Orbital pseudotumour

Orbital pseudotumour is an idiopathic inflammatory condition that usually involves the extraocular muscles although, in some cases there is inflammatory change involving the uvea, sclera, lacrimal gland and retrobulbar soft tissues. The exact aetiology is not known.

Clinical presentation

Patients typically present with rapid onset usually unilateral (approximaltely 90% of cases) orbital pain, proptosis and diplopia. 

Pathology

Histologically acute lesions demonstrate lymphocyte (which can be mistaken for orbital lymphoma), plasma cell, and giant cell infiltration. Most cases resolve rapidly with treatment (usually steroids 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.

Classification

Division into a number of sub groups 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)

Associations

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

• sarcoidosis
• Wegener's granulomatosis
• systemic lupus erythematosus (SLE)
• polyarteritis nodosa (PAN)
• dermatomyositis
• rheumatoid arthritis (RA)
• sclerosing cholangitis
• Riedel thyroiditis
• mediastinal fibrosis
• retroperitoneal fibrosis

Radiographic features

Imaging demonstrates enlargement of the muscle belly of one (or more) occular muscles with involvement of the tendinous insertion. 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.

MRI

Reported signal characteristics include

• T1: affected region typically iso ⁴ to hypo intense ³
• T2: affected region typically hypo intense ³

Differential diagnosis

General imaging differential considerations include

• Tolosa-Hunt syndrome: related condition with involvement of the cranial nerves in the cavernous sinus and resulting ophthalmoplegia.
• thyroid associated orbitopathy (TAO): spares the tendinous insertions and not usually painful
• orbital sarcoidosis
• orbital lymphoma
• orbital rhabdomyosarcoma
• orbital metastases
• orbital cellulitis