The causes of psoriasis.

Psoriasis is a chronic inflammatory skin disease, affecting approximately 2% of the French population. Non-contagious , it manifests as the appearance of thick red patches covered with white dead skin, known as scales. Generally, these lesions can be located on the scalp, elbows, knees, the lumbo-sacral region (lower back) or even on the nails. Psoriasis lesions can also develop on the site of a trauma or injury: this is known as the Koebner phenomenon . It describes the development of new lesions at the sites of mechanical trauma.

This dermatosis progresses in flare-ups, and the frequency and intensity of the lesions are unpredictable. The remission periods present highly variable durations. It affects both men and women, and is more common in Caucasians. Approximately 30% of cases are said to be familial and appear during adolescence.

In the majority of instances, psoriasis is a mild condition. However, in 20% of cases, the forms of psoriasis are severe and are associated with widespread effects across the entire body (erythroderma, pustular psoriasis…) and/or joint-related (psoriatic arthritis).

Psoriasis can have a significant negative impact on the quality of life of patients. It is proven that patients suffering from psoriasis feel stigmatised by their condition. In its most severe forms, psoriasis leads to daily disability, resulting in depression and suicidal thoughts.

Thus, psoriasis is a disease that has a significant negative impact on the quality of life of those affected. For optimal management of this disease, it is necessary to understand its causes. Today, there are highly effective treatments that have been developed thanks to a better understanding of the pathophysiology of the disease, yielding spectacular results.

Psoriasis can occur at any age and the genetic factors play a significant role in the pathophysiology of this skin condition.

At least 30% of psoriasis cases correspond to familial forms involving a genetic predisposition.

The major gene involved is the PSORS1 locus. Other minor genes also come into play. The genetic variants associated with psoriasis are located in the genes involved in immunity, which is the cause of a immune system disorder. Thus, without infection and for reasons still unknown, immune cells find themselves in the skin and secrete inflammatory molecules. These stimulate the proliferation of keratinocytes, thus causing an increase in the thickness of the horny layer: this is hyperkeratosis.

In 30% of cases, psoriasis is linked to the presence of genetic factors. These genetic variants are responsible for an immune system imbalance, causing chronic skin inflammation and an overproduction of keratinocytes. However, both extrinsic and intrinsic factors have been identified as potential triggers or exacerbators of psoriasis. Therefore, psoriasis is a multifactorial disease.

• Mechanical stress.

  As mentioned earlier, psoriasis can be triggered by mechanical stress, such as radiotherapy, UV radiation, or simple irritation: this is known as the Koebner phenomenon.

• Infections.

  In scientific literature, the link between streptococcal infections and psoriasis is clearly established. Indeed, psoriasis can appear after a streptococcal sore throat (which is a bacterial infection) in most cases : this is referred to as guttate psoriasis. An infection can also trigger a flare-up in a patient who has previously had psoriasis or worsen pre-existing plaques.

  "Don't panic, we sometimes observe in patients who have never had psoriasis, a flare-up a few days after a sore throat. In most cases, things spontaneously return to normal or with the help of treatments. The occurrence of subsequent flare-ups is not the rule," according to Dr. B. LEVY GAREL.

• The medications.

  Some medications can also trigger a flare-up of psoriasis. This is the case with drugs for treatment and prevention of malaria, for lowering blood pressure (beta-blockers and ACE inhibitors), for treating bipolar disorders (lithium), and for treating Hepatitis C and multiple sclerosis.

• Cellular oxidants.

  Pollution and UV rays are responsible for oxidative stress that damages the body's tissues, including the skin. The cadmium is an airborne pollutant that affects the pathogenesis of psoriasis. Indeed, studies have highlighted a significant concentration of cadmium in the blood of affected patients compared to the normal population. These are data to be handled with caution as they are extremely rare and need to be confirmed.

  Phototherapy has been used for over 30 years in the treatment of psoriasis. Although UV rays are beneficial for psoriasis, there is a certain subgroup of people who exhibit a photosensitive psoriasis . In other words, these individuals develop psoriasis following UV exposure and the lesions are particularly severe in the summer. Generally, those suffering from photosensitive psoriasis are predominantly women with a family history and a low age of onset. However, this scenario is very rare.

• Lifestyle habits: tobacco and alcohol.

  Many studies have highlighted the link between psoriasis and tobacco. Indeed, smoking appears to be associated with a high risk of developing psoriasis, particularly the pustular form. Furthermore, the longer the duration of tobacco consumption, the greater the risk of psoriasis.

  Regarding alcohol, the link with psoriasis is not as clear. However, excessive alcohol consumption is correlated with the severity of psoriasis and with a reduction in the effectiveness of treatments. However, studies on this subject are still too contradictory and further research needs to be conducted.

• The metabolic syndrome.

  There is a significant association between metabolic syndrome and psoriasis. Metabolic syndrome is defined by diabetes or pre-diabetes, combined with at least two of the following criteria: obesity/overweight, high blood pressure, lipid balance disorder, presence of an abnormally high level of protein in the urine. It is crucial to detect it, as it constitutes a major risk factor for developing cardiovascular disease (heart attack, stroke, etc.). Therefore, patients suffering from psoriasis and metabolic syndrome should be referred to specialist doctors for better management.

  This association between psoriasis and metabolic syndrome has been extensively studied and there are multiple pathophysiological hypotheses, among these we can mention:

  • l’hypothèse génétique avec un terrain génétique commun au développement de ces deux maladies ;

  • l’hypothèse environnementale : certains médicaments pour traiter le psoriasis favorisent les anomalies du bilan des graisses ; du fait du retentissement psychique de leur maladie de peau affichant les patients atteints de psoriasis sont sédentaires, en surpoids voire obèse, fument ce qui augmente leur risque cardiovasculaire, etc. ;

  • l’hypothèse inflammatoire : dans le psoriasis, de nombreuses molécules pro-inflammatoires sont sécrétées (cytokines). Celles-ci ont une toxicité directe sur les vaisseaux et favorisent le développement d’athérome et donc de maladies cardiovasculaires. Cette hypothèse est retenue pour d’autres maladies inflammatoires (polyarthrite rhumatoïde, maladie de Crohn, etc.).

• The stress.

  Many patients and clinicians have observed that stress and anxiety exacerbate psoriasis.

• GRIFFITHS C. E. M. & al. Psoriasis : epidemiology, clinical features, and quality of life. Annals of the Rheumatic Diseases (2005).

• OHTSUKI M. & al. Risk factors for the development of psoriasis. International Journal of Molecular Sciences (2019).