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.
Besides the constitutional conditions just mentioned, the eruptive fevers, syphilis, pyemia, septicemia, scurvy and malaria present characteristic cutaneous eruptions. Chronic or severe forms of malaria may produce melanotic staining of the skin, herpes and urticaria; less often, erythema and purpura; and rarely, furuncles and carbuncles. The gastrointestinal tract is the seat of many digestive disturbances which develop ferments and toxins. These processes constitute auto-intoxication and bear a direct causal relation to acne, rosacea, seborrhea, eczema, psoriasis, the erythemata and many other reflex disorders. The relation of food in quality and quantity to disorders of the digestive system and the resulting skin eruptions is most important. Infant foods, containing undigested starch, are frequent sources of cutaneous eruptions in young children; but, at any age, food may be an important factor. individual idiosyncrasies in respect to one or more articles of diet may also have a direct causal relation. Diabetes predisposes strongly to the formation of furuncles, carbuncles, cachectic acne, urticaria and erythema. It may produce a characteristic papulopustular eruption (xanthoma diabeticorum) without subjective symptoms, but which rapidly disappears with the relief of the underlying disease. Pruritus, especially about the genital regions, is frequently caused by diabetes. Of rarer occurrence are the superficial and terminal gangrenes of the skin, which occur sometimes in the advanced stages of the same disease. Eczema, purpura, urticaria and general pruritus are occasionally observed in the course of chronic nephritis in old people. In the advanced stages, there sometimes appears upon the skin an erythema, which, at first, may resemble measles or scarlet fever, but the patches of eruption soon coalesce and may become generally diffused. Desquamation in large flakes follows, leaving the skin infiltrated, red and sometimes eczematous. This uremic eruption is of grave prognostic significance and resembles dermatitis exfoliativa.
Spasmodic asthma is so frequently observed in association with eczema, acne, urticaria and ichthyosis that it may be considered causal. Lithemia and jaundice are common causes of persistent pruritus, with or without a secondary dermatitis. Chronic jaundice, from whatever origin, frequently stands in etiological relation to existing multiple xanthoma; while simple xanthoma of the eyelids, in the majority of cases, is secondary to migraine. Incidentally, this shows how the same pathological disease in different clinical forms may arise from widely divergent causes.
Chronic constipation may cause chloasma or the rare affection, chromidrosis. Chloasmatic patches occur not infrequently as above stated, in pregnancy, from uterine derangements, and occasionally as symptoms of an abdominal cancer or a contracted liver. The general staining of the skin from Addison’s disease or from Graves’ disease (exophthalmic goitre) is well known. Senile pigmented patches are sometimes due to degeneration of the skin. While neurotic disturbances frequently determine the location and extent of a cutaneous eruption, the latter expression is by no means constant, nor its character uniform, if we except the varieties of zoster which are believed to be due to an inflammation of some part of the nerve trunk or ganglion, having terminations in the affected area. Nervous shocks, such as great fright, grief, etc., occasionally transmit of the trophic nerve impressions powerful enough to cause bleaching of the hair, baldness, eczema and psoriasis. However, all of these diseases may follow other and quite different causes. Marked variation in the degree of nutrition resulting in plethora or anemia, if persistent, may cause a predisposition to disease. Plethora tends to superficial congestions and inflammations of the skin, and is apt to make them less tractable to treatment. Overfed, indolent and stagnant individuals are more liable to attacks of eczema, pruritus, etc., from trivial causes. The debilitated or anemic are more prone to seborrhea, furuncles, carbuncles ecthyma and impetigo than those who are well nourished. Defective nutrition and impaired functions of the skin probably have much to do with the lodgment and growth of parasites, the invasion of pathogenic germs or the absorptions of contagion. Thus some persons are easily infected by contact, while others remain wholly immune after exposure. This difference in the avoidance of disease can only be explained by general or local loss of vigor, by some anatomical peculiarity or by a change in quantity or quality of the protective secretions of the skin.
The ingestion of certain drugs, in small or large doses, may produce nearly all the elementary eruptions of the skin. With few exceptions, drug eruptions are not a constant effect or uniformly characteristic. Eruptions caused by quinine, copaiba, belladonna, iodine, bromine, chloral and many others drugs, will be referred to in detail under dermatitis medicamentosa. The element of personal idiosyncrasy should be constantly borne in mind when the consideration of a drug eruption is necessary.
The etiological relations of climate, seasons and habitation have already been briefly mentioned but they may be considered as exciting factors in some dermatoses although for the most part their action is upon the health or nutrition.
Personal Hygiene. Filth and uncleanness are undoubtedly exciting factors in dermatological practice but when compared with skin diseases arising from occupation, overindulgences of all sorts, mental and nervous worry and the manifold responsibilities of modern life, they are not as frequently noted as supposed. So it happens that defects in personal hygiene are not limited to the poor and neglected but involve the cleanly and well nourished, not to mention the luxurious in as great a proportion, although the items of unhygienic living are those of plenty rather than want. The lack of soap and water bathing is noticeable in the causation of acne, seborrhea and keratosis pilaris, while the too free use of the same agents may excite milaria, erythema, intertrigo, dermatitis and eczema. The indiscriminate use of water, especially when combined with impure soaps and vigorous scrubbing, is not only to be condemned for the vigorous but may be expected to aggravate almost any pre-existing skin eruption. Clothing may assume an exciting role. Too many or too thick clothes easily causes a miliaria in infants, especially during hot weather. Soiled underclothing, long worn, favors the development of eczema and seborrhea of the body. Too light or too coarse garments, or the presence of irritating dyes in clothing, may excite erythema, dermatitis, excoriations and pruritus.
Chemical and Mechanical Factors. Chemical substances, applied externally, may excite irritation, inflammation or destroy the superficial and deep layers of the skin. The degree of injury will depend upon the nature of the irritant, the duration of the application and the sensitiveness of the part involved. Agents of this class are very numerous; they include many plants which contain an active principle deleterious to the skin such as arnica, ivy, sumac, mustard, etc.; most of the etheral oils and resins; the poisons introduced into the skin by the stings and bites of insects and reptiles such as bees, mosquitoes and snakes; the various antiseptic preparations, when carelessly or freely used, such as iodoform, mercuric chloride, phenol and creolin; substances used in the process of manufacturing dyes as the aniline preparations; strong acids as nitric, muriatic, sulphuric, acetic and lactic; the stronger alkalies as caustic potash; the effects of heat from unusual exposure to sun-rays, overexposure to X-rays, radiation from heaters or flames, or actual contact, and also the extremes of cold. To these might be added the effects of chemicals used in various trades, frequently unavoidable, and especially the many medicated applications, often unnecessary. It should always be remembered that chemical irritants may not be limited in their effects to the area of the skin directly acted upon; but, through their influence on innervation, may cause disturbances at distant points or, by lessening the resistance of the skin, permit other agents to act more readily. Regarding this relation of medicinal irritants, Kaposi has well said that “these relations are altogether too little known for, if they were, physicians would not use cutaneous irritants so indiscriminately.”
Mechanically acting agents are often chemical in their action at the same time. They are incidental to nearly every active employment of mankind and may act to produce changes in the epidermis or deeper portions of the skin. In fact, the effects of occupation trade eruptions, professional or occupations dermatitis are quite common. Plain water may cause a surface irritation if used for long periods or often. Active exercise which commonly promotes health, if too long continued without protection of the surface, may cause chafing and consequent dermatitis. Fissured eczema is common in plasterers, masons and wash-women, whose hands are frequently brought in contact with alkalies. Those who handle animals or animal substances, as herdsmen, tanners and butchers, are most liable to anthrax and ecthyma. Occupations necessitating exposure to heat, as with cooks, blacksmiths and firemen, are favorable to attacks of erythema, eczema and dermatitis; while workers at oil refining, tar distillation and in aniline color making, etc., are peculiarly subject to dermatitis. Intermittent pressure, incident to the trade of shoemakers, blacksmiths, etc., produces thickening of the corneous layer of the epidermis, known as callous; while prolonged pressure over a point, as from a tight shoe, may produce a corn. Tight bands and garters may obstruct the local circulation, causing swelling of the skin or enlargement of the veins. Among the many other unmentioned causes of mechanical irritation, one group needs special notice excoriations from scratching with the finger- nails and all sorts of accidental abrasions, bruises and lacerations of the skin.
Parasites. It is hardly necessary to mention the large number of diseases that might be classified as parasitic from an etiological standpoint. Parasites are animal or vegetable and the diseases caused by these factors may be so grouped. Affections, such as pediculosis, scabies, ringworm and favus, caused by the grosser animal and vegetable parasites need no explanation. A large group of diseases, including impetigo contagiosa, leprosy, tuberculosis cutis, etc., are caused by lower microorganisms, but conclusive proof of the exact relations existing between dermatoses supposedly due to bacteria and the various cocci, bacilli, etc., supposedly causative, is lacking at the present time. The subject just mentioned offers a large field of possibility in the way of future investigation. The subject of contagion naturally presents itself along with the consideration of parasites. Even admitting the probable parasitic origin of many diseases and the inferential belief as to their communicability, it is a fact that in many instances contagiousness does not exist with any regularity or certainty. Probably the normal and healthy skin does not afford the proper soil for the habitat of most vegetable or animal parasites; hence, some predisposition may be the first element in the etiology of these diseases. Conversely, some peculiarity of the skin, such as the odor of its secretions, may serve to protect an individual from parasitic attacks.