01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Acanthosis nigricans describes a reaction pattern rather than one disease. The skin becomes thick, soft or papillomatous and grey-brown to dark brown, classically across the back and sides of the neck, axillae, groins and other folds. Skin tags often coexist. Similar colour without texture can reflect friction or post-inflammatory change; palpation and comparison with less affected skin reduce overdiagnosis. The finding can occur at any body size and should trigger a measured history rather than an assumption about behaviour.
High circulating insulin can activate insulin-like growth pathways in keratinocytes and fibroblasts, linking the common phenotype to insulin resistance. Diabetes risk, adiposity, PCOS, family tendency and selected medicines are relevant. Rare paraneoplastic acanthosis is usually abrupt, extensive and symptomatic and may involve mouth or palms. Its rarity argues against indiscriminate cancer imaging in a young person with a stable metabolic pattern, while its clinical severity argues against ignoring a convincing red-flag presentation.
The consultation should address both the medical association and the visible skin. Ask what the person wants changed, offer evidence-based diabetes and cardiovascular risk assessment, and provide practical fold and friction care. Treating insulin resistance improves health whether or not pigmentation clears; cosmetic response must not be used as a surrogate for HbA1c or metabolic success.
Key points
- Acanthosis nigricans is velvety thickened hyperpigmented skin, usually on the posterior neck, axillae or groins; texture and distribution are more diagnostic than colour alone.
- Insulin resistance associated with higher adiposity, type 2 diabetes or polycystic ovary syndrome is the commonest acquired context, but the sign is not itself a glucose diagnosis.
- Examine respectfully in good light and ask permission before folds are exposed; in dark skin the change may be prominent, but baseline pigmentation must not be pathologised.
- Check blood pressure and metabolic risk, then use HbA1c or glucose testing through the appropriate diabetes pathway rather than assuming normality or disease from appearance.
- Irregular cycles, hirsutism or fertility concerns support a PCOS assessment, while hypothyroid, Cushing or medicine testing should follow specific clinical clues.
- Sudden extensive itchy disease, mucosal papillomatosis, tripe palms and unexplained weight loss are uncommon but important malignant-acanthosis features.
- Treating the driver may soften and lighten skin gradually; weight-management support should be person-centred and never framed as blame or a guaranteed cosmetic cure.
- Keratolytics and retinoids offer inconsistent cosmetic benefit and can irritate folds, so underlying risk reduction and realistic expectations remain central.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Insulin resistance
Hyperinsulinaemia associated with adiposity, type 2 diabetes or PCOS is the commonest acquired driver of the characteristic flexural skin response.
Familial or syndromic disease
Stable childhood or adolescent acanthosis can reflect inherited susceptibility or a rarer genetic insulin-resistance syndrome, especially when severe at low adiposity.
Medicine-associated change
Systemic glucocorticoids, high-dose nicotinic acid, insulin and selected hormonal medicines may provoke or amplify acanthosis through metabolic or growth signalling.
Paraneoplastic signalling
Rare internal tumours release growth factors that produce abrupt extensive acanthosis, often with severe itch, mucosal papillomatosis and palmar thickening.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Growth receptors are activated
High insulin or tumour-derived growth signals stimulate keratinocyte and dermal fibroblast receptors, increasing epidermal thickness and papillomatosis.
- 2Skin markings deepen
Hyperkeratosis and papillomatosis accentuate natural flexural lines and shadowing, while increased pigment contributes variably to the visible colour.
- 3Friction amplifies texture
Occlusion and rubbing in folds can thicken an established plaque, although mechanical friction alone does not explain every metabolic or malignant case.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Symmetrical thickened skin with accentuated markings over posterior or lateral neck is the commonest readily visible presentation.
Axillary, groin, inframammary, elbow or knuckle change and accompanying acrochordons support the reaction pattern when texture is convincing.
Central adiposity, hypertension, dysglycaemia, irregular cycles or hirsutism makes insulin resistance and PCOS clinically relevant without proving either diagnosis.
Onset after systemic glucocorticoids, high-dose nicotinic acid, insulin or selected hormonal treatment warrants a benefit-risk review with the prescriber.
Rapid widespread itch, mucosal papillomatosis, tripe-like palms and weight loss in an older person demand expedited systemic assessment.
Retained keratin or terra-firma-forme dermatosis may wipe away with alcohol, while ordinary dirt should never be assumed from skin colour.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
HbA1c or fasting plasma glucoseFirst step - Why
- Detect prediabetes or type 2 diabetes in a metabolically plausible phenotype.
- Interpretation and limitations
- Use diagnostic thresholds and repeat confirmation rules from diabetes guidance; an isolated normal result does not erase future risk or justify fasting insulin measurement.
- 02
Blood pressure, lipids and adiposity assessment - Why
- Measure associated cardiovascular risk and guide preventive treatment.
- Interpretation and limitations
- Use appropriate cuff and person-centred BMI or waist discussion; each risk factor has its own threshold and should not be inferred from neck appearance.
- 03
PCOS-focused assessment - Why
- Investigate ovulatory and androgen excess when cycles, hair or fertility history supports it.
- Interpretation and limitations
- Exclude pregnancy and selected endocrine mimics before applying PCOS criteria; ultrasound is not automatically necessary and acanthosis alone is insufficient.
- 04
Targeted endocrine tests - Why
- Evaluate thyroid disease, Cushing syndrome or another hormonal driver when discriminatory features exist.
- Interpretation and limitations
- Select TSH or validated cortisol testing from clinical evidence; random cortisol and broad hormone panels generate misleading incidental results.
- 05
Skin biopsy - Why
- Confirm an atypical keratotic or papillomatous eruption when morphology remains uncertain.
- Interpretation and limitations
- Histology can support the reaction pattern but does not identify its systemic cause, so it cannot replace metabolic and red-flag assessment.
- 06
Urgent malignancy evaluation - Why
- Identify an internal cancer when the sudden extensive paraneoplastic phenotype is present.
- Interpretation and limitations
- Investigation is symptom-, age- and examination-directed through an urgent specialist pathway; a routine tumour-marker panel is not an adequate screen.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Frictional hyperpigmentation
Colour follows rubbing or pressure without the characteristic velvety papillomatous surface and may improve when the mechanical driver stops.
Terra-firma-forme dermatosis
Brown retained keratin can resist soap yet wipe away with isopropyl alcohol, avoiding unnecessary endocrine investigation when morphology fits.
Confluent papillomatosis
Scaly reticulate papules over central trunk coalesce differently from the smooth velvety neck and fold plaques of acanthosis.
Epidermal naevus
A longstanding unilateral linear or Blaschkoid papillomatous plaque suggests mosaic epidermal change rather than a systemic insulin-resistance marker.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01Typical metabolic patternMeasure risk without making assumptionsFirst stepStable flexural acanthosis occurs with plausible insulin resistance and no malignant warning features.+
- 1Confirm velvety thickening by examination, document onset and family tendency, and ask about cycles, hair change, medicines, sleep and symptoms of hyperglycaemia.
- 2Measure blood pressure and arrange HbA1c or fasting glucose plus lipids and other cardiovascular assessment according to age and risk.
- 3Offer person-centred nutrition, activity and weight support where wanted, and treat diagnosed diabetes, hypertension, dyslipidaemia or PCOS through their own evidence-based pathways.
02Atypical or suddenExclude serious acquired driversDisease is abrupt, rapidly spreading, intensely itchy, mucosal or associated with weight loss or tripe palms.+
- 1Take a full systemic review, medicine and smoking history and examine mouth, palms, nodes, abdomen and other clinically indicated systems.
- 2Arrange expedited dermatology and medical assessment, using age- and symptom-directed imaging or endoscopy rather than relying on nonspecific tumour markers.
- 3Continue investigation even if a metabolic risk factor coexists, because common insulin resistance does not neutralise a strong paraneoplastic phenotype.
03Skin symptom careReduce friction and treatment injuryTexture, itch, odour or appearance causes symptoms or distress after systemic assessment.+
- 1Use gentle washing, dry folds carefully and reduce avoidable friction without implying that ordinary hygiene caused the pigmentation.
- 2Consider a cautious trial of a low-strength keratolytic or dermatologist-selected topical retinoid, starting infrequently and stopping inflammation.
- 3Review photographs and comfort after several months, explaining that treatment of the underlying driver may improve texture without restoring an exact previous colour.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Undiagnosed dysglycaemia
Failure to assess the common metabolic phenotype can delay type 2 diabetes diagnosis and cardiovascular risk reduction.
Missed internal malignancy
Dismissing an abrupt extensive mucosal phenotype as ordinary metabolic disease can postpone diagnosis of an underlying gastrointestinal or other cancer.
Fold discomfort
Thickened occluded skin may itch, macerate, smell or become secondarily inflamed, particularly when friction and sweating are substantial.
Stigma and treatment injury
Blame, forceful scrubbing and caustic lighteners cause shame, irritant dermatitis and additional post-inflammatory pigmentation without treating the driver.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- Repeat glycaemic assessment at the interval determined by baseline result and diabetes risk rather than waiting for skin to change.
- Track blood pressure, lipids and diagnosed metabolic conditions independently of pigmentation response.
- Review menstrual pattern, fertility goals and androgen symptoms when PCOS is suspected or established.
- Re-examine if acanthosis accelerates, becomes pruritic, reaches mucosa or palms, or is accompanied by unintentional weight loss.
- Stop cosmetic products that cause burning, fissuring or new post-inflammatory pigmentation in folds.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Texture carries the diagnosis
Hyperpigmentation alone is nonspecific; palpable velvety thickening and a characteristic flexural distribution identify acanthosis more reliably.
The sign is not the test
Acanthosis raises the probability of insulin resistance, but HbA1c or glucose establishes dysglycaemia and guides treatment.
Common and serious can coexist
An older adult may have obesity and a malignancy, so abrupt mucosal or palmar progression should not be dismissed as metabolic.
Pigment change lags biology
Improved glycaemia or weight-related health may precede visible softening by months, and some pigmentation persists despite excellent metabolic control.
Respect improves detection
Permission, privacy and neutral language make full fold examination more acceptable and reduce missed extent or a wrongly attributed cause.
11Common pitfallsFrequent interpretation and management errors.
- 01
Diagnosing diabetes solely from a dark neck without biochemical testing.
- 02
Equating deeply pigmented normal skin or retained keratin with acanthosis nigricans.
- 03
Using stigmatising language or presenting body weight as a moral cause.
- 04
Ordering broad cancer imaging for every stable young patient while missing the genuinely abrupt malignant phenotype.
- 05
Treating colour cosmetically and failing to measure blood pressure, glucose and cardiovascular risk.
- 06
Assuming improved pigmentation proves that diabetes or PCOS has resolved.