Imaging

• Enlarged extraocular muscles; tendinous insertion often spared

• Orbital fat normal, diffusely increased opacity or increased in quantity

• Occasional slight bowing of the medial orbital wall (lamina papyracea); the "Coca-Cola bottle" sign

• Frequent inferior rectus enlargement on axial scan, the mass of which may simulate an orbital tumour

• Crowding of the optic nerve, at the orbital apex, by enlarged extraocular muscles

• Lacrimal gland rarely enlarged, but often prolapsed forwards

• Fat prolapse from the orbit into the cranium at the superior orbital fissure

• Absence of orbital masses, vascular anomaly or sinus involvement orbital walls (Box 11.1). Enlargement of the posterior part of the medial rectus is most likely to crowd the orbital apex and cause optic neuropathy (Figure 11.2a) and direct coronal CT scans are valuable for showing "crowding" of the optic nerve at the orbital apex, with loss of the fat spaces, in compressive optic neuropathy (Figure 11.2b). MRI scans, particularly STIR (short-tau inversion recovery) sequences, may provide an indication of the water content of extraocular muscles - this being a reflection of the degree of inflammatory myositis - but the relatively costly investigation adds little to clinical examination. Likewise, B-mode ultrasonography may be used to assess the size of the anterior part of the extraocular muscles, but provides poor images of the posterior orbital structure.

Figure 11.2 (a) Axial and (b) coronal CT for a patient with compressive optic neuropathy, shown in Figure 11.1. All extraocular muscles are enlarged and there is loss of the fat planes around the optic nerve at the orbital apex.

Figure 11.2 (a) Axial and (b) coronal CT for a patient with compressive optic neuropathy, shown in Figure 11.1. All extraocular muscles are enlarged and there is loss of the fat planes around the optic nerve at the orbital apex.

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