Idiopathic orbital inflammation

Last revised by Tariq Walizai on 15 Jun 2024

Idiopathic orbital inflammation, also known as orbital pseudotumor, or idiopathic orbital inflammatory syndrome and non-specific orbital inflammation, is an idiopathic inflammatory condition that most commonly involves the extraocular muscles. Less commonly there is inflammatory change involving the uvea, sclera, lacrimal gland, and retrobulbar soft tissue.

The exact etiology is not known but an association with many inflammatory/autoimmune diseases is reported.

Many terms are used interchangeably in the literature to refer to idiopathic orbital inflammation including orbital pseudotumor, non-specific orbital inflammation and orbital inflammatory syndrome.

The condition has been associated with other inflammatory and autoimmune conditions:

Patients typically present with rapid-onset, usually unilateral, painful proptosis and diplopia. In a minority of cases (8-20%) bilateral involvement may occur either simultaneously or metachronously 8-10. Idiopathic orbital inflammation is a diagnosis of exclusion; atypical presentation, poor response to treatment with corticosteroid and recurrence should prompt biopsy to exclude other diseases.

Histologically acute lesions demonstrate lymphocytes (which can be mistaken for orbital lymphoma), plasma cells, and giant cell infiltration.

Division into a number of subgroups according to location has been proposed:

Imaging demonstrates enlargement of the muscle belly of one (or more) extraocular muscles typically with the involvement of tendinous insertions. Involvement of the tendinous insertion distinguishes the idiopathic orbital inflammation from thyroid-associated orbitopathy in which the insertion point is spared. However, sparing of the anterior tendon does not exclude the diagnosis of idiopathic orbital myositis 7.

Additional inflammation can be seen in surrounding tissues, including the orbital fat, lacrimal gland, and optic nerve sheath 9.

It can appear as an infiltrative mass and extends outside of the orbit via superior or inferior orbital fissures. Extension into the cavernous sinus, meninges, and dura can occur. It is most commonly unilateral but can be bilateral (either at the same time or sequentially) in 8-20% of cases 8-10.

Reported signal characteristics include:

  • T1: affected region typically isointense (to extraocular muscles) 1 but can also be hypointense 1-3

  • T2: affected region typically hypointense due to fibrosis and with more progression of fibrosis it becomes more hypointense, but the signal can also be iso- to hyperintense to extra-ocular muscles 2

  • T1 C+ (Gd): moderate to a marked diffuse enhancement 

Most cases resolve rapidly with treatment (usually corticosteroids suffice) although in a subset with more chronic progression chemotherapy and radiotherapy may be required. A degree of residual fibrosis can be demonstrated, especially in the more refractory cases.

The disease was first described by Birch-Hirschfeld et al. in 1905 6. They also introduced the term orbital pseudotumor afterward in 1930 7.

One of the main differential diagnoses of idiopathic orbital inflammation is orbital lymphoma. There is considerable overlap between these entities both clinically and radiologically. However, orbital lymphoma usually presents as a progressive orbitopathy rather than acutely, is more often bilateral, shows lower values on ADC, and does not respond to corticosteroid.

Other imaging differential considerations include:

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