The consideration of the cause of disease, as they individualize the dermatoses, will be found under their respective heads. At this place, however, a brief discussion of the more common and important etiological influences will be given. Causes may be divided for convenience into the predisposing which are generally of an internal origin, and the exciting or direct which are of external origin. Predisposing causes include inherited conditions of the system or acquired conditions which may be manifested by the presence of general or local internal disease. Eruptions arising under such conditions may be incidental but they are usually essential and hence are called symptomatic skin diseases. Among these may be mentioned the exanthemata, diseases due to constitutional causes or infections as syphilis, leprosy and purpura, or those following the ingestion of drugs, drug eruptions. These conditions are symptomatic of a constitutional agent, whether it be known or unknown, bacterial, chemical or toxic. They may be caused directly by some underlying disturbances or indirectly through some action on the nervous system or the vasomotor apparatus.
Exciting or direct causes are often of external origin and include those agencies which act directly on the skin itself, actually or apparently. Diseases produced in this manner, together with those originating in or confined to the skin, even if their causes are not determined, are designated idiopathic skin diseases. The local parasitic diseases, many hypertrophies and dermatitis venenata are examples of this group. It is well to remember, however, that in many instances the local causative factors in idiopathic diseases are influenced or even made possible by underlying conditions of organic disturbances. It is also possible that the same disease at one time may be symptomatic and at another time, idiopathic. The direct relation between internal diseases and the cutaneous disturbances, apparently caused by them, is often difficult to explain. In a measure this may be due to the activity of one or many factors operating at the same time to produce diseased conditions of the system, of separate organs or of parts not easily isolated or defined.
Climate and Seasons are well known causative factors. It is probable, however, that they alone are not so productive of disease as their combination with habits of living and other personal factors. Nevertheless, it is a fact that prurigo and lupus vulgaris are common in Austria, favus and pellagra in Italy, gangosa in Guam, rosacea in England and Germany and leprosy in many widely scattered points. The endemic feature of many diseases is well understood. Warm weather favors outbreaks of urticaria, miliaria and intertrigo, and cold weather aggravates psoriasis, seborrhoea, lupus vulgaris, ichthyosis and pruritus. Chilblains and many forms of dermatitis originate in cold weather while furuncules and the erythemata are most frequent in the spring and autumn. Sudden changes in temperature may cause greater activity in many existing eruptions or a fresh outbreak of the same condition. Diseases behaving in this manner are called recurrent when there appears to be some definite course.
Abode or Habitation. If unhygienic, these factors may contribute their quota in disease-giving influences. Thus the air of houses and rooms, polluted with sewer gas and other noxious emanations, are favorable conditions for the occurrence of pemphigus (in infants), furuncles, strumous and catarrhal diseases; and associated with uncleanliness of the person, largely diminish the resistance of the skin to the invasion and multiplication of animal and vegetable parasites.
Heredity. There are only a few diseases such as syphilis and ichthyosis that are actually transmissible, but in many persons there exists a tissue weakness or predisposition to disease conditions such as psoriasis, alopecia prematura, lepra, xanthoma and possibly eczema. Many eruptions, long thought to be hereditary, have been traced to direct infection, and others to the transmission of a diathesis not uniformly productive of skin disease. Hereditary tendencies may show a marked preference for one sex of a family through succeeding generations. Racial tendencies have been noted in the various nationalities, constituting the different races. Thus favus remains with Italians and Hungarians even when they settle in the United States, while the American-born, living in the same localities, will show in proportion an insignificant number of cases. The tendency that some races have to the development of certain diseases has been cursorily mentioned under the remarks concerning endemic dermatoses. The negro seems more liable to keloid, dermatitis papillaris capillitii and ainhum and he seldom presents acne and the animal parasitic disturbances. The Hebrew race seem more liable to xeroderma pigmentosum and idiopathic multiple pigmented sarcoma while they are less liable to epithelial cancers than their Gentle neighbors.
Age. Certain dermatoses belong typically to the different periods of life. In youth and early manhood, the more acute inflammations and hypertrophies are more apt to occur; in late life, less acute inflammations, atrophies and degenerations. Beginning with infancy, miliaria, congenital syphilis, ichthyosis, etc., may appear. Intertrigo, impetigo contagiosa and ringworm of the scalp are common in childhood, but acne, seborrhea and psoriasis seldom develop before puberty. Tinea versicolor, rosacea, lupus erythematosus, etc., are diseases of adult life, while cancer and affections due to degenerative changes are usually seen in late middle or old age.
Dentition, occurring at an age when the skin as well as other organs are susceptible to influences of any sort, acts as a disturbing element. Its specific action on the skin, through the digestive and nervous systems, is much exaggerated but the erythemata, eczema and urticaria may be noted at this period.
Vaccination must be considered as an event in the life of most children. While its causal relationship to subsequent disease is not clear, there is little doubt that such relation exists as regards some skin diseases. Probably its influence in such instances is largely that of stimulating some latent tendency rather than a direct effect. I have seen cases of psoriasis, eczema, pemphigus and impetigo which apparently originated from vaccination. Louis Frank has cited twenty-two skin diseases which he classified as due to the vaccine virus or to mixed inoculation, and as sequelae of vaccination (see Vaccinal Eruptions).
Sex alone exerts little influence until the period of puberty. There-after, the differences in the habits of the two sexes has a modifying effect only less pronounced than the physiological differences of the mature male and female. These latter differences reach the point of exclusion in only two diseases: sycosis, which does not occur in the female, and Paget’s disease of the nipple, which does not often occur in the male. As causal factors peculiar to women, the physiological crises menstruation, pregnancy, lactation and the menopause require mention.
Menstruation often aggravates existing eruptions by its regular and recurring disturbance of the circulatory or nervous systems. If excessive, it may lower nutrition and create a predisposition to cutaneous disorders. Such diseases as eczema, urticaria, acne and rosacea are frequently worse shortly before or just after the appearance of the menses, while simple erythema, herpes simplex and purpura may appear only at the menstrual period. Bloody sweat (hematidrosis) has been observed in amenorrhoea; and excessive local or general sweating, with or without resulting erythema or eczema, is not uncommon.
Pregnancy may be attended with a distressing pruritus vulvae or the itching may be generalized, sometimes associated with urticaria. Chloasma and herpes simplex are common, dermatitic herpetiformis is quite rare and impetigo herpetiformis, a fatal disease, is fortunately extremely rare. With the end of pregnancy, most of the eruptions incident thereto soon disappear or aggravations, apparently caused by gestation, subside.
Lactation, like the menstrual flow, may tend to aggravate chronic eruptions rather than to excite fresh outbreaks. This is probably due to a lack of cutaneous vitality from a lowered or perverted nutrition. Thus, psoriasis, eczema or lichen planus which may have subsided during pregnancy, are liable to return during lactation.
The menopause usually occurs at an age when even moderate derangements of circulation or innervation are apt to cause an aggravation of existing eruptions or contribute to the development of new ones. Rosacea, eczema, dermatitis seborrhoica, and seborrhea of the scalp are among the diseases which may originate or become more pronounced at this crisis.
Diathesis; Visceral and Constitutional Diseases. Diathesis may be defined as an underlying condition usually of some duration, affording a tendency to definite forms of disease. Diathesis may be active or latent, persistent in their effects or transient, acquired or inherited. The tuberculous, scrofulosus or strumous diathesis may encourage the development of any form of tuberculosis cutis, lupus erythematosus, erythema induration, lichen scrofulosus, acne varioliformis, etc. The rheumatic, gouty, uric acid or arthritic diathesis may lead to psoriasis, eczema, pruritus, erythemata, purpura, etc. The catarrhal diathesis seems to predispose to eczema, seborrhea, dermatitis seborrhoica, etc. As regards the relation of these predisposing conditions or diathesis to cutaneous diseases, from the standpoint of the duration of the latter, it may be said that they are more noticeable in the chronic dermatoses, less so in the subacute and recurrent manifestations and rarest in the purely acute type.