Smooth Muscle Hamartoma

An 18-month-old, healthy, white boy presented for evaluation of a raised, hairy growth on his left lower back. His mother had first noticed the lesion when he was 2 months old. It had started as pea-sized but grew proportionally with the patient and occasionally caused back pain.

On physical examination, the patient had a 1.4 × 2-cm, skin-colored, firm, dermal plaque studded with fine blond hair (A). Stroking of the lesion did not result in a pseudo-Darier sign. Differential diagnosis included smooth muscle hamartoma, benign congenital nevus, unspecified vascular lesion, mastocytoma, and fibromatosis. His parents deferred biopsy, but at 6-month follow-up, the lesion had grown to 2 × 2.3 cm, and a biopsy was performed.

Histologic sections revealed well-defined smooth muscle bundles in the deep dermis and superficial subcutis, with fascicles oriented in random directions. These aggregates typically surround or attach to follicular units (B-D). The cells are well differentiated and indistinguishable from normal smooth muscle fibers. Positive immunohistochemical staining for muscle-specific actin confirms the smooth muscle nature of the proliferation (E).

Leiomyoma is in the clinical differential; however, unlike smooth muscle hamartomas, which contain scattered bundles of smooth muscle fibers embedded within dermal collagen, a leiomyoma is a discrete mass of interconnecting smooth muscle in wind-swept or whorled arrangements.

Smooth muscle hamartoma (SMH) is a rare, benign, congenital or acquired neoplasm characterized by randomly oriented, hyperplastic smooth muscle bundles in the dermis. Congenital SMH presents in infancy as skin-colored or hyperpigmented, irregularly shaped patches or plaques, usually with hypertrichosis or perifollicular papules.¹ Congenital lesions may be solitary or multiple. They are usually sporadic but have been associated with PTEN-mutation neoplastic syndromes and the Michelin tire baby syndrome.^2,3

Transient piloerection or induration with stroking as a result of smooth muscle contraction (positive pseudo-Darier sign) can help differentiate SMH from other lesions.⁴ Unlike congenital melanocytic nevi, SMHs have no malignant potential.

Atypical presentations may complicate diagnosis.^5,6 No diagnostic criteria exist because of the varied presentations and low prevalence. Biopsy remains the gold standard for diagnosis.

Treatment is not necessary for SMH, but lesions may be surgically excised for cosmetic reasons. In this case, surgery was recommended over further observation because of the hamartoma’s persistent growth and the patient’s occasional back pain. A plastic surgeon excised the lesion without complication. n

References

1. Johnson MD, Jacobs AH. Congenital smooth muscle hamartoma: a report of six cases and a review of the literature. Arch Dermatol. 1989;125(6):820-822.

2. Pilarski R, Burt R, Kohlman W, Pho L, Shannon KM, Swisher E. Cowden syndrome and the PTEN hamartoma tumor syndrome: systematic review and revised diagnostic criteria. J Natl Cancer Inst. 2013;105(21):1607-1616.

3. Nomura Y, Ota M, Tochimaru H. Self-healing congenital generalized skin creases: Michelin tire baby syndrome. J Am Acad Dermatol. 2010;63(6):1110-1111.

4. Schmidt CS, Bentz ML. Congenital smooth muscle hamartoma: the importance of differentiation from melanocytic nevi. J Craniofac Surg. 2005;16(5):926-929.

5. Vivehanantha S, Browne F, Bowen C, et al. A congenital smooth muscle hamartoma masquerading as a reticulate vascular naevus. Clin Exp Dermatol. 2013;38(7):751-753.

6. Holst VA, Junkins-Hopkins JM, Elenitsas R. Cutaneous smooth muscle neoplasms: clinical features, histologic findings, and treatment options. J Am Acad Dermatol. 2002;46(4):477-490.