Ichthyosis vulgaris, X-linked ichthyosis, and congenital ichthyoses

Meet the patient

A 26-year-old man is referred for dry skin that has troubled him since infancy. Fine white scales carpet his shins and forearms, worse every winter, yet his elbows and the backs of his knees are conspicuously clear. His palms are deeply creased, and he reels off a childhood of eczema and then asthma.^[2][13]

The next cubicle holds a boy of six whose calves carry large, dark, firmly stuck scales that climb into his popliteal fossae. His mother adds, almost in passing, that she needed a caesarean section for failure to progress in labour. That is the second disease on this page. Hold both patients, because every paragraph below is the machinery to tell them apart.^[6][1]

What ichthyosis is — and the one split that earns marks

Ichthyosis is a disorder of cornification: corneocytes are produced faster than they can desquamate, so they accumulate as visible scale. Sort the family once, at the bedside, and the rest of the viva falls into place:^[1][3]

• Common inherited — ichthyosis vulgaris (FLG) and X-linked recessive ichthyosis (STS). These two are the staples, and both are tested as one-line answers.
• Rare congenital — harlequin, lamellar, congenital ichthyosiform erythroderma, epidermolytic, Netherton, plus the syndromic forms. These declare themselves at birth and carry systemic disease.
• Acquired — adult-onset scaling that was never there in childhood. This one hunts a malignancy.^[5]

The classic trap: lump every scaly child as eczema and you miss X-linked ichthyosis hiding in the flexures; dismiss new adult scaling as dry skin and you miss Hodgkin lymphoma. The flexure sign sorts the children; the age of onset sorts the adults.^[1][5]

Etymology for viva gold: ichthys is Greek for fish — the scale of ichthyosis vulgaris was likened to fish scales two thousand years before anyone sequenced FLG. The word survived because the morphology never changed.^[1]

How common, who, and which gene — the numbers you own

Prevalence is one of the few things in genodermatoses that examiners quote as single figures, so learn them cold.^[1][2]

^[1]

The why behind the numbers. Ichthyosis vulgaris is common because one FLG copy merely gives mild dryness and the atopic march — natural selection barely saw it, so it drifted to roughly one in 250 and a carrier frequency near 10 per cent in Europeans. X-linked ichthyosis is fully penetrant in hemizygous males with no male-to-male transmission; carrier females stay silent through lyonisation. The congenital forms are individually rare but collectively the ones that present in the neonatal unit.^[2][6]

Pathophysiology — one mechanism per gene

The beauty of the ichthyoses is that each gene maps to one rate-limiting step in building the stratum corneum, and the step predicts the morphology. Learn the chain, not the list — cause, then the stuck intermediate, then the visible scale.^[1][3]

Ichthyosis vulgaris — the barrier that lost its mortar

FLG (1q21.3) encodes profilaggrin, processed to filaggrin, which bundles keratin into the cornified envelope and then breaks down into natural moisturising factor — the humectant amino acids that hold water in the stratum corneum. Lose FLG and the barrier leaks, water escapes, and scale forms. The same leak lets allergens through, which is why FLG is the strongest single genetic risk factor for atopic dermatitis and the engine of the atopic march — eczema, then asthma, then allergic rhinitis.^[2][13]

X-linked ichthyosis — cholesterol sulfate that will not let go

STS (Xp22.31) encodes steroid sulfatase, the enzyme that strips the sulfate from cholesterol sulfate. Without it, cholesterol sulfate accumulates and brakes the serine proteases and transglutaminases that drive desquamation, so corneocytes cling on as large dark scales. Roughly nine in ten cases are a complete STS deletion, often dragging neighbouring genes with it — hence the contiguous gene syndromes: KAL1 (Kallmann — anosmia plus hypogonadotrophic hypogonadism), ARSE (chondrodysplasia punctata), and VCX3A (intellectual disability).^[6][3]

The confession every consultant makes: the placenta needs steroid sulfatase to make oestriol, so a carrier mother carrying an affected fetus returns low or absent unconjugated oestriol on the triple or quad test and often fails to progress in labour — a perinatal clue worth a mark and the reason to flag a planned caesarean.^[6]

Harlequin ichthyosis — the lipid lamellae never form

ABCA12 (2q34) is the lipid transporter that loads glucosylceramides into lamellar granules for the extracellular lipid mortar of the stratum corneum. Knock it out and the mortar is absent, so the skin responds with massive hyperkeratotic plates and total barrier failure. The genotype-phenotype link is loose: hypomorphic missense ABCA12 alleles produce the milder congenital ichthyosiform erythroderma rather than the full harlequin picture, which is why two children with ABCA12 variants can look utterly different.^[10][18]

Netherton syndrome — the protease brake is off

SPINK5 (5q33.1) encodes LEKTI, the serine-protease inhibitor that restrains kallikrein 5 and 7. Without LEKTI the kallikreins run riot, chewing through desmoglein 1 and corneodesmosin, shedding corneocytes prematurely and unleashing a Th2-skewed, high-IgE inflammatory state. Variants in the 5-prime half of SPINK5, which knock out more inhibitor domains, cause the severer phenotype with failure to thrive.^[9]

The rest of the congenital family

Each remaining gene is one stuck enzyme and one recognisable child. Hold them as a single table and the viva answer writes itself.^[3][1]

^[1]

Lamellar ichthyosis and the bathing-suit variant

TGM1 (14q12) catalyses the cross-linking that rivets the cornified envelope together; biallelic loss leaves the envelope unstable and the neonate arrives as a collodion baby. Once the membrane sheds at two to four weeks, the classic phenotype appears — large, dark, plate-like scales over the whole body with facial tightness and ectropion that softens with age. A temperature-sensitive subset, bathing-suit ichthyosis, carries TGM1 missense variants whose enzyme folds correctly only on cooler skin, so scaling maps neatly to the warmer trunk.^[3][16]

Congenital ichthyosiform erythroderma (CIE)

ALOX12B, ALOXE3 and NIPAL4 run the lipoxygenase-ichthyin pathway that makes the barrier lipids. The infant is born a collodion baby and then develops fine white scaling overlying generalised erythroderma. Erythema is the discriminator from lamellar ichthyosis: CIE scales less but flushes more, and the flush persists.^[3][16]

Epidermolytic ichthyosis — bullous, then warty

KRT1 and KRT10 are autosomal-dominant keratin defects, roughly half arising de novo. Dominant-negative mutations in the helix boundaries collapse the suprabasal cytoskeleton, so the newborn blisters widely — mimicking staphylococcal scalded skin or epidermolysis bullosa — and then, as blistering remits in early childhood, verrucous hyperkeratosis takes over the flexures. The genotype-phenotype link is unusually tight: KRT1 means severe palmoplantar keratoderma; KRT10 spares palms and soles. Biopsy shows epidermolytic hyperkeratosis — clumped keratin in a vacuolated upper epidermis.^[3]

Sjögren-Larsson syndrome — the one with glistening dots

ALDH3A2 encodes fatty aldehyde dehydrogenase; its loss disrupts epidermal lipids and brain myelin. The triad is congenital ichthyosis (often pruritic, which is unusual among ichthyoses), spastic diplegia or tetraplegia with intellectual disability, and a crystalline maculopathy of glistening white retinal dots present in over 95 per cent by age three or four. MR spectroscopy shows a characteristic lipid peak, and the diagnosis is sealed by deficient enzyme activity in fibroblasts.^[14]

Refsum disease — the diet is the treatment

PHYH (over 90 per cent) or PEX7 runs alpha-oxidation of phytanic acid, a branched-chain fatty acid that comes only from the diet — ruminant fat, dairy, certain fish. Lose the enzyme and phytanic acid climbs past 200 micromol per litre, poisoning myelin and photoreceptors. The clinical tetrad is retinitis pigmentosa, peripheral neuropathy, cerebellar ataxia, and anosmia, with deafness, ichthyosis and cardiac arrhythmia completing the picture. The treatment is the diet: lifelong phytanic-acid restriction, with plasmapheresis for acute deteriorations.^[15]

Clinical features — read the flexures first

When the skin is scaly, the fastest single sign is where the flexures sit. Spared points to ichthyosis vulgaris; involved points to X-linked ichthyosis. Everything else confirms.^[1][2]

Ichthyosis vulgaris declares itself after three months with fine, semi-adherent, polygonal scales on shins, forearms and thighs, hyperlinear palms and keratosis pilaris, and the atopic history that practically makes the diagnosis for you. X-linked ichthyosis starts earlier, lays down larger brown scales that deliberately involve the popliteal and antecubital fossae while sparing the face, and brings the two extracutaneous clues — stromal corneal opacities on slit-lamp that never affect vision, and cryptorchidism in about a fifth.^[2][6]

Collodion baby — a presentation, never a diagnosis

Collodion baby is what you see at birth, not what the child has. The neonate is cased in a tight, shiny, parchment-like membrane that forces ectropion, eclabium, pseudo-clawing of the digits and restricted movement. The membrane desquamates over two to four weeks to reveal the truth underneath — usually lamellar ichthyosis or CIE, occasionally normal skin in the self-healing variant. The danger window is those weeks: evaporative heat and water loss, electrolyte imbalance, sepsis through the broken barrier, and corneal exposure from the ectropion. Manage in a humidified incubator with emollients, ocular lubrication and infection prophylaxis.^[8]

Harlequin ichthyosis — the neonatal emergency

At birth the infant is encased in massive, thick, armour-like hyperkeratotic plates split by deep erythematous fissures, with severe ectropion, eclabium, flattened ears, hypoplastic nose and digits, and a restricted chest wall that threatens breathing. Historically fatal in days, survival now sits around half to four-fifths when systemic retinoids are started within the first week alongside intensive neonatal care — humidified incubator, fluid and electrolyte replacement, ocular lubrication, emollient baths and sepsis prevention. Residual ectropion, contractures and temperature dysregulation are lifelong.^[7][10][12]

Netherton syndrome — bamboo hair is pathognomonic

The skin runs a migratory, polycyclic, serpiginous rash — ichthyosis linearis circumflexa with its double border — or an atopic-like dermatitis. The pathognomonic sign is in the hair: trichorrhexis invaginata, the bamboo or golf-tee hair, brittle and spiky and best found under the microscope. Serum IgE is markedly elevated, food allergy (classically peanut) and asthma are routine, and the barrier defect is so profound that topical calcineurin inhibitors carry real systemic-absorption risk.^[9]

Epidermolytic ichthyosis — bullous at birth

The newborn blisters widely with erosions that risk sepsis and electrolyte loss, mimicking staphylococcal scalded skin or epidermolysis bullosa — so take skin swabs to exclude infection. Blistering ceases in early childhood and is replaced by verrucous hyperkeratosis, worst in the flexures.^[3]

Acquired ichthyosis — the malignancy hunt

Acquired ichthyosis is adult-onset scaling that was never there in childhood, and it is always a hunt for the cause. The single most testable association is Hodgkin lymphoma, which can precede the cancer diagnosis by months; the wider net also catches non-Hodgkin lymphoma, mycosis fungoides and multiple myeloma.^[5]

^[5]

Histopathology — the granular layer sorts the common two

One line on H and E separates ichthyosis vulgaris from X-linked ichthyosis: the granular layer. Ichthyosis vulgaris shows a reduced or absent granular layer with compact hyperkeratosis; X-linked ichthyosis keeps a normal or thickened granular layer. Epidermolytic ichthyosis is unmistakable — epidermolytic hyperkeratosis, with vacuolar degeneration of the upper spinous and granular layers and clumped eosinophilic keratin. Netherton may show parakeratosis and eosinophilic spongiosis with reduced LEKTI on immunohistochemistry.^[3][10]

Investigations — diagnosis is clinical, tests confirm the gene

Most ichthyoses are diagnosed at the bedside from onset, distribution and family history; the laboratory confirms which gene. Skin biopsy (H and E) nails the common two through the granular layer. Genetic testing assigns the gene — FLG sequencing for IV, STS deletion analysis (MLPA or array CGH) for XLI, ABCA12 for harlequin, TGM1 for lamellar, SPINK5 for Netherton, KRT1 and KRT10 for epidermolytic. Two cheap surrogates earn easy marks: raised cholesterol sulfate on lipoprotein electrophoresis flags STS deficiency, and low or absent unconjugated oestriol on maternal screening flags a carrier mother carrying an XLI fetus. Add slit-lamp for corneal opacities, hair microscopy for bamboo hair in Netherton, and serum IgE which is markedly elevated in Netherton.^[4][9]

Management — emollients first, retinoids for the severe, biologics for the refractory

The mantra for ichthyosis: hydrate, keratolyse, then escalate by severity. Every form starts the same way — generous, frequent emollients (ceramide-dominant or petrolatum-based) plus a keratolytic, bath oils and soap substitutes to keep the skin acidic.^[4]

The general ladder, applied to all:^[4]

• Emollients — ceramide-dominant creams, petrolatum ointments, and humectant creams with urea 5 to 10 per cent, lactic acid or glycerol, applied generously and often.
• Keratolytics — salicylic acid 2 to 6 per cent, urea 10 to 20 per cent, lactic acid 12 per cent, or propylene glycol 50 per cent under occlusion for severe scaling.
• Soap substitutes and bath oils — avoid alkaline soaps; preserve the acidic skin pH.^[4]

Disease-specific escalation

• Ichthyosis vulgaris and XLI — topical therapy alone; treat the associated atopic dermatitis in IV. No systemic drug is needed for either.
• Lamellar, CIE and harlequin — oral retinoids, acitretin about 0.5 mg per kg per day or isotretinoin, to strip hyperkeratosis and restore mobility; for harlequin this is survival-critical and must start within days of birth.
• Netherton — topical calcineurin inhibitors (tacrolimus, pimecrolimus) calm the inflammation, but watch for systemic absorption because the barrier defect is profound; IVIG for severe disease, and biologic therapy (dupilumab anti-IL-4 receptor alpha, anti-IL-17) for refractory cases.
• Epidermolytic ichthyosis — oral retinoids flatten the hyperkeratosis but may increase blistering, so use low doses, with topical antiseptics through the bullous phase.^[7][10][11]

Emerging therapies

Biologics are the active frontier for the congenital forms. Dupilumab (anti-IL-4 receptor alpha) is used off-label in Netherton, anti-IL-17 agents such as secukinumab are tried in lamellar and CIE, and anti-kallikrein-5 antibodies are under investigation for Netherton — a logical target, given that LEKTI normally restrains kallikrein 5. Gene therapy and ABCA12 replacement remain preclinical.^[11]

Differential diagnosis — and the one that is not scaly skin disease

The common confusions are dry-skin look-alikes rather than other ichthyoses. Atopic dermatitis overlaps through FLG and shared xerosis but is eczematous and itchy, not ichthyotic. Xerosis is age- or climate-related dryness without true scale. Psoriasis gives well-demarcated erythematous plaques with silvery scale, not ichthyotic scaling. Two rarer mimics earn marks: Refsum disease if the ichthyosis rides with retinitis pigmentosa, ataxia and anosmia, and pityriasis rotunda — perfectly circular scaly hyperpigmented patches that in some populations flag underlying malignancy.^[5]

The preventable-harm list

^[1]

Ward-round test

A 30-year-old with fine white shin scales, clear flexures, hyperlinear palms and childhood eczema. Gene, inheritance, and the single strongest associated risk?ShowHide

Ichthyosis vulgaris — FLG loss-of-function, autosomal semidominant. The flexures are spared and the palms hyperlinear, which separates it from X-linked ichthyosis at the bedside. The strongest associated risk is atopic dermatitis: FLG loss is the single biggest genetic risk factor for the atopic march. First-line is emollients plus a keratolytic such as urea 10 per cent.^[2][13]

A boy with large brown scales climbing into his popliteal fossae, whose mother had a low-oestriol screen and a caesarean for failure to progress. Diagnosis, gene, and two extracutaneous signs?ShowHide

X-linked recessive ichthyosis — STS deletion at Xp22.31, fully penetrant in hemizygous males. The flexures are involved (the opposite of IV) because cholesterol sulfate accumulation brakes desquamation. Two extracutaneous signs: stromal corneal opacities on slit-lamp (about half, vision spared) and cryptorchidism (about a fifth). The low maternal oestriol and failure to progress in labour come from placental steroid-sulfatase deficiency.^[6]

A newborn encased in a shiny parchment membrane with ectropion and eclabium. What is this, what is it not, and what is the danger window?ShowHide

Collodion baby — a presentation at birth, never a diagnosis; the membrane desquamates over two to four weeks to reveal lamellar ichthyosis or CIE (occasionally normal skin). The danger window is those weeks: evaporative heat and water loss, electrolyte imbalance, sepsis through the broken barrier, and corneal exposure from ectropion. Manage in a humidified incubator with emollients, ocular lubrication and infection prophylaxis.^[8]

A neonate with armour-like plates, severe ectropion and respiratory compromise. Gene, and the single survival-changing intervention with its timing?ShowHide

Harlequin ichthyosis — ABCA12, autosomal recessive; the lipid lamellae never form, so the barrier fails. The survival-changing intervention is an oral retinoid (acitretin) started within the first week of life, with intensive neonatal care — humidified incubator, fluid and electrolyte replacement, ocular lubrication, and sepsis prevention. Modern survival is around half to four-fifths.^[7][17]

A child with a migratory double-bordered scaly rash, brittle spiky hair, asthma and a very high IgE. Diagnosis and the pathognomonic hair sign?ShowHide

Netherton syndrome — SPINK5, encoding LEKTI; loss of the protease brake drives the rash and the Th2-skewed, high-IgE inflammation. The pathognomonic sign is trichorrhexis invaginata — bamboo (golf-tee) hair on microscopy. Beware profound systemic absorption of topical calcineurin inhibitors through the defective barrier.^[9]

A 58-year-old develops generalised ichthyosiform scaling over eight weeks with drenching night sweats and a palpable cervical node. What must you do, and why?ShowHide

Treat this as paraneoplastic acquired ichthyosis until proven otherwise — the classic association is Hodgkin lymphoma, and the ichthyosis can precede the cancer by months. The action is a malignancy workup now: full history for B symptoms, lymph-node and abdominal examination, baseline bloods including LDH, chest imaging, and biopsy of the palpable node. Emollients help the skin; they do not find the lymphoma.^[5]

A young adult with ichthyosis, night blindness, progressive ataxia and anosmia. Gene, the accumulating metabolite, and the treatment that is also the diet?ShowHide

Refsum disease — PHYH (over 90 per cent) or PEX7; alpha-oxidation of phytanic acid fails, so phytanic acid climbs past 200 micromol per litre and poisons myelin and photoreceptors. The tetrad is retinitis pigmentosa, neuropathy, ataxia and anosmia. Treatment is the diet — lifelong restriction of phytanic acid (ruminant fat, dairy, certain fish) — with plasmapheresis for acute deteriorations.^[15]

References18ShowHide

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