Psoriasis causes inflammation in the skin , which results in a reactive abnormal epidermal differentiation and proliferation . The increased rate of production of epidermal cells outweighs the rate of exfoliation of the skin , causing thick scaling plaques . Psoriasis was previously thought to be a skin disease only . However , more and more evidence suggests that it has several systemic implications , including depression , myocardial infarctions and arthritis .

AETIOLOGY There are a number of factors implicated in the aetiology of psoriasis , with autosomal dominant patterns of inheritance being the most significant of all . HLA-B13 , -B17 and -Cw6 are all associated with plaque psoriasis . Several putative genetic susceptibility loci have also been identified , including psoriasis susceptibility 1 ( PSOR1 ) on chromosome 6 , which is associated with up to 50 % of cases .

Other factors that precipitate or aggravate the disease includes , trauma , sun burns , infections , HIV infection and psychological stress . All types of trauma have been associated with the development of plaque psoriasis . Even excessive scratching can aggravate or precipitate localised psoriasis by a process called as Koebner reaction . Even a severe sunburn can lead to an exacerbation of the disease by the same reaction .

Pharyngeal streptococcal infections have been shown to produce a clinically distinctive disease flare known as guttate psoriasis . Furthermore , there is an increase in psoriasis activity in patients who are infected or become infected with HIV . At first , the extent and severity of the skin disease appears to parallel that of the HIV

stage with the psoriasis becoming less active in advanced HIV . A number of medications have been shown to cause an exacerbation of psoriasis . Lithium and withdrawal from systemic corticosteroids are well known to cause flares of the disease . Beta-blockers , antimalarials and non-steroidal antiinflammatory drugs ( NSAIDs ) have also been implicated . Moreover , most patients report an increase in psoriasis severity with psychological stress .

PATHOGENESIS The pathogenesis of psoriasis is complex and involves an interplay of the immune cells with the keratinocytes in the skin . Activation of T cells in the dermis and the epidermis and the local effects of cytokines such as tumour necrosis factor lead to inflammation , cell-mediated immune responses and epidermal hyperproliferation observed in persons with psoriasis . An interleukin ( IL ) -12-related cytokine , IL-23 , is involved in the establishment of chronic inflammation and in the development of a T-helper ( Th ) -cell subset producing IL-17 . These cells which are designated Th-17 are the T-effector cell subset together with IL-23 / IL-17 are implicated in the induction and progression of a number of inflammatory diseases , including psoriasis .

TYPES The plaque type of psoriasis is the most common , although several other distinctive clinical variants are recognised ( e . g . Guttate psoriasis , psoriasis of the nails , pustular psoriasis and psoriatic arthritis ). Plaque psoriasis is characterised by raised and easily palpable lesions , owing to the thickened epidermis , expanded vascular compartment as well as infiltrate of neutrophils and lymphocytes . The

plaques are well defined with sharply demarcated boundaries . The scales tend to be silvery-white , which when removed lead to some capillary bleeding . The lesions are more commonly found on the scalp , trunk , intergluteal cleft and limbs with a predilection for the extensor surfaces such as the elbows and knees . Pruritus can be severe in some of the cases .

Guttate psoriasis is described as a raindrop-type rash of multiple psoriasis areas appearing on the trunk of patients , usually some 2-3 weeks after an upper respiratory tract infection . It occurs in people younger than 30 years of age and often resolves within 20 weeks , but a significant number will progress to plaque psoriasis . Most studies have found evidence of recent streptococcal infection in the majority ( up to 93 %) of patients with acute guttate psoriasis .

Patients with these lesions may sometimes need to be treated with antibiotics to eliminate carrier states .

Pustular psoriasis is an uncommon form of psoriasis consisting of widespread pustules on an erythematous background . It may affect the palms and soles or rarely lead to a generalised erythroderma . Cutaneous lesions characteristic of psoriasis vulgaris may be present before , during , or after an acute pustular episode . Pruritus , intense burning , fever , erythroderma , hypocalcaemia and cachexia are usually observed . Other systemic complications may include acute respiratory distress syndrome , pneumonia , congestive heart failure and hepatitis . Episodes of pustular psoriasis can be evoked by withdrawal of systemic corticosteroids as well the use of certain drugs , like iodides , coal tar , minocycline and salicylates . The treatment of choice is oral retinoids which can result in quick relief . Other oral treatments are also very effective .

Nail psoriasis can be easily mistaken for onychomycosis . The nails may exhibit pitting , onycholysis , subungual hyperkeratosis or the oil drop sign . The disease tends to affect most nails and is usually associated with plaque psoriasis elsewhere . Thus a good history and examination are necessary to distinguish it from a fungal infection . The treatment mainly entails the use of oral medications as topical creams

cannot be applied to the nail bed . Some authorities also advocate the use of scalp preparations of corticosteroids to be applied around the nails , which can be effective in very mild cases .

Psoriatic arthritis can occur in up to 30 % of patients with skin disease . It affects any age and typically starts around a decade after the onset of skin lesions . The symptoms can range from mild to severe and there is no correlation between the severity of skin lesions and that of the joints . The patients complain of morning stiffness , pain , swelling of joints , pain in ligaments and tendons that is variable and unpredictable with flares and remissions .

There is a definite association between HLA-B27 and radiological evidence of sacroiliitis . However , there are probably other genes that are associated with psoriatic arthropathy . When these symptoms are present , patients need to be treated aggressively and referred to a rheumatologist to stabilise their condition , before the onset of permanent joint damage .

TREATMENTS As the understanding of the disease evolved over the years , research has produced various medications that target different pathways of the disease .

Up till the 1980s , it was known to be a disease of keratinocyte dysfunction . Thus various treatments targeting keratinocyte differentiation were used : methotrexate , retinoids , UVB and PUVA . The immunologic component of the disease was identified in the 1980s whereby the use of ciclosporin was introduced for the treatment . In the following decade , the targeted approach to IL12 , led to the development of various biologics , including tumour necrosis factor alpha blockers . Finally , as from 2005 , new medications that target even more specific pathways , the IL-23 and Th17 , have found their place in the treatment of severe skin and joint disease . These include ustekinumab and secukinumab . It must be added that even though we have many new therapies for psoriasis , the older medications are still widely and safely used with great success .

References available on request .

44 MAY 2017 | MEDICAL CHRONICLE