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.
Box 24.1
Functions of the skin
Physical barrier against friction and shearing forces
Protection against infection (immune and innate), chemicals, ultraviolet irradiation
Prevention of excessive water loss or absorption
Ultraviolet-induced synthesis of vitamin D
Sensation (pain, touch and temperature)
Antigen presentation/immunological reactions/wound healing
Basement membrane zone
The basement membrane zone (see Fig. 24.27) is a complex proteinaceous structure consisting of type IV, VII and XVII collagen, hemidesmosomal proteins, integrins and laminin. Collectively, they hold the skin together, keeping the epidermis firmly attached to the dermis. Inherited or autoimmune-induced deficiencies of these proteins can cause skin fragility and a variety of blistering diseases (see p. 1221).
The dermis
The sweat glands
Eccrine sweat glands are found throughout the skin except the mucosal surfaces.
The sweat glands and vasculature are involved in temperature control.
Hair
Terminal: medullated coarse hair, e.g. scalp, beard, pubic
Vellus: non-medullated fine downy hairs seen on the face of women and in prepubertal children
Lanugo: non-medullated soft hair on newborns (most marked in premature babies) and occasionally in people with anorexia nervosa.
The subcutaneous layer
FURTHER READING
Borkowski AW, Gallo RL. The coordinated response of the physical and antimicrobial peptide barriers of the skin. J Invest Dermatol 2011; 131:285–287.
Lin JY, Fisher DE. Melanocyte biology and skin pigmentation. Nature 2007; 445:843–850.
Shimomura Y, Christiano AM. Biology and genetics of hair. Annu Rev Genomics Hum Genet 2010; 11:109–132.
Approach to the patient
The history should aim to elicit the following points:
Table 24.2 Investigations used in skin disorders
Test | Use | Clinical example |
---|---|---|
Skin swabs | Bacterial culture | Impetigo |
Blister fluid | Electron microscopy, viral culture and PCR | Herpes simplex |
Skin scrapes | Fungal culture | Tinea pedis |
Microscopy | Scabies | |
Nail sampling | Fungal culture | Onychomycosis |
Wood’s light | Fungal fluorescence | Scalp ringworm |
Erythrasma | ||
Blood tests | Serology | Streptococcal cellulitis |
Autoantibodies | Systemic lupus erythematosus | |
HLA typing | Dermatitis herpetiformis | |
DNA analysis | Epidermolysis bullosa | |
Skin biopsy | Histology | General diagnosis |
Immunohistochemistry | Cutaneous lymphoma | |
Immunofluorescence | Immunobullous disease | |
Culture | Mycobacteria/fungi | |
Patch tests | Allergic contact eczema | Hand eczema |
Urine | Dipstick (glucose) | Diabetes mellitus |
Cytology (red cells) | Vasculitis | |
Dermatoscopy (direct microscopy of skin) | Assessment of pigmented lesions | Malignant melanoma |
Infections
Bacterial infections (see also p. 114)
Hidradenitis suppurativa
Treatment is difficult but weight loss, ‘prophylactic’ antibiotics, oral retinoids, zinc and co-cyprindiol (2 mg cyproterone acetate + 35 µg ethinylestradiol in females only) have been tried. They should be used as for acne vulgaris (p. 1212). Severe recalcitrant cases have been treated with intravenous infliximab, a monoclonal antibody (p. 72). Surgery and skin grafting is sometimes required.
Mycobacterial infections
Skin manifestations of tuberculosis
Tuberculosis can occasionally cause skin manifestations:
Lupus vulgaris usually arises as a post-primary infection. It usually presents on the head or neck with red-brown nodules that look like apple jelly when pressed with a glass slide (‘diascopy’). They heal with scarring and new lesions slowly spread out to form a chronic solitary erythematous plaque. Chronic lesions are at high risk of developing squamous cell carcinoma.
Tuberculosis verrucosa cutis arises in people who are partially immune to tuberculosis but who suffer a further direct inoculation in the skin. It presents as warty lesions on a ‘cold’ erythematous base.
Scrofuloderma arises when an infected lymph node spreads to the skin causing ulceration, scarring and discharge.
The tuberculides are a group of rashes caused by an immune manifestation of tuberculosis rather than direct infection. Erythema nodosum is the commonest and is discussed on page 1216. Erythema induratum (‘Bazin’s disease’) produces similar deep red nodules but these are usually found on the calves rather than the shins and they often ulcerate.
Viral infections
Herpes zoster (shingles)
‘Shingles’ results from a reactivation of VZV. It may be preceded by a prodromal phase of tingling or pain, which is then followed by a painful and tender blistering eruption in a dermatomal distribution (Fig. 24.4 and Fig. 4.15, p. 98). The blisters occur in crops, may become pustular and then crust over. The rash lasts 2–4 weeks and is usually more severe in the elderly. Occasionally more than one dermatome is involved.
Human papilloma virus
Human papilloma virus (HPV) is responsible for the common cutaneous infection of ‘viral warts’.