Ichthyosis and Inherited Keratinisation Disorders
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Direct answer
Ichthyosis is a group of inherited disorders of keratinisation in which the stratum corneum becomes dry, thick and scaly. Ichthyosis vulgaris is the commonest form and is usually autosomal dominant, linked to filaggrin gene mutations, whereas X-linked ichthyosis results from steroid sulfatase deficiency. Lamellar ichthyosis is autosomal recessive and often presents as a collodion baby at birth.
What you must remember
- Ichthyosis vulgaris: autosomal dominant filaggrin mutation, onset in early childhood, fine white scales that worsen in dry weather, often with keratosis pilaris and atopy.
- X-linked recessive ichthyosis: steroid sulfatase deficiency in males, large dark brown polygonal scales on the neck and trunk, and corneal opacities.
- Lamellar ichthyosis: autosomal recessive TGM1 mutation, collodion baby at birth, large plate-like scales, ectropion and eclabium.
- Epidermolytic ichthyosis: autosomal dominant keratin 1 or keratin 10 mutation, blistering at birth and hyperkeratosis mainly over flexures.
- Collodion baby: born encased in a shiny, tight membrane that later cracks and sheds, seen in lamellar ichthyosis and congenital ichthyosiform erythroderma.
- Associated features: cryptorchidism and corneal opacities in X-linked disease, and prolonged labour reported in some affected pregnancies.
- Management: emollients, urea or salicylic acid keratolytics, and oral retinoids for severe disease, with teratogenicity counselling.
Matching the type to inheritance and the clinical clue
The pattern of inheritance, age at onset and appearance of the scale usually identify the type. The table sorts the common forms by these clues.
| Type | Inheritance and gene | Clinical clue |
|---|---|---|
| Ichthyosis vulgaris | Autosomal dominant, filaggrin | Fine white scales, flexures spared |
| X-linked ichthyosis | X-linked recessive, STS | Large dark scales, corneal opacity |
| Lamellar ichthyosis | Autosomal recessive, TGM1 | Collodion baby, plate scales, ectropion |
| Epidermolytic ichthyosis | Autosomal dominant, keratin 1 and 10 | Blisters at birth, flexural hyperkeratosis |
A collodion birth points to a neonatal form, while childhood onset with fine scales suggests ichthyosis vulgaris. Genetic testing aids counselling but is not needed for routine examination answers.
Where the examiner tests genetics and onset
Most questions here turn on inheritance and the timing of onset.
- Trap: calling X-linked ichthyosis autosomal recessive Correct: it is X-linked recessive, so carrier mothers transmit it and an affected father does not pass it to his sons.
- Trap: expecting ichthyosis vulgaris to present at birth Correct: it usually starts in early childhood, whereas collodion birth and bullous forms are neonatal presentations.
- Trap: prescribing an oral retinoid to a young woman without precautions Correct: acitretin and isotretinoin are teratogenic, so pregnancy must be excluded and contraception counselled.
- Trap: treating ichthyosis as curable Correct: these are lifelong disorders, so management aims at controlling scale, comfort and quality of life.
Frequently asked questions
Which ichthyosis is inherited in an X-linked recessive pattern?
X-linked ichthyosis is inherited in an X-linked recessive pattern because of steroid sulfatase deficiency, so it mainly affects males. It causes large, dark, polygonal scales on the neck and trunk, and corneal opacities are a recognised feature.
What is a collodion baby and which ichthyosis causes it?
A collodion baby is born encased in a shiny, tight membrane that later cracks and sheds. Lamellar ichthyosis and congenital ichthyosiform erythroderma are the main causes, and the newborn needs skin care and careful temperature and fluid management.
Which ichthyosis is associated with atopic dermatitis and keratosis pilaris?
Ichthyosis vulgaris is associated with atopic dermatitis and keratosis pilaris, largely through filaggrin gene mutations that impair the skin barrier. Fine white scales, worse in dry weather, and relative flexural sparing help distinguish it from the other forms.
Why are oral retinoids avoided in women of childbearing age with ichthyosis?
Oral retinoids such as acitretin and isotretinoin are teratogenic, so they are avoided or used only with strict contraception and pregnancy exclusion in women who could become pregnant. This is a key safety point when treating severe forms of ichthyosis.