Mastocytosis (cutaneous and systemic) — urticaria pigmentosa

Meet the patient

A 4-year-old is sent to clinic with "funny brown spots" on his trunk since infancy; his mother has noticed they go red and raised after a hot bath. A 42-year-old woman presents separately with new flushing, diarrhoea and similar brown macules that wheal when you stroke one. Same lesion, two patients, opposite exam questions.^[1][4]

The central viva question this whole page answers: is this child reassured and discharged, and is this adult sent for a bone marrow biopsy? The answer turns on age, tryptase, and one mutation.^[2][5]

One cell, one mutation, two compartments

Mast cells are loaded granule-bombs that live in your tissues; in mastocytosis there are too many of them and they detonate too easily. Normally they are long-lived, tissue-resident, SCF/KIT-dependent defenders that release histamine, tryptase, heparin, chymase and freshly-made prostaglandin D2 and leukotrienes on demand. In disease, their sheer mass plus a hair-trigger degranulation threshold turns a useful cell into a source of chronic symptoms and episodic danger.^[2]

The split is anatomical, not biological. Cutaneous mastocytosis (CM) is clonal mast cells confined to skin; systemic mastocytosis (SM) is the same clone in marrow and extracutaneous organs — gut, liver, spleen, nodes, skeleton. Children usually have CM only; most adults with skin lesions harbour occult SM. The same KIT mutation drives both, so the divider is burden and distribution, not different biology.^[2][6]

Two facts anchor the whole assessment. First, the lesion is unmistakable: reddish-brown to yellow-tan macules, papules or nodules that, stroked firmly, develop an urtical wheal, erythema and itch within minutes — the Darier sign, pathognomonic because lesional mast cells are mechanically fragile and degranulate on trivial trauma.^[1] Second, the symptoms are mediator-release, not infiltration: flushing, itch, cramps, diarrhoea, palpitations, syncope and anaphylaxis — all triggerable by heat, friction, exercise, alcohol, opioids, NSAIDs, iodinated contrast and Hymenoptera venom.^[9]

The mantra for the round: rub it — if it wheals, it is mast cells; a tryptase over 20 sends you to the marrow.^[5]

Four cutaneous faces, one systemic tree

Classify by where the clone lives; in adults, then stratify systemic disease by burden (B) and damage (C). The current framework is the 2016 WHO criteria harmonised with the 2022 International Consensus Classification and the ECNM or AIM proposals — the three have now converged for clinical use.^[6][10]

Cutaneous mastocytosis — the four faces

A cluster rule fixes them in memory: common, solitary, diffuse, telangiectatic.^[1][4]

^[1]

The classic trap: TMEP looks trivial and behaves systemic. A negative Darier sign never excludes mastocytosis — it just means the mast cell burden per lesion is low.^[1]

Systemic mastocytosis — the burden-to-damage ladder

Systemic disease is subtyped by how much clone is present (B-findings) and whether it has begun to harm organs (C-findings). C-findings are the trigger for cytoreductive drugs; without them, you observe.^[2][6]

^{[2] [6]}

The B and C markers themselves are worth memorising — they decide observe versus treat:^[2][6]

^[2]

Who gets it, and who is in danger

Mastocytosis is age-stratified, and the age line is the prognostic line. Children carry most cutaneous disease — about 65% of paediatric CM begins before age two — with an equal sex ratio and no racial predilection.^[1][4]

^[4]

The decisive divide is paediatric versus adult-onset. In children the disease is overwhelmingly cutaneous, systemic spread is uncommon, and progression to aggressive disease is rare.^[4] In adults the reverse holds: new-onset skin lesions almost always mean underlying SM, and adult disease is chronic and persistent rather than self-resolving.^[2][5] The strongest predictors of a bad outcome are an elevated serum tryptase, advanced age at onset, C-findings or an associated haematological neoplasm, and Hymenoptera venom allergy — the single most important provokable cause of life-threatening anaphylaxis in these patients.^[9] Familial clustering exists but is exceptional; most cases are sporadic.^[1]

One mutation rewrites the pharmacology — KIT D816V

The unifying molecular event in systemic disease is an activating somatic mutation in KIT, and the dominant one is D816V. KIT encodes the transmembrane receptor tyrosine kinase for stem cell factor (SCF). D816V substitutes valine for aspartate at codon 816 in the activation loop of the kinase domain, locking the receptor into constitutive, ligand-independent signalling.^[2][6] That single change removes the proliferative brake: clonal mast cells survive, divide and accumulate in marrow and extracutaneous tissue.

Normal mast cell development is SCF/KIT-dependent — CD34+ marrow progenitors commit, circulate, and finish differentiating in SCF-rich tissues (skin, gut mucosa, airway, marrow). This is exactly why an activating KIT mutation produces mast cell excess so efficiently: the brake is gone at the receptor itself.^[2]

Why D816V decides the drug

Imatinib does not work here — and that is the most testable fact in the disease. The D816V activation-loop conformation cannot be bound by imatinib, so the drug is useless against the common mutation and reserved only for the rare wild-type KIT or non-D816V disease (and a few KIT rearrangements).^[2][7] This is why modern cytoreduction for advanced SM is midostaurin (a multi-kinase inhibitor active against D816V) and avapritinib (a selective D816V inhibitor), both built to circumvent imatinib resistance.^[7]

Why one cell makes so many symptoms

The mast cells in this disease are not merely numerous — they are hypersensitive, degranulating to trivial triggers. Symptoms reflect the collective action of several mediators, not a tidy one-mediator-per-symptom map:^[8][9]

• Histamine (preformed) — H1 effects give flushing, pruritus, urticaria, bronchospasm; H2 effects give gastric acid hypersecretion, peptic ulceration, cramps, diarrhoea, and contribute to tachycardia and hypotension.
• Prostaglandin D2 (newly made) — potent vasodilator (flushing, headache, hypotension) and bronchoconstrictor (wheeze).
• Leukotrienes (LTC4, LTD4, LTE4) — bronchoconstriction, vascular leak, gut motility (wheeze, cramps, anaphylaxis).
• Heparin (preformed) — anticoagulant effect; bruising and bleeding tendency.
• Platelet-activating factor — amplifies permeability and bronchoconstriction in anaphylaxis.
• Tryptase (preformed) — released with histamine, but its job is diagnostic, not symptomatic: serum tryptase measures total-body mast cell burden, which is why it rises in systemic disease.^[8]

This multimediator basis is why combined H1 and H2 blockade is needed (histamine hits both receptor families, and half the symptoms are not histamine at all), and why leukotriene antagonists and mast cell stabilisers add incremental benefit in refractory mediator symptoms.^[1][9]

Why rubbing a lesion raises a wheal

The Darier sign is this biology made visible. Lesional dermis is packed with fragile, granule-laden mast cells; firm stroking ruptures them, dumping histamine and other vasoactive mediators locally. Within two to five minutes the overlying vessels dilate and leak — erythematous wheal and surrounding flare — and histamine fires local C-fibres to produce itch.^[1] The same mechanism explains why hot baths, clothing friction, exercise, alcohol, opioids and iodinated contrast provoke systemic flushing: each is a degranulation stimulus acting on an enlarged, hair-trigger mast cell population.^[9]

Etymology for viva gold: Darier sign honours Ferdinand-Jean Darier, the French dermatologist who described it in 1905. TMEP — telangiectasia macularis eruptiva perstans — is its own definition: persistent, eruptive, macular telangiectasia. The name is the lesion.^[1]

Rub it and watch — eliciting the Darier sign

There is no substitute for stroking a lesion at the bedside. A focused assessment answers three questions in order: (1) is this cutaneous mastocytosis? (2) could it be systemic? (3) what is the anaphylaxis risk?^[1][5]

The MPCM lesion is the most diagnostically powerful finding in the disease:^[1][4]

• Reddish-brown to yellow-tan macules and papules (occasionally small nodules), chiefly on trunk, thighs and extremities, sparing palms, soles and scalp.
• Discrete and scattered, or numerous and confluent; monomorphic and small in adults, polymorphic in children.
• A bedside clue: lesions blanch only partially on diascopy, and bullae may form in infants and in diffuse cutaneous disease from massive subepidermal mast cell load.^[1]

To elicit the sign: pick a representative trunk lesion, stroke it firmly (not traumatically) with the blunt end of a pen or a tongue depressor, and watch for two to five minutes. A positive Darier sign — urtical wheal with erythema and pruritus over the stroked lesion — is pathognomonic.^[1] Two cautions earn marks: stroke gently (vigorous rubbing can provoke a systemic mediator response in a heavily burdened patient), and remember the sign may be absent in TMEP — its absence does not exclude the diagnosis.

Triggerable, episodic mediator symptoms

The symptom complex is provoked by identifiable triggers — name them and you name half the management:^[9]

^[9]

Ask directly about the classic triggers — hot baths, sudden temperature change, vigorous exercise, emotional stress, alcohol, spicy foods, NSAIDs, opioids (morphine, codeine), iodinated contrast and Hymenoptera venom — because identifying and avoiding them is the cornerstone of management.^[1][9]

When the clone is in the organs

In systemic disease, organ-infiltration signs join the mediator complex:^[2][6]

• Gut — cramping, diarrhoea, nausea, vomiting, peptic ulcer disease from histamine-driven acid hypersecretion; malabsorption with weight loss in aggressive disease.
• Liver and spleen — hepatosplenomegaly; portal hypertension, ascites and transaminitis in aggressive disease.
• Lymph nodes — lymphadenopathy, often found on imaging.
• Skeleton — bone pain, osteoporosis, osteosclerosis, osteolyses; a pathological fracture may be the presentation, and unexplained osteoporosis in an adult warrants a tryptase.
• Blood — cytopenias (anaemia, leucopenia, thrombocytopenia) in aggressive disease or SM-AHN; circulating mast cells mean mast cell leukaemia.
• Neuropsychiatric — headache, fatigue, poor concentration, depression; under-recognised and disabling.^[1]

The classic trap: recurrent idiopathic anaphylaxis with a normal-looking skin exam may be the only clue to bone marrow mastocytosis — send tryptase and KIT D816V before labelling it idiopathic.^[9] Unexplained osteoporosis or a pathological fracture in a middle-aged adult may be indolent SM. Adult-onset cutaneous lesions almost always mean systemic disease and must be worked up. In pregnancy, plan the anaesthetic early; in infants with diffuse cutaneous disease, the risks are fluid loss and anaphylaxis.^[3]

The pigmented papule face-off

The pigmented, urticating papule has a finite differential, and each mimic has one discriminator. The logic is mechanical: ask first whether the lesions urticate when rubbed — only cutaneous mastocytosis does, so a positive Darier sign essentially settles it.^[1][4]

^[1]

Among Darier-negative pigmented lesions, sort next by shape: elevated and granuloma-like (juvenile xanthogranuloma, histiocytosis), flat and uniform from birth (cafe-au-lait, naevi), or transient and migratory (ordinary urticaria, papular urticaria). When doubt persists, a punch biopsy with Giemsa or tryptase stain resolves it by demonstrating the mast cell infiltrate.^[1]

Two biopsies, one number — skin then marrow, tryptase 20

The investigation strategy is two-tiered. Cutaneous disease is confirmed by skin biopsy; systemic disease is confirmed and sub-categorised by bone marrow biopsy with mutational and laboratory assessment.^[5][6]

Skin biopsy — confirms cutaneous disease

A punch biopsy of a representative lesion shows the diagnostic infiltrate:^[1]

• Increased mast cells in the papillary and upper dermis — perivascular and periadnexal aggregates, or sheets in dense lesions; cells cuboidal or spindle-shaped (atypical).
• Threshold for CM is a dense infiltrate (typically more than 15 mast cells per high-power field in an aggregate, or perivascular clusters); scattered cells are non-specific.
• Stains: metachromatic granules on Giemsa or toluidine blue; immunohistochemistry for tryptase, CD117 (KIT) and CD25 confirms lineage and the aberrant clonal phenotype (CD25 is the clonal marker).^[1]

Systemic workup — mandatory in adults

When CM is confirmed in an adult, or a child has high burden or mediator symptoms, the systemic screen is undertaken:^[2][5]

• Serum tryptase — the best non-invasive screen. A baseline value persistently above 20 ng/mL is a WHO minor criterion for SM and correlates with total-body mast cell burden; use it to monitor activity.^[6]
• Bone marrow aspirate and trephine — the gold standard. Shows the major criterion (multifocal dense mast-cell infiltrates, 15 or more in aggregate) and allows sub-categorisation; mast cells identified by tryptase/CD117/CD25 stain and often atypical (spindle or bilobed).^[10]
• KIT D816V mutation analysis — on marrow aspirate or peripheral blood (allele-specific PCR). A positive result is a WHO minor criterion and confirms clonality.^[6]
• Full blood count, film and metabolic panel — to detect cytopenias (C-finding), transaminitis and electrolyte disturbance.
• Skeletal survey and DEXA — in bone pain, high tryptase or advanced disease, for osteolyses, osteosclerosis or osteoporosis; DEXA is now recommended at baseline in systemic disease.^[2]
• Abdominal imaging (ultrasound or CT) — to define hepatosplenomegaly, lymphadenopathy, ascites or portal hypertension.
• Venom allergy testing — specific IgE and skin testing to Hymenoptera venom in any sting-anaphylaxis history, because venom allergy is the largest provokable cause of life-threatening anaphylaxis here and indicates venom immunotherapy.^[9]

The WHO diagnostic criteria for SM are examinable, so reproduce them. Diagnosis requires the major plus one minor, or three minor in the absence of the major:^[5][6]

^[6]

Stop the degranulation first — anaphylaxis is the death

Severe mast cell mediator release presents as anaphylaxis, and it kills — treat it as anaphylaxis without delay. It is rapid-onset flushing, urticaria, angioedema, bronchospasm, abdominal cramping, hypotension and syncope, most often triggered by a Hymenoptera sting, an opioid or iodinated contrast, or an NSAID in a sensitised patient.^[9]

The single most important anticipatory step is to make sure every patient with systemic mastocytosis — and anyone with a prior mediator-induced anaphylaxis — carries two adrenaline auto-injectors at all times and knows how to use them.^[9] Wherever possible, switch patients off beta-blockers: beta-blockade both increases mast cell mediator release and renders adrenaline ineffective in anaphylaxis.

Three layers, symptom-titrated — avoid, block, cytoreduce

Definitive management is three layers: trigger avoidance, anti-mediator pharmacotherapy, and — only for advanced systemic disease — cytoreduction. Cutaneous disease in children is mostly layers one and two, often nothing more than reassurance.^[1][5]

Layer 1 — trigger avoidance (universal)

Every patient gets structured counselling to avoid degranulation triggers:^[1][9]

• Heat and friction — no very hot baths or showers, sauna or hot tub; pat (do not rub) the skin dry.
• Medications — avoid NSAIDs (if previously reactive; challenge-test if essential), opioids (especially morphine and codeine; prefer fentanyl or pethidine if an opioid is unavoidable), iodinated contrast (premedicate if essential), and alcohol tinctures.
• Alcohol and spicy foods — common flushing triggers.
• Hymenoptera stings — insect-avoidance measures; carry adrenaline; venom immunotherapy if venom-allergic.
• Perioperative planning — alert anaesthesia, premedicate with antihistamines, avoid histamine-releasing agents, and monitor for intraoperative mediator crisis.^[1]

Layer 2 — anti-mediator pharmacotherapy (the standard ladder)

The pharmacological ladder attacks mediator symptoms across multiple receptor and mediator pathways. Therapy is titrated to symptoms, not to tryptase.^[1][5]

^{[1] [11]}

Cutaneous disease — topical and physical

For troublesome lesions resistant to systemic anti-mediator therapy, two modalities help:^[1]

• Topical corticosteroids — a very potent agent (e.g. clobetasol propionate 0.05%) under occlusion reduces lesional mast cell load and improves pruritus and pigmentation, especially in localised or paediatric disease; use time-limited courses to avoid atrophy.
• Phototherapy — PUVA or narrowband UVB over several weeks reduces lesional burden, pruritus and cosmetic pigment. Improvement is gratifying but temporary — relapse within months is typical, so reserve it for symptomatic disease, not cosmesis. NB-UVB is preferred in children for its safety profile.^[1]

Anaphylaxis prevention

• Adrenaline auto-injector — two devices carried at all times; teach patient and family. Essential for any systemic mastocytosis, any prior anaphylaxis, or extensive cutaneous disease in children.^[9]
• Venom immunotherapy — for Hymenoptera venom allergy with systemic mastocytosis, lifelong VIT dramatically cuts sting-anaphylaxis risk and is strongly recommended.^[9]
• Omalizumab (anti-IgE) — reduces recurrent anaphylaxis and refractory mediator symptoms in selected patients; off-label but increasingly used.^[8]
• MedicAlert identification and a written anaphylaxis action plan.^[9]

Layer 3 — cytoreductive therapy (advanced systemic disease only)

Cytoreduction is reserved for advanced SM — aggressive SM, SM-AHN and mast cell leukaemia — defined by C-findings. It is NOT used for indolent disease, where it adds toxicity without benefit.^[2][7]

^{[7] [11] [12]}

For SM-AHN, therapy targets the associated haematological neoplasm (azacitidine or decitabine for MDS, AML-type regimens), often combined with midostaurin for the mast cell component.^[2] Bone disease (osteoporosis, fractures) is managed with bisphosphonates or denosumab alongside calcium and vitamin D optimisation.^[2]

The named subtypes that earn extra marks

• Solitary mastocytoma — a single tan-yellow or reddish nodule in an infant, trunk or wrist, Darier positive, occasional blistering; almost uniformly self-resolving over years. Management is reassurance, trigger avoidance and symptomatic H1 antihistamines; excision is rarely needed.^[4]
• Diffuse cutaneous mastocytosis — the rarest and most severe cutaneous form, presenting in infancy with diffusely thickened, doughy skin, prominent folds and high burden. Blistering, erosions, fluid and electrolyte loss, and high anaphylaxis risk dominate; bullae are managed as burns. Prognosis is guarded in infancy but often improves with age.^[3]
• TMEP — adult-onset small telangiectatic, lightly pigmented trunk macules, minimal Darier sign; cutaneous burden looks trivial while systemic involvement is common, so tryptase and systemic assessment are mandatory in every case.^[1]
• Mast cell leukaemia — the most aggressive category, defined by at least 20% mast cells on marrow aspirate smears or circulating mast cells. Acute MCL carries C-findings and has a very poor prognosis (survival in months); chronic MCL lacks C-findings and runs a more indolent but still serious course. Manage with intensive cytoreduction and consider allogeneic transplant.^[2]
• Bone marrow mastocytosis and recurrent idiopathic anaphylaxis — a patient with recurrent anaphylaxis and a normal skin exam may have BMM. The key is to check tryptase and KIT D816V before labelling the episodes idiopathic; marrow biopsy confirms the clone. These patients need anti-mediator therapy, two adrenaline auto-injectors, and venom immunotherapy if a trigger is identified.^[9][10]

Complications, and where candidates go wrong

The complications are the complications of mediator release, and in advanced disease of organ infiltration.^[2][9]

• Anaphylaxis — the most feared complication, most often triggered by Hymenoptera venom in adults and carrying real mortality. Every systemic mastocytosis patient needs an adrenaline auto-injector and, if venom-allergic, venom immunotherapy.^[9]
• Peptic ulcer disease and GI bleeding — histamine-driven acid hypersecretion; prevent with H2 blockers and PPIs.
• Osteoporosis and pathological fractures — common and under-recognised in systemic disease; screen with DEXA and treat with bisphosphonates or denosumab.^[2]
• Malabsorption and weight loss — in aggressive disease with gut infiltration; a C-finding that prompts cytoreduction.
• Portal hypertension and ascites — in aggressive disease with liver or spleen infiltration.
• Cytopenias — in aggressive SM, SM-AHN and MCL.^[2]

The classic pitfalls deserve their own list:^[5][9]

• Mislabelling recurrent idiopathic anaphylaxis without checking tryptase for occult systemic mastocytosis.
• Missing adult-onset cutaneous mastocytosis as systemic disease by failing to do a systemic workup.
• Overlooking osteoporosis as the presenting feature of indolent systemic mastocytosis in a middle-aged adult.
• Perioperative mediator crisis from histamine-releasing anaesthetic agents (opioids, muscle relaxants) — avoid them and premedicate.
• Aggressive rubbing to elicit the Darier sign provoking systemic symptoms — stroke gently, test one lesion.
• Prescribing ranitidine — withdrawn in many markets for NDMA contamination; use famotidine or nizatidine.^[1]

Children usually win, adults usually do not (but live long)

Prognosis is sharply age-dependent and, in adults, burden-dependent.^[1][2]

• Childhood cutaneous mastocytosis — benign. Up to 50% resolve by puberty, and most of the rest improve substantially; progression to systemic or aggressive disease is rare.^[4]
• Adult-onset disease — chronic and persistent; cutaneous lesions rarely resolve. Indolent SM has a near-normal life expectancy, but the patient carries a lifelong anaphylaxis risk and needs monitoring.^[2]
• Smouldering SM — intermediate; a minority progress to aggressive disease or SM-AHN.
• Aggressive SM, SM-AHN and mast cell leukaemia — poor prognosis, with median survival from roughly two to four years for aggressive SM down to months for acute MCL, though midostaurin and avapritinib have improved outcomes.^[7]

^[2]

Disposition. A child with cutaneous mastocytosis is managed jointly by primary care and dermatology or paediatrics — reassurance, trigger advice, symptomatic antihistamines; a baseline tryptase is reasonable, marrow biopsy reserved for high-burden or atypical cases.^[4] An adult with cutaneous mastocytosis goes to haematology or allergy for systemic assessment (tryptase, KIT D816V, marrow biopsy if tryptase is elevated or symptoms suggest organ involvement).^[5] Anyone with prior anaphylaxis, high tryptase or Hymenoptera venom allergy is enrolled in an anaphylaxis-prevention programme — auto-injector, venom immunotherapy, MedicAlert.^[9]

The patient who is pregnant, venom-stung, or arriving for surgery

Children — weight-based dosing

Childhood cutaneous mastocytosis is the commonest scenario and is usually benign. Weight-based dosing applies to antihistamines and adrenaline: children use the lower-strength (0.15 mg) adrenaline auto-injector, stepping up to the 0.3 mg adult device as they reach adult weight, and antihistamines are dosed to body weight.^[3][4][14] Diffuse cutaneous mastocytosis and extensive blistering disease in infants warrant specialist paediatric dermatology input because of the fluid, electrolyte and anaphylaxis risks.

Counsel parents that the disease usually improves and often resolves, that the child should avoid known triggers and carry adrenaline if there has been any mediator event, and that school and childcare staff must be trained in auto-injector use. A MedicAlert is advisable for children with a history of anaphylaxis.^[4]

Pregnancy

Mastocytosis symptoms may intensify or, less often, remit in pregnancy. The principal risk is perioperative anaphylaxis during labour and delivery: a written anaesthetic plan, avoidance of histamine-releasing agents (prefer fentanyl or remifentanil over morphine; propofol-based induction), and adrenaline availability are essential. Mastocytosis is not a contraindication to pregnancy or vaginal delivery, and the disease does not cross the placenta. Adrenaline auto-injectors are safe in pregnancy and should be continued.^[9]

Adult-onset cutaneous disease

Approach adult-onset cutaneous mastocytosis as systemic until proven otherwise: serum tryptase, KIT D816V, and — if tryptase is elevated — a bone marrow biopsy are mandatory. Enrol the patient in long-term monitoring (annual tryptase, blood count, symptom review; DEXA at baseline).^[2][5]

SM-AHN and the immunocompromised

Patients with SM-AHN are managed jointly with haematology, with therapy directed at the associated neoplasm (MDS, MPN, AML) combined with midostaurin for the mast cell component. Immunosuppression for comorbid disease does not by itself worsen mastocytosis but complicates cytoreductive choices.^[2]

Regional — same biology, different bottle

The diagnostic and therapeutic framework has been progressively refined by the WHO (2001, 2008, 2016), the International Consensus Classification (2022), and the ECNM and AIM harmonisation proposals, which have converged the WHO and ICC criteria for clinical use.^[6][10]

The landmark therapy trial is the Gotlib et al NEJM 2016 study of midostaurin in advanced SM, which showed clinically meaningful responses (overall response around 60%, including organ-damage responses) and established midostaurin as the standard cytoreductive agent for advanced disease.^[7] Avapritinib extended effective targeted inhibition to D816V-driven disease in both advanced and selected indolent or smouldering settings. The ECNM diagnostic algorithm guides the workup and the decision to biopsy marrow.^[5]

^{[5] [11] [1] [1] [1]}

Controversies. The boundary between indolent systemic mastocytosis and mast cell activation syndrome (MCAS) is debated — MCAS describes recurrent mediator symptoms with a clonal mast cell abnormality that does not meet full SM criteria and a response to anti-mediator therapy.^[8] The roles of omalizumab and phototherapy rest on observational rather than randomised data; ketotifen evidence is largely historical. The tryptase threshold for marrow biopsy is conventionally 20 ng/mL, but this is calibrated to the SM population and may over-investigate patients with isolated anaphylaxis.^[5]

High-yield points for the exam

^[9]

Ward-round test

Stem 1. A 4-year-old has reddish-brown trunk macules present since infancy that wheal when rubbed. He is otherwise well, and his baseline tryptase is 8 ng/mL. What is the diagnosis, the prognosis, and the single safety instruction to the parents?^[4]

AnswerShowHide

Cutaneous mastocytosis (maculopapular CM, urticaria pigmentosa) with a positive Darier sign and a normal tryptase — the typical benign paediatric presentation. Prognosis is excellent: up to 50% resolve by puberty and most of the rest improve. Safety instruction: avoid known triggers (heat, friction), and carry an adrenaline auto-injector only if there has been any mediator event; train school and childcare staff in its use.^[4]

Stem 2. A 42-year-old woman has new-onset flushing, diarrhoea and reddish-brown trunk macules that wheal when rubbed. Tryptase is 38 ng/mL. What is the diagnosis, and what is the next step?^[1]

AnswerShowHide

Systemic mastocytosis: adult-onset cutaneous lesions with a positive Darier sign, mediator symptoms, and a serum tryptase persistently above 20 ng/mL (a WHO minor criterion). Next step is bone marrow aspirate and trephine biopsy with KIT D816V mutation analysis, plus full blood count, biochemistry and DEXA. Start trigger avoidance plus combined H1 and H2 antihistamine therapy and prescribe an adrenaline auto-injector; arrange Hymenoptera venom testing. Cytoreduction is NOT indicated unless C-findings are present.^[1][6][11]

Stem 3. A 55-year-old beekeeper with known systemic mastocytosis is stung and collapses with flushing, wheeze and hypotension. What is the first drug, its route, and the essential follow-up step?^[9]

AnswerShowHide

IM adrenaline into the anterolateral thigh immediately — it is the first-line drug in anaphylaxis — repeated if there is no response. The accepted weight-based dose is 0.01 mg/kg; auto-injectors come in fixed strengths up to 0.5 mg for heavy adults, matched to body weight. After recovery, refer to allergy: venom immunotherapy is mandatory in a sting-allergic mastocytosis patient.^[13][14][11]

Stem 4. A 60-year-old presents with recurrent idiopathic anaphylaxis and a completely normal skin examination. Tryptase is 34 ng/mL. What is the likely diagnosis, and what test confirms it?^[9]

AnswerShowHide

Bone marrow mastocytosis — marrow involvement without skin lesions and low burden, a classic cause of recurrent idiopathic anaphylaxis with a normal skin exam. Confirmation is bone marrow biopsy with KIT D816V analysis demonstrating the clonal mast cell population. Do not label anaphylaxis idiopathic until tryptase and KIT D816V have been checked.^[9][10]

Stem 5. A patient with aggressive systemic mastocytosis and KIT D816V is referred for cytoreductive therapy. The haematology team proposes imatinib. What do you say, and what do you offer instead?^[7]

AnswerShowHide

Imatinib does not inhibit the D816V-mutant kinase — it has a therapeutic role only in the rare patient with an imatinib-sensitive KIT mutation. Offer midostaurin 100 mg PO BD (multi-kinase, active against D816V; established by the Gotlib NEJM 2016 trial) or avapritinib (potent selective KIT D816V inhibitor) instead.^[7][12]

Stem 6. A 30-year-old with systemic mastocytosis wants to start a family and asks whether her disease is a contraindication to vaginal delivery. What is the key perioperative consideration, and which drugs do you avoid in labour?^[9]

AnswerShowHide

Mastocytosis is not a contraindication to pregnancy or vaginal delivery, and the disease does not cross the placenta; continue her adrenaline auto-injectors (safe in pregnancy). The key risk is perioperative anaphylaxis, so agree a written anaesthetic plan that avoids histamine-releasing agents — prefer fentanyl or remifentanil over morphine, and propofol-based induction — with adrenaline drawn up and ready.^[9]

References14ShowHide

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13. [13]Cardona V, Ansotegui IJ, Ebisawa M, et al. World allergy organization anaphylaxis guidance 2020 World Allergy Organ J, 2020.PMID 33204386
14. [14]Dreborg S, Walter G, Kim H International recommendations on epinephrine auto-injector doses often differ from standard weight-based guidance: a review and clinical proposals Allergy Asthma Clin Immunol, 2022.PMID 36471385