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Soft tissue tumors

Skeletal muscle

Neuromuscular hamartoma

Reviewer: Vijay Shankar, M.D. (see Reviewers page)
Revised: 18 February 2013, last major update November 2012
Copyright: (c) 2003-2013, PathologyOutlines.com, Inc.


● Rare developmental lesion of mature skeletal muscle and nerve


● Not a WHO diagnosis
● Also called benign Triton tumor (malignant Triton tumor refers to rhabdomyosarcoma plus MPNST), neuromuscular choristoma

Clinical features

● Usually < 2 years old, affects brachial plexus or sciatic nerve
● May involve large cranial nerve trunks and present as intracranial mass
● Very rarely involves peripheral nerve and presents as small nodules
● Aggressive behavior

Case reports

● 15 year old boy with tumor of trigeminal nerve (AJNR Am J Neuroradiol 2005;26:967)
● 36 year old woman with tumor of trigeminal nerve (Acta Neurochir (Wien) 2006;148:83)
● 51 year old man with migraine (Cephalalgia 2012;32:171)
● 61 year old man with bilateral exophthalmos and lid retraction (Orbit 2010;29:168)

Treatment and prognosis

● Biopsy for diagnosis plus observation, may develop fibromatosis after biopsy or complete excision
● May recur, mandating regular followup after excision

Gross description

● Circumscribed, firm, gray-brown-white, multinodular, attached to nerve

Micro description

● Multiple nodules, each 3-5 mm, separated by narrow bands of connective tissue
● Nodules are composed of fascicles of striated muscle of varying size with nerve fibers (myelinated or not) within same perimysial fibrous sheath
● Stroma may be more cellular with bland spindle cells and resemble fibromatosis

Micro images

Nodules of skeletal muscle and neural elements that subdivide into smaller nodules by narrow bands of connective tissue

Nodules contain fascicles of striated muscle fibers of variable size and nerve fibers (with or without myelination) in same perimysial fibrous sheath

Rare case with haphazardly distributed skeletal muscle and nerve fibers, cellular stroma and bland spindle cells with scant cytoplasm and oval nuclei

Nerve fibers are S100+

Positive stains

Striated muscle: desmin and muscle specific actin
Nerve: S100, neuron specific enolase, neurofilament protein

Differential diagnosis

Embryonal rhabdomyosarcoma: poorly and moderately differentiated cells
Fetal rhabdomyoma: usually not intimately associated with nerves, S100 negative

End of Soft tissue tumors > Skeletal muscle > Neuromuscular hamartoma

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