Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Recognise painless photosensitive hand fragility, confirm a porphyrin rather than acute-porphyria pattern, identify hepatic and iron drivers, and use venesection or low-dose hydroxychloroquine safely.
Porphyria cutanea tarda is the commonest porphyria and is a hepatic, predominantly acquired disorder rather than an acute neurovisceral porphyria. Uroporphyrins build up in liver, circulate and deposit around superficial dermal vessels. Light in the near-ultraviolet and visible violet range activates them, producing oxidative vessel and junctional injury. The result is painless fragility, vesicles and bullae on dorsal hands, fingers and forearms, often after trivial trauma. Healing leaves milia, scars and mottled hypo- or hyperpigmentation.
The enzyme inhibition usually requires a combination of hepatic oxidative stress and iron. Alcohol, hepatitis C, HIV, oestrogen, smoking, iron overload and haemochromatosis variants are important associations. A minority has inherited partial enzyme deficiency, but penetrance still depends on acquired factors. Because PCT can reveal chronic viral hepatitis, fibrosis or iron-loading disease, dermatological confirmation opens a liver-health pathway rather than ending at blister treatment.
Treatment lowers hepatic iron or mobilises excess porphyrin slowly. Serial venesection is usually preferred when feasible. Very low-dose hydroxychloroquine is an alternative, not the standard daily anti-inflammatory dose. Skin can remain fragile for months while biochemical remission develops, and relapse follows return of an untreated driver. Liver-fibrosis assessment and appropriate hepatocellular-carcinoma surveillance are based on underlying liver disease, not on hand blisters alone.
Most disease arises when hepatic oxidative stress inhibits uroporphyrinogen decarboxylase despite no inherited enzyme deficit in other tissues.
Hepatic iron loading and sustained alcohol exposure promote oxidative conditions that reduce enzyme activity and amplify porphyrin accumulation within the liver.
Hepatitis C, HIV and oestrogen exposure increase susceptibility and should be sought because each has a distinct modifiable pathway.
A minority inherits reduced enzyme activity in all tissues, but clinical PCT still usually requires iron or another acquired hepatic factor.
Reduced uroporphyrinogen decarboxylase activity prevents orderly haem synthesis and permits highly carboxylated porphyrins to accumulate in liver and plasma.
Near-ultraviolet and visible violet energy excites porphyrins in superficial skin, generating reactive oxygen species around dermal vessels.
Repeated photo-oxidative damage weakens structural proteins at the dermal–epidermal interface, so relatively minor hand trauma forms cell-poor subepidermal blisters.
Iron-driven oxidation maintains inhibitory metabolites inside hepatocytes, explaining why controlled depletion restores enzyme function and reduces porphyrins.
Minor knocks produce erosions, crust or peeling over backs of hands and fingers, often before the patient notices intact bullae.
Clear tense vesicles and blisters form on sun-exposed hands or forearms without the intense itch of autoimmune pemphigoid.
Small white keratin cysts, mottled pigment and thin scars remain after repeated blisters and provide evidence of chronic recurrence.
New coarse temple, cheek or forehead hair with facial pigmentation can accompany PCT and should be distinguished from androgen-pattern hirsutism.
Porphyrin-rich urine may appear red-brown or darken on standing, but urine colour is neither required nor sufficiently specific for diagnosis.
Jaundice, ascites, hepatomegaly, splenomegaly, weight loss, bleeding or encephalopathy demands expedited liver assessment beyond routine PCT care.
Trauma-site mechanobullous lesions, milia and scar overlap, but type VII collagen immunopathology and absent porphyrin excess distinguish acquired epidermolysis.
Intense itch, urticarial inflammation and generalised flexural bullae with linear IgG or C3 differ from painless light-exposed fragility.
A similar hand eruption associated with medicines or renal failure has normal porphyrin studies and therefore requires exposure or dialysis assessment.
Acute erythema and burning in a medication-shaped light distribution usually lacks the chronic milia, fragility and biochemical porphyrin pattern.
Variegate or hereditary coproporphyria can blister and also cause acute neurovisceral attacks, requiring full fractionation rather than morphology alone.
Repeated trauma and blistering leave milia, dyspigmentation and scars, while open hands can develop painful bacterial infection.
Alcohol, viral hepatitis and iron overload can independently and jointly progress to advanced fibrosis or cirrhosis.
Cancer risk reflects chronic hepatic injury and fibrosis, requiring surveillance according to liver stage rather than cutaneous activity alone.
Excessive venesection depletes iron and haemoglobin, while overly rapid antimalarial porphyrin mobilisation can acutely injure liver.
PCT primarily affects skin and liver; acute abdominal pain, neuropathy and psychiatric attacks suggest a different hepatic porphyria and need porphobilinogen testing.
Porphyrins absorb visible violet wavelengths that pass through ordinary glass, explaining hand disease in drivers and incomplete protection from standard UV filters.
Even modest iron excess can support oxidative enzyme inhibition, so venesection works through hepatic biochemistry rather than by treating blood viscosity.
Hydroxychloroquine slowly releases porphyrin for excretion; normal daily dosing can mobilise too much and acutely worsen liver injury.
Fragility improves after porphyrins fall, while scars and milia persist, so residual marks do not justify continuing iron removal beyond the target.
Ordering urine porphobilinogen alone and declaring a normal result excludes PCT.
Diagnosing autoimmune pemphigoid from tense blisters without recognising exposed-hand fragility and milia.
Starting venesection without haemoglobin, iron and liver assessment or continuing into symptomatic deficiency.
Prescribing hydroxychloroquine at an ordinary daily lupus dose for PCT.
Treating hand lesions while failing to test hepatitis C, HIV, iron overload and liver fibrosis.
Relying on ordinary ultraviolet sunscreen without physical protection from visible violet light.
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A patient has painless fragile dorsal hands, recurrent clear bullae, milia and new facial hypertrichosis. Which investigation best confirms the suspected metabolic diagnosis?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
This dose is deliberately far below ordinary daily inflammatory-disease dosing; excessive dosing can cause abrupt porphyrin release and hepatotoxicity. Check liver, kidney, eye, QT and interaction risks and avoid unsupervised use in pregnancy.