Ichthyosis is a common hereditary skin keratosis disease, known as tinea pedis in traditional Chinese medicine. The clinical features are dry and rough skin on the extensor sides of the limbs or trunk, with obvious markings, accompanied by rhombus or polygonal scales, with an appearance like fish scales or snake skin. There may also be hyperlinearity of the palms and soles of the feet, atopy and heat intolerance, or keratosis pilaris.

Due to different causes of the disease, it is divided into vulgaris, sex-linked ichthyosis, congenital ichthyosis-like erythroderma and lamellar ichthyosis.

Ichthyosis vulgaris is a common type, and its genetic characteristics are considered to be a completely penetrant autosomal dominant disease with no gender differences. The skin lesions were symmetrically distributed on the extensor sides of the limbs and trunk, especially on the extensor sides of the elbows and knees. The onset usually occurs within one year after birth, and worsens with age. The symptoms are most significant in adolescence, and then stop developing. Skin lesions vary in severity. In mild cases, the skin is only dry in winter without obvious scales. There are powdery flakes after scratching, which is called xeroderma. In severe cases, it can spread to the whole body, accompanied by hyperkeratosis of the palms and soles, chapped and nail changes. Generally, the face, scalp, cubital fossa, armpits, cheek fossa, vulva, and abdominal and gluteal grooves are not affected, or only have mild scales. Usually there are no symptoms. In winter, due to the reduction of sweat glands and sebaceous glands, the skin becomes dry and may cause itching and discomfort. If it is chapped, there may be pain.

Sex-linked ichthyosis is rare. The disease-causing gene is on the X chromosome, and women are carriers. Generally, the disease does not occur or sometimes only thin scales appear on the forearms and calves. The patient is male and develops the disease after birth or in infancy. The skin lesions are slightly different from the ordinary type. The scales are large and obvious, and they are yellowish brown or black in appearance. The skin is dry and rough, often spread all over the body. The armpits, fossae, elbow fossa, etc. can also be affected, especially the abdomen than the back. . If the face is affected, it is limited to the front of the ears and the sides of the face. Follicular keratosis generally does not occur. The skin on the palms and soles is mostly normal. Skin lesions do not diminish with age, but sometimes worsen. This disease may have small turbid spots on the posterior wall of the cornea and Descemet's membrane, which does not affect vision.

Lamellar ichthyosis is an autosomal recessive inheritance and is even rarer. After birth, the whole body is tightly wrapped by an extensive collodion-like membrane, which often causes the eyelids and lips to ectropion and is called a collodion baby. After a few weeks, the membrane fell off, and the skin showed extensive and chronic flushing with gray-white or gray-brown polygonal or diamond-shaped large scales, fixed in the center and free at the edges. It often occurs symmetrically all over the body, and is more obvious in the flexors of the limbs, cubital fossa, fossae, armpits, and vulva. Hyperkeratosis of the palms and soles, excessive growth of nails and hair. The course of the disease is slow and can persist throughout life. The erythroderma can be reduced in adulthood, but the scales still exist.