Medical brain notes 28

  Causes The skin is the boundary between ourselves and the world around us. It is an important sense organ, and controls heat and water loss. It reflects internal changes and reacts to external ones. Usually, it adapts easily and returns to a normal state, but some-times it fails to do so and a skin disorder appears. Some of the internal and external factors that are important causes of skin disease are shown in Fig. 1.1. Often several will be operating at the same time; just as often, no obvious cause for a skin abnormality can be foundaand here lies much of the difficulty of dermatology. Nevertheless, when a cause is obvious, such as the washing of dishes and the appearance of irritant hand dermatitis, or sunburn and the develop-ment of melanoma, education and prevention are just as important as treatment.

  Prevalence No one who has worked in any branch of medicine will doubt the importance of diseases of the skin. A neurologist, for example, will know all about the Sturge–Weber syndrome, a gastroenterologist about the Peutz–Jeghers syndrome, and a cardiologist about the LEOPARD syndrome; but even in their own wards they will see far more of other common skin conditions such as drug eruptions, asteatotic eczema and scabies. They should know about these too. In primary care, skin problems are even more important, and the prevalence of some common skin conditions, such as skin cancer and atopic eczema, is undoubtedly rising. Currently, skin disorders account for about 15% of all consultations in general practice in the UK, but this is only the tip of an iceberg of skin disease, the sunken part of which consists of problems that never get to doctors, being dealt with or ignored in the community. How large is this problem? No one quite knows, as those who are not keen to see their docto...

  Impact Much of this book is taken up with ways in which skin diseases can do harm. Most fit into the five Ds shown in Fig. 1.5; others are more subtle. Topical treatment, for example, can seem illogical to those who think that their skin disease is emotional in origin; it has been shown recently that psoriatics with great disability comply especially poorly with topical treatment. In addition, the problems created by skin disease do not necessarily tally with the extent and severity of the eruption as judged by an outside observer. Quality-of-life studies give a different, patient-based, view of skin conditions. Questionnaires have been designed to compare the impact of skin diseases with those of other conditions; patients with bad psoriasis, for example, have at least as great a disability as those with angina. In the background lurk problems due to the costs of treatment and time lost from work.   Disfigurement They range from a leper  (e.g. some patients with psoria...

  The function and structure of the skin The skinathe interface between humans and their environmentais the largest organ in the body. It weighs an average of 4 kg and covers an area of 2 m 2 . It acts as a barrier, protecting the body from harsh external conditions and preventing the loss of important body constituents, especially water. A death from destruction of skin, as in a burn, or in toxic epidermal necrolysis, and the misery of unpleasant acne, remind us of its many important functions, which range from  the vital to the cosmetic (Table 2.1). The skin has two layers. The outer is epithelial, the  epidermis , which is firmly attached to, and supportedby connective tissue in the underlying  dermis . Beneath the dermis is loose connective tissue, the  subcutis / hypo-dermis  which usually contains abundant fat (Fig. 2.1).

  Epidermis The epidermis is formed from many layers of closely packed cells, the most superficial of which are flattened and filled with keratins; it is therefore a stratified squam-ous epithelium. It adheres to the dermis partly by the interlocking of its downward projections ( epidermalridges  or  pegs ) with upward projections of the dermis( dermal papillae ) (Fig. 2.1). The epidermis contains no blood vessels. It varies in thickness from less than 0.1 mm on the eyelids to nearly 1 mm on the palms and soles. As dead surface squames are shed (accounting for some of the dust in our houses), the thickness is kept constant by cells dividing in the deepest ( basal  or  germinative ) layer. A generated cell moves, or is pushed by underlying mitotic activity, to the surface, passing through the  prickle  and  granular cell layers  before dying in the  horny layer . Thejourney from the basal layer to the surface (epidermal turnover or transi...

  The dermo-epidermal junction The basement membrane lies at the interface between the epidermis and dermis. With light microscopy it can be highlighted using a periodic acid–Schiff (PAS) stain, because of its abundance of neutral mucopolysac-charides. Electron microscopy (Fig. 2.9) shows that the  lamina densa  (rich in type IV collagen) is separatedfrom the basal cells by an electron-lucent area, the  lamina lucida . The plasma membrane of basal cellshas  hemidesmosomes  (containing bullous pemphigoid antigens, collagen XVII and  α 6  β 4 integrin). The lamina lucida contains the adhesive macromolecules, laminin-1, laminin-5 and entactin. Fine  anchoringfilaments  (of laminin-5) cross the lamina lucida andconnect the lamina densa to the plasma membrane of the basal cells.  Anchoring fibrils  (of type VII collagen), dermal microfibril bundles and single small collagen fibres (types I and III), extend from the papillary dermis ...