ARTICLE
Auteur(s) : Junna Okada1,
Yoichi Moroi1, Jun Tsujita2, Masakazu
Takahara1, Kazunori Urabe1, Hiromaro
Kiryu3, Masutaka Furue1
1Kyushu University Graduate School of Medicine,
Department of Dermatology, Graduate School of Medical Sciences,
Kyushu University, Maedashi 3-1-1, Higashi-ku, 812-8582 Fukuoka,
Japan
2Department of Dermatology, Inatsuki Hospital, Kama,
Japan
3Kiryu Clinic, Department of Dermatopathology, Chihaya
5-14-25, Higashi-ku, 813-0044 Fukuoka, Japan
accepté le 27 Novembre 2007
Hair follicle nevus is an extremely rare hamartoma with
follicular differentiation [1-3]. It is often congenital and
characterized histologically by many tiny mature hair follicles [1,
3]. Trichofolliculoma is also a hamartoma that differentiates into
hair follicles, and its histological features are very similar to
those of hair follicle nevus. Therefore, controversy still exists
among dermatopathologists whether hair follicle nevus is a distinct
entity or a simple variant of trichofolliculoma [4, 5]. In the
past, the term “hair follicle nevus” was often used for what we now
call trichofolliculoma and accessory ears [6-10]. A central cyst
and radiating small hair follicles are the main and indispensable
characteristics of trichofolliculoma [6, 11, 12]. In 1984, Pippione
et al. [1] first reported a case of hair follicle nevus in which
the histopathological findings in serial sections were distinct
from those of trichofolliculoma. However, no clinical or
dermoscopic characteristics of hair follicle nevus have until now
been reported in the literature. We encountered a case of hair
follicle nevus diagnosed by dermoscopic and histological
appearance. This is the first report to demonstrate the dermoscopic
manifestation of this clinical entity.
Case report
A 26-year-old Japanese man was seen for treatment for two small
nodules on his nose that had been present since childhood. There
was no tendency to enlarge and they did not cause pain. The patient
had suffered from atopic dermatitis since he was 3 years old, and
underwent an operation for bilateral cataract at the age of 23.
Physical examination revealed two brown soft nodules of 4 mm
and 3 mm in diameter, respectively, located on the right nasal
ala (figure 1A).
Dermoscopy showed a large number of uniform hair follicles and an
interfollicular ‘pseudo-pigment network’ (figure 1B). The lesions
were completely excised and examined histologically.
Hematoxylin-eosin staining showed mild epidermal acanthosis,
proliferation of well-differentiated hair follicles and sebaceous
glands of various sizes in the dermis, and an increase in melanin
pigment in the upper dermis (figures 2A and B). Nevus
cells were completely absent both in the epidermis and in the
dermis. Serial sections did not reveal a central cyst or cartilage.
These findings allowed us to diagnose this case as hair follicle
nevus.
Discussion
Hair follicle nevus is a very rare hamartoma that presents all
stages of hair follicles in tissue sections [1-3, 11, 12]. Thus
far, about 20 cases have been reported. The hamartoma is often
present at birth, mostly on the face. It is defined as a crowding
of small, well-differentiated vellus hair follicles, sometimes
accompanied by sebaceous glands, smooth muscles, and sweat glands
[1, 4, 11, 12]. Since these histological features also are seen in
accessory ears and trichofolliculoma, hair follicle nevus has been
confused histologically with them [4-6, 13] (table 1). Accessory ear appears as a
normal-colored papule or nodule, mostly in the preauricular area,
and occasionally a central cartilaginous core can be palpated. Its
histological appearance shows a more complex structure, not only
with pilar complexes but also with cutaneous appendages in
different stages of development, and even with adipose tissue [12,
14]. When no cartilage is found, it is quite difficult to make a
distinction from hair follicle nevus. In such cases, diagnosis can
be made only by anatomic location. In our case, skin lesions were
not located in the pre- or peri-auricular areas but on the nose, so
a differential diagnosis of accessory ear could be rejected.
Trichofolliculoma is a benign hamartoma that arises mostly in
adults on the face, scalp and neck [13]. Clinically, it has one or
more silky, white, thread-like hairs growing out of the central
opening [6]. Histopathological findings are characterized by a
cystic cavity lined by stratified squamous epithelium (including a
granular layer) connecting to the surface epithelium [4, 11]. The
cavity has keratinous debris [13] and hair shaft fragments inside
[15]. Also, secondary budding with immature hair differentiation
can be observed [12, 15]. Our case did not satisfy the
characteristics of trichofolliculoma clinically or histologically,
in that there was neither a central opening nor a cyst. Therefore
we ruled out trichofolliculoma as a differential diagnosis. The
terminology of “hair follicle nevus” is very confusing. In 1924,
Fessler [16] observed a 12-year-old girl with a papule above the
tragus of the left ear, which showed many hair follicles
accompanying a small piece of cartilage histologically. In 1928,
Gans [17] defined Fessler’s case as ‘hair follicle nevus’ or ‘hair
nevus’ (“Haarnaevus”). Supposedly, that histological pattern is now
called accessory ear. Later, in 1944, Miescher described a case and
gave a new name, ‘trichofolliculoma,’ for the lesions, which
previously had been referred to simply as “folliculoma” [4]. In
1984, Pippione et al. [1] described a typical case of hair follicle
nevus and showed the difference from trichofolliculoma by examining
the specimen in systematically prepared serial sections at various
orientations [4]. In 1993, however, Ackerman suggested that hair
follicle nevus was only a derivative of the periphery of
trichofolliculoma [4]. In other words, he argued that all of the
small follicles in hair follicle nevus correspond to the radiated
secondary follicles in trichofolliculoma, and that there therefore
must be a central cyst or canal present in hair follicle nevus in
the rest of the tissues. However, it should be emphasized that hair
follicles are more regularly located or distributed in hair
follicle nevus than in trichofolliculoma, in which each follicle’s
direction is random. Moreover, in trichofolliculoma only the single
central hair follicle is connected to the outside, whereas in hair
follicle nevus many follicles are open to the skin surface. This
histological difference is apparently evident clinically and by
dermoscopy. According to the Consensus Net Meeting on Dermoscopy
2001, the term “pigment network” stands for a honeycomb grid of
brown to black ‘lines’ over a diffuse lighter background
(hypopigmented ‘holes’), and its dermoscopic image correlates
histologically either with melanin pigment in keratinocytes or in
melanocytes along the dermoepidermal junction [18, 19]. In
contrast, the term “pseudo-pigment network” or “pseudonetwork” is
used for pigmented lesions of the face in which the pigmented
“lines” are thick and hypopigmented hair follicles are prominent.
If this pattern is observed, it means that there are many follicles
open to the skin surface. In our case, a ‘pseudo-pigment network’
with many follicular openings was clearly observed by dermoscopy.
Although both trichofolliculoma and hair follicle nevus are
hamartomas with hair follicle differentiation and with similar
histological features, it appears reasonable to distinguish them.
We do not agree with Ackerman’s argument that there must always be
a central cyst or canal in each hamartoma. To identify the presence
or absence of the central cyst, we propose that dermoscopy is a
more practical and helpful method than serial sectioning.
Dermoscopy may improve diagnostic accuracy in the clinical
evaluation of hair-related hamartomas.
Table 1 The clinical and histological characteristics
of hair follicle nevus, trichofolliculoma and accessory ear
|
Hair follicle nevus
|
Trichofolliculoma
|
Accessory ear
|
|
Definition
|
Hamartoma
|
Hamartoma
|
Malformation
|
|
Clinical presentation
|
Dome-shaped or pedunculated papule to nodule
|
Dome-shaped small nodule with central cavity
|
Subcutaneous nodule occurring mostly in preauricular area
|
|
Histological findings
|
Many small hair follicles in the dermis
|
A dilated hair follicle and radiated secondary hair follicles
|
Crowding of hair follicles, sebaceous glands and cartilage
|
Acknowledgements
Conflict of interest: none. Financial support: none.
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