PSEUDOTUMOR OF THE ORBIT

Idiopathic orbital inflammation or pseudotumor represents a nongranulomatous inflammatory process in the orbit or eye with no known local or systemic causes.8, 31 It is a diagnosis by exclusion based on history, clinical course, response to steroid therapy, laboratory tests, and biopsy in a limited number of cases.⁵² There is a group of diverse disease entities that can mimick pseudotumor,³⁷ which are discussed in the section on differential diagnosis.

Among orbital disorders pseudotumor, after Graves' disease and lymphoproliferative disease, is a common ophthalmologic disease. In three large series of orbital disorders, pseudotumor accounted for 6.3%,²⁵ 5.2%,³⁷ and 4.7%.⁵² The disease usually occurs in adults but may also affect children. Pediatric orbital pseudotumor encompasses about 6% to 16% of orbital pseudotumors.8, 46 In children, there is a higher incidence of bilateral orbital involvement without evidence of underlying systemic disease.

The disease may present acutely, subacutely, or chronically in one orbit or may occur bilaterally. There may be recurrent disease after a 10-year interval.8, 9 The disease can be categorized according to which orbital structure is predominantly involved. According to location, we distinguish myositis (one or more extraocular muscles); dacryoadenitis (lacrimal gland); periscleritis including Tenon's space; trochleitis⁶⁰; and perineuritis (outer dural sheath of the optic nerve and adjacent fat). The disease may be localized to these orbital structures, but frequently there is associated fatty infiltration. The inflammatory process within the orbital fat may be localized simulating a tumor or may be diffuse within the fatty tissue. The disease may occur predominantly anteriorly or posteriorly.

The radiologic evaluation consists of CT and MR imaging. The imaging findings, correlated with the clinical findings, allow a diagnosis in most cases and hence obviate the need for a biopsy.

For elucidation and confirmation of the suspected clinical diagnosis, in our experience CT is the preferred method because of the inherent contrast by different attenuation values of the orbital fat, muscle, bony structures, and air in the adjacent paranasal sinuses. Extraorbital extension, however, especially to the cavernous sinuses is better delineated on MR imaging. At times orbital fatty infiltration and perineuritis are better delineated on fat suppression T1-weighted MR images than on CT scans.