Neuromuscular choristoma

Benign Triton tumours or neuromuscular choristomas (NMC) are benign expansile peripheral nerve sheath tumours featuring featuring a tight interconnection of skeletal muscle and nerve fibres within the endoneurial sheath. It seemsThey are also known as nerve rhabdomyoma,neuromuscular or ectomesenchymal harmatoma and seem to have a precursor role in desmoid-type fibromatosis.

Terminology

Other acceptable terminologies for the entity include neuromuscular or ectomesenchymal harmatoma or nerve rhabdomyoma¹.

Epidemiology

Benign Triton tumours are extremely rare and are usually seen in childhood or adolescence. There is no gender predilection ¹.

Associations

Neuromuscular choristoma is associated wit desmoid-type fibromatosis ^1-3.

Clinical presentation

Typical clinical features include signs of neuropathy or plexopathy with muscular atrophy as well as progressive pain ^1,2.

Pathology

Benign Triton tumours are peripheral nerve tumours where skeletal muscle fibres are positioned within a nerve and its fibres ^1-3.

Aetiology

Hypotheses about the aetiology of benign Triton tumour include the following ^1,3:

• harmatous overgrowth of muscle spindles
• intraneural entrapment of skeletal muscle during embryogenesis
• myogenic metaplasia of nerve sheath components
• intraneural rhabdomyoma

Location

Neuromuscular choristomas have been reported in large nerves or within the plexus as the sciatic nerve and the brachial plexus. They have been also described in cranial nerves or intraorbital ^1-5.

Macroscopic appearance

Macroscopically benign Triton tumours show a multi fascicular fusiform nerve enlargement with some nerve fascicles showing a red muscle-like appearance. Nerve stimulation causes contraction of the whole nerve in addition to the innervated muscle ¹.

Microscopic appearance

Microscopically benign Triton tumours are characterised by disorganized fascicles of skeletal muscle intertwined with nerve fascicles or neural elements ^1-4.

Immunophenotype

Immunohistochemistry stains are positive for desmin and muscle-specific actin as well as S100, and possibly epithelial membrane antigen (EMA) or β-catenin ^1-3.

Genetics

Most cases of neuromuscular choristomas show a CTNNB1 mutation ^1,2.

Radiographic features

MRI

Typical magnetic resonance imaging features include fusiform enlargement of the nerve with characteristics resembling skeletal musculature, as long as there is no associated desmoid tumour ^1-4.

Signal characteristics

• T1: isointense or slightly hypointense to muscle
• T2: isointense or slightly hypointense to muscle
• T1 C+ (Gd): mild heterogeneous contrast enhancement

Radiology report

The radiological report should include a description of the following:

• form, location and size
• tumour margins and transition zone
• affected nerve/plexus and relations to adjacent neurovascular structures
• interfascicular fat
• associated desmoid tumour if present
• relations to bones

Treatment and prognosis

Neuromuscular choristoma is benign and therefore does not require an overly aggressive approach. Since a trauma or surgery related stimulation of desmoid-type fibromatosis has been considered and the latter requires an aggressive management a do not touch approach has been advocated in patients in whom the diagnosis is clinically and radiologically clear ^1,4.

History and etymology

Neuromuscular choristoma was first described by E. Orlandi in 1895 ^3,4.

Differential diagnosis

Tumours or conditions which can mimic the presentation and/or appearance of a benign Triton tumour include ¹:

• lipofibromatous harmatoma: interfascicular fat
• neurofibroma: contrast enhancement
• schwannoma: contrast enhancement
• rhabdomyoma
• embryonal rhabdomyosarcoma