Pellagra
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Direct answer
Pellagra is a deficiency of niacin, or of its precursor tryptophan, that produces photosensitive dermatitis, diarrhoea and dementia, the classic three Ds. The rash is symmetrical and sits on sun-exposed skin, with the Casal necklace around the neck as its signature sign. Monotonous maize-based diets, alcoholism, carcinoid tumours and Hartnup disease are the main causes, and treatment is oral nicotinamide with other B vitamins.
What you must remember
- Deficiency: niacin (nicotinic acid, vitamin B3) or tryptophan, which the body converts to niacin.
- Three Ds plus death: dermatitis, diarrhoea and dementia, with death as the fourth D when the disease goes untreated.
- Casal necklace: a symmetrical, rough, hyperpigmented dermatitis encircling the neck, the bedside sign examiners favour.
- Photosensitive distribution: the dorsum of the hands and feet, face and the V of the neck, with glossitis and stomatitis often present.
- Dietary cause: maize-based diets, because niacin in maize is bound and largely unavailable unless the grain is alkali-treated.
- Other causes: alcoholism, carcinoid syndrome (tryptophan diverted to serotonin), Hartnup disease (impaired tryptophan absorption) and malabsorption.
- Key test for carcinoid: urinary 5-hydroxyindoleacetic acid, when flushing and diarrhoea raise the suspicion.
- Treatment: oral nicotinamide with a balanced diet and replacement of other B vitamins, with neurological features tracked over time.
A stepwise approach to a sun-exposed dermatitis
A patient with a symmetrical rash on sun-exposed skin and diarrhoea needs a diet history before any cream is chosen. The steps below move from history to replacement.
- Take the diet and drug history: ask about a monotonous maize or sorghum diet, alcohol intake and any prolonged medication.
- Map the rash: confirm a symmetrical, sun-exposed dermatitis with a Casal necklace rather than a random eruption.
- Look for the other Ds: check for diarrhoea, glossitis and cheilitis, then assess cognition for early dementia or confusion.
- Exclude mimics: consider porphyria, lupus, dermatomyositis and zinc deficiency when the pattern is unclear.
- Replace niacin: give oral nicotinamide in standard doses with a balanced diet, alongside other B vitamins.
- Track the nervous system: neurological features may improve over weeks, but residual deficits can remain.
Replacement alone does not prevent relapse. If the diet or the alcohol use that caused the deficiency continues, the disease returns, so the dietary correction is part of the treatment rather than an optional extra.
Where the exam tries to catch you
Stems turn on recognising the classic pattern and then chasing the cause.
- Trap: choosing pellagra for any photosensitive rash in a child with diarrhoea. Correct: look for the symmetrical, sun-exposed dermatitis with the Casal necklace and a dietary or metabolic cause before naming it.
- Trap: prescribing niacin without asking about the diet. Correct: the cause, whether maize, alcohol or carcinoid, decides where the search goes next.
- Trap: missing carcinoid in an older adult with flushing, diarrhoea and pellagra. Correct: check urinary 5-hydroxyindoleacetic acid, since carcinoid diverts tryptophan away from niacin production.
Frequently asked questions
What are the three Ds of pellagra?
The three Ds of pellagra are dermatitis, diarrhoea and dementia, which together define the classic syndrome. Death is added as a fourth D when the disease goes untreated, so early recognition and replacement matter.
Why does a maize diet cause pellagra?
A maize diet causes pellagra because niacin in maize is bound and largely unavailable to the body. Traditional alkali treatment releases the niacin, which explains why pellagra appeared where maize replaced more varied foods.
What is the Casal necklace?
The Casal necklace is a symmetrical, rough, hyperpigmented dermatitis encircling the neck in pellagra. It reflects photosensitive involvement of exposed skin and is the classic bedside sign that examiners expect candidates to name.
Which test points to carcinoid as a cause of pellagra?
Urinary 5-hydroxyindoleacetic acid points to carcinoid as a cause of pellagra. Carcinoid tumours divert tryptophan to serotonin and reduce niacin production, so a raised level in a patient with flushing and diarrhoea makes carcinoid a leading consideration.