Pruritis

Itch and pain share the same nerve pathway: both are abolished by analgesia/ anaesthesia, reduced by counter-irritation (cold/heat/vibration), and can be referred to other sites. Anti-histamines are commonly not effective -> other pathways are involved.

Localised pruritis

Caused by common and easily diagnosable skin conditions; an intense localised itch is suggestive of scabies. Causes: atopic dermatitis, contact dermatitis, urticaria, dermatitis herpetiformis (coeliac disease), Grover disease, scabies, insect bites, pediculosis, asteatosis (dry skin), lichen planus (4 Ps), chicken pox.

Generalised pruritis

More likely to be caused by systemic disease; common in the 3rd trimester of pregnancy.

Causes: pregnancy; chronic kidney failure; liver disorders (cholestatic jaundice — e.g. cancer of the head of pancreas, primary biliary cirrhosis, drugs such as chlorpromazine/antibiotics — or hepatic failure); malignancy (Hodgkin lymphoma, leukaemia — especially CLL — multiple myeloma, disseminated carcinoma); haematological disorders (polycythaemia rubra vera — “bath itch” — iron-deficiency anaemia, pernicious anaemia, macroglobulinaemia); endocrine disorders (diabetes, hypo/hyperthyroidism, carcinoid syndrome, hyperparathyroidism); malabsorption (gluten sensitivity); parasitic infestation (scabies, ascariasis, filariasis, hookworm); drugs (alkaloids, aspirin/other NSAIDs, diuretics, ACE inhibitors, opiates, cocaine, antibiotics e.g. penicillin/sulfonamides, quinidine, chloroquine, CNS stimulants); autoimmune disorders (polyarteritis nodosa, sicca syndromes e.g. Sjögren syndrome); neurological disorders (Parkinson disease, brain tumour, multiple sclerosis); irritants (fibreglass, soaps, detergents, chlorine); aquagenic pruritus; xerosis (dry skin, winter itch); HIV/AIDS; and psychological/emotional causes (anxiety/depression, OCD, psychosis, parasitophobia).

Diagnostic strategy (generalised pruritis):

• Probability: xerosis (especially elderly), dermographism, urticaria, psychological/emotional, varicella.
• Serious, not to miss: neoplasia (lymphoma/Hodgkin, leukaemia especially CLL, other cancers), HIV/AIDS, CKD, primary biliary cirrhosis.
• Often missed: pregnancy, tropical infection/infestation/scabies, polycythaemia rubra vera (triggered by hot bath, lasts 1hr), polyarteritis nodosa, lichen planus, topical irritants (fibreglass, bubble bath), drugs, iron deficiency, diabetes, hyper/hypothyroidism.

Nodular prurigo

The end point of chronic scratching: intensely itchy, firm, leathery, dome-shaped nodules. Any itchy condition can lead to it — atopic dermatitis is the most common association. Investigate for (and treat) an underlying cause; exclude scabies. General measures: soap-free wash, frequent emollient. Treatment aims to break the itch-scratch cycle: potent topical steroid (betamethasone dipropionate 0.05% ointment) applied to nodules twice daily for 2–4 weeks, with or without an occlusive dressing; treat secondary bacterial infection if present. Rarely affects the face — if it does, refer to dermatology. If not clearing, refer for consideration of intralesional steroids, systemic therapy, or UVB phototherapy.