A case of diffuse pilar leiomyoma or acquired smooth muscle hamartoma?

A 62-year-old woman presented with a skin-coloured indurated asymptomatic plaque, 150 mm in diameter on the knee. Histological analysis of a skin biopsy taken from the lesion showed haphazardly arranged bundles of smooth muscle in the deep dermis, characteristic of a pilar leiomyoma. Cutaneous pilar leiomyomas are rare, benign smooth muscle tumours arising from the arrector pili muscle, which usually appear as red or brown papules < 15 mm in diameter. This case is unusual in its clinical appearance and size, being at least 10 times larger than the largest previously documented cutaneous pilar leiomyoma. Similar lesions have also been previously reported under the term 'acquired smooth muscle hamartoma' (ASMH). These cases are also rare and have no known consistent clinical features. Histologically it is difficult to differentiate between ASMH and leiomyomas, and some authors would consider this case as a new report of ASMH. We feel, however, that as the lesion is composed of only one element and has grown out of proportion with the growth of the limb, it is more in keeping with a true tumour rather than a hamartoma. We suggest the term 'diffuse pilar leiomyoma' may be more appropriate in this case, representing a new clinical variant of cutaneous pilar leiomyoma.