Smooth Muscle Hamartoma (Congenital Smooth Muscle Hamartoma, Acquired Smooth Muscle Hamartoma)

Are You Confident of the Diagnosis?

What you should be alert for in the history

Smooth muscle hamartoma is an uncommon benign cutaneous lesion, characterized histologically by a proliferation of smooth muscle bundles within the dermis. The lesion is typically noted at birth or during infancy, although several cases of acquired smooth muscle hamartoma have been reported.

To establish the diagnosis, it is important to determine the time of onset to help differentiate the lesion from Becker’s nevus, which typically presents peripubertally.

Although lesions appear to grow slightly after onset, hyperpigmentation and hypertrichosis have been noted to diminish with time. Transient induration and piloerection is often observed after rubbing the lesion (pseudo-Darier’s sign) and intermittent myokymia (spontaneous undulations of muscle fibers) was reported as the presenting complaint in one case.

Most lesions are asymptomatic, with occasional reports of pruritus and one acquired case on the sole that was associated with pain and numbness. Almost all reported cases were sporadic, although multiple lesions have been described among three members of the same family.

Characteristic findings on physical examination

The vast majority of lesions present as an irregular well-circumscribed hypertrichotic flesh-colored or lightly-pigmented patch or plaque. Lesions are usually solitary, and are preferentially located on the lumbosacral trunk, buttocks, or proximal extremities (Figure 1, Figure 2). Numerous other locations have been reported, including scrotum, scalp, cheek, sole, and elbow, with rare cases involving multiple sites. Size typically ranges from 1 to 10cm in diameter.

Figure 1.

Smooth muscle hamartoma of the proximal left upper extremity, presenting as coalescing skin-colored papules (The central red plaque is a punch biopsy site). (Courtesy of Sarah Dill, MD)

Figure 2.

Higher magnification view demonstrates the small skin-colored papules. (Courtesy of Sarah Dill, MD)

The lesions less commonly present as perifollicular papules, and rarely as generalized hypertrichosis and excess skin folds. Few reported cases presented in a linear arrangement, and two cases presented as an atrophic plaque. Approximately 80% of reported lesions show pseudo-Darier’s sign. This temporary induration that occurs after rubbing is due to mechanically stimulated arrector pili muscles, whereas true Darier’s sign seen in mastocytosis is caused by the release of histamine, causing localized urticaria.

Expected results of diagnostic studies

On histologic examination, there are numerous well-defined thick smooth muscle bundles of varying orientation, with or without connection to hair follicles, and distributed throughout the dermis. There is usually some retraction from adjacent collagen surrounding the bundles. Bundles of nerve fibers (both myelinated and unmyelinated) may also be present. Acanthosis, slight elongation of the rete ridges, and mild basal hyperpigmentation may be seen (Figure 3, Figure 4). The Masson trichrome stain and muscle actin stains can be used to confirm the presence of the smooth muscle bundles.

Figure 3.

Section of smooth muscle hamartoma demonstrating smooth muscle bundles of varying size and orientation, distributed throughout the dermis (H&E). (Courtesy of Gary Kantor, MD)

Figure 4.

High magnification highlights numerous irregularly distributed smooth muscle bundles in the dermis with retraction from adjacent collagen bundles (H&E). (Courtesy of Gary Kantor, MD)

Diagnosis confirmation

The differential diagnosis includes congenital melanocytic nevus, Becker’s nevus, pilar leiomyoma, mastocytoma, café au lait macules, and nevus pilosus.

Congenital melanocytic nevi may be difficult to differentiate from smooth muscle hamartoma clinically, and therefore biopsy is often recommended. Upon histologic examination, smooth muscle hamartoma demonstrates proliferation of smooth muscle within the dermis, and absence of melanocytic nevus cells. It is important to differentiate these two lesions because congenital melanocytic nevi may carry the risk of malignant transformation and often require surgical excision.

Some authors suggest that Becker’s nevus and smooth muscle hamartoma represent varying phenotypic presentations of the same developmental process. Becker’s nevus is characterized by its peripubertal onset, unlike smooth muscle hamartoma, which typically presents at birth or infancy. However, both congenital cases of Becker’s nevus and acquired cases of smooth muscle hamartoma have been reported in the literature.

There is significant overlap between the lesions both clinically and histologically. Clinically, smooth muscle hamartoma is raised and has less prominent pigmentation and hypertrichosis when compared to Becker’s nevus. Additionally, the geographic predilection for smooth muscle hamartoma differs from that of Becker’s nevus, which is most commonly located on the shoulders, anterior chest, and scapular region. Histologically, Becker’s nevus has more pronounced epidermal changes, and often minimal to varied amounts of dermal smooth muscle.

Leiomyomas typically present in the second or third decades as painful papulonodules rather than asymptomatic plaques. Upon histologic examination, the smooth muscle fibers coalesce to form tumors, unlike the haphazardly scattered smooth muscle bundles seen in smooth muscle hamartoma.

Solitary mastocytoma typically presents at birth or infancy as a yellow-brown nodule that becomes erythematous, raised, and pruritic in response to stimulation (Darier’s sign). The Darier’s sign of solitary mastocytoma is delayed and lasts longer than the pseudo-Darier’s sign seen in smooth muscle hamartoma.

Café au lait macules lack terminal hairs, firmness, and pseudo-Darier’s sign.

Nevus pilosus is a skin-colored hypertrichotic patch that lacks induration or proliferation of smooth muscle, histologically.

Who is at Risk for Developing this Disease?

The incidence of smooth muscle hamartoma has been estimated to range from 1:26,000 to 1:1000 births. The male-to-female ratio, based upon reported cases in one review, was approximately 1.5:1.

The vast majority of lesions are present at birth or are first noted during infancy. Ten cases of acquired smooth muscle hamartoma have been reported in the English language, with age of onset ranging from 16 to 64. Almost all reported lesions were sporadic, with the exception of one family that had three members with multiple lesions.

What is the Cause of the Disease?

The underlying etiology of smooth muscle hamartoma remains unclear.


The lesion most likely represents abnormal proliferation of the arrector pili muscles during fetal maturation. One study found a significant increase in the number of CD34-positive dendritic cells throughout the reticular dermis in three cases of smooth muscle hamartoma. This study speculates that these dendritic cells may play a role in the aberrant development of the arrector pili muscles.

In addition to increased smooth muscle and hair follicles, lesions also demonstrate bundles of both unmyelinated and myelinated nerve fibers. These nerve bundles are associated with the smooth muscle and are likely responsible for the contraction that occurs with mechanical stimulation.

Systemic Implications and Complications

Associations with systemic disease have been reported in cases of diffuse smooth muscle hamartoma. Nine such confirmed cases have been reported in the literature, and presented clinically with diffuse hypertrichosis, skin-colored to slightly hyperpigmented patches, and variable redundant skin folds.

The term “Michelin tire baby syndrome” has been used previously to describe this clinical appearance. Seven of the nine reported cases had associated systemic abnormalities, including skeletal and facial anomalies, growth retardation, lax joints, seizures, inguinal hernias, and pigmentary retinopathy.

Treatment Options

Treatment options are summarized in Table I.

Table I.
Medical Treatment Surgical Procedures Physical Modalities
Surgical excision Cosmetic laser therapy to reduce hypertrichosis, hyperpigmentation, or both (see Becker’s nevus treatment).

Optimal Therapeutic Approach for this Disease

The first step in the treatment of smooth muscle hamartoma is to confirm the diagnosis. Because most lesions present at birth or infancy, they are difficult to differentiate from congenital melanocytic nevus, which may carry risk of malignant transformation. It is therefore necessary to biopsy suspect lesions in order to confirm the diagnosis of smooth muscle hamartoma histologically.

Smooth muscle hamartoma is a benign skin lesion and no risk of malignant transformation has been reported. Almost all cases are asymptomatic and therefore treatment is typically unnecessary.

Most lesions are only lightly hyperpigmented with mild hypertrichosis and often do not require cosmetic therapy. Inform the patient that the hypertrichosis and hyperpigmentation often diminish over time.

For cosmetically unacceptable lesions, treatment can be divided into removal of excess hair and reduction of hyperpigmentation. Cosmetic treatment approach of congenital smooth muscle hamartoma parallels that of Becker’s nevus. See Becker’s nevus chapter for specific modalities.

In cases of diffuse smooth muscle hamartoma, the patient should be evaluated carefully for any associated developmental anomalies. Such anomalies should be investigated and managed with the relevant specialist, where appropriate.

Surgical excision of this benign lesion is not recommended.

Patient Management

Congenital smooth muscle hamartoma is a benign skin lesion. Once the diagnosis is confirmed histologically, regular follow-up is unnecessary.

In approaching cosmetic therapy, the patient should be informed that pigmentation and hypertrichosis diminish with time. Before cosmetic therapy, the patient should also be informed of the various risks and expected outcomes associated with each specific treatment modality.

Unusual Clinical Scenarios to Consider in Patient Management

Cases of diffuse smooth muscle hamartoma are commonly associated with developmental anomalies, including skeletal and facial anomalies, growth retardation, lax joints, seizures, inguinal hernias, and pigmentary retinopathy. If such anomalies are suspected or identified, it is important to consult the relevant specialist for proper diagnosis and treatment.

What is the Evidence?

Holland, KE, Galbraith, SS. “Generalized congenital smooth muscle hamartoma presenting with hypertrichosis excess skin folds, and follicular dimpling”. Pediatr Dermatol. vol. 25. 2008. pp. 236-9. (Case report and review of generalized smooth muscle hamartoma)

Zvulunov, A, Rotem, A, Merlob, P, Metzker, A. “Congenital smooth muscle hamartoma: prevalence, clinical findings, and follow-up in 15 patients”. Am J Dis Child. vol. 144. 1990. pp. 782-4. (Case series of fifteen patients with congenital smooth muscle hamartoma followed over a 9-year period, and review of the literature)

Koizumi, H, Kodama, K, Tsuji, Y, Matsumura, T, Nabeshima, M, Ohkawara, A. “CD-34 positive dendritic cells are an intrinsic part of smooth muscle hamartoma”. Br J Dermatol. vol. 140. 1999. pp. 172-4. (Case letter discussing the association of CD-34 positive dendritic cells with smooth muscle hamartoma and the potential relationship to its underlying etiology)

Schmidt, CS, Bentz, ML. “Congenital smooth muscle hamartoma: the importance of differentiation from melanocytic nevi”. J Craniofac Surg. vol. 16. 2005. pp. 926-9. (Case report with comprehensive discussion of the differential diagnosis of smooth muscle hamartoma)

Gualandri, L, Cambiaghi, S, Ermacora, E, Tadini, G, Gianotti, R, Caputo, R. “Multiple familial smooth muscle hamartomas”. Pediatr Dermatol. vol. 18. 2001. pp. 17-20. (Case report of multiple familial smooth muscle hamartomas)

Lee, D, Kim, SH, Hong, SK, Sung, HS, Hwang, SW. “A case of acquired smooth muscle hamartoma on the sole”. Ann Dermatol. vol. 21. 2009. pp. 78-80. (Case report and review of acquired cases of smooth muscle hamartoma)

Kienast, A, Maerker, J, Hoeger, PH. “Myokymia as a presenting sign of congenital smooth muscle hamartoma”. Pediatr Dermatol. vol. 24. 2007. pp. 628-31. (Case report of smooth muscle hamartoma with myokymia [spontaneous intermittent skin movement caused by smooth muscle contraction] as the presenting complaint)