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Summary Table

Category	Details
Risk Factors	Chronic alcoholism, corn-based diet (maize), carcinoid syndrome, Hartnup disease, isoniazid therapy without pyridoxine supplementation, malabsorption syndromes, anorexia nervosa
Etiology	Deficiency of niacin (nicotinic acid) or its precursor tryptophan, leading to impaired NAD/NADH and NADP/NADPH synthesis
Patient Presentation	The classic triad: diarrhea, dermatitis, dementia ("the 3 Ds"), progressing to death ("the 4th D") if untreated
Classic Physical Exam	Bilateral, symmetric, photosensitive dermatitis in sun-exposed areas; Casal necklace (hyperpigmented scaling rash around the neck); glossitis with a beefy red tongue
Key Diagnostic Results	Clinical diagnosis primarily; low urinary N1-methylnicotinamide levels; decreased NAD/NADP in erythrocytes
Management	Nicotinamide (niacinamide) 100 mg orally three times daily; treat underlying cause
Keywords	"Casal necklace," "photosensitive dermatitis," "3 Ds," "corn-based diet," "carcinoid syndrome," "isoniazid use," "Hartnup disease"

1. Pathophysiology

Niacin (vitamin B3) serves as the precursor for two essential coenzymes: NAD (nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate). These coenzymes function as electron carriers in over 400 enzymatic reactions across the body, including glycolysis, the citric acid cycle, oxidative phosphorylation, fatty acid synthesis, and DNA repair pathways. When niacin is deficient, cellular energy production fails broadly, but tissues with high turnover rates and high metabolic demands are affected first.

The body can synthesize niacin endogenously from tryptophan through the kynurenine pathway. Approximately 60 mg of tryptophan yields 1 mg of niacin, and this conversion requires adequate vitamin B6, riboflavin (B2), and iron as cofactors. This is why conditions that deplete tryptophan or block its conversion also produce pellagra. Carcinoid syndrome diverts tryptophan toward serotonin synthesis, leaving insufficient substrate for niacin production. Hartnup disease, an autosomal recessive disorder, impairs neutral amino acid absorption in the intestine and renal tubules, reducing tryptophan availability. Isoniazid depletes pyridoxine (vitamin B6), which is a required cofactor in the tryptophan-to-niacin conversion pathway.

The dermatitis occurs because skin cells exposed to UV radiation sustain DNA damage that requires poly-ADP-ribose polymerase (PARP), an NAD-dependent DNA repair enzyme. Without adequate NAD, UV-damaged keratinocytes cannot repair and undergo apoptosis, producing the characteristic photosensitive, bilateral, symmetric rash in sun-exposed areas. The diarrhea results from mucosal atrophy and inflammation of the gastrointestinal tract, since enterocytes are among the most rapidly dividing cells in the body. The dementia reflects neuronal dysfunction from impaired energy metabolism and neurotransmitter synthesis in the central nervous system, as NAD is also involved in serotonin production from tryptophan.

The reason a corn-based diet causes pellagra is twofold. First, the niacin in corn exists in a bound form (niacytin) that is not bioavailable unless treated with alkali (as in the traditional nixtamalization process used in Mesoamerican cultures). Second, corn is low in tryptophan, eliminating the alternative biosynthetic route. This explains why pellagra was historically endemic in populations relying on untreated maize as a dietary staple.

2. Classification and Clinical Manifestation

System	Manifestation	Details
Dermatologic	Photosensitive dermatitis	Bilateral, symmetric distribution on sun-exposed areas (dorsum of hands, forearms, face, neck, upper chest); initially resembles sunburn, then progresses to hyperpigmented, thickened, scaling plaques
Dermatologic	Casal necklace	Broad band of hyperpigmented, scaling dermatitis encircling the neck sharply along the collar line; pathognomonic finding
Dermatologic	Glossitis and stomatitis	Beefy red, swollen tongue; angular cheilitis; oral mucosal erythema
Gastrointestinal	Diarrhea	Watery, sometimes bloody diarrhea from mucosal inflammation and villous atrophy; nausea and vomiting
Neuropsychiatric	Early: irritability, insomnia, poor concentration	Subtle cognitive changes that may be mistaken for depression or anxiety
Neuropsychiatric	Late: dementia, psychosis, delirium	Confusion, hallucinations, memory loss, eventually encephalopathy; can mimic other causes of organic brain syndrome
Neuropsychiatric	Peripheral neuropathy	Less common; may present with paresthesias

The clinical progression typically follows the sequence of dermatitis first, then diarrhea, then dementia, though not all three need to be present for diagnosis. In chronic alcoholics, the neuropsychiatric features may predominate and overlap with Wernicke encephalopathy (thiamine deficiency), making differentiation important.

3. Diagnostic Workup

Test	Role	Expected Finding
Clinical assessment	Best initial approach	Recognition of the classic triad plus risk factor identification
Urinary N1-methylnicotinamide	Most commonly used biochemical marker	Low levels (below 5.8 micromol/day) confirm deficiency
Urinary 2-pyridone	Supporting marker	Low excretion
Serum NAD/NADP ratio in erythrocytes	Confirmatory when available	Decreased ratio indicates depleted niacin stores
Therapeutic trial	Practical confirmatory approach	Rapid clinical improvement within 24 to 48 hours of niacin supplementation strongly supports the diagnosis
Skin biopsy	Rarely needed	Nonspecific findings of hyperkeratosis, dermal inflammation, and epidermal atrophy

Pellagra is fundamentally a clinical diagnosis. The best initial step is recognizing the pattern: a patient with a known risk factor (alcoholism, corn-dependent diet, carcinoid syndrome, isoniazid therapy, or Hartnup disease) presenting with some combination of photosensitive dermatitis, diarrhea, and neuropsychiatric symptoms.

Laboratory confirmation is often pursued but not always necessary in a classic presentation. The most practical biochemical test is measuring urinary N1-methylnicotinamide, a downstream metabolite of niacin. A 24-hour urine collection showing levels below 5.8 micromol/day supports deficiency. However, this test is not universally available, and results take time.

In clinical practice and on exams, the therapeutic trial functions as the most practical confirmatory method. Niacin supplementation produces dramatic improvement in dermatitis and diarrhea within 24 to 48 hours and neuropsychiatric symptoms within days to weeks. This rapid response is itself diagnostic and is a commonly tested concept.

When the dermatitis is the dominant feature and the clinical picture is ambiguous, a skin biopsy may be performed to exclude other photosensitive dermatoses (such as systemic lupus erythematosus or porphyria cutanea tarda). However, the histologic findings in pellagra are nonspecific and not diagnostic in isolation.

One important workup consideration: because pellagra often occurs in the context of global malnutrition or alcoholism, always evaluate for concurrent deficiencies. Thiamine (B1), riboflavin (B2), pyridoxine (B6), folate, and zinc deficiencies frequently coexist and require simultaneous treatment.

4. Management and Treatment

Setting	Treatment	Details
First-line therapy	Nicotinamide (niacinamide)	100 mg orally three times daily for several weeks
Alternative	Nicotinic acid (niacin)	Same dosing, but causes flushing; nicotinamide preferred because it lacks vasodilatory side effects
Severe or oral-intolerant	Parenteral nicotinamide	50 to 100 mg intramuscularly or intravenously if patient cannot take oral medication
Adjunctive	Multivitamin supplementation	B-complex vitamins, especially pyridoxine and riboflavin, to support tryptophan-to-niacin conversion
Adjunctive	High-protein diet	Tryptophan-rich foods (meat, fish, eggs, dairy) to restore the endogenous synthesis pathway
Cause-directed	Treat underlying etiology	Alcohol cessation; add pyridoxine (25 to 50 mg/day) to isoniazid regimen; manage carcinoid tumor; dietary modification

Nicotinamide is preferred over nicotinic acid for treatment because it does not cause the prostaglandin-mediated flushing reaction (facial warmth, redness, pruritus) that nicotinic acid produces. Both forms are equally effective at replenishing NAD and NADP stores, but nicotinamide is better tolerated. The standard dose is 100 mg three times daily, continued for three to four weeks or until clinical resolution.

For patients who cannot tolerate oral intake due to severe vomiting or gastrointestinal disease, parenteral nicotinamide at 50 to 100 mg can be administered intramuscularly or intravenously.

The dermatitis responds fastest, often showing improvement within 24 to 48 hours. Diarrhea typically resolves within days. Neuropsychiatric symptoms, particularly dementia, improve more slowly and may take weeks. Long-standing neurologic damage may be only partially reversible, which underscores the importance of early recognition.

Addressing the underlying cause is essential for long-term management. In patients on isoniazid, prophylactic pyridoxine (25 to 50 mg daily) should always be co-administered to prevent niacin depletion through the tryptophan pathway. In carcinoid syndrome, the serotonin-producing tumor must be managed (somatostatin analogs such as octreotide reduce tryptophan diversion). In patients with Hartnup disease, long-term niacin supplementation and a high-protein diet are the cornerstones of prevention.

A key distinction for exam purposes: nicotinic acid at pharmacologic doses (1 to 3 g/day) is used as a lipid-lowering agent to raise HDL and lower triglycerides, which is a completely different clinical context from treating pellagra. The lipid-lowering dose is associated with hepatotoxicity, hyperglycemia, hyperuricemia, and flushing. Do not confuse the therapeutic replacement dose with the pharmacologic lipid dose.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Zinc deficiency (acrodermatitis enteropathica)	Dermatitis affecting extremities and perioral/perianal areas; diarrhea	Zinc deficiency rash is perioral, perianal, and acral (not photosensitive and not in sun-exposed distribution); no Casal necklace; alopecia is prominent
Systemic lupus erythematosus	Photosensitive facial rash	SLE produces a malar (butterfly) rash sparing nasolabial folds; bilateral symmetric but limited to face; associated with joint pain, serositis, and positive ANA/anti-dsDNA
Porphyria cutanea tarda	Photosensitive skin lesions, blistering on sun-exposed areas	PCT presents with blistering and skin fragility (vesicles and bullae) on dorsal hands, not scaling/hyperpigmented plaques; associated with elevated urine porphyrins and iron overload
Thiamine (B1) deficiency (Wernicke encephalopathy)	Neuropsychiatric symptoms in an alcoholic patient	Wernicke triad is confusion, ophthalmoplegia, and ataxia; no photosensitive dermatitis; no diarrhea
Riboflavin (B2) deficiency	Glossitis, angular cheilitis, oral mucosal changes	B2 deficiency produces cheilosis and corneal vascularization; no photosensitive dermatitis; no dementia
Kwashiorkor	Dermatitis, edema, diarrhea in malnourished patient	Kwashiorkor shows flaky paint dermatitis (desquamating, non-photosensitive), edema, fatty liver, and depigmented hair; a protein-energy malnutrition picture rather than vitamin deficiency
Hartnup disease	Pellagra-like rash and symptoms	Hartnup is actually a cause of pellagra (not a mimic); the key is recognizing the autosomal recessive inheritance, onset in childhood, and neutral aminoaciduria on urine amino acid analysis
Drug reaction (phototoxic)	Photosensitive dermatitis on sun-exposed areas	Drug-induced phototoxicity has a temporal relationship to medication initiation (e.g., tetracyclines, fluoroquinolones); no diarrhea or dementia; resolves with drug withdrawal

6. Traps and High-Yield Pearls

The most common way students miss pellagra questions is by failing to connect the underlying cause to the deficiency. A vignette will rarely hand you a malnourished patient on a corn diet. Instead, it will present an alcoholic patient with a photosensitive rash and confusion, a tuberculosis patient on isoniazid who develops diarrhea and dermatitis, or a patient with flushing episodes (carcinoid) who later develops a neck rash and cognitive decline. The core competency being tested is your ability to trace the pathophysiologic link from the risk factor to tryptophan/niacin depletion.

Another frequent trap involves confusing pellagra with Wernicke encephalopathy in the alcoholic patient. Both present with neuropsychiatric changes in a malnourished alcoholic. The discriminator is the photosensitive dermatitis and diarrhea in pellagra versus the ophthalmoplegia and ataxia in Wernicke. If the vignette describes a rash, the answer is pellagra. If it describes eye movement abnormalities, think thiamine.

Students also confuse the treatment dose of niacin (100 mg three times daily for pellagra) with the pharmacologic lipid-lowering dose (1 to 3 g/day). A question may ask about side effects: flushing, hepatotoxicity, and hyperglycemia are associated with the high lipid-lowering dose of nicotinic acid, not with the replacement dose of nicotinamide used for pellagra.

Remember that Casal necklace is pathognomonic. If any vignette describes a hyperpigmented, scaling rash distributed around the neck in a collar-like pattern, the answer is pellagra regardless of what other information is presented.

Finally, always recall the tryptophan-to-niacin conversion requires B6 as a cofactor. This is why isoniazid (which depletes B6) causes pellagra indirectly, and why the prevention strategy is to supplement pyridoxine alongside isoniazid, not to supplement niacin directly.

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