Cutaneous smooth muscle tumors

Benign tumors and hamartomas

Smooth Muscle Hamartoma


  • A hamartomatous lesion composed of a dermal proliferation of mature smooth muscle cells growing in haphazardly arranged bundles

Clinical features


  • Slight male predominance

  • Prevalence of 1 in about 2700 live births

  • The majority of the lesions are congenital

  • Familial occurrence exceptional, with autosomal-dominant inheritance reported in some families


  • Skin-colored indurated patch, plaque, follicular papules, or variably pigmented macules; occasionally associated with hypertrichosis

  • Usually asymptomatic, but can be pruritic

  • Stroking of the lesion can result in transient elevation of the affected area and piloerection due to smooth muscle contraction (pseudo-Darier sign)

  • Induration, hyperpigmentation, and hypertrichosis diminish over time

  • Unusual clinical presentations include linear, atrophic, or a morphea-like plaque

  • Multiple lesions or a generalized presentation very rare

  • The generalized variant occasionally part of the “Michelin-tire baby” syndrome, characterized by deep circumferential skin folds over the trunk and limbs with distinctive clinical appearance, mental retardation, and other developmental abnormalities

  • Predilection for lumbosacral area and proximal extremities

  • Uncommon sites include the head and neck, conjunctiva, nipple, and genital area

  • Rare associated conditions include Becker nevus, congenital melanocytic nevus, and basal cell carcinoma, with the latter two conditions likely to be coincidental

Prognosis and treatment

  • No treatment generally required

  • Surgical excision or laser treatment usually for cosmetic reasons



  • Localized in the dermis, with occasional extension into the subcutis

  • Mature smooth muscle cells, growing in haphazardly arranged bundles

  • Mitotic activity absent

  • Association with morphologically normal hair follicles in about 40%

  • Prominent nerve fibers and collagen bundles usually present among smooth muscle bundles

  • Overlying epidermis normal or displays mild hyperkeratosis, acanthosis, and hyperpigmentation of basal keratinocytes

Immunohistochemistry/special stains

  • Smooth muscle actin, calponin, desmin, and h-Caldesmon positive

Main differential diagnoses

  • Becker nevus

  • Pilar leiomyoma (more orderly proliferation of smooth muscle bundles arranged in ramifying fascicles, intervening collagen more discrete)

Fig. 1

Smooth muscle hamartoma.

Low-power magnification showing multiple dermal bundles of smooth muscle cells identical to arrector pili muscles.

Fig. 2

Smooth muscle hamartoma.

Higher magnification depicting multiple arrector pili muscles.

Pilar Leiomyoma


  • A benign smooth muscle tumor composed of interlacing fascicles of bland spindle cells displaying typical features of smooth muscle differentiation (e.g., cigar-shaped or blunt-ended nuclei and bright eosinophilic cytoplasm)

  • Believed to arise from or differentiate toward the arrector pili muscle

  • Lesions can be solitary or multiple and develop either as a sporadic or inherited condition

Clinical features


  • Most commonly develops in the second and third decades of life

  • Female predominance for multiple lesions, whereas solitary tumors occur more commonly in males

  • Congenital occurrence exceptional, likely overlapping with or representing a variant of congenital smooth muscle hamartoma

  • A subset of pilar leiomyoma(s) occurs in the familial setting, the so-called Reed syndrome or multiple leiomyomatosis

    • Autosomal-dominant mode of inheritance

    • Associated with germline mutation(s) of the fumarate hydratase ( FH ) gene on chromosome 1q42.3-43 acting as a tumor suppressor gene

    • Combination of multiple pilar and uterine leiomyomas, additionally complicated by development of a solitary multifocal or bilateral papillary renal cell carcinoma in roughly up to 20% of patients


  • Firm and smooth, skin-colored, red-brown to violaceous papule(s) or nodule(s)

  • Multiple lesions

    • More common than solitary lesions

    • Size of individual lesion usually up to 1 cm in diameter

    • Often tender or painful, either spontaneously or in response to cold, pressure, or emotional stimuli

    • Show predilection for extensor surface(s) of the extremities and trunk

    • Distribution can be segmental, dermatomal, linear, zosteriform, or more widespread/diffuse

  • Solitary lesions

    • Most commonly develop on the limbs

    • Tend to be larger than multiple tumors

    • Frequently asymptomatic

    • Rarely presentation may be with an indurated plaque

Prognosis and treatment

  • Complete surgical excision curative, but difficult to achieve in multiple tumors

  • Recurrence rate as high as 50% for multiple lesions



  • Dermal-based proliferation, extension into the subcutis infrequent

  • Poorly circumscribed and infiltrative, less often nodular and well circumscribed

  • Interweaving/interlacing fascicles of bland spindle cells displaying typical features of smooth muscle differentiation

    • Cigar-shaped or blunt-ended nuclei

    • Low mitotic activity present in roughly 30% of the lesions (fewer than 1 mitosis per 10 high-power fields)

    • Bright eosinophilic cytoplasm

  • Histological variants

    • Epithelioid

    • Symplastic

      • Pleomorphic cells with large, multilobated, hyperchromatic nuclei; macronucleoli; and intranuclear cytoplasmic pseudoinclusions, usually scattered among bland spindle cells

      • Multinucleated giant cells not uncommon

      • Mitotic activity low (usually fewer than 1 per 10 high-power fields)

      • Necrosis absent

      • Represents degenerative phenomenon

Immunohistochemistry/special stains

  • Positive for smooth muscle actin, desmin, calponin, and h-Caldesmon

  • A small percentage of cases express keratin

Main differential diagnoses

  • Cutaneous leiomyosarcoma

  • Metastatic leiomyosarcoma

  • Cellular dermatofibroma

  • Dermatofibrosarcoma protuberans with myoid nodules

Fig. 1

Pilar leiomyoma.

Low-power magnification showing poor circumscription of smooth muscle bundles.

Fig. 2

Pilar leiomyoma.

Poor circumscription is particularly evident at the periphery of the neoplasm.

Fig. 3

Pilar leiomyoma.

Low-power magnification demonstrating haphazardly arranged bundles of smooth muscle cells in the dermis.

Fig. 4

Pilar leiomyoma.

Note bundles of tumor cells with well-defined, pink cytoplasm and bland vesicular nuclei.

Fig. 5

Pilar leiomyoma.

Tumor cells with characteristic cigar-shaped vesicular nuclei and pink cytoplasm.

Genital Leiomyoma


  • A variant of leiomyoma arising from superficial smooth muscles at genital locations, including scrotum, vulva, and nipple

Clinical features


  • Predilection for middle-aged adults


  • Solitary, slowly growing papule or nodule, less often a pedunculated lesion with surface ulceration

  • Multiple lesions most uncommon

  • Lesions can increase in size during pregnancy

  • Most of the lesions are asymptomatic

  • Nipple leiomyomas

    • Tend to be smaller (generally less than 2 cm)

    • Poorly circumscribed

  • Vulvar and scrotal leiomyomas

    • Size generally between 3 and 5 cm in diameter

    • Well circumscribed

  • Vulvar leiomyoma(s) can be associated with synchronous or metachronous development of esophageal leiomyoma(s), a condition known as esophagovulvar syndrome

Prognosis and treatment

  • Complete surgical excision curative



  • Histological features essentially similar to pilar leiomyomas, including the so-called symplastic change representing a degenerative phenomenon (see corresponding section on pilar leiomyoma)

  • Certain morphological features occur more commonly at genital sites (see below)

  • Vulvar and scrotal leiomyomas

    • More cellular than pilar leiomyomas

    • Tend to be better circumscribed and more nodular but not encapsulated

    • Focal or more widespread epithelioid morphology common

    • Myxoid change frequent, especially during pregnancy

    • Hyalinization of the stroma can be prominent

    • Variably intense perivascular inflammatory cell infiltrate composed of lymphocytes, histiocytes, and eosinophils

Immunohistochemistry/special stains/cytogenetics

  • Smooth muscle actin, calponin, desmin, and h-Caldesmon positive

  • Variable positivity (over 50%) for estrogen and progesterone receptors

  • Androgen receptor positivity in scrotal leiomyomas

  • No consistent cytogenetic change in vulvar leiomyomas, yet the data is limited

Main differential diagnoses

  • Angiomyofibroblastoma

  • Aggressive angiomyxoma

Fig. 1

Nipple leiomyoma.

Numerous variably sized bundles of smooth muscle cells are seen throughout the dermis.

Fig. 2

Nipple leiomyoma.

Bundles are composed of bland smooth muscle cells with round to cigar-shaped nuclei and pink cytoplasm.

Fig. 3

Nipple leiomyoma.

Perpendicular section through smooth muscle bundles reveals tumor cells with round to oval nuclei and abundant well-defined, pink cytoplasm. Note the absence of nuclear atypia and lack of mitotic activity.

Fig. 4

Nipple leiomyoma.

Parallel section through smooth muscle bundles with characteristic bland, cigar-shaped nuclei and brightly eosinophilic cytoplasm.



  • A benign tumor composed of vascular smooth muscle cells and blood vessels

  • Alternatively designated as a vascular leiomyoma or angiomyoma

  • This tumor is now regarded as part of the spectrum of pericytic (perivascular) tumors that include glomus tumor and its variants, and myopericytoma and its variants

Clinical features


  • Slightly more common in females

  • Wide age distribution from 12 to 84 years, the majority present between the fourth and sixth decades of life

  • Congenital presentation exceptional


  • A solitary, slowly growing, and firm subcutaneous nodule

  • Tenderness and pain, often paroxysmal, reported in up to 60% of the lesions, with the remaining lesions asymptomatic

  • Size usually less than 2 cm in about 80% of the lesions

  • Predilection for extremities, in particular, lower extremities (about 70%), followed by upper extremities and head and neck

  • Unusual locations include intracranial site (dura) and intraosseous occurrence

Prognosis and treatment

  • Complete or marginal surgical excision generally sufficient

  • Recurrences exceptional, usually related to incomplete excision



  • Well-circumscribed proliferation in the subcutis, less often in the dermis

  • Two main components are (1) bland smooth muscle cells and (2) blood vessels

  • Smooth muscle cells

    • Grow in interlacing or disorganized bundles

    • Appear to originate from the vessel walls

    • Perivascular concentric arrangement overlapping with myopericytoma occasionally present

    • Pointed nuclei, tapering eosinophilic cytoplasm

    • No cytological atypia or, when present, focal and degenerative in nature (see later)

    • Mitoses generally absent or very limited in numbers

  • Three subtypes of angioleiomyoma can be separated on the basis of the morphology of the vascular channels

    • Solid type

      • Most common type

      • Closely packed bundles of spindled smooth muscle cells

      • Intervening vascular channels with small slitlike lumina and thin vessel walls

    • Venous type

      • Intervascular smooth muscle cells blend with smooth muscle cells of the vascular walls

      • Thick muscular vessel walls

    • Cavernous type

      • Least common type

      • Small amounts of intervascular smooth muscle cells

      • Intervascular smooth muscle cells difficult to separate from smooth muscles cells of the vascular walls

      • Dilated vascular channels

  • Histological variants

    • Epithelioid

    • Clear cell

    • Symplastic

      • Marked degenerative cellular atypia(s) and prominent nuclear pleomorphism

      • Absence of mitotic activity in pleomorphic cells

  • Additional histological features present in a small proportion of the cases

    • Intralesional fat

    • Inflammatory cell infiltrate, composed predominantly of lymphocytes with occasional formation of aggregates

    • Collagen hyalinization

    • Foci of calcification

    • Areas of hemorrhage and/or deposition of hemosiderin pigment

    • A feeding medium-sized artery

Immunohistochemistry/special stains

  • Smooth muscle actin, muscle specific actin, calponin, and h-Caldesmon positive

  • Variable desmin positivity

  • Negative for keratins, S100 protein, CD34, CD31, and HMB45

Genetic profile

  • No consistent genetic changes have been detected

  • Various gains and losses of chromosomes in about one-third of the cases

Main differential diagnoses

  • Leiomyoma

  • Myofibroma

  • Myopericytoma (morphological and immunohistochemical overlap with angioleiomyoma)

Fig. 1


Low-power magnification reveals a well-circumscribed proliferation of smooth muscle cells growing in haphazardly arranged fascicles in the dermis and subcutis.

Fig. 2


In addition to smooth muscle cell proliferation, numerous small blood vessels are present within the lesion with encircling smooth muscle and represent an integral part of the tumor.

Fig. 3


Smooth muscle cell and vascular proliferation—higher magnification.

Cutaneous Leiomyosarcoma (Atypical Intradermal Smooth Muscle Neoplasm)


  • A distinctive smooth muscle tumor localized in the dermis or associated with limited extension into the superficial subcutis (generally less than 5 mm), characterized histologically by mitotic activity, cellular pleomorphism, and, much less often, tumoral necrosis

  • Although such lesions have been classified as cutaneous leiomyosarcoma, they invariably follow a benign clinical course, with possible local recurrence due to incomplete/marginal excision

  • Nevertheless, metastatic risk is absent when confined to dermis

  • Needs to be distinguished/separated from leiomyosarcoma arising predominantly in the subcutis due to more aggressive biological behavior of the latter (see corresponding section)

Clinical features


  • Males predominantly affected (M:F = 2–4:1)

  • Although the age distribution is wide, about 90% of the patients are older than 40 years

  • Extremely rare in childhood

  • Most common in the sixth decade of life


  • Solitary, slowly growing nodule or indurated plaque

  • The majority appear to be asymptomatic, rare lesions painful

  • Size usually less than 2 cm

  • Predilection for trunk and lower extremities, followed by the head and neck and upper extremities

  • Isolated examples developed in the background of traumatic injury, ionizing radiation, lupus vulgaris, smallpox and scrofuloderma scar, tattoo, nevus sebaceous, and chronic venous ulcer

Prognosis and treatment

  • Status of resection margins the most important predictive factor for local recurrence(s)

  • Complete wide surgical excision generally curative, with very low risk of possible local recurrence

  • Recurrence rate about 20% after incomplete/marginal excision, multiple recurrences possible

  • Recurrent tumors can display increased cellularity and deeper localization



  • Pure dermal-based proliferation, occasionally with very limited extension into the subcutis, generally of less than 5 mm in thickness

  • Infiltrative/poorly defined (diffuse) and/or nodular growth pattern

  • Intersecting fascicles of spindle cells ramifying between dermal collagen fibers

    • Nuclear atypia mild to moderate, can be severe on occasion

    • Nuclei cigar shaped or blunt ended with abundant eosinophilic cytoplasm

    • Mitoses present, usually from 2 to 5 mitoses per 10 high-power fields, but their number can vary greatly among lesions

    • Atypical mitoses not uncommon (in up to 50% of lesions)

    • Eccentric perinuclear intracytoplasmic vacuoles occasionally present

    • Tumoral necrosis rare (in less than 5% of the lesions), generally focal

    • Multinucleated giant cells few and scattered

  • A tumor-free, subepidermal grenz zone of variable thickness present in the majority of the lesions

  • Additional occasional histological features

    • Focal epithelioid cell morphology

    • Granular cell change

    • Myxoid stromal degeneration and hyalinization/sclerosis

    • Multinucleated osteoclast-like giant cells

    • Perivascular inflammatory cell infiltrate composed of lymphocytes and histiocytes

  • Desmoplastic variant of leiomyosarcoma

    • Predominance of hyalinized/sclerotic stroma over neoplastic cells

Immunohistochemistry/special stains

  • Smooth muscle actin, calponin, desmin, and h-Caldesmon positive in decreasing order of frequency

  • Up to 50% display focal cytokeratin positivity

  • Rare examples with focal S100 protein positivity also reported

Main differential diagnoses

  • Pilar leiomyoma

  • Cellular fibrous histiocytoma

  • Nodular fasciitis

  • Sarcomatoid carcinoma

Oct 29, 2019 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Cutaneous smooth muscle tumors

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