Neuromuscular choristoma

Neuromuscular choristomas (NMC), also known as benign triton tumours, nerve rhabdomyomas and neuromuscular or ectomesenchymal hamartomas are benign expansile peripheral nerve sheath tumours featuring a tight interconnection of skeletal muscle and nerve fibres within the endoneurial sheath. They also seem to have a precursor role in desmoid-type fibromatosis.

Epidemiology

Neuromuscular choristomas are extremely rare and are usually seen in childhood or adolescence. There is no gender predilection ¹.

Associations

Neuromuscular choristoma is associated with 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

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

Aetiology

Hypotheses about the aetiology of neuromuscular choristomas include the following ^1,3:

• hamartomatous 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 neuromuscular choristoma shows a multifascicular 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 neuromuscular choristoma is characterised by disorganizeddisorganised 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 ^1-4,6.

Signal characteristics

• T1: isointense or slightly hypointense to muscle
• T2: isointense or slightly hypointense to muscle
• T1 C+ (Gd): mild heterogeneous contrast
  • ​no or minimal enhancement in neuromuscular choristoma alone ¹
  • strong enhancement in cases with superimposed desmoid fibromatosis ^1,6

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-related stimulation of desmoid-type fibromatosis has been considered and the latter requires 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

NeuromuscularA case of a neuromuscular choristoma of the sciatic nerve was first described by the Italian physician E. Orlandi in 1895 ^3,4,7.

Differential diagnosis

Conditions that may mimic the presentation and/or appearance of a neuromuscular choristoma include ^1,6:

• lipofibromatous hamartoma: interfascicular fat
• rhabdomyoma
• embryonal rhabdomyosarcoma