Ichthyosis & Disorders of Keratinisation

Ichthyoses are a heterogeneous group of genodermatoses (and a few acquired conditions) characterised by persistent, generalised scaling of the skin owing to defective epidermal cornification. For NEET PG the high-yield axis is inheritance pattern + enzyme/protein defect + scaling morphology + histology, plus the classic "collodion baby" and Darier's disease. This note ties those threads together so a single distinguishing clue in the stem points you to the answer.

Basic concepts: how cornification fails

Normal epidermal turnover takes ~28 days; keratinocytes mature from the basal layer through the spinous and granular layers and are shed as anucleate corneocytes embedded in an intercellular lipid mortar. "Ichthyosis" results when either:

• Retention hyperkeratosis — reduced shedding (desquamation) of corneocytes with normal/low proliferation (e.g., ichthyosis vulgaris, X-linked ichthyosis).
• Proliferative (epidermolytic) hyperkeratosis — increased keratinocyte production with a thickened, often fragile epidermis (e.g., epidermolytic hyperkeratosis, lamellar ichthyosis variants).

Two histology words decide many MCQs:

• Orthokeratosis / hyperkeratosis = thickened stratum corneum with anucleate cells (basket-weave). The granular layer status is the discriminator.
• Parakeratosis = retained nuclei in the stratum corneum (seen in psoriasis, not classic ichthyosis vulgaris).
• Acanthosis = thickened spinous (stratum spinosum) layer.

High-yield: Ichthyosis vulgaris shows hyperkeratosis with a reduced or absent granular layer; X-linked ichthyosis shows hyperkeratosis with a normal or thickened granular layer. This single line is the most repeated histology MCQ in this group.

Classification

Type	Inheritance	Defect	Onset	Scale character
Ichthyosis vulgaris	Autosomal semi-dominant	Filaggrin (FLG) loss-of-function	3–12 months (after birth)	Fine, white, flaky; spares flexures
X-linked recessive ichthyosis	X-linked recessive (males)	Steroid sulfatase (STS) deficiency	Birth / first weeks	Large, dark brown / dirty polygonal scales; involves flexures
Lamellar ichthyosis	Autosomal recessive	TGM1 (transglutaminase-1)	Birth (collodion baby)	Large, plate-like, dark scales; ectropion
Congenital ichthyosiform erythroderma (CIE / non-bullous)	Autosomal recessive	TGM1, ALOX12B, NIPAL4	Birth (collodion baby)	Fine white scales on erythroderma
Epidermolytic hyperkeratosis (bullous CIE)	Autosomal dominant	Keratin 1 / Keratin 10	Birth	Blistering then warty/cobblestone scale
Harlequin ichthyosis	Autosomal recessive	ABCA12 transporter	Birth (severe)	Thick armour plates with deep fissures
Darier's disease	Autosomal dominant	ATP2A2 (SERCA2 Ca²⁺ pump)	2nd decade	Greasy keratotic papules, seborrhoeic distribution

High-yield: Lock the enzyme/protein to the disease — Filaggrin → IV; Steroid sulfatase → XLI; Transglutaminase-1 → lamellar; Keratin 1/10 → epidermolytic hyperkeratosis; ABCA12 → Harlequin; ATP2A2 → Darier's.

Ichthyosis vulgaris (the commonest)

The most common inherited ichthyosis (≈1 in 250–300). Caused by loss-of-function mutations in filaggrin (filament aggregating protein), which bundles keratin filaments and is the precursor of "natural moisturising factor."

Clinical features

• Onset after birth (typically 2–6 months), worse in winter / dry climates.
• Fine, white to grey scales, most prominent on extensor surfaces and shins.
• Flexures (antecubital/popliteal fossae) are SPARED — a classic exam discriminator from XLI.
• Hyperlinear palms and soles and keratosis pilaris (follicular plugging on outer arms/thighs).
• Strong association with atopy (atopic dermatitis, asthma, allergic rhinitis) — filaggrin defects impair the skin barrier.

Histology: mild hyperkeratosis with a thinned or absent stratum granulosum (reduced keratohyaline granules — because filaggrin's precursor profilaggrin forms keratohyalin).

High-yield: Hyperlinear palms + keratosis pilaris + flexural sparing + atopy = ichthyosis vulgaris. Filaggrin mutation is also the key barrier defect linking it to atopic dermatitis.

X-linked recessive ichthyosis (XLI)

Due to deficiency of steroid sulfatase (arylsulfatase C), the gene (STS) lies on Xp22.3. Accumulated cholesterol sulfate in the stratum corneum acts as a "biological glue," impairing corneocyte shedding (retention hyperkeratosis).

Clinical features

• Affects males; mothers are carriers.
• Onset within the first weeks of life.
• Large, dark/dirty brown, adherent polygonal scales, prominent on neck ("unwashed neck"), trunk, extremities.
• Flexures may be INVOLVED; palms/soles and face (central) relatively spared. Pre-auricular area often involved.
• Comma-shaped corneal opacities (asymptomatic) in patients and carrier mothers.
• Cryptorchidism in ~20% (raised risk of testicular cancer).
• Failure of labour to progress / prolonged labour — placental steroid sulfatase deficiency → low maternal urinary/serum oestriol (picked up on antenatal screening).

Diagnosis: ↑ cholesterol sulfate in blood (lipoprotein electrophoresis shows increased mobility of LDL), low/absent steroid sulfatase activity in fibroblasts/leukocytes; FISH/MLPA for STS deletion. Histology: hyperkeratosis with a normal or thickened granular layer.

High-yield: XLI clues — male child, dark scales involving flexures, comma-shaped corneal opacities, cryptorchidism, low maternal oestriol / prolonged labour, steroid sulfatase deficiency. Contrast every point with ichthyosis vulgaris.

Ichthyosis vulgaris vs X-linked ichthyosis (the head-to-head table)

Feature	Ichthyosis vulgaris	X-linked ichthyosis
Inheritance	Autosomal semi-dominant	X-linked recessive (males)
Defect	Filaggrin (FLG)	Steroid sulfatase (STS)
Onset	After birth (2–6 mo)	At birth / first weeks
Scale colour & size	Fine, white, small	Large, dark/dirty brown
Flexures	Spared	Involved
Palms/soles	Hyperlinear palms	Usually normal
Granular layer	Reduced/absent	Normal/thick
Eye	—	Comma-shaped corneal opacities
Other	Keratosis pilaris, atopy	Cryptorchidism, low oestriol

Lamellar ichthyosis & the collodion baby

Collodion baby is a presentation, not a diagnosis: a neonate encased in a taut, shiny, parchment-like / cellophane membrane that cracks and peels over days. It most often evolves into lamellar ichthyosis or CIE (autosomal recessive congenital ichthyoses), occasionally resolving as a "self-healing collodion baby."

• Complications of collodion membrane: ectropion (everted eyelids), eclabium (everted lips), temperature instability, dehydration, hypernatraemia, and risk of sepsis through the impaired barrier — managed in a humidified incubator with emollients, fluid/electrolyte monitoring.

Lamellar ichthyosis — autosomal recessive, commonly TGM1 (transglutaminase-1) mutation (enzyme cross-links the cornified envelope).

• Large, plate-like, dark brown scales over the whole body (including flexures).
• Ectropion, eclabium, scarring alopecia, palmoplantar keratoderma.

Harlequin ichthyosis — most severe; ABCA12 lipid transporter defect. Thick "armour" plates with deep fissures, severe ectropion/eclabium, restricted breathing/feeding. Modern care with oral retinoids (acitretin) has improved survival.

High-yield: Collodion baby → think recessive (lamellar/CIE); TGM1 is the prototypic gene. Harlequin = ABCA12, treated with systemic retinoids.

Epidermolytic hyperkeratosis (bullous ichthyosiform erythroderma)

• Autosomal dominant, mutations in keratin 1 or keratin 10.
• Neonatal blistering and erythroderma → later thick, warty (verrucous), cobblestone hyperkeratosis, especially in flexures, with a characteristic odour from secondary infection.
• Histology = epidermolytic hyperkeratosis: hyperkeratosis, vacuolar degeneration of the upper spinous/granular layers, and coarse clumped keratohyaline granules with perinuclear vacuolisation.
• Note: an epidermal naevus can show identical histology; a parent with a keratin-mutated germline mosaic naevus can have children with generalised disease.

Darier's disease (keratosis follicularis)

A disorder of keratinisation with acantholysis + dyskeratosis, not a true ichthyosis but grouped here and frequently asked.

• Autosomal dominant, mutation in ATP2A2 encoding the SERCA2 calcium ATPase of the endoplasmic reticulum → impaired desmosomal assembly.
• Onset in adolescence; worsens with UV light, heat and sweating (summer flares).
• Greasy, warty, crusted keratotic papules in a seborrhoeic distribution (chest, back, scalp margins, forehead, flexures).
• Nail signs (pathognomonic-ish): red and white longitudinal bands with V-shaped distal notching (nicks).
• Palmar pits and acrokeratosis verruciformis of Hopf (flat warty papules on dorsa of hands).
• Mucosal cobblestone papules.

Histology of Darier's: suprabasal acantholysis with two dyskeratotic cell types — corps ronds (in the granular/spinous layer, with a central pyknotic nucleus and a clear halo) and grains (parakeratotic, flattened cells in the stratum corneum). Villi = elongated dermal papillae lined by a single layer of basal cells projecting into the suprabasal cleft.

High-yield: Corps ronds and grains + suprabasal acantholysis = Darier's disease (ATP2A2/SERCA2). Don't confuse with Hailey–Hailey disease (ATP2C1, SPCA1 pump) which shows a "dilapidated brick wall" full-thickness acantholysis in flexures.

Darier's vs Hailey–Hailey vs Grover's (acantholytic disorders)

Feature	Darier's	Hailey–Hailey	Grover's (transient acantholytic dermatosis)
Gene/pump	ATP2A2 / SERCA2 (ER)	ATP2C1 / SPCA1 (Golgi)	Acquired, none
Acantholysis	Focal suprabasal + dyskeratosis	Extensive "dilapidated brick wall"	Focal, several patterns
Distribution	Seborrhoeic areas, nails	Flexures (axilla, groin, neck)	Trunk of middle-aged men
Dyskeratosis	Corps ronds & grains	Minimal	Variable

Diagnosis & investigation of choice

Approach: Clinical morphology + family history + sex of child → targeted test.

1. History/exam — onset (birth vs later), scale colour/size, flexural involvement, palmar lines, atopy, family tree.
2. Skin biopsy — granular layer status, epidermolytic vs retention pattern, corps ronds/grains.
3. Targeted biochemistry/genetics:
   • XLI → steroid sulfatase assay (leukocytes/fibroblasts), serum cholesterol sulfate, STS deletion testing — investigation of choice for XLI.
   • Ichthyosis vulgaris → largely clinical; FLG genotyping if needed.
   • Lamellar/CIE/Harlequin → next-generation gene panels (TGM1, ABCA12, ALOX12B…).
4. Prenatal/antenatal clues — low maternal serum unconjugated oestriol flags XLI; molecular prenatal diagnosis for severe recessive forms.

Stepwise reasoning flow: Scaling neonate → collodion membrane present? → yes: recessive (lamellar/CIE) → no, male child with dark flexural scales → XLI (check steroid sulfatase) → older child, fine white scales sparing flexures + hyperlinear palms → ichthyosis vulgaris (filaggrin).

Management & drug of choice

There is no cure; management restores the barrier and reduces scaling.

• Topical first line: liberal emollients; keratolytics — urea (5–10%), lactic acid / alpha-hydroxy acids, salicylic acid, propylene glycol. (Avoid extensive salicylic acid in infants — risk of salicylism.)
• Topical retinoids (tazarotene) and topical vitamin D analogues (calcipotriol) for thicker forms.
• Systemic retinoids — acitretin (drug of choice for severe ichthyoses, lamellar, Harlequin); isotretinoin also used. Teratogenic — strict contraception; monitor lipids, LFTs, and skeletal toxicity (DISH) on long-term use.
• XLI specific: topical cholesterol ± statin combinations can reduce cholesterol-sulfate–driven scaling.
• Darier's: sun protection, antiseptics for secondary infection, oral retinoids for severe disease; ciclosporin/antibiotics for flares.
• Collodion baby: humidified incubator, bland emollients, fluid–electrolyte balance, ophthalmology for ectropion, watch for sepsis.

High-yield: Acitretin is the systemic drug of choice for severe ichthyoses (lamellar, Harlequin, epidermolytic HK). Remember its teratogenicity and the bone (DISH/hyperostosis) and lipid side effects.

Complications

• Neonatal: hypernatraemic dehydration, temperature dysregulation, sepsis (impaired barrier), ectropion, eclabium, respiratory/feeding compromise (Harlequin).
• Skin: secondary bacterial/fungal infection, painful fissures, scarring alopecia, restricted joint movement, heat intolerance (impaired sweating).
• XLI: cryptorchidism and testicular cancer risk; contiguous gene deletions may cause Kallmann syndrome, X-linked recessive chondrodysplasia punctata, intellectual disability.
• Darier's: widespread HSV superinfection (eczema/dermatitis herpeticum-like Kaposi varicelliform eruption), secondary bacterial infection, social/psychological impact.

Key differentials

• Psoriasis — well-defined erythematous plaques with silvery scale, parakeratosis, Munro microabscesses, Auspitz sign; not generalised retention scaling.
• Atopic dermatitis — shares filaggrin biology with ichthyosis vulgaris; look for itchy flexural eczema.
• Acquired ichthyosis — new-onset diffuse scaling in an adult → screen for Hodgkin lymphoma (classic association), other malignancies, hypothyroidism, sarcoidosis, HIV, drugs (nicotinic acid, statins).
• Pityriasis rubra pilaris / erythroderma in the neonate.
• Netherton syndrome — SPINK5 (LEKTI) defect: ichthyosis linearis circumflexa + bamboo hair (trichorrhexis invaginata) + atopy; severe, can present as collodion-like erythroderma.

High-yield: Acquired ichthyosis in an adult = paraneoplastic until proven otherwise → think Hodgkin lymphoma. Bamboo hair + ichthyosis linearis circumflexa = Netherton (SPINK5).

Recently asked / exam angle

• "Granular layer reduced/absent" → ichthyosis vulgaris; "granular layer normal/thick" → X-linked ichthyosis (single most repeated histology two-liner).
• Enzyme-to-disease single best answer: steroid sulfatase → XLI; transglutaminase-1 → lamellar; filaggrin → IV/atopy; ATP2A2/SERCA2 → Darier's; ABCA12 → Harlequin.
• Image/clinical stems: collodion baby (recessive CI), comma-shaped corneal opacities (XLI), corps ronds & grains (Darier's), dilapidated brick wall (Hailey–Hailey).
• Obstetric crossover: low maternal serum oestriol / failure of labour to progress → placental steroid sulfatase deficiency → XLI baby.
• Pharmacology: drug of choice for severe ichthyosis = acitretin; teratogenicity + DISH.
• Association traps: filaggrin–atopic dermatitis, XLI–cryptorchidism/Kallmann, acquired ichthyosis–Hodgkin lymphoma.
• Mnemonic for inheritance: ichthyosis Vulgaris is the Very common dominant one; the X-linked one affects boys and has eXtra features (corneal opacity, cryptorchidism).

High-yield: When a stem gives sex of the child + flexural status + scale colour, you can usually answer without histology: male + dark flexural scales = XLI; either sex + fine white flexure-sparing scales + hyperlinear palms = ichthyosis vulgaris.

Rapid revision

1. Ichthyosis vulgaris = commonest; filaggrin; AD-semidominant; fine white scales sparing flexures; reduced granular layer; atopy + hyperlinear palms + keratosis pilaris.
2. X-linked ichthyosis = steroid sulfatase deficiency; boys; dark scales involving flexures; normal/thick granular layer.
3. XLI extras: comma-shaped corneal opacities, cryptorchidism, low maternal oestriol, prolonged labour, possible Kallmann (contiguous deletion).
4. Accumulated cholesterol sulfate glues corneocytes in XLI; topical cholesterol/statin helps.
5. Collodion baby usually evolves into lamellar ichthyosis / CIE; risks = hypernatraemia, sepsis, ectropion, eclabium.
6. Lamellar ichthyosis = TGM1 (transglutaminase-1), AR, plate-like dark scales, ectropion.
7. Harlequin ichthyosis = ABCA12; armour plates; treat with acitretin.
8. Epidermolytic hyperkeratosis = keratin 1/10, AD; histology shows vacuolar degeneration + clumped keratohyalin.
9. Darier's disease = ATP2A2 (SERCA2); corps ronds & grains, suprabasal acantholysis; UV/heat-aggravated; V-notched nails, palmar pits; risk of Kaposi varicelliform eruption.
10. Hailey–Hailey = ATP2C1, flexural, "dilapidated brick wall" acantholysis.
11. Acitretin = drug of choice for severe ichthyoses; teratogenic, monitor lipids/LFTs/bones (DISH).
12. Acquired ichthyosis in an adult → screen for Hodgkin lymphoma; Netherton = SPINK5, bamboo hair + ichthyosis linearis circumflexa.