Congenital onychodysplasia of the index finger (COIF, or Iso-Kikuchi disease) is a rare disorder characterized by the following five criteria:
Unilateral or bilateral involvement of the index fingers
Possible hereditary component
Affected individuals and their family members can be reassured that there is usually little functional impairment and that for most affected individuals, the disorder is mainly of cosmetic concern. Sometimes, bone or nail abnormalities need to be addressed with surgery. Certain nail devices can help treat associated pincer deformity. More research needs to be performed to better understand the etiology and hereditary basis of some cases.
Congenital onychodystrophy of the index finger (COIF), a term Kikuchi et al coined in 1974,  identifies a clinical syndrome consisting of nail dysplasias of the index fingers associated with underlying bone abnormalities. The first case report of this condition is attributed to Kamei.  Iso collected a series of patients with this condition and made the first attempt to define the clinical syndrome. Kikuchi et al observed additional variations in the manifestations of the nail dysplasia and noted the associated radiographic abnormalities of the distal phalanges of the index fingers. The spectrum of underlying bone abnormalities has since been expanded to include the following:
A Y-shaped bifurcation of the distal phalanx on lateral view
Narrowing and terminal enlargement of the distal phalanx on anteroposterior view
Brachymesophalangia (shortening of the middle phalanx of the fifth digit)
Syndactyly of the index and other fingers
The clinical spectrum of nail abnormalities has expanded to include malalignment and deformities of nails other than the nails of the index fingers.
Although no evidence from the Iso's series indicates hereditary transmission of congenital onychodystrophy of the index finger,  Millman and Strier reported a five-generation family in which nine individuals had congenital onychodystrophy of the index finger.  The investigators observed an autosomal dominant pattern of inheritance with variable penetrance.
If data from small series, case reports, and retrospective reviews over 30 years are summarized, the four cardinal features of congenital onychodystrophy of the index finger are the following:
Unilateral or bilateral hypoplasia of the index fingernails
Deformities of the nails on other fingers
Radiographic abnormalities of the distal bony phalanx on the affected fingers
Congenital occurrence, which can be either hereditary or sporadic
A genetic mechanism most likely underlies the pathophysiology of congenital onychodystrophy of the index finger (COIF). [5, 6] . Most cases are sporadic, but autosomal dominant transmission has been described. Two other proposed theories are (1) in utero ischemic injury and (2) in utero exposure to teratogens.
In utero ischemic injury
On the basis of two cases of syndactyly associated with digital artery hypoplasia and another case of traumatic injury leading to arterial hypoplasia and permanent nail loss, Kitayama and Tsukada proposed that congenital onychodystrophy of the index finger is due to in utero ischemic injury.  Kikuchi proposed that ischemia occurs because of an abnormal fetal grip in which the thumbs are pressed on the index fingers during the critical period in the development of the distal phalanx and nail.
Although the association of ischemic injury and nail malformation is of considerable interest, the abnormal grip theory is not consistent with the current understanding of human developmental biology. Bones are mesodermal derivatives, and limbs are the result of the apical ectodermal ridge exerting an inductive influence on limb mesenchyme. This development occurs early in fetal life and therefore seems to exclude the possibility Kikuchi suggested, that is, a grip of the fingers from the fetus causes malformation. Such bony malformations would most likely be present prior to the fetus' ability to grip.
In utero exposure to teratogens
Franceschini et al reported a boy with bilateral hypoplasia of the index fingers associated with mild mental retardation, inguinal hernia, macrocephaly, medial flaring of the eyebrows, esotropia of the left eye, long palpebral fissures, malar hypoplasia, a high-arched palate, clinodactyly of the fifth fingers, and a simian palmar crease.  Radiographs of the hands revealed a hypoplastic terminal tuft on both index fingers. They reported this case as congenital onychodystrophy of the index finger and suggested that the other associated anomalies expanded the definition of the congenital onychodystrophy of the index finger syndrome.
Although maternal exposure to antiepileptic drugs (AEDs) was not specifically mentioned in this case report, the findings are consistent with fetal AED syndrome, and, in fact, hypoplasia of the distal digits is a well-recognized finding in the broad spectrum of fetal AED syndrome. The teratogenicity of AEDs is probably related to either their antifolate effects or the metabolic generation of toxic metabolites such as epoxides (arene oxides), especially in individuals who lack epoxide hydrolase. Although no data link in utero AED exposure to the narrow spectrum of congenital onychodystrophy of the index finger, other, as yet unidentified, in utero exposures remain a possible cause of the genetic damage that allows expression of congenital onychodystrophy of the index finger, especially in mothers with epoxide hydrolase deficiency.
A report from 2014 suggests that hepatitis B may have played a role in the development of congenital onychodysplasia of the index fingers.  The role of infection is unclear but warrants further consideration.
The true prevalence of congenital onychodystrophy of the index fingers (COIFs) in the United States is unknown.
Most case reports of congenital onychodystrophy of the index finger are from Japan, but whether this predominance is because of increased awareness of the syndrome in Japan or actual prevalence is unclear. A 2008 case report from Brazil may be the first case of congenital onychodystrophy of the index finger reported from South America.  An Israeli registry that monitors all congenital anomalies included three cases among approximately 71,000 live births between 1977 and 1997 (4.2 cases per 100,000 live births). Whether this rate is accurate for other areas of the world is unknown. 
Most case reports of congenital onychodystrophy of the index fingers are from Japan. However, whether Japanese individuals are affected more often than others or whether Japanese clinicians are simply more aware of congenital onychodystrophy of the index finger syndrome than others (given where the syndrome was first identified) is unclear. Cases have also been reported in whites and in people of East Indian descent.
Both sexes are equally affected by congenital onychodystrophy of the index fingers.
Congenital onychodystrophy of the index fingers is congenital.
The prognosis is excellent, and patients and their families should be reassured that limitations usually are minimal. The absence of an index fingernail rarely affects the patient's ability to perform functions. Often, the middle fingernail can be used with the thumb if a pincer grip is required. Congenital onychodystrophy of the index finger (COIF) is of cosmetic significance only.
Identification of the syndrome and reassurance for the patient are all that is required. Affected individuals should be advised to clip nails short so as to avoid needless trauma as dystropic nails catch on surfaces while performing activities of daily living. Use of artificial nails to camouflage the problem or using devices composed of shape-memory alloys can help minimize problems resulting from nail curvature.
For patient education resources, visit the Skin Conditions and Beauty Center.
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