The characteristic features include:
- Abnormally thick curved nails (onychogryposis)
- Thickening of the skin (hyperkeratosis) of the palms, soles, knees and elbows
- White plaques (leukoplakia) in the mouth
- Excess sweating (hyperhidrosis) of the hands and feet
- Teeth are already erupted at birth (natal teeth)
Generation after generation in a family may show the syndrome. It is an autosomal dominant trait. The gene responsible for the syndrome is on chromosome 12 (in band 12q13) and a single copy of the gene (named PD1) is capable of causing the disease. The basic abnormality is a mutation (change) in a gene for keratin, a primary constituent of nails, hair, and skin. Alternate names for the syndrome include pachyonychia congenita of the Jadassohn-Lewandowski type and type 1 pachyonychia congenita. The syndrome is named for the professor of dermatology at the University of Bern in Switzerland, Josef Jadassohn (1860-1936), and his colleague, Felix Lewandowski (1879-1921), who first described the syndrome in 1906. About their patient, a 15-year-old girl, they wrote: "The nail plates of all the fingers and toes are extremely thickened, and so hard that they cannot be cut with a scissors; the father has to trim them with a hammer and chisel."