Idiopathic orbital inflammation

Idiopathic orbital inflammation (IOI), also known as orbital pseudotumour and nonspecific 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 aetiology is not known but an association with many inflammatory/autoimmune diseases is reported.

Many terms are used interchangeably in the literature to refer to IOI including orbital pseudotumour, nonspecific orbital inflammation and orbital inflammatory syndrome.

Patients typically present with rapid-onset, usually unilateral (~90% of cases), painful proptosis and diplopia. IOI 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:

  1. lacrimal pseudotumour
  2. anterior pseudotumour: immediately behind the globe
  3. posterior pseudotumour: distinguished from Tolosa-Hunt syndrome in that the cavernous sinus is spared
  4. diffuse pseudotumour
  5. myositic pseudotumour: predominantly involve the EOMs and therefore mimic thyroid associated orbitopathy (TAO) but unlike TAO it also involves the tendons 

The condition has been associated with many wider inflammatory and autoimmune conditions including:

Imaging demonstrates enlargement of the muscle belly of one (or more) extraocular muscles with involvement of their tendinous insertions. Involvement of the tendinous insertion distinguishes it from thyroid-associated orbitopathy (TAO) in which the insertion point is spared. Additional inflammation can be seen in surrounding tissues, including the lacrimal gland.

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 in 25% of cases.

Reported signal characteristics include:

  • T1: affected region typically isointense (to extra-ocular 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 marked difusse 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 pseudotumour afterwards in 1930 7.

One of the main differential diagnosis of idiopathic orbital inflammation is orbital lymphoma. There is a 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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Article information

rID: 1781
System: Head & Neck
Synonyms or Alternate Spellings:
  • Idiopathic orbital inflammatory disease
  • ISIO
  • Idiopathic Sclerotic Inflammation of the Orbit (ISIO)
  • OIS
  • Orbital inflammatory syndrome
  • Pseudotumour of orbit
  • Orbital pseudotumours
  • Orbital pseudotumors
  • Orbital pseudotumor
  • Inflammatory pseudotumor of the orbit
  • Orbital inflammatory pseudotumour
  • Nonspecific orbital inflammation
  • NOIS
  • IOI

Cases and figures

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    Case 1: coronal
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    Case 1: axial
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    Case 2
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    Case 3
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    Case 4
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    Case 5
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    Case 6: myositic pseudotumour
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