Lichen spinulosus represents a type of follicular reaction. The individual lesion presents grossly a flat to conical, slightly inflammatory or noninflammatory element 1 mm. in diameter, bearing a hairlike, horny spine (spinulus) often containing a lanugo hair. Removal of the parakeratotic spine reveals a tiny funnel-like orifice. The lesions appear simultaneously or in crops, and involve more commonly the nape of the neck, buttocks, trochanteric regions, flexor surfaces of the knees, extensor and flexor surfaces of the arms and the abdomen. The face, hands and feet are usually spared.
Microscopic examination shows the process to be limited to the follicle. In the early stages, the vessels are slightly congested, and slight effusion may be noted. The follicle usually exhibits marked atrophy; the epithelium is parakeratotic, as is the lamellated corneous plug.
The acute type may be a form of trichophytid (lichen trichophyticus), a form of arsphenamine dermatitis,1 or due