14.5 — The Common Skin Diseases

Skin conditions are the fourth leading cause of non-fatal disease burden worldwide, and they are unusual in medicine in one respect: they are visible. A liver problem is private; a facial rash is not, and the psychological impact of skin disease consistently exceeds what its physical severity would predict.

The encouraging counterpart is that dermatology has improved as much as any specialty in the last twenty years, and several conditions that were managed indefinitely are now controlled almost completely.

Acne

Affects up to 85 percent of people aged 12 to 24, and it is not confined to adolescence — adult acne, particularly in women, is common and frequently untreated because people assume they should have grown out of it.

Four things happen together in a follicle, and every treatment targets one of them.

Increased sebum, driven by androgens (Chapter 12.6) — which is why it begins at puberty in both sexes.

Abnormal keratin plugging of the follicle opening.

Cutibacterium acnes proliferating in the blocked, sebum-rich follicle.

Inflammation — and this is what produces the redness, the pain and the scarring.

A blocked follicle is a comedone: closed (whitehead) or open (blackhead).

And blackheads are not dirt. They are oxidised melanin and lipid, which is why scrubbing does not remove them and why aggressive washing makes acne worse by damaging the barrier.

Treatment, in a clear ladder:

Topical retinoids — normalise the keratin plugging. First-line for essentially everyone, and they must be used continuously rather than on spots. They cause irritation initially and people stop too early.

Benzoyl peroxide — antibacterial, and bacteria do not develop resistance to it, which is why it is combined with topical antibiotics.

Topical antibiotics — always with benzoyl peroxide, never alone, to limit resistance.

Oral antibiotics — usually a tetracycline, for 3 to 6 months, and their benefit is anti-inflammatory as much as antibacterial.

Hormonal treatment in women — the combined pill, or spironolactone, which blocks androgen receptors.

Isotretinoin — the only treatment that addresses all four mechanisms, and it produces long-term remission in the majority after a single course.

Its requirements are strict and non-negotiable: it is highly teratogenic (Chapter 4.6), so pregnancy prevention is mandatory with documented testing. Dryness of the skin, lips and eyes is universal. The reported association with mood changes has been studied extensively and remains contested; the practical position is that mood is monitored and any concern is taken seriously.

And the single most important message about acne is to treat it early enough to prevent scarring. Scars are permanent and far harder to treat than the acne was. A person with inflammatory acne should not be told to wait and see.

Eczema (atopic dermatitis)

Affects 15 to 20 percent of children and 2 to 10 percent of adults.

A barrier defect first, and an immune problem second (Chapter 14.1) — the filaggrin story reversed the direction of thinking about it.

Itchy, dry, red, scaly skin, in a distribution that changes with age: face and outer limbs in infants, flexures — elbow creases, behind the knees — in children and adults.

And the itch–scratch cycle is the central problem. Scratching damages the barrier, which worsens the inflammation, which worsens the itch. Breaking that cycle matters more than any single treatment.

Treatment:

Emollients — the foundation, and consistently under-used. Applied generously and frequently, including when the skin looks clear. Quantities matter: an adult with widespread eczema needs 500 grams a week, which is far more than most people are given or use.

Topical steroids for flares — and here there is a genuine and harmful problem.

Steroid phobia is widespread and it causes undertreatment. The risks of appropriate topical steroid use are small; the risks of untreated eczema — infection, sleep loss, growth impairment in children, and years of misery — are not.

The rule is to use an adequate potency for a short period to gain control, rather than a weak preparation for months. Potency is matched to the site: mild on the face, stronger on the body, strongest on palms and soles.

Topical calcineurin inhibitors — steroid-sparing, particularly useful on the face and eyelids.

And the newer treatments have transformed severe disease. Dupilumab, an antibody blocking the key Th2 cytokines, and oral JAK inhibitors both produce clearance rates that were unattainable a decade ago — for a condition previously managed with immunosuppression and phototherapy.

Two practical points. Bleach baths — very dilute sodium hypochlorite, at swimming-pool concentration — reduce bacterial colonisation and flare frequency. And soap substitutes rather than soap, because alkaline soap strips the barrier (Chapter 14.1).

And eczema herpeticum is the emergency. Herpes simplex infecting eczematous skin spreads rapidly, producing painful punched-out erosions with fever. It needs urgent antiviral treatment.

Psoriasis

Affects 2 to 3 percent of people.

An immune-driven condition in which skin cells turn over in about 4 days instead of 28, so cells reach the surface before maturing and pile up as scale.

Well-defined red plaques with thick silvery scale, characteristically on the extensor surfaces — elbows, knees, scalp, lower back — which is the opposite distribution to eczema and is the quickest way to tell them apart.

Nail pitting and onycholysis in around half (Chapter 14.3).

And it is a systemic disease rather than a skin one.

Around 30 percent develop psoriatic arthritis, which can be destructive and which is frequently diagnosed late. Anyone with psoriasis and joint pain should be asked about it specifically.

And psoriasis is independently associated with cardiovascular disease, metabolic syndrome, inflammatory bowel disease and depression — the chronic inflammation appears to be the link.

Treatment has been transformed more than almost any other condition in this chapter.

Topical — vitamin D analogues with steroids; coal tar; salicylic acid to remove scale.

Phototherapy — narrowband UVB.

Systemic — methotrexate, ciclosporin, acitretin.

Biologics — TNF, IL-17 and IL-23 inhibitors. Complete or near-complete clearance in a majority of patients with severe disease, from a starting point where "control" meant plaques that were smaller.

Triggers worth knowing: streptococcal throat infection, which classically triggers guttate psoriasis in young people; stress; skin injury, which produces new plaques at the site — the Koebner phenomenon; and drugs: beta-blockers, lithium, antimalarials, and — importantly — withdrawal of oral steroids, which can precipitate a severe generalised flare. Oral steroids are therefore avoided in psoriasis.

Skin infections

Bacterial:

Impetigo — superficial, with golden crusts, common in children, highly contagious. Topical or oral antibiotics.

Cellulitis — infection of the deeper dermis and subcutaneous tissue. Hot, red, swollen, tender, with a poorly defined edge, usually on a leg, often with fever.

And it is over-diagnosed. Around 30 percent of cellulitis diagnoses are wrong, and the commonest mimic is venous eczema or stasis dermatitis — which is usually bilateral, itchy rather than painful, and afebrile. Cellulitis is almost always one-sided, and bilateral leg redness is rarely cellulitis.

Necrotising fasciitis — the emergency.

Infection spreading rapidly along the fascial planes, destroying tissue.

The features that distinguish it: pain out of all proportion to the appearance, rapid progression over hours, systemic illness that is worse than the skin suggests, and later, skin anaesthesia, blistering and crepitus.

Early on the skin can look almost normal, which is exactly why it is missed.

It requires immediate surgical debridement. Antibiotics alone do not save the limb or the life, and mortality rises steeply with every hour of delay. Severe pain out of proportion to appearance is the finding that should prompt immediate surgical review.

Viral:

Warts — human papillomavirus. Most resolve spontaneously within two years, which is worth knowing before embarking on aggressive treatment.

Cold sores — herpes simplex, reactivating from a nerve ganglion.

Shingles — Chapter 11.8.

Molluscum contagiosum — small pearly umbilicated papules in children. Self-limiting over months to a couple of years, and treatment is usually unnecessary.

Hand, foot and mouth disease — coxsackievirus, common in young children, mild.

Fungal:

Tinea — ringworm, athlete's foot, jock itch. Named by site rather than by organism.

The characteristic feature is an advancing scaly edge with central clearing.

And there is a trap worth knowing. Applying a topical steroid to a fungal infection reduces the redness and itch while allowing the fungus to spread — producing an atypical, poorly demarcated rash called tinea incognito that is then very hard to recognise. This is why combination steroid–antifungal creams are used cautiously and why an unclear rash should be scraped and tested rather than treated blindly.

Candida — in moist skin folds, in nappy areas, and in the mouth. More common in diabetes, after antibiotics, and in immunosuppression (Chapter 13.1).

Infestations:

Scabies — a mite burrowing in the skin. Intense itching, worse at night, with burrows in the finger webs, wrists and genitals.

The itch is an allergic response to the mite, which is why it begins weeks after infestation and why it continues for two to four weeks after successful treatment. Patients must be told this, or they conclude the treatment failed and re-treat repeatedly.

All household contacts are treated simultaneously, whether or not they itch, and bedding and clothing are washed hot.

Head lice — treated with physical methods or insecticides. Exclusion from school is not recommended and does not reduce spread.

Skin cancer

The commonest cancer in humans, and the most preventable.

Basal cell carcinoma — most common by a wide margin.

A slow-growing pearly nodule with visible fine vessels, often with a rolled edge and a central ulcer.

It essentially never spreads to distant sites and it destroys tissue locally, which is why it is called a rodent ulcer. Curable by excision.

Squamous cell carcinoma — a scaly, crusted or ulcerated lesion, often on sun-exposed sites.

It can spread, and the risk is higher on the lip and ear and in immunosuppressed people (Chapter 13.8).

Actinic keratoses — rough scaly patches on sun-damaged skin — are precursors, and treating them prevents progression.

Melanoma — less common, and responsible for the great majority of skin cancer deaths.

And this is the one everyone should be able to recognise, because early melanoma is essentially curable and late melanoma was, until recently, not.

The ABCDE rule:

A — Asymmetry. One half unlike the other. B — Border. Irregular, notched or blurred. C — Colour. More than one colour, or uneven. D — Diameter. Over 6 millimetres, though small melanomas exist. E — Evolving. Changing in size, shape, colour or symptoms — and this is the most important letter.

Plus the "ugly duckling" sign, which is arguably more useful than the letters: a mole that looks different from all the person's other moles. Most people's moles resemble each other; the odd one out deserves attention.

And two forms are missed disproportionately.

Nodular melanoma — grows downward rather than outward, so it fails several of the ABCDE criteria. A new, firm, growing, often uniformly coloured nodule — sometimes red or skin-coloured. Any new growing nodule warrants assessment regardless of colour.

Acral melanoma — on the palms, soles and under the nails (Chapter 14.3). It is not related to sun exposure, occurs at similar rates across all skin types, and is diagnosed late in people with darker skin because skin cancer is assumed not to occur — which is a genuine and documented disparity in outcomes.

Prevention:

Sun protection, particularly avoiding burning in childhood — sunburn in childhood roughly doubles later melanoma risk.

No sunbeds. Classified as a Group 1 carcinogen, alongside tobacco and asbestos. Use before age 35 increases melanoma risk by around 75 percent, and several countries have banned commercial sunbeds for under-18s.

Regular self-examination, and skin checks for anyone at higher risk.

And the encouraging half:

Melanoma caught early — thin, confined to the epidermis or the upper dermis — has a five-year survival above 95 percent, and simple excision is curative.

And advanced melanoma, which had a median survival of under a year a decade ago, has been transformed by immunotherapy. Checkpoint inhibitors (Chapter 19.7) and targeted drugs against specific mutations have produced long-term survival in a substantial proportion of patients with metastatic disease — some of whom appear to be cured.

Melanoma was the disease where immunotherapy was first shown to work, and it changed the outlook for a cancer that had defeated everything else.

Urticaria and drug reactions

Urticaria (hives) — raised itchy weals that come and go within 24 hours in any one spot. Histamine release from mast cells (Chapter 13.6).

Acute urticaria is usually viral or drug-related and settles.

Chronic urticaria — lasting over six weeks — is usually not allergic at all, despite the assumption, and is often autoimmune. Extensive allergy testing in chronic urticaria is generally unrewarding, and treatment is high-dose antihistamines, with omalizumab for resistant cases.

Angioedema — deeper swelling of the lips, tongue, eyelids or airway. Associated with urticaria in allergic cases; occurring alone, it raises two other possibilities: ACE inhibitors (Chapter 7.6) and hereditary angioedema (Chapter 13.1), neither of which responds to antihistamines or adrenaline.

Severe drug reactions — the ones that must be recognised.

Stevens–Johnson syndrome and toxic epidermal necrolysis — a spectrum, with widespread skin and mucosal detachment.

The warning features: a painful rather than itchy rash, mucosal involvement — mouth, eyes, genitals — blistering, skin that separates when pressed, fever, and systemic illness.

Mortality reaches 30 percent in the most extensive form, and the treatment is to stop the drug immediately and manage the person as a burns patient (Chapter 23.7).

Common culprits: allopurinol, some anticonvulsants, sulfonamides, and NSAIDs.

DRESS — a drug reaction with widespread rash, fever, facial swelling, raised eosinophils and internal organ involvement, appearing 2 to 8 weeks after starting a drug. The long delay is why the drug is often not suspected.

Any rash with fever, mucosal involvement, blistering, facial swelling or systemic illness is treated as a serious drug reaction until proven otherwise.

When to see someone about a skin problem

A short practical list.

A new or changing mole, or any lesion that bleeds, does not heal, or grows.

A rash with fever, blistering, mouth or eye involvement, or systemic illness.

Severe pain out of proportion to appearance.

A rash that does not fade under pressure — Chapter 11.10.

Rapidly spreading redness with fever.

And any skin condition that is affecting sleep, work or mood, which is not a lesser reason. Skin disease has a measurable psychological impact, and "it's only skin" is a phrase that has kept people from effective treatment for decades.

What Part 15 does next

One system remains before the volume turns to disease. Part 15 covers reproduction and the human life cycle — male and female anatomy in full, the cycle, sex, conception, pregnancy trimester by trimester, birth, lactation, puberty, menopause, contraception, fertility and sexual health.