Dermatological conditions are very common (10–15% of general practice consultations) and present to doctors in all specialties. In the UK, 50% are lesions (‘lumps and bumps’), including skin cancers, and most of the remainder are acute and chronic inflammatory disorders (‘rashes’), including infections, with genetic conditions accounting for a small minority.
Dermatological diagnosis can be challenging: not only is there a vast number of distinct skin diseases, but also each may present with a great variety of morphologies and patterns determined by intrinsic genetic factors, with the diagnostic waters muddied still further by external influences such as rubbing and scratching, infection, and well-meaning attempts at topical and systemic treatment. Even in one individual, lesions with the same pathology can have a very variable appearance (for example, melanocytic naevi, seborrhoeic keratoses and basal cell carcinomas).
Many skin findings will have no medical significance, but it is important to be able to examine the skin properly in order to identify tumours and rashes, and also to recognise cutaneous signs of underlying systemic conditions. The adage that the skin is a window into the inner workings of the body is entirely true, and an examination of the integument will often provide the discerning clinician with important clues about internal disease processes, as well as with information about the physical and psychological wellbeing of an individual.
Anatomy and physiology
Skin
The skin is the largest of the human organs, with a complex anatomy ( Fig. 14.1 ) and a number of essential functions ( Box 14.1 ). It has three layers, the most superficial of which is the epidermis, a stratified squamous epithelium, containing melanocytes (pigment-producing cells) within its basal layer, and Langerhans cells (antigen-presenting immune cells) throughout.
- •
Protection against physical injury and injurious substances, including ultraviolet radiation
- •
Anatomical barrier against pathogens
- •
Immunological defence
- •
Retention of moisture
- •
Thermoregulation
- •
Calorie reserve
- •
Appreciation of sensation (touch, temperature, pain)
- •
Vitamin D production
- •
Absorption – particularly fetal and neonatal skin
- •
Psychosexual and social interaction
The dermis is the middle and most anatomically complex layer, containing vascular channels, sensory nerve endings, numerous cell types (including fibroblasts, macrophages, adipocytes and smooth muscle), hair follicles and glandular structures (eccrine, sebaceous and apocrine), all enmeshed in collagen and elastic tissue within a matrix comprising glycosaminoglycan, proteoglycan and glycoprotein.
The deep subcutis contains adipose and connective tissue.
Dermatoses (diseases of the skin) may affect all three layers and, to a greater or lesser extent, the various functions of the skin.
Hair
Hair plays a role in the protective, thermoregulatory and sensory functions of skin, and also in psychosexual and social interactions. There are two main types of hair in adults:
- •
vellus hair, which is short and fine, and covers most of the body surface
- •
terminal hair, which is longer and thicker, and is found on trunk and limbs, as well as scalp, eyebrows, eyelashes, and pubic, axillary and beard areas.
Abnormalities in hair distribution can occur when there is transitioning between vellus and terminal hair types (for example, hirsutism in women) or vice versa (androgenic alopecia). Hairs undergo regular asynchronous cycles of growth and thus, in health, mass shedding of hair is unusual. Hair loss can occur as a result of disorders of hair cycling, conditions resulting in damage to hair follicles (such as purposeful removal in trichotillomania), or structural (fragile) hair disorders.
Nails
The nail is a plate of densely packed, hardened, keratinised cells produced by the nail matrix. It serves to protect the fingertip and aid grasp and fingertip sensitivity. The white lunula at the base of the nail is the visible distal aspect of the nail matrix ( Fig. 14.2 ). Fingernail regrowth takes approximately 6 months, and toenail regrowth 12–18 months.
