Accessory nail of the fifth toe

The accessory nail of the fifth toe, also known as a double nail of the fifth toe or a petaloid toenail, is a physical trait of the small toe, where a minuscule sixth toenail is present in the outer corner of the nail situated on the smallest toe. Although understudied and underreported, its prevalence is common.

Description

The accessory nail was first described in a 1969 paper by M. Hundeiker. The trait can be observed on either one or both feet where there is a lengthwise separation of the toenail on the corner of smallest toe. The smaller sixth toenail separates from the main toenail on the outermost side of the foot, and protrudes outwards from the corner. This nail averages in length. The additional nail can be cut with a nail clipper. Its occurrence is considered common but underreported.
The entire nail of the small toe is usually short and wide, and both nails often share the same nail bed.

Causes

The cause is poorly understood due to a lack of research, but genome-wide scans indicate that it is a heritable trait, and could be autosomal dominant. However, the wide variance in the size and structure of the accessory nail indicates that the trait may not follow a Mendelian pattern of inheritance, and may instead be a complex trait affected by multiple genes with minor genetic effects.
In some cases, the accessory nail presents with an abnormal protrusion or distortion of the fifth distal phalanx, or the splaying or rotation of the toe or entire foot. The trait may be a mild form of hexadactyly.
The condition may also appear following trauma to the toenail.

Diagnosis

Accessory toenails are often observed by the patient and diagnosed upon examination by a physician. X-rays are sometimes used to investigate the underlying bone structure for deformity. Ena et al. proposed the diagnostic criteria have 4 components: presence at birth, potential inheritance, involvement of the fifth toe, and bilaterality.
Differential diagnoses include trauma-induced short-term splits, ectopic nails, onychophosis, and calluses.

Treatment

Most people do not seek treatment because the accessory nail typically does not cause pain or other symptoms that interfere with everyday activities. However, the condition can sometimes cause discomfort or pain, particularly when the accessory nail protrudes vertically from the nail bed. For those who do choose treatment, an option is surgical or chemical matricectomy.

Prevalence

Accessory toenails are observed in people from all ethnicities and races. They are stereotypically endemic to Han Chinese. Studies have suggested its prevalence in certain parts of China to be between 51% and 79%. The condition is equally as common in both males and females.

In mythology

Chinese mythology has it that during the time of the Yellow Emperor, there were two types of people living in China: those who were the descendants of the Yellow Emperor, and those who were nomadic Qiang people. The Henan people, led by Yin Wang, attacked the Qiang and abducted a Qiang woman, who later tried to escape. Yin Wang stabbed her in the abdomen as she tried to escape, so she gave birth to two children with a scar on the small toe of the foot. The children were taken back by Yin Wang for adoption, and their descendants were born with double nails on their small toes.