Ainhum (from Portuguese, pronounced /aj.ˈɲũ/[1]), also known as dactylolysis spontanea,[2] is a painful constriction of the base of the fifth toe frequently followed by bilateral spontaneous autoamputation a few years later.

The groove begins on the lower and internal side of the base of the fifth toe, usually according to the plantar-digital fold.

The rate of spread is variable, and the disease may progress to a full circle in a few months, or still be incomplete after years.

Slight pain is present in the earliest stage of ainhum, caused by pressure on the underlying nerves.

[2] Histology shows a change in the prickle cell layer, and this is responsible for the laying down of condensed keratin causing the groove.

Soft tissue constriction on the medial aspect of the fifth toe is the most frequently presented radiological sign in the early stages.

In grade III lesions osteolysis is seen in the region of the proximal interphalangeal joint with a characteristic tapering effect.

Ainhum has been much confused with similar constrictions caused by other diseases such as leprosy, diabetic gangrene, syringomyelia, scleroderma or Vohwinkel syndrome.

[5] Wearing shoes to protect barefoot trauma has shown decrease in incidence in ainhum.

[8] The first description of ainhum in the West appears to have been provided by English surgeon Robert Clarke, who made a passing reference to "dry gangrene of the little toe" as a common occurrence in the Gold Coast in an 1860 report to the Epidemiological Society of London, but did not recognize it as a distinct entity and believed it to be a consequence of "suppressed yaws".

[9][10][11] Ainhum was first recognized as a distinct disease and described as such in detail by Brazilian physician José Francisco da Silva Lima (1826–1910), in 1867.