Approach to Pruritus
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Direct answer
Pruritus lasting six weeks or more without a primary eruption is systemic, neuropathic or psychogenic until proved otherwise — and the work-up is a fixed panel, not guesswork. History and examination hunt for dry skin, drugs and pattern clues; baseline investigations are full blood count with smear (polycythaemia vera, lymphoma, eosinophilia), ESR, liver function with alkaline phosphatase (cholestasis), renal function, TSH, fasting glucose, ferritin, HIV and hepatitis B and C serology, urinalysis and chest radiograph. Pattern recognition shortens the list: aquagenic pruritus after hot baths suggests polycythaemia vera; nocturnal itch with weight loss suggests Hodgkin lymphoma; itch with jaundice suggests cholestasis. Treatment is cause-directed: cholestyramine for cholestatic itch, gabapentinoids and UVB for uraemic itch, SSRIs for aquagenic pruritus.
What you must remember
- The six-week threshold: acute itch is usually dermatological; chronic pruritus without primary lesions triggers the systemic panel — the entry-level distinction every exam stem uses.
- Work-up panel to recite: FBC with peripheral smear, ESR, LFTs with ALP and GGT, urea/creatinine, TSH, glucose/HbA1c, ferritin, HIV, hepatitis B and C, urinalysis, chest radiograph — add serum tryptase if mastocytosis is suspected and pregnancy tests where relevant.
- Haematological classics: aquagenic pruritus (intense prickling within minutes of a warm bath, no visible lesions) — polycythaemia vera, and a recognised prodrome of myeloproliferative disease; refractory pruritus with B symptoms — Hodgkin lymphoma; iron deficiency alone causes pruritus and hair loss in Indian women disproportionately.
- Hepatic and renal: cholestatic itch (primary biliary cholangitis, cholestasis of pregnancy) worst on palms and soles, first line cholestyramine 4 g up to four times daily, alternatives rifampicin (liver-monitored), sertraline and naltrexone; uraemic itch — gabapentin (renally dose-adjusted) or pregabalin, plus narrowband UVB.
- Neuropathic pattern set: brachioradial pruritus — dorsolateral forearms, sun-exposed, driven by cervical spine disease, ice-pack relief; notalgia paraesthetica — medial scapular patch from thoracic nerve involvement; post-herpetic itch; both respond to gabapentinoids or capsaicin.
- Itch pharmacology to bank: antihistamines work only in histaminergic (urticarial) itch — non-sedating agents fail in most systemic itch, a recurring exam correction; newer targets include kappa-opioid agonists (difelikefalin for dialysis itch) and JAK inhibitors.
- Skin-safe general measures: emollients as the universal base, menthol 0.5-1% for cooling, capsaicin for localised neuropathic itch, and treating xerosis (the most common genuine cause) before any exotic diagnosis.
- Red flags that escalate: weight loss, night sweats, lymphadenopathy, jaundice, anaemia or polycythaemia, and new drugs — each converts a "simple itch" into a same-week work-up.
From bath-tickling to bone-marrow: a structured pass
A 58-year-old reports a year of intense, whole-body prickling that erupts minutes after warm baths and lasts up to an hour, with no rash. Aquagenic pruritus is almost a physical sign of polycythaemia vera: the FBC returns a haemoglobin of 18 g/dL, and the JAK2 V617F mutation assay confirms the myeloproliferative neoplasm. His itch — histamine-independent — will not respond to antihistamines; phlebotomy and cytoreduction reduce it, SSRI therapy (paroxetine or sertraline) is the symptomatic bridge, and phototherapy is a backup. The lesson generalises: itch with a paradoxical trigger (water, cold, exercise) is a pointer to systemic or neuropathic disease, never to a moisturiser deficit.
Contrast the 70-year-old with excoriated shins whose panel returns an alkaline phosphatase thrice normal with positive antimitochondrial antibodies — primary biliary cholangitis announcing itself through itch before jaundice. Cholestyramine 4 g before and after breakfast, spaced from other drugs, is first line; rifampicin 150-300 mg daily second with liver monitoring. Between the two sits the largest real group — asteatotic xerosis of the elderly — cured by emollients, but only after the panel is clear, because a benign commonest cause does not license skipping the screen.
Where the exam scratches
The stem machinery is predictable: an elderly patient with nocturnal generalised itch, normal skin except excoriations, and the question asks the next investigation — the answer is the systemic panel, not an antihistamine. The aquagenic-pruritus-bath clue for polycythaemia vera is a perennial single-best-answer. Pharmacology questions exploit the antihistamine fallacy — non-sedating antihistamines help urticarial itch, not cholestatic or uraemic itch — and pair cholestatic itch with cholestyramine and uraemic itch with gabapentin or UVB. The neuropathic pair (brachioradial pruritus with cervical spine disease, notalgia paraesthetica with the medial scapula) appears as a "localised itch" stem. Finally, know that refractory iron-deficiency itch responds to iron — the single commonest correctable cause in Indian women.
Frequently asked questions
What defines chronic pruritus and what first-line panel does it trigger?
Itch persisting beyond six weeks; full blood count with smear, ESR, liver and renal function, TSH, glucose, ferritin, HIV and hepatitis serology, urinalysis and chest radiograph.
Which haematological disease classically causes aquagenic pruritus?
Polycythaemia vera — intense prickling within minutes of warm water contact, histamine-independent and antihistamine-resistant.
What is first-line therapy for cholestatic pruritus?
The bile-acid sequestrant cholestyramine (4 g up to four times daily), with rifampicin, sertraline or naltrexone as alternatives.
Why do non-sedating antihistamines fail in systemic itch?
Most systemic, neuropathic and cholestatic itch is transmitted through non-histaminergic C-fibre pathways, so H1 blockade has little effect — sedating agents help only sleep.
What is brachioradial pruritus?
Neuropathic itch of the dorsolateral forearms linked to cervical spine disease and sun exposure, characteristically relieved by ice packs and treated with gabapentinoids or capsaicin.