Inherited as an autosomal semidominant disorder, this condition is common, affecting about 1 in 250 people in the United Kingdom. Mutations in the filaggrin gene lead to a loss or reduction of profilaggrin, the major component of the keratohyalin granule. Profilaggrin is cleaved to filaggrin, which in turn is responsible for aggregating keratin filaments in the cornified cell envelope. The breakdown of filaggrin results in the formation of filaggrin degradation products, which reduce transepidermal water loss. The reduction in keratohyalin granules gives icthyosis vulgaris its characteristic histology; a paucity or absence of the granular layer of the epidermis. Mutant alleles of the filaggrin gene have a frequency of 4% in European populations, which accounts for it being such a common disorder. Heterozygotes have milder disease than compound heterozygotes and homozygotes.

The dryness is usually mild and symptoms are few. The scales are small and branny, being most obvious on the limbs and least obvious in the major flexures. The skin creases of the palm may be accentuated. Keratosis pilaris (p. 48) is often present on the limbs.

The skin changes are not usually present at birth but develop over the first few years of life. Some patients improve in adult life, particularly during warm weather, but the condition seldom clears completely.

The already dry skin chaps in winter and is easily irritated by degreasing agents. This should be taken into account in the choice of a career. Ichthyosis of this type is apt to appear in a stubborn combination with atopic eczema, as mutations in the filaggrin gene are strong predisposing factors for atopic eczema (p. 88).

It can usually be distinguished from less common types of ichthyosis on the basis of the pattern of inheritance and the type and distribution of the scaling. Mild variants may be difficult to differentiate from the xerosis of atopic eczema, which is not surprising given their common genetic basis.

None are usually needed.

This is palliative. The dryness can be helped by the regular use of emollients, which are best applied after a shower or bath. Emulsifying ointment, soft white paraffin, E45 and Unguentum Merck are all quite suitable (Formulary 1, p. 397) and the selection depends on the patient’s preference. Many find proprietary bath oils and creams containing humectants such as glycerin, urea or lactic acid helpful (Formulary 1, p. 397).

This less common type of ichthyosis is inherited as an X-linked recessive trait and therefore, in its complete form, is seen only in males, although some female carriers show mild scaling. The condition affects about 1 in 6000 males in the United Kingdom and is associated with a deficiency of the enzyme steroid sulfatase, which hydrolyses cholesterol sulfate. The responsible gene has been localized to the end of the short arm of the X chromosome, at Xp 22.3 (see Chapter 24).

Affected babies may be born after a prolonged labour. Corneal opacities may appear in adult life. Kallmann’s syndrome (hypogonadotrophic hypogonadism and anosmia) is caused by the deletion of a part of the X chromosome that includes the gene for X-linked recessive ichthyosis, which is therefore one of its features. Neurological defects may also occur in this contiguous gene disorder.

This is as for ichthyosis vulgaris. It is helpful to remember that only males are affected. Bear Kallmann’s syndrome in mind if there are other congenital abnormalities.

None are usually needed. Steroid sulfatase gene deletions can be identified by fluorescence in situ hybridization (FISH; see Chapter 24). Electron microscopy shows retained corneodesmosomes within the stratum corneum (see Chapter 2).

Oral aromatic retinoids are probably best avoided. Topical measures are as for ichthyosis vulgaris.