Ichthyosis is a malformation of the skin characterized by dryness, roughness, and thickening of the epidermis which is cast off in dandruffy to plate-like scales. Inflammatory phenomena are absent. The condition is congenital, though it may not be sufficiently pronounced to attract attention for a year or more after birth.
There are various grades of the deformity. The mildest is described as xerosis or xeroderma; in this there is mere harshness and branny scaliness of the skin mostly affecting the extremities. Next in intensity is ichthyosis simplex in which the skin is rough, dry, slightly lustrous and crinkly; the upper layers of the epidermis crack to form pea- to fingernail-sized, mica-like polygonal scales which adhere at their centers but loosen and often warp outward at the edges. The plate-like scaliness recalls the appearance of fish skin, as the name indicates, but the scales of ichthyosis never overlap. The lines and furrows of the skin are more widely separated and deeper than normal.
In more marked cases, the scales are thicker and often discolored a dirty or greenish gray or brown, and regions seldom affected by the milder forms, such as the face, scalp, and flexor surfaces of the joints, may be more or less involved. But even with a universal ichthyosis the skin of the face is usually only moderately changed, so that it appears somewhat shiny and tense, abnormally dry, and in places cracked and rough. Ichthyosis hystrix is a very rare form in which occur circumscribed patches of dark-colored, warty, spiny or bark-like masses of thickened epidermis. This type is never universal.
In any case of ichthyosis the skin deformity increases somewhat from childhood to adult life, and them remains about stationary. Ichthyosis simplex never develops into ichthyosis hystrix. As a rule, the condition causes no subjective symptoms other than occasional slight itching, though sometimes the elasticity of the skin is so impaired that painful fissures are formed at the joints. No cause for ichthyosis is known other than heredity. The malformation frequently occurs in various members of a family. It may skip one or more generations. There are many cases where hereditary influence cannot be discovered.