The anterior scalene muscle in thoracic outlet compression syndrome. Histochemical and morphometric studies.

Histochemical studies and morphometric fiber type analysis were done on biopsy specimens of anterior scalene muscle (ASM) from patients with thoracic outlet compression syndrome (TOS), without structural abnormality. Hypertrophy and atrophy factors were determined from muscle fiber histograms of ASM from controls, patients with TOS, and after scalene tenotomy. Scalene muscle from patients with TOS showed marked type 1 (tonic contracting) fiber predominance (85.1% +/- 5.1%) and type 1 fiber hypertrophy (55.6 +/- 2.7 microns). After tenotomy there is a reduction of type 1 fiber representation, selective atrophy in the type 1 fiber system (atrophy factor, 0.66 +/- 0.24), and increase of type 2 fibers. These distinctive changes indicate that ASM is uniquely structured in fiber composition to sustain prolonged contraction. The ASM in patients with TOS demonstrates an extraordinary adaptive transformation and recruitment response in the type 1 fiber system reflecting chronic increased tone or motor neuron stimulation. These observations form a basis for clarifying the structural and pathophysiologic changes in TOS.