A hypertrichotic, lightly hyperpigmented patch on the flank

Issue: October 2012

ByMarissa J. Perman, MD

A 6-month-old, healthy male presents to your clinic with a birthmark on the right flank. His parents became aware of the birthmark shortly after birth due to the increased hair density. There have been no significant changes in size or appearance since presentation. It does appear to be growing proportionately with him and is asymptomatic. On physical examination, you appreciate a 4-cm, fairly well-defined, slightly hyperpigmented thin plaque with subtle overlying textural change and hypertrichosis (Figure 1). Upon further stroking, you notice the lesion becomes slightly more indurated and pink. The patient does not appear bothered by the changes you induced when rubbing.

Can you spot the rash?

Diagnosis: Congenital smooth muscle hamartoma

Congenital smooth muscle hamartomas (CSMH) are benign skin tumors seen at or shortly following birth, most commonly located on the trunk or proximal extremities. CSMH consist of proliferations of smooth muscle bundles around the arrector pili muscles. CSMH are most commonly confused with congenital melanocytic nevi due to their variable hyperpigmentation and hypertrichosis.

Marissa J. Perman

It is important to distinguish these two entities, as CSMH confer no malignant potential and are not routinely excised. In contrast, some congenital melanocytic nevi require excision due to potential lifetime malignant risk. The redundant skin folds seen in children with Michelin tire syndrome also demonstrate a smooth muscle hamartoma histologically when biopsied.

CSMH are characteristically 1 cm to 6 cm in size, skin-colored, tan or light-brown patches or thin plaques with overlying prominent vellus hairs. The hairs may be dark or light. Rarely, perifollicular papules have been described. There is often a subtle textural component or induration present, which may be highlighted with tangential lighting. Upon stroking, most lesions demonstrate elevation, erythema or papulation, known as the pseudo-Darier’s sign due to transient piloerection (see Figure 2). This is in contrast to the Darier’s sign seen with mastocytomas, in which stroking of a lesion produces histamine release causing a delayed wheal and flare that often leads to pruritus of the lesion. The pseudo-Darier’s sign tends to diminish with age.

In Figure 1, a fairly well-defined, several centimeter, slightly elevated, hypertrichotic, thin pink plaque on the flank.

Source: Perman MJ

Histologically, CSMH are composed of fascicles of well-defined smooth muscle bundles in the dermis, often arranged haphazardly and in close proximity to hair follicles. Epidermal thickening (acanthosis) and epidermal hyperpigmentation may also be seen. The histologic features seen in CSMH are also seen in Becker’s melanosis, also known as a Becker’s nevus. A Becker’s nevus typically presents around puberty in males as a hyperpigmented, hypertrichotic patch usually located on the shoulder, chest or scapular region. Whether Becker’s nevi and CSMH are part of the same entity remains controversial. Many consider the two hamartomas to be at opposite ends of a spectrum.

In Figure 2, the pseudo-Darier’s sign after stroking a congenital smooth muscle hamartoma.

Other lesions on the clinical differential include congenital melanocytic nevi, café-au-lait macules and a solitary mastocytoma. Café-au-lait macules and mastocytomas are not associated with hypertrichosis. Mastocytomas tend to be more brown or reddish-brown and exhibit the Darier’s sign, as described above. Café-au-lait macules are usually tan and do not have any associated textural changes. Congenital melanocytic nevi are often associated with hypertrichosis and textural change but do not demonstrate the pseudo-Darier’s sign. In most cases, congenital melanocytic nevi tend to have pigmentation that is less subtle than CSMH.

It is important for physicians to be aware of and recognize CSMH to educate families about their benign nature and avoid referral for excision in most cases.

For more information:

Holst VA. J Am Acad Dermatol. 2002;46:477-490.
Johnson MD. Arch Dermatol. 1989;125:820-822.
Kharfi M. Pediatr Dermatol. 2005;22:245-249.
Schmidt CS. J Craniofac Surg. 2005;16:926-929.

Marissa J. Perman, MD, is an attending physician at The Children’s Hospital of Philadelphia. She can be reached at permanm@email.chop.edu.
Disclosure: Perman reports no relevant financial disclosures.