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- Immune-mediated disorder
- Associated with hyperthyroidism in Graves disease
- Can be seen in hypo or euthyroid individuals
- Orbitopathy often precedes actual thyroid disease
- M:F = 1:4
- 4th to 5th decade
- Most common cause of bilateral or unilateral proptosis
- Deposition of mucopolysaccharides (hyaluronic acid) within extra-ocular muscles
Imaging
- Fusiform muscle enlargement with sparing of tendon
- Inferior > medial > superior > lateral recti involved
- Isolated lateral rectus involvement (RARE – look for other causes)
- Increase of intra-orbital fat and fat stranding giving a dirty appearance