Skin disease

Chapter 24 Skin disease

Structure and function of the skin

The skin consists of four distinct layers:

The functions of the skin are summarized in Box 24.1.

The epidermis

The epidermis is a stratified epithelium of ectodermal origin that arises from dividing basal keratinocytes. The downward projections of the epidermis into the dermis are called the ‘rete ridges’. The lower epidermal cells (basal layer) produce a variety of keratin filaments and desmosomal proteins (e.g. desmoglein and desmoplakin), which make up the ‘cytoskeleton’. This confers strength to the epidermis preventing it shedding. Higher up in the granular layer, complex lipids are secreted by the keratinocytes and these form into intercellular lipid bilayers, which act as a semipermeable skin barrier. The upper cells (stratum corneum) lose their nuclei and become surrounded by a tough impermeable ‘envelope’ of various proteins (loricrin, involucrin, filaggrin and keratin). Changes in lipid metabolism and protein expression in the outer layers allow normal shedding of keratinocytes.

Keratinocytes can secrete a variety of cytokines (e.g. interleukins, gamma-interferon, tumour necrosis factor alpha) in response to tissue injury or in certain skin diseases. These play a role in specific immune function, cutaneous inflammation and tissue repair. There is a further layer of protection against microbial invasion – the innate immune system of the skin. This comprises neutrophils and macrophages as well as keratinocyte-produced antimicrobial peptides (called β-defensins and cathelicidins). Expression of these peptides is both constitutive and induced by skin inflammation and they are active against infection and play a role in wound healing. A deficiency of these peptides may account for the susceptibility of people with atopic eczema to skin infection.

The dermis

The dermis is of mesodermal origin and contains blood and lymphatic vessels, nerves, muscle, appendages (e.g. sweat glands, sebaceous glands and hair follicles) and a variety of immune cells such as mast cells and lymphocytes. It is a matrix of collagen and elastin in a ground substance.

Approach to the patient

The history should aim to elicit the following points:

Examination entails looking and feeling a rash (for terminology, see Table 24.1). It should include an assessment of nails, hair and mucosal surfaces even if these are recorded as unaffected. The following terms are used to describe distribution: flexural, extensor, acral (hands and feet), symmetrical, localized, widespread, facial, unilateral, linear, centripetal (trunk more than limbs), annular and reticulate (lacey network or mesh like).

Investigation. With regard to investigation, clinical acumen remains the most useful tool in dermatology but a variety of tests are useful in confirming a diagnosis (Table 24.2).

Table 24.2 Investigations used in skin disorders

Test Use Clinical example

Skin swabs

Bacterial culture


Blister fluid

Electron microscopy, viral culture and PCR

Herpes simplex

Skin scrapes

Fungal culture

Tinea pedis



Nail sampling

Fungal culture


Wood’s light

Fungal fluorescence

Scalp ringworm


Blood tests


Streptococcal cellulitis


Systemic lupus erythematosus

HLA typing

Dermatitis herpetiformis

DNA analysis

Epidermolysis bullosa

Skin biopsy


General diagnosis


Cutaneous lymphoma


Immunobullous disease



Patch tests

Allergic contact eczema

Hand eczema


Dipstick (glucose)

Diabetes mellitus

Cytology (red cells)


Dermatoscopy (direct microscopy of skin)

Assessment of pigmented lesions

Malignant melanoma


Bacterial infections (see also p. 114)

The skin’s normal bacterial flora prevents colonization by pathogenic organisms. A break in epidermal integrity by trauma, leg ulcers, fungal infections (e.g. athlete’s foot) or abnormal scaling of the skin (e.g. in eczema) can allow infection. Nasal carriage of bacteria can be a source of reinfection.


Impetigo is a highly infectious skin disease most common in children (Fig. 24.2). It presents as weeping, exudative areas with a typical honey-coloured crust on the surface. It is spread by direct contact. The term ‘scrum pox’ refers to impetigo spread between rugby players. Staphylococci or group A β-haemolytic streptococci are the causative agents: skin swabs should be taken.


Cellulitis presents as a hot, sometimes tender area of confluent erythema of the skin due to infection of the deep subcutaneous layer. It often affects the lower leg, causing an upward-spreading, hot erythema, and occasionally will blister, especially if oedema is prominent. It may also be seen affecting one side of the face. Patients are often unwell with a high temperature. It is usually caused by a β-haemolytic streptococcus, rarely a staphylococcus, and sometimes community-acquired MRSA (in chapter 4). In the immunosuppressed or diabetic patient Gram-negative organisms or anaerobes should be suspected.

There may be an obvious portal of entry for infection such as a recent abrasion or a venous leg ulcer. The web spaces of the toes should be examined for fungal infection. Skin swabs are usually unhelpful. Confirmation of infection is best done serologically: antistreptolysin O titre (ASOT) and antiDNAse B titre (ADB).

Erysipelas is the term used for a more superficial infection (often on the face) of the dermis and upper subcutaneous layer that clinically presents with a well-defined edge. However, erysipelas and cellulitis overlap so it is often impossible to make a meaningful distinction.

Necrotizing fasciitis (see p. 116).

Mycobacterial infections

Viral infections

Human papilloma virus

Human papilloma virus (HPV) is responsible for the common cutaneous infection of ‘viral warts’.

Common warts are papular lesions with a coarse roughened surface, often seen on the hands and feet, but also on other sites. Small black dots (bleeding points) are often seen within the lesion (Fig. 24.5). If they occur on the face they are often elongated (‘filiform’) Children and adolescents are usually affected. Spread is by direct contact and is also associated with trauma.

Plantar warts (verrucae) is the term used for lesions on the soles of the feet. They often appear flat (‘inward growing’) although they have the same papillomatous surface change and black dots are often revealed if the skin is pared down (unlike callosities). Warts may be painful or tender if they are over pressure points or around nail folds.

Plane warts are much less common and are caused by certain HPV subtypes. They are clinically different and appear as very small, flesh-coloured or pigmented, flat-topped lesions (best seen with side-on lighting) with little in the way of surface change and no black dots within them. They are usually multiple and are frequently found on the face or the backs of the hands.

Anogenital warts are usually seen in adults and are normally transmitted by sexual contact. They are rare in childhood and, whilst child sex abuse should always be considered, it should be remembered they may well have been transmitted through non-sexual contact. HPV subtypes 16 and 18 are potentially oncogenic and are associated with cervical and anal carcinomas.

Mar 31, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Skin disease

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