Psoriasis: everything you need to know about this skin condition.

Psoriasis is a chronic inflammatory disease that primarily manifests in the skin and joints. It presents as red, thick plaques that shed skin. Psoriasis is characterised by an excessive renewal and accumulation of epidermal cells, leading to a local inflammation. Approximately 30% of cases are said to be familial and appear during adolescence. When psoriasis appears after the age of 40, it is referred to as isolated or sporadic forms. However, it affects both sexes equally.

Histological analyses show an increase in the thickness of the epidermis (acanthosis) and incomplete differentiation of keratinocytes (parakeratosis).Its manifestation is triggered by the combination of several risk factors. These include the immune system, genetic predisposition, and environmental factors.The skin symptoms of psoriasis can affect any area of the body, from the ears to the tongue, including the genital areas, leading to various clinical forms. However, the scaly plaques are generally located on the scalp, elbows, and knees. They are sometimes accompanied by a rheumatism.

For optimal management of this disease, it is necessary to understand its causes. The dysfunction of the immune system is one of the main causes of psoriasis. Indeed, immune cells (T lymphocytes) are found in the skin in an unusual way and produce inflammatory molecules (cytokines IL-17, IL-22, TNF-alpha). These stimulate the proliferation of keratinocytes, which leads to a faster cell renewal process, usually going from three weeks to three days. This results in an accumulation of immature keratinocytes on the skin's surface, increasing the thickness of the outer layer: this is referred to as hyperkeratosis.

A study has also revealed that 30% of cases exhibit a family history of this inflammatory skin condition.

Genetic factors also play a role in the onset of psoriasis. The major gene involved is the PSORS1 locus located on chromosome 6p21. HLA-Cw6 is the susceptibility allele in PSORS1 and is thought to be associated with the onset of this disease. Other minor genes also come into play, although their mode of action is not yet well understood.

Other factors can also come into play in the onset of psoriasis, including the mechanical stress, streptococcal infections, medications, cellular oxidants, as well as tobacco and alcohol. Psoriasis is therefore a multifactorial skin disease causing numerous inconveniences.

The psoriasis is characterised by the development of thick, red, inflammatory patches on the skin, topped with scales, which subsequently peel off like white flakes. The patches can develop on various parts of the body, including:

• On the scalp;

• On the skin folds such as the groin or the armpits;

• On the soles of the feet and the palms of the hands, in the case of a palmoplantar psoriasis;

• At the level of the nails.

Beyond their unpleasant appearance, these red patches can be painful when they occur in certain areas such as the palms of the hands, skin folds, and soles of the feet. Furthermore, 7% of people affected by psoriasis suffer from psoriatic arthritis , which is characterised by joint pain, accompanied by swelling and stiffness.

There are numerous treatments for psoriasis including topical treatments to be applied on the skin, oral systemic treatments, or non-drug treatments.

Among them, we find the dermocorticoids, which, when applied topically, help to soothe the skin by combating inflammation. They inhibit the NF-kB transcription factor by activating the transcription of the IkB gene, thus conferring anti-inflammatory effects. Dermocorticoids are available in the form of ointment, lotion, or cream.

Vitamin D analogues,such as calcipotriol or calcitriol, are anti-proliferative, meaning they combat the excessive multiplication of certain skin cells. They therefore act against hyperkeratosis.

Phototherapy is a technique that has been used for many years in the treatment of psoriasis. It is a treatment using ultraviolet rays. There are two types: PUVA therapy based on UVA rays and UVB phototherapy. This technique helps to alleviate psoriasis but does not cure it. The products based on keratolytic actives: they promote desquamation. Among these actives, we can mention salicylic acid, lactic acid or even urea.

• LEBRUN-VIGNES B. & al. Dermocorticoïdes. Annales de Dermatologie et de Venereologie (2004).

• ZANOLLI M. Phototherapy arsenal in the treatment of psoriasis. Dermatologic Clinics (2004).

• SAURAT J. & al. Psoriasis. Dermatologie et Infections Sexuellement Transmissibles (2016).