Is there a difference between habit tic and median canaliform nail dystrophy of Heller?

Autoaggressive behaviours targeting the nails are a well-defined group of conditions in the large field of body-focused repetitive behaviours (BFRB). They are usually divided into habit tic deformity (HTD), nail chewing (onychophagia), the group of onychotillomania – onychoteiromania – onychotemnomania, and overzealous manicure.

HTD often includes picking of the perionychial skin.^[1] The characteristic habit-tic involves a persistent pushing back of the proximal nail fold by pressing the nail of another finger (usually the thumbnail of the opposite hand) onto the proximal nail plate, and then moving it forcefully proximally to push the cuticle back. This leads to a series of changes in the nail and the proximal nail fold [Figure 1]. There is a loss of the sharp angle of the proximal nail fold’s free margin, with the disappearance of the cuticle and apparent thickening of the nail fold. This results in a detachment of the nail fold’s free margin from the underlying nail plate. Subsequent changes include an excessively long and large lunula (mega lunula) and a thumbnail that appears markedly longer than wide [Figure 2]. Serial median transverse furrows appear over the nail plate, with a central depression, giving the nail surface the appearance of a washboard. The medial depression and the shallow longitudinal furrows occur due to chronically repeated trauma on the nail plate, which is still soft when it emerges from under the proximal nail fold.

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Figure 1:

Schematic illustration of (a) a normal nail plate surface for reference; and (b) the appearance in cases with habit tic deformity; (c) median canaliform dystrophy; and (d) solenychia.

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Figure 2:

Nails with longitudinal median or paramedian dystrophy. (a) Thumbnail showing median canaliform dystrophy demonstrating a median split in the proximal part of the nail plate with oblique furrows giving the aspect of a fir tree. (b) Another thumbnail showing habit-tic deformity with enormous enlargement of the lunula, loss of the cuticle and wash-board appearance due to serial short transverse furrows and signs of skin picking. (c) Thumbnail with habit-tic deformity seen from the side exhibits a deep depression in the proximal nail region, thickening of the free margin of the proximal nail fold, loss of cuticle and severe perionychotillomania. (d) Great toenail showing a paramedian intraungual fibrokeratoma causing ‘solenonychia’. (e) Thumbnail showing onychopapilloma in median location. (f) Traumatic nail split.

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These usually cover at least one-third of the nail’s width. The proximal nail plate just in front of the nail fold may appear sinking inwards, accentuating the transition from the nail fold, which appears thickened. In advanced cases, the proximal nail fold’s free margin may be completely pushed back to the depth of the nail pocket, completely obliterating it. This results in roughness involving the medial one-third to one-half of the nail plate because the most apical matrix, responsible for producing the shiny superficial nail plate, is damaged. Intense manipulation may even lead to superficial lamellar nail plate defects, similar to elkonyxis. This HTD is often associated with skin picking, also known as perionychotillomania.

In contrast, Heller’s median canaliform (thumb) nail dystrophy (MCD) is characterised by a single longitudinal fissure or break in the thumbnail, with no depression in the plate. It starts at the cuticle, and slowly grows distally. Obliquely oriented, proximally directed furrows originate from the median fissure, giving the nail the appearance of a “fir-tree” [Figure 1]. This longitudinal crack often does not reach the free margin of the nail plate. In MCD, transverse furrows are not a characteristic feature, the nail is not longitudinally depressed, and the lunula may or may not be increased in size. Obvious pushing back or thickening of the free margin of the proximal nail fold is not apparent. In most cases, the cuticle is intact [Figure 2].

Signs of perionychotillomania are usually lacking, although some cracking of the cuticle may be seen.^[2] Thus, these two conditions can be distinguished largely based on their clear morphological differences.^[3] Unfortunately, this is often not the case as depicted by the two manuscripts submitted to this issue of JONS.^[4,5] A thorough literature search of these two conditions showed that the confusion has been prevalent in a number of other publications [Table 1].^[2,6-43] All the references were not available as full text^[44-57], also the images were not clear.^[58]

On searching ‘median canaliform nail dystrophy’ on Pubmed, the term solenonychia also appears. However, on further search, it was found to be coined for a nail disorder characterised by a paramedian canal in the plate of one nail.^[10] The figures in this publication show a narrow, sharply demarcated canal in the big toenail. Evaluating the photographs revealed an intraungual fibrokeratoma, a diagnosis not related to Heller’s MCD or HTD. The author in this report had discussed the term MCDN, elaborating that his term “solenonychia” is more representative. Unfortunately, the correct diagnosis seems to have been missed.

A regular longitudinal depression in the nail may also be seen as a result of pressure on the nail matrix by myxoid pseudocysts. In type B lesions, a rupture of the pseudocyst into the nail pocket is frequent, leading to a temporary decrease in the pressure on the matrix. This results in interruptions in the longitudinal depression, intermittently. However, observations show this type of midline depression to be completely different from the transverse furrows of the HTD. Distal nail splits are also commonly seen in cases of onychopapilloma. However, these are also different from the proximally beginning crack of Heller’s MCD. A variety of subungual tumours may also cause a longitudinal split in the nail plate due to the volume of the neoplasm. Similarly, longitudinal scars as well as cicatricial pterygium may also occasionally cause a (para)median split in the nail plate; however, the characteristic oblique furrows are lacking.

Both HTD and the MCD are caused by repeated trauma. This is more obvious in cases with HTD, but less so in Heller’s MCD. Virtually all patients with BFRB tend to reject the autoaggressive etiology of their complaints; however, HTD patients may eventually acknowledge this after counseling. The type of mechanical trauma in MCD is much more difficult to ascertain. Most patients are truly not aware of a specific behaviour leading to this outcome. Consistent protection of the nail region with a foam tube, an occlusive dressing such as skin-colored suture strips, or an artificial nail, invariably leads to improvement and, finally, the disappearance of the median nail split. Sometimes, even therapies not specifically indicated, such as antivirals or retinoids, have been reported to produce improvement, as the patient is made aware of the condition, and takes care not to traumatise the thumbnail [Table 1]. In contrast, stopping the habit-tic is more difficult. The patient may not be aware of the habit and it requires consistent, empathetic explanation to recognise this behaviour, without causing offense. Many different measures have been proposed. These include simple mechanical devices, behaviour-altering psychotherapy, and even psychopharmacologic drugs. Recently, N-acetyl cysteine has been used for obsessive-compulsive disorders with some effect. The usual dose for adults is 1800–2400 mg per day.^[59]

To conclude, habit-tic deformity and median canaliform dystrophy are distinct entities, which can be recognized clinically, based on subtle differences. This recognition aids appropriate management of these cases.