Skin

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281

26.2 Immunological disorders283
26.3 Genetic disorders283
26.4 Neoplasia284

Self-assessment: questions288
Self-assessment: answers289

Chapter overview
The skin is a multifunctional organ involved in structural support, protection from injury and infection and temperature regulation. When these functions are dramatically disturbed – in extensive burn injuries, for example – patients are at risk of fatal metabolic and fluid homeostasis disruption or overwhelming bacterial infection. Skin neoplasms are very common in white-skinned races, with malignant melanoma rising rapidly in incidence. There are many inflammatory diseases of the skin that cause significant morbidity and cosmetic problems.

26.1. Inflammation and infection

Learning objectives
You should:

• understand the terminology used to described skin lesions macroscopically
• know how inflammatory skin diseases can be classified according to the pattern of microscopic changes; this will help your understanding of clinical dermatology
• understand the classification and complications of burns.
One of the common difficulties students encounter when learning about skin diseases is the large number of specific terms used to describe both macroscopic and microscopic appearances. Common naked-eye and microscopic descriptive terms are shown in Box 31.
Box 31

Naked-eye:

Macule – flat area of altered colour
Papule – small raised lesion
Plaque – slightly raised, flat-topped lesion
Erythema – reddening of the skin
Vesicle – small fluid-filled lesion
Bulla – larger fluid-filled lesion
Blister – non-specific term that includes vesicle and bulla
Microscopic:

Hyperkeratosis – increased thickness of the superficial keratin
Parakeratosis – presence of residual nuclear material within the superficial keratin (implies abnormal maturation)
Spongiosis – intercellular oedema in the epidermis, characteristically seen in eczematous lesions
Acanthosis – thickening of the epidermis
Acantholysis – discohesion, ‘falling apart’, of epidermal cells
Inflammatory skin diseases can be classified according to the pattern of histological changes (Table 60).
Table 60 Major skin inflammatory disease patterns
Spongiotic Spongiosis – intraepidermal oedema Eczema
Psoriasiform Regular epidermal hyperplasia (thickening) Psoriasis
Lichenoid Damage to basal epidermis, often with chronic inflammatory infiltrate along dermo-epidermal junction Lichen planus
Lupus erythematosus
Erythema multiforme
Graft-versus-host disease
Vesico-bullous Vesicle or blister (bulla) formation Pemphigoid
Dermatitis herpetiformis
Pemphigus
Granulomatous Chronic inflammation with aggregates of enlarged (epithelioid) histiocytes Sarcoidosis
Infection (tuberculosis, fungal)
Reaction to foreign material
Vasculitis Inflammation of vessel walls Primary cutaneous vasculitis
Skin involvement by systemic vasculitis
Panniculitis Inflammation of the subcutaneous fat Erythema nodosum

Eczema

Eczema is a common inflammatory skin disease presenting clinically with red papules or plaques containing small vesicles, which may itch, ooze and crust. There are several clinical subtypes. The characteristic histological feature is intercellular oedema in the epidermis (spongiosis), which separates the keratinocytes and can cause vesicle formation. Chronic inflammation with lymphocytic infiltration of the epidermis and dermis is often seen. Pathogenesis is variable and includes:

• direct toxic effects (irritant contact dermatitis)
• delayed hypersensitivity reaction (allergic contact dermatitis)
• combination of IgE-mediated and T cell-mediated reaction (atopic eczema).

Psoriasis

Psoriasis is a common chronic relapsing dermatitis that classically manifests as circumscribed red skin plaques often several centimetres in diameter, with silvery surface scale. Extensor surfaces of knees and elbows, the sacral region and the scalp are common sites. Nail changes are common and arthritis develops in a small percentage of patients. Onset is usually in early adult life. The pathogenesis is unclear but there is hyperproliferation of keratinocytes, with the epidermal turnover time reduced from the normal 13days to 3–4days. The histological features of classical psoriasis are:

• regular epidermal hyperplasia (thickening)
• parakeratosis (retained nuclei within the surface keratin layer)
• thinning of the granular layer
• neutrophil microabscesses
• increased mitotic activity (cell turnover).

Lichen planus

Lichen planus presents clinically as multiple small itchy violet papules, around the wrist and elbows. Oral and genital lesions also occur. Skin lesions often resolve after several months or years. Histological features include:

• irregular epidermal hyperplasia
• damage to the basal epidermis with vacuolation and apoptotic epidermal cells
• band-like (‘lichenoid’) chronic inflammatory infiltrate at the dermo-epidermal junction.

Erythema multiforme

Erythema multiforme (EM) is an uncommon but potentially serious hypersensitivity response to certain infections (herpes simplex and Mycoplasma) and drugs (including sulphonamides, penicillin and aspirin). Clinical lesions include red macules, papules and vesicles. The characteristic ‘target’ lesion has a pale, raised or eroded centre and erythematous (red) rim. Symmetrical involvement of the limbs is most commonly seen. Stevens–Johnson syndrome is severe erythema multiforme with oral mucosal lesions. Toxic epidermal necrosis is the most serious manifestation of EM, with a high risk of systemic sepsis and fluid imbalance due to extensive loss of skin and mucosal epithelium; mortality is 35%. Histological examination shows a lichenoid inflammatory infiltrate with epidermal cell apoptosis.

Vesico-bullous diseases

Vesico-bullous diseases are classified according to the site of the vesicle formation, which may be within the epidermis or at the dermo-epidermal junction. The pathogenesis in many cases involves immune complex formation with autoantibodies. Examples are shown in Table 61. Many of these diseases are described further below.
Table 61 Vesico-bullous skin diseases
Site of blistering Disease example Pathogenesis
Within epidermis Impetigo Bacterial infection
Staphylococcal scalded skin syndrome
Pemphigus Autoantibody formation
Spongiotic dermatitis Extreme intercellular oedema
At dermo-epidermal junction Epidermolysis bullosa Hereditary defect in proteins that anchor basal epidermis cells to basement membrane
Porphyria Enzyme deficiency causing skin fragility
Dermatitis herpetiformis IgA antibody deposition
Pemphigoid Autoantibody formation

Burns

Burns can be caused by hot liquids (scalds), gases or solids, as well as fire. Burn injuries are classified as:

• partial thickness (first and second degree burns)
• full thickness (third degree burns).
In first degree burns there is endothelial injury and leakage of intravascular fluid into the surrounding tissue. There is vascular congestion with pain and erythema, but no skin necrosis. Second degree burns show epidermal necrosis with blistering as the epidermis separates from the dermis. Regeneration of the skin surface cells can occur from adjacent viable epidermis or from surviving skin adnexal structures. More severe damage occurs in third degree burns, with necrosis of the epidermis and dermis. As the extent of severe burn injuries increases, so does the risk of serious or fatal complications, including:

• septic shock
• renal failure
• adult respiratory distress syndrome (ARDS)
• scarring and contractures.
Skin grafting is often required for healing of third degree burns.

Infectious disease

Skin infections are common, with a wide variety of potential pathogens, including:

• Bacteria

impetigo (staphylococcal or streptococcal infection, particularly in children)
staphylococcal scalded skin syndrome (infants and children)
furuncles (‘boils’)
cellulitis
cutaneous tuberculosis.
• Viruses

varicella zoster (chicken pox and shingles)
verrucae (common warts – human papilloma virus infection)
molluscum contagiosum (poxvirus).
• Fungi

• Candida
tinea (ringworm, athlete’s foot).
• Arthropods

scabies
lice.

26.2. Immunological disorders

Learning objective
You should:

• appreciate that certain skin diseases are immunologically mediated (specialist immunofluorescence tests may be needed for precise diagnosis in these lesions).

Lupus erythematosus

Lichenoid inflammatory pattern skin lesions may be part of systemic lupus erythematosus or more commonly isolated cutaneous disease, called discoid lupus erythematosus. Immunoglobulins and complement are deposited along the dermo-epidermal junction.

Graft-versus-host disease

Recipients of immunocompetent bone marrow transplants often develop lesions in the skin, gastrointestinal tract and liver, when grafted lymphocytes attack host tissue. Cutaneous graft-versus-host disease has a lichenoid pattern.

Bullous pemphigoid

Bullous pemphigoid is a blistering skin disease that occurs in the elderly. Up to 90% have IgG autoantibodies against the bullous pemphigoid major antigen located in the epidermal basement membrane. The vesicles and bullae usually contain eosinophils.

Pemphigus

Pemphigus is an intraepidermal blistering disease with autoantibodies to various structural proteins involved in intercellular adhesion.

Dermatitis herpetiformis

Dermatitis herpetiformis is a subepidermal blistering disease characterised by IgA antibody deposition, although the exact pathogenesis is unclear. There is a strong association with coeliac disease.

26.3. Genetic disorders

Learning objective
You should:

• understand that inherited skin disease can manifest as inflammatory disease or neoplastic disease.

Epidermolysis bullosa

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