Phorbol ester-induced orbital myositis.

A single injection of TPA (12-0-tetradecanoyl-phorbol-13-acetate) into the superior rectus muscle of New Zealand white rabbits produced inflammation and edema, followed by fibrosis and muscle restriction. Strain gauge measurements showed a 60% increase in muscle restriction at 24 hours and an increase greater than 400% at 12 weeks. Clinical and histologic characteristics paralleled those of human idiopathic orbital myositis and thyroid orbital myopathy. The combination of initial muscular inflammation and enlargement, subsequent fibrosis and restricted extraocular motility, and histologic evidence of these changes creates a useful model of orbital myositis.