Abstract

A rudimentary accessory or double nail of the little toe is not rare, although only described three times before. Most cases are accidentally detected and only few patients seek help because they have discomfort or pain. Some have a positive family history, but most patients cannot give any information concerning heredity. Clinically, the nail of the little toe is abnormally wide and is split or shows a longitudinal depression corresponding to a slight protuberance of the cuticle. Histopathology shows a complete though short nail. The treatment of choice is segmental excision of the entire accessory nail unit with mobilization of the lateral skin and primary suture.

Introduction

Modern civilization has resulted in poor foot health with many painful conditions and acquired malformations. These are often taken for granted by the affected individuals until they cause discomfort and pain. Often, one condition leads to another and they mutually aggravate each other. Flat and splayfoot are extremely common leading to callus formation in the middle of the forefoot sole, hallux valgus and external rotation of the little toe. Some patients develop a callus or even a corn on the lateral aspect of the little toe that may become painful, particularly when wearing pointed shoes.

In the last 20 years, many patients who had a callus on the lateral aspect of the distal phalanx of the little toe and a peculiar wide little toenail with a split between the lateral and median third were incidentally seen.

Materials and Methods

The author's photographic archive from April 2000 until October 2013 was searched for the diagnosis of ‘double nail’ or ‘double little toenail’. The charts of the patients seen after 2005 were retrieved and evaluated for gender, age, nationality, race, familiarity and treatment. Surgery, either excision or phenolization, was recommended for symptomatic cases. All surgical specimens were examined histopathologically with hematoxylin and eosin and PAS stains. Immunohistochemistry was performed in three cases.

Results

Patients with double little toenails were seen in all countries visited, irrespective of skin complexion and race (table1). Most cases were detected during an examination for another reason, some patients showed their little toes during foot examination, and only very few consulted because of this condition.

The age of the patients ranged from 5 to 68 years; those individuals that were aware of their condition stated that their toes had always been like this or that they could not remember when it had started. Familial occurrence was only seen when the patient was accompanied by a family member, except for 2 patients with symptomatic double nails who reported that their father and grandfather, or mother and aunt, respectively, also had the same condition.

Clinically, a wide little nail was by far the commonest presentation (fig.1,2). Close inspection and dermatoscopy revealed that the nail consisted of two parts with the medial being bigger than the lateral one. They were divided by a longitudinal depression or a real split and the cuticle protruded slightly distally in this particular location. Two really distinct nails were rare (fig.3,4). The expression was usually symmetrical on both sides, although roughly one fourth showed a more marked picture on one side.

Discomfort was noted by approximately one third and pain by one fifth of the, mostly elderly, patients. Most of them had a splayfoot with external rotation of the little toe, with an almost vertically growing nail exerting pain (fig.5). They also often had a circumscribed callus lateral to the nail (fig.1), which was usually misdiagnosed as a hard corn. Palpation and probing showed that it was the nail, not the callus, that was painful. A radiograph was taken in 10 patients; however, the quality of the X-ray films of the little toe did usually not allow specific details to be detected. Three patients exhibited a lazy Y at the tip of the distal phalanx and 1 a thorn-like bony excrescence. In 2 patients, a Y-shaped tip of the terminal phalangeal bone was identified during surgery.

Most patients did not request treatment. However, for those who accepted treatment, either phenolization of the accessory matrix or complete resection of the double nail was performed. Healing was uneventful in all cases, taking about 10 days for the surgical excision and 3 weeks for the chemocautery. Normal physical activity was taken up 2-3 days after phenolization (fig.6,7).

Differential diagnosis comprises traumatic double nail, ectopic nail and nail spiculum after incomplete extirpation of the lateral matrix horn. Histologic examination of the surgical specimens showed virtually all components of a normal nail in the most pronounced cases and only an invagination of a thick hyperkeratotic epidermis with a broad stratum granulosum and a narrow cone of typical nail plate in the center in small double nails (fig.8). There were all transitions in between these two extremes. The fully developed double nail had a normal or moderately thickened matrix epithelium and a normal though short nail bed with a disproportionately long isthmus. The hyponychium was clearly discernable. The proximal nail fold was well developed, with a relatively thick cuticle and a pronounced true and false eponychium. Fungal elements were seen in PAS-stained sections in 1 patient. Immunohistochemistry showed a CD10-positive onychodermis and a weaker CD34 positivity in the matrix and proximal nail bed dermis. Glomus bodies and neural structures were seen as in normal nails.

Discussion

A hereditary dysplasia of the fifth toenail identical to our observations was first published in 1969 [1]. Surprisingly, descriptions of a double little toenail are rare [2,3], despite its relatively common occurrence. It was thought to be a feature of Han Chinese [3], but among our patients, there was just one Han Chinese colleague who showed his feet during an observational stage. However, double little toenails were observed in dermatological practices in Germany, Norway, the Netherlands, Switzerland, Belgium, Portugal, and Thailand as well as in immigrants from a variety of other countries including Africa (Benin). Thus, this is certainly not a special racial or ethnic feature, although it has to be stated that our results do not allow conclusions to be drawn as to the frequency of double little toenails in different countries, continents and races.

Although rarely stated, the condition appears to be autosomal dominant and occurring in families, both in males and in females. In our study, women predominated, but this is probably due to the fact that they have more often acquired foot problems and wear narrow pointed fashionable shoes. Two mothers came with their healthy children, one daughter and one son, who both had hitherto unnoticed asymptomatic double little toenails like their mothers. However, the expression was variable; whether this is a genetically variable expression or just due to the age is not clear. As most patients did not even mention their double little toenails, it can be assumed that they are not embarrassed by them. In our cases, they were between 2 and 7 mm wide. Symptoms did not necessarily depend on the width of the nail but rather on the severity of accompanying foot anomalies.

Treatment, if requested, is easy. We either performed a selective matrix phenolization using 88% liquefied phenol for a total of 3 times for 1 min under complete anemia with a tourniquet or by segmental excision of the accessory nail including the entire double nail with the matrix, proximal fold and hyponychium. The defect was closed primarily with triple sutures after mobilization of the lateral aspect of the distal phalanx. Triple sutures allow 5-0 suture material to be used as much of the tension is exerted and distributed while tying a single knot.

Pathomechanisms and genetics are interesting aspects to be considered. Fully developed supernumerary digits develop nails. It is known that a bone core has to be present at the distal phalanx for the development of a nail anlage; atelephalangia is associated with lack of nail formation or nail hypoplasia [4,5], although total complete anonychia also occurs without bone changes [6]. Whether or not an additional bone core, which may be found in the bifid tip of the distal phalanx of the little toe, is sufficient to induce an accessory nail remains a matter of speculation at this moment. On the other hand, nail epithelium also has a special role in bone formation and digit regeneration. Nail cells were indeed found to influence underlying mesenchymal cells to regenerate digit bone [7].

Another interesting question is how this condition develops. Whether the double little toenail represents the most initial form of a hexadactyly has not been investigated. If it is a rudimentary hexadactyly, it may induce an extra nail anlage. One patient stated that a radiograph taken for a hallux valgus operation had shown a spine-like extension at the lateral aspect of the terminal phalanx of the little toe. Hexadactyly is sometimes hereditary [8]. Another interesting point is that in congenital onychodysplasia of the index finger (Iso-Kikuchi syndrome), also known as COIF, a Y-shaped distal phalanx of the index finger is associated with a defective index nail, such as hemimicronychia and several other types of onychodysplasia [9,10].

In conclusion, a double or accessory little toenail is a rather common though underdiagnosed and rarely reported condition. It is autosomal dominant with variable expression. Symptoms such as pain and discomfort are infrequent. Histological examination of these double little toenails shows short but otherwise normal nails or less well developed nail structures.

Statement of Ethics

The study was performed in accordance with the Helsinki Declaration and Good Clinical Practice.

Disclosure Statement

The authors have no conflicts of interest to disclose.

References