Skin Disorders Guide

Diagnostic Hallmarks

1. Distribution: cheeks, arms, legs, and groin (infants), feet and antecubital and popliteal fossae (children, adolescents), hands, feet, ankles and groin (adults)
2. Presence of the itch-scratch cycle
3. Minor Criteria - atopy, xerosis, keratosis pilaris, pityriasis alba, and two prominent lower eyelid folds

Clinical Presentation

The distribution pattern, appears to be determined by irritant factors that lead to stimulation of the superficial cutaneous nerves. Sweat retention is the most important of these and probably accounts for development of lesions on the genitalia, in the groin, over the occiput, on the dorsal surface of the feet, around the ankles, and in the antecubital and popliteal fossae. lrritation from other fluids is also important, drooling during sleep and urine-soaked diapers greatly influence the localization of diseases in infants.

Excess dryness, on the other hand, is also a major factor because the microscopic cracks allow easier-than-normal simulation of the cutaneous nerve endings. Thus, xerosis secondary to soap and water exposure, low environmental humidity, and the asteatosis that accompanies aging may well account for dorsal hand and lower leg involvement in adults.

The most highly characteristic feature of atopic dermatitis is the presence of uncontrolled scratching in a pattern termed the itch-scratch cycle. In this pattern, stimulation of cutaneous nerve endings leads to central nervous system recognition of itching, which, in turn, creates the response of vigorous rubbing and scratching. The cycle is initiated because the scratching leads to even greater peripheral nerve stimulation, heightened appreciation of itching, and ever more vigorous scratching. The cycle then continues for minutes until the pain of nail-induced skin damage finally supplants the sensation of itching.

Clinical recognition of the itch-scratch cycle is usually not difficult. Most patients acknowledge its presence and will voluntarily discuss the role it plays in their disease. Sometimes, however, the scratching occurs almost entirely at the subconscious level. In such instances, identification of the cycle depends on a family member’s description of habitual scratching, particularly at night. The importance of nighttime scratching can hardly be overemphasized. Many patients can control, at least to some degree, the amount of scratching that occurs during the day but then virtually destroy their skin as a result of scratching during the lighter stages of sleep.

Frustration over the inability to control this scratching and the fatigue that occurs as a result of scratching-induced sleep disturbance lead to daytime irritability and a continued worsening of the problem. In my view, the importance of nighttime scratching is such that identification of its presence (and the ruling out of scabies and dermatitis herpetiformis) is usually sufficient to warrant a diagnosis of atopic dermatitis even in the absence of the other features listed as diagnostic hallmarks.

The presence of numerous excoriations can, in itself, be a clue to the existence of the itch-scratch cycle. Most other pruntlc diseases (urticaria and lichen planus, for example) are not accompanied by prominent excoriations, even though patients complain vigorously about the severity of itching. From this observation one can hypothesize that the itching in atopic dermatitis qualitatively differs from that which exists in most other pruritic processes. However, there are at least two diseases, scabies and dermatitis herpetiformis, wherein all patients, regardless of whether or not they are atopic, scratch to the point of prominent excoriation.

Lichenification is to rubbing as exconation is to scratching. Not surprisingly, then, lichenification can also be used as a clue to the existence of the itch-scratch (or itch-rub) cycle and, in fact, is probably an even more reliable indicator of atopic dermatitis, since it is not often encountered in scabies and dermatitis herpetiformis.

The reason the itch-scratch cycle is so closely related to atopic dermatitis is not known, but it probably depends at least in part on the marked sensation of pleasure noted by atopics when they scratch or rub. This sensation of pleasure probably acts in a Pavlovian way to reinforce the habituation of scratching. Adults can be directly queried about the degree of pleasure associated with scratching, but a similar response is observable in infant. They are quiet while scratching but cry vigorously when their hands are even gently restrained.

The third and, to my mind, the least useful hallmark of atopic dermatitis is the identification of the patient as possessing the atopic diathesis. The features that are said to identify atopy include -

(1) Drier-than-normal skin (xerosis)

(2) Keratosis pilaris

(3) Pityriasis alba

(4) A second wrinkle, the Dennie-Morgan fold, on the lower eyelid

(5) Abnormal cutaneous vascular reaction (white dermographism) to mechanical and pharmacologic stimuli

(6) A personal history of hay fever or asthma

(7) Idiopathic peripheral eosinophilia

(8) Elevated levels of immunoglobulin E (lgE) antibodies.

The problem with the use of these featness is the variability with which they are present and the fact that even if present, they only identify the patient as being atopic witholt identitying the disease in question as being atopic dermatitis. For example, since about 20% of the population is atopic approximately one-fifth of all psoriatics will possess some or all of these characteristics,

The diagnosis of atopic dermatitis is made on a clinical basis. Biopsy is not often required or helpful.

Pathogenesis

The autosomal dominant trait responsible for the development of atopy occurs in about 20% of the population, The expression of this trait in the form of atopic dermatitis is seen in only about one-fourth of those who have inherited the atopic diathesis. This results is a lifetime incidence rate of about 5% of the American population.

Several factors seem to be important in determining who will develop atopic dermatitis and at what point in their lives it will appear. These “precipitating” factors include adverse environmental conditions such as sweat retention, maceration resulting from other fluids, and excess dryness, all of which result in stimulation of cutaneous nerve endings. This stimulation might lead to irritation or mild discomfort in nonatopics but is perceived as pruritus in those who are atopic. This, in turn, results in scratching and initiation of the itch-scratch cycle.

Psychologic factors are also very important. Most atopic individuals experience considerable itching during times of stress and fatigue. The mechanism through which this occurs is unknown. Some believe that atopic individuals have distinctive personalities characterized by increased activity levels, high productivity, obsessive-compulsive (type A) behavior, and suppressed hostility toward parents and authority figures. This may well be so, but even if true, it offers no explanation as to why it should be expressed in the form of a pruritic eruption. Atopic disease does seem to occur more commonly in those of higher socioeconomic standing, but it is not clear whether this is a result of personality traits generally associated with high achievement or whether it is simply a reflection of greater access to medical care.

In any event, the presence of one or more of these precipitating factors in atopic individuals frequently results in scratching of what appears to be normal skin. This sequence of events had led to the tongue-in-cheek definition of atopic dermatitis as the itch that rashes. However, it seems likely to me that this clinically normal skin is not truly normal but rather has undergone some subclinical cellular or molecular changes that are responsible for the itching. Certainly, once scratching has occurred, various inflammatory mediators are released, these, in turn, no doubt intensify the pruritus.

Immunologic changes in both the humoral and cell-me­diated systems are regularly present, but their role in the pathogenesis of atopic dermatitis is not known. The mild but consistent depression of cutaneous T-cell responsiveness does lead to more frequent viral and fungal infections of the skin, but no direct relationship can be shown between this propensity and the appearance of atopic dermatitis. Serum levels of IgE (especially to house dust mites and staphylococcal organisms) are frequently elevated in those who have severe atopic dermatitis. It would be tempting to relate this immunoglobulin elevation to the presence of a causative (inhaled, ingested, or contacted) antigen, since antigenic stimulation seems to play such an important role in the associated atopic diseases of hay fever and asthma. Unfortunately, with the exception of some evidence incriminating food antigens in infancy, few bard data support this view. Interest in an immunologic explanation as the cause of atopic dermatitis is further stimulated by the observation that an eruption indistinguishable from atopic dermatitis occurs in infants with certain congenital immunodeficiency diseases, but here, too, no clear-cut cause and effect mechanisms are recognized.

Tags:atopic dermatitis, dermatitis herpetiformis, Eczematous Disease, irritant factors, itch scratch, Lesions, pityriasis alba, scabies scratching