BASIC CONSIDERATIONS

Embryologically, the skin and its appendages develop from ectoderm.

Anatomically, skin is the largest organ of body, containing three layers-epidermis, dermis and hypodermis (subcutaneous fat), along with its appendages (Fig.

Epidermis, the barrier layer, is a stratified epithelial tissue made up of specialized cells, i.e. keratinocytes. The deepest epidermal layer, i.e. stratum basalis is the germinal layer from where keratinocytes continuously develop and move superficially to mature further to form other epidermal layers, i.e. stratum spinosum, stratum granulosum and finally the most superficial layer, i.e. stratum corneum. Top-most layer of S. corneum contain plenty of keratin, i.e. dead keratinocytes, which are continuously shed and replaced by lower cell layers. Thus, epidermis is in a state of dynamic balance between

Fig. 25.1: Basic structure of skin

formation of immature keratinocyte in deepest S. basalis and shedding of keratin from upper-most S. corneum- whole cycle taking about 28 days.

Other important cells in epidermis are melanocytes (producing melanin pigment), Merkel cells (related to sensory function) and Langerhans cells (related to immune function).

Dermis, the supporting layer, consists of collagen and elastic fibers to provide strength and resilience to skin, along with blood vessels, lymphatics, neural structures and skin appendages. Predominant dermal cell is a spindle-shaped fibroblast, which produces collagen, elastic fibers and mucopolysaccharidoses, i.e. ground matrix.

Hypodermis (subcutaneous fat) consists of fat cells (adipocytes) arranged in lobules by dividing fibrous septa and serves mainly as heat insulator or trauma protector.

Skin appendages include hair follicles, sebaceous glands, apocrine and eccrine sweat glands and nails, all present mainly in dermis.

Clinical evaluation of skin disease: While most skin disorders are easily identifiable on examination of skin lesion under adequate illumination, careful history and systemic examination is important for etiological confirmation. Many skin lesions are a part of systemic illnesses, e.g. malnutrition, vitamin deficiency or endocrinal diseases.

Clinically, skin lesions should be examined and described according to their: (a) morphology, (b) size,

(c) color, (d) texture, (e) location or distribution, and (f) configuration. A commonly used descriptive nomenclature in dermatology is given in Table 25.1 which categorizes skin lesions into primary lesions and secondary lesions, developing after or over primary lesions. Using these terms helps not only in differential diagnosis but also to communicate better between care providers. However, many lesions are too altered in their morphology after therapy, infection or trauma, to have a diagnostic value.

*due to extravasated blood, do not blanch on pressure (d/d macule)

Laboratory investigations: Dermatological diagnosis in difficult cases may be supported by:

• Wood's lamp examination (ultraviolet rays) for hypo- pigmented and fungal infections, etc.

• KOH preparation of skin scrapping for fungal infections

• Tzanck smear from presumably viral blisters, specially for chickenpox or HSV lesions

• Skin-punch biopsy for diagnostic confirmation in many diseases.

• Immunofluorescence studies to detect tissue-fixed antibodies, e.g. in pemphigus, dermatitis herpetiformis and connective tissue disorders.

Topical therapy in dermatology: Topical therapy is the mainstay of treatment in most dermatological disorders, either alone or along with systemic therapy.

Topical applicants are available in various forms, depending on specific agent (steroid, antimicrobials or keratolytic agents) and the vehicle (lotion, creams, ointments, powders and pastes).

Vehicles: Selection of proper vehicle is as important in topical therapy as the specific therapeutic agents. Lotions are pourable mixture of water with some oil while creams are non-pourable mixtures of water with more oil. Ointments have either oil alone or very small amount of water. Lotions and creams are more useful for acute weepy lesions while ointments are preferable over dry, thickened and scaly lesions. Powders are hygroscopic vehicles and are more useful as absorptive agents in high-moisture areas, e.g. intertriginous area and groin. Pastes are mixtures of powders in ointment base, often used as non-specific bland agents to cover friction areas, e.g. in diaper dermatitis.

Specific therapeutic agents for topical therapy may be bland, e.g. wet dressing or lubricants or with active pharmacological properties. Some commonly used therapeutic agents in dermatology are as follows:

Wet dressing is often the initial treatment in acutely inflamed or moist oozing lesions to decrease pruritis and burning sensation. These dressings may be provided by simply dipping a soft cotton/gauze piece in normal saline, plain water or Burrow's solution (2-5% aluminum acetate in water) and applying over the lesion for 10-20 minutes. Closed wet dressings are needed over abscesses.

Lubricants, i.e. bland lotions, creams and ointments and petroleum jelly, are used as vehicle for other drugs, as well as emollients for dry and pruritic skin. These preparations are most effective when applied over damp skin immediately after bath or when contain evaporation agents, e.g. camphor or menthol.

Keratolytic agents are used to soften and humidify thickened skin, for easy removal of scales and crusts. Common keratolytic agents include salicylic acid, urea and #945;-hydroxy acids, e.g. lactic or glycolic acid. Whitfield ointment (6% benzoic acid +3% salicylic acid) is a commonly used keratolytic as well as antifungal agent.

Sunscreens reflect all sunrays or selectively harmful ultraviolet rays and mainly contain zinc oxide or titanium oxide. These agents are used for prevention of photosensitive rashes or dermatitis.

Tar compounds have antipruritic and astringent properties as well as promote normal keratinization. These compounds are used for psoriasis and chronic eczema and efficacy is increased on post-application exposure to sunlight. Tar compounds should never be used on acutely inflamed lesions.

Topical antibiotics should not contain antibiotics that are commonly used for systemic therapy for fear of sensitization. Mupirocin is perhaps the most effective topical antibiotics for cutaneous bacterial infections.

Topical antifungal agents may be broad spectrum, i.e. azoles-miconazole, clotrimazole, ketoconazole or selectively acting, e.g. nystatin (candida), tolnaftate (dermatophytes), etc.

Topical steroids are potent anti-inflammatory and antipruritic agents, classified as: (a) non-fluorinated, e.g. hydrocortisone and mometasone, and (b) fluorinated, e.g. betamethasone, fluocinolone and triamcinolone. Fluorinated compounds are more potent but toxic, and should be used with caution.

Apart from vehicles and special therapeutic agents, many topical preparations contain preservatives and stabilizing agents, which may cause local allergic reactions.

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