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Benign hamartomaCongenitalICD-10 Q82.x

Smooth muscle hamartoma

Congenital smooth muscle hamartoma · CSMH · smooth muscle naevus

Congenital smooth muscle hamartoma is a benign hamartoma of the arrector pili / dermal smooth muscle, presenting at birth or in early childhood as a slightly hyperpigmented, hypertrichotic patch with a characteristic transient pseudo-Darier sign (rippling / induration on stroking). It is typically lumbosacral or sacroiliac. Becker naevus is its acquired counterpart, often appearing in adolescence. Recognition prevents over-investigation as Becker, congenital melanocytic naevus or other pigmented hamartomata.

CurrentLast reviewed 16 May 2026

Pathology

  • Increased number of mature smooth-muscle bundles in the dermis, arising from arrector pili muscle.
  • Slight epidermal hyperpigmentation; thickened keratinocytes.
  • Increased vellus / terminal hairs (hypertrichosis).
  • No atypia, no mitoses, no melanocytic proliferation.
  • IHC: SMA+, desmin+; bland morphology.

Clinical features

  • Congenital (or early childhood onset) hyperpigmented patch / plaque.
  • 1-10 cm diameter, well-demarcated.
  • Hypertrichosis / increased vellus hair common.
  • Sites:
    • Lumbosacral (commonest).
    • Sacroiliac.
    • Trunk, proximal limbs.
  • Pseudo-Darier sign: transient induration, rippling or piloerection on stroking the lesion — pathognomonic.
  • Asymptomatic; cosmetic concern.
  • Stable over time; usually unchanged into adulthood.

Differentials

  • Becker naevus — acquired, adolescent onset; typically shoulder / upper trunk; can be associated with Becker naevus syndrome.
  • Congenital melanocytic naevus — pigmented, hairy patch from birth; biopsy distinguishes.
  • Naevoid hypertrichosis — hypertrichosis only.
  • Café-au-lait macule — flat, uniform pigment, no hypertrichosis or pseudo-Darier.
  • Cutaneous mastocytosis — positive Darier sign (urticarial wheal); biopsy with mast-cell IHC.
  • Hairy faun-tail — lumbosacral hair tuft over spinal dysraphism — important to exclude with MRI.

Investigations

  • Clinical diagnosis with pseudo-Darier sign.
  • Skin biopsy if diagnostic uncertainty (e.g. to differentiate from CMN, mastocytosis).
  • Spinal MRI if midline lumbosacral lesion with risk features of dysraphism (faun tail, dimple, capillary malformation, lipoma).
  • Photography for follow-up.

Management

  • Reassurance — benign hamartoma.
  • No malignant potential; no malignant transformation reported.
  • Counsel about pseudo-Darier sign — explain phenomenon.
  • Cosmetic management:
    • Laser hair removal for hypertrichosis.
    • Q-switched / picosecond laser for pigment.
    • Surgical excision rarely indicated (cosmetic).
  • If midline lumbosacral + dysraphism risk features: refer for spinal MRI / paediatric neurosurgery.

References

  1. Stokes JH. Nevus pilaris with hyperplasia of nonstriated muscle. Arch Dermatol Syph. 1923;7:479-481.
  2. Berger TG, Levin MW. Congenital smooth muscle hamartoma. J Am Acad Dermatol. 1984;11:709-712.
  3. Zvulunov A et al. Congenital smooth muscle hamartoma: prevalence, clinical findings, and follow-up in 15 patients. Am J Dis Child. 1990;144:782-784.
  4. Schmidt CS, Bentz ML. Congenital smooth muscle hamartoma. J Pediatr Surg. 2004;39:E15-E17.

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