The first manifestation of psoriasis may occur at any age. Early-onset psoriasis (age <40 years old) accounts for 75% of patients with psoriasis. Late onset psoriasis (age >40 years old) accounts for 25% of patients. The course is unpredictable and the variations numerous. Its duration may vary from a few weeks to a whole lifetime.
Post-traumatic psoriasis is not uncommon in young, athletic men, who develop psoriasis initially at the site of injury. Psoriasis is one of several conditions in which various types of trauma may elicit the disease in previously uninvolved skin (Koebner reaction). The Koebner reaction is thought to be more frequent in actively spreading, severe psoriasis. The reaction appears to be a marker for a subgroup of patients with a tendency to early onset and early relapse after various forms of therapy.
The Koebner reaction usually occurs 7–14 days after injury. It obeys an all-or-none rule in a given patient (that is if psoriasis occurs at one site of injury it does so at all sites of injury). Clearing of existing psoriasis following injury has been observed and termed the reverse Koebner reaction. This reaction also obeys an all-or-none rule, and the Koebner and reverse Koebner reactions are mutually exclusive.
|Post-traumatic psoriasis in a young, athletic man, who has developed psoriasis at the site of injury on the right shin. The rest of the physical examination was unrevealing.|