Koebner phenomenon



The Koebner phenomenon, also called the "Koebner response" or the "isomorphic response", refers to skin lesions appearing on lines of trauma. The Koebner phenomenon may result from either a linear exposure or irritation. Conditions demonstrating linear lesions after a linear exposure to a causative agent include: molluscum contagiosum, warts and toxicodendron dermatitis (a dermatitis caused by a genus of plants including poison ivy). Warts and molluscum contagiosum lesions can be spread in linear patterns by self-scratching ("auto-inoculation"). Toxicodendron dermatitis lesions are often linear from brushing up against the plant. Causes of the Koebner phenomenon that are secondary to scratching rather than an infective or chemical cause include vitiligo, psoriasis, lichen planus, lichen nitidus, pityriasis rubra pilaris, and keratosis follicularis (Darier disease).

The Koebner phenomenon describes skin lesions which appear at the site of injury. It is seen in:
 * Psoriasis
 * Pityriasis rubra pilaris
 * Lichen planus
 * Lichen nitidus
 * Vitiligo
 * Lichen sclerosus
 * Elastosis perforans serpiginosa

A similar response occurs in pyoderma gangrenosum and Adamantiades-Behcet's syndrome, and is referred to as pathergy.

Warts and molluscum contagiosum are often listed as causing a Koebner reaction, but this is by direct inoculation of viral particles.

Namesake dermatologist
The Koebner phenomenon was named after a rather eccentric, renowned German dermatologist, Heinrich Koebner (1838–1904). Koebner is best known for his work in mycology. Here is one story to illustrate his intense nature: in a medical meeting he proudly exhibited on his arms and chest three different fungus infections, which he self-inoculated in order to prove the infectiousness of the organisms he was studying. The Koebner phenomenon was the generalized term applied to his discovery that on psoriasis patients new lesions often appear on lines of trauma, which are often linear.

Contrasted distributions
The linear arrangement of skin lesions in the Koebner phenomenon can be contrasted to both lines of Blaschko and dermatomal distributions. Blaschko lines follow embryotic cell migration patterns and are seen in some mosaic genetic disorders such as incontinentia pigmenti and pigment mosaicism. Dermatomal distributions are lines on the skin surface following the distribution of spinal nerve roots. The rash caused by herpes zoster (Shingles) follows such dermatomal lines.