Linear atrophoderma of Moulin: an underrecognized entity

LAM is a rare disorder with reports published only in dermatologic literature. The
classic cutaneous findings of LAM most commonly arise during childhood or adolescence
and primarily affect the trunk and limbs. These pigmented and atrophic band-like lesions
follow the lines of Blaschko and develop over a finite time period after which they
cease to progress though new lesions may still develop. Typically, the lesions develop
without preceding inflammation and are devoid of subsequent induration or sclerosis.
However, Browne and Fisher have reported 2 cases of LAM preceded by a clinical and
histologic inflammatory phase and have proposed that the disease has inflammatory
and non-inflammatory variants 3].

Histologically, LAM is characterized by hyperpigmentation of the basal cells along
with slight thickening of the collagen fibers in the dermis and a sparse perivascular
lymphocytic infiltrate. No clear signs of dermal atrophy are typically seen on histologic
examination of routine LAM specimens. Rather, the atrophic appearance on clinical
exam is likely secondary to a reduction in subcutaneous tissue which has been demonstrated
by ultrasound imaging 4]. In contrast, microscopic examination of linear scleroderma lesions reveals thickened
and closely packed collagen bundles accompanied by the hallmark finding of atrophic
eccrine glands, hair follicles and periappendageal fat. It should be mentioned that
Atrophoderma of Pasini and Pierini clinically and histologically resembles LAM except
that it does not adhere to Blaschko’s lines.

Clinically, LAM is an important entity to consider in the differential diagnosis of
linear scleroderma. This subtype of morphea, mainly seen in children and adolescents,
presents with linear streaks that typically involve the limbs or trunk 5]. The lesions eventually evolve into atrophic and hyperpigmented bands that may become
difficult to distinguish from the lesions seen in LAM. However, the preceding inflammation,
sclerosis and induration that accompany linear scleroderma appear to be absent in
LAM.

Like many other diseases that follow Blaschko’s lines, LAM is postulated to result
from somatic mosaicism whereby a mutation in early embryogenesis gives rise to two
or more genetically distinct cell populations in the same individual 6]. This mosaic state with disease likely resulting from subsequent exposure to an initiating
trigger may also represent the underlying pathogenesis of linear scleroderma. The
exact post-zygotic mutation(s) and secondary events that give rise to the cutaneous
changes associated with LAM and linear scleroderma are still unknown and the nosological
relationship between LAM, linear scleroderma and APP remains to be determined.

Thus far, no effective treatment options have been discovered for LAM. High-dose penicillin,
topical steroids, heparin and oral potassium benzoate have all been tried without
success 6]. Recently, methotrexate was reported to partially improve the clinical appearance
of widespread LAM in a 20 year old female 7]. However, its true effectiveness still remains to be ascertained. In contrast, the
lesions of linear scleroderma are routinely treated with systemic steroids and methotrexate
with measurable clinical improvement 8]. This therapeutic distinction is important as LAM is a benign self-limited disorder,
confined to the skin, with mainly cosmetic concern whereas linear scleroderma may
extend beyond the skin and affect the underlying muscle and bone. Consequently, unlike
linear scleroderma, LAM lesions that overly joints are not worrisome as they do not
lead to joint contractures due to the lack of sclerosis and subsequent tightening
of overlying tissues.