Lichen circumscriptus is an aggravated form of lichen simplex, and is characterized by a circular arrangement of the pimples. The circumference which marks the limit of the patch is sharply defined. This form of lichen usually appears on the chest, hips, or limbs, and is not unfrequently mistaken for ringworm.
Lichen strophulosus is a variety peculiar to infants. Dermatologists recognize several subdivisions of this species, but the general characteristics are the same in all. The pimples are much larger than in the other forms of lichen, of a vivid red color and the duration of the eruption is limited to two or three weeks.
Lichen urticatus is also an infantile affection and begins with inflammation, which is soon succeeded by the eruption. In a few days the pimples shrink, the redness disappears, and the skin has a peculiar bleached appearance. The eruption is attended by an intense itching sensation and, if the skin is ruptured, a small quantity of blood is discharged and a black scab formed. This variety of lichen is very obstinate and of long duration.
Lichen tropicus, popularly known as prickly heat, is an affection which attacks Europeans in hot climates. It is characterized by the appearance of numerous red pimples of an irregular form, distributed over those portions of the body usually covered by the clothing. It is attended with a fierce, burning, itching sensation, which is aggravated by warm drinks, friction of the clothing, and the heat of the bed. The eruption indicates a healthy condition of the system; its suppression or retrocession is an unfavorable symptom, denoting some internal affection such as deranged nutrition.
In lichen planus, as the term indicates, the pimples are flattened. There is no sensation of itching or formation of scabs. The pimples are solitary and have an angular base, and the fresh pimples formed appear on the spaces between the former eruptions. This affection usually attacks some particular region, such as the abdomen, hips, or chest. Instances are recorded in which it has appeared on the tongue and the lining membrane of the mouth. Sometimes it appears in patches, but even then, the margin of each pimple can be discerned.
Lichen pilaris and lividus are modifications of lichen simplex, the former being so named to describe the location of the pimples, i.e., surrounding the minute hairs which cover the body, especially the lower limbs. The term lichen lividus indicates the dark purplish hue caused by a torpid circulation and the consequent change of arterial into venous blood before leaving the pimples. Lichen circinatus is a modified form of lichen circumspectus. The pimples in the center of the circular patch subside and a ring is formed which gradually increases in size. When the rings become broken or extend in regular forms, the affection is termed lichen gyratus.
Causes. Constitutional debility predisposes the system to this eruption. The exciting causes are irritation of the skin, strumous diathesis, dentition, and any violation of hygienic rules. Although lichen is not a fatal disease, yet it tends to reduce the vitality of the system.
Impetigo. (Crusted Tetter or Scall.) Impetigo is a term applied to an inflammation of the skin, more severe and energetic in its character than the preceding affection. We have found the predominating characteristics of eczema and lichen to be the presence of exudation in the former, and the absence of it in the latter.
Impetigo is marked by the formation of yellow pus, which raises the cuticle into pustules. There is a slight swelling, redness, and the pus gradually dries up, forming an amber-colored crust, a representation of which is given in Colored Plate I, Fig. 5. It soon falls, leaving the skin slightly inflammed, but with no scar. The pustules are sometimes surrounded by a cluster of smaller ones.