic, chronic, inflammatory disorder that can be transmitted genetically and is provoked by environmental factors. It primarily affects the skin and joints. The nails are affected in 40-50% of patients. It is associated with other inflammatory disorders and autoimmune diseases (eg psoriatic arthritis, inflammatory bowel disease [IBD], coronary artery disease) and is characterized by recurrent exacerbations and remissions.
Epidemiology
Psoriasis is found in approximately 2% of the population. Its
prevalence ranged from 0.5-11% in adults and 0-1.4% in children.
In the
United States, the prevalence is from 51-79/100,000 cases. In Canada, the
prevalence differs by age ranging from 44-225/10,000 cases.
According
to the 2019 Global Burden of Disease (GBD) report, the prevalence of psoriasis
was approximately 129/100,000 cases in Southeast Asia. The prevalence in China
was consistently below 1% as reported by the Chinese Medical Association from a
survey in 2008 and the GBD in 2019. Several studies in India reported that the
prevalence in the country was around 0.4-2.8% affecting men more than women. A Korean-based study that utilized the Korean National Health Insurance
Database from 2011-2015 mentioned that the prevalence of psoriasis was 450/100,000
cases. In Malaysia, a study claimed that the prevalence was 2-6%.
Pathophysiology
Psoriasis is a complex immune-mediated inflammatory disease involving T lymphocytes, dendritic cells, and cytokines (eg interleukin [IL]-23, IL-17, and tumor necrosis factor [TNF]) causing hyperproliferation and abnormal differentiation of the epidermis, infiltration of inflammatory cells, and vascular dilatation in genetically susceptible patients.
Risk Factors
Risk and
aggravating factors include alcohol consumption and drugs. It has been noted
that high alcohol consumption (>224 g/week) is associated with increased
risk for psoriasis. Drugs like angiotensin-converting enzyme (ACE) inhibitors,
antibiotics (eg tetracyclines), anti-tumor necrosis factor alpha (anti-TNF alpha),
beta-blockers, interferons, Lithium, nonsteroidal anti-inflammatory drugs
(NSAIDs), and synthetic antimalarial drugs may aggravate psoriasis. With regard
to exercise and physical activity, vigorous or regular physical activity
reduces the risk of psoriasis while low cardiorespiratory fitness or <1 hour
of exercise per week is associated with increased risk of psoriasis.
Additionally, with environmental factors, pollution has been shown to increase
the risk and likelihood of having a flare.
With
regard to genetic and family history, genetic predisposition to psoriasis
involves multiple genes, with 109 loci having been identified. Among these, PSORS1,
which includes the HLA-Cw6 allele, is a significant locus that accounts
for 35-50% of psoriasis heritability. Furthermore, genes involved in the
pathogenesis of psoriasis include antigen presentation (eg HLA-C, ERAP1),
T17 cell activation (eg IL-23R, IL-23A, IL-12B, TYK2, TRAF3IP2), innate
antiviral immunity/type 1 interferon signaling (eg RNF114, IFIH1), and
skin barrier function (eg LCE3B/3D). A positive family history of
psoriasis is a significant risk factor for psoriasis. Lastly, maternal-only
psoriasis and paternal-only psoriasis are also risk factors for psoriasis
development.
In the
case of infections, it has been noted that respiratory tract infections and
infectious skin diseases in the previous month are associated with increased
risk for psoriasis. Infections caused by Streptococcus pyogenes,
Staphylococcus aureus, Malassezia spp, Candida spp, Epstein-Barr
virus (EBV), varicella zoster virus (VZV), cytomegalovirus (CMV), and human
immunodeficiency virus (HIV) are also linked to the onset and flare-up of
psoriasis. With regard to obesity, body mass index (BMI), waist circumference,
and weight gain increase the risk of psoriasis. Injury to the skin is also
associated with increased risk of psoriasis. The Koebner phenomenon is the
development of skin lesions at the site of injury and is seen in 5% of
early-onset guttate psoriasis. Smoking increases the risk of psoriasis, while
stress and depression increase both the risk and severity of psoriasis.