Neuroradiology

Orbital pseudotumour

(also called inflammatory orbital pseudotumour), idiopathic inflammatory disorders of the orbit, defined as nonspecific inflammatory conditions for which no identifiable cause or systemic disease can be found. The histopathology of these disorders ranges from polymorphous inflammatory cells and fibrosis, with a matrix of granulation tissue, eosinophils, plasma cells histiocytes, germinal follicles and lymphocytes to a predominantly lymphocytic form. The lymphocytic form has been found to harbour sometimes malignant lymphomas in particular when it is steroid resistant. The morphological appearance of pseudotumour includes a variety of forms with related different clinical pictures. A classification based on structures involved distinguishes six different types:

  • anterior, when only the anterior orbit and the globe are involved by inflammation;

  • diffuse, when all the retrobulbar fat is filled;

  • myositic, characterized by the uni- och bilateral enlargement of one or more of the extraocular muscles in particular mono- or bilaterally;

  • apical, when infiltration involves selectively the orbital apex;

  • dacryoadenitic, when the lacrimal gland is selectively involved; and

  • perineuritic, when the inflammation is particularly concentrated around the optic nerve.

    Classical clinical signs of pseudotumour include pain, proptosis, lid swelling, painful ophthalmoplegia, decreased vision and chemosis, varying according to the area affected. Usually the symptoms and signs are responsive to steroid therapy.

    Imaging

    From an imaging point of view both CT and MR show pathological tissue with marked enhancement. The morphostructural characteristics are not specific and problems of differential diagnosis arise according to the site of involvement: the diffuse tumefactive type is very difficult to differentiate from lymphoma on the basis of imaging alone, and also from other tumours such as cavernous haemangioma, haemangiopericytoma and optic nerve sheath tumours. Absense of deformation of the globe and/or bony structure and iso- to hypointensity of MR signal on long TR images with respect to orbital fat in comparison with a brighter appearance of tumours may be factors to consider in the differential diagnosis. The myositic type of pseudotumour must be differentiated from orbit thyroidopathy which is slower in onset, not associated with pain and usually associated with a systemic diathesis although it may be absent in 10% of cases (euthyroid ophthalmopathy). Other helpful morphological distinguishing features include a medial bowing appearance, extension of the inflammatory involvement to the tendon insertion of the muscle on the globes and fluffy and ragged margins of inflammation of pseudotumour myositis with respect to a more fusiform appearance of thyroid myositis with sharply defined borders. When the lacrimal gland is affected the differential diagnosis ranges from lymphoma to viral and bacterial dacryoadenitis and specific lacrimal gland inflammation such as in Sjogren's syndrome and sarcoidosis. The accessibility of the lacrimal gland to biopsy makes this the easiest and most reliable tool for diagnosis. Finally the perineuritic type of pseudotumour must be distinguished from meningioma of the nerve sheaths in which pain is usually absent as well as ophthalmoplegia, and also from optic neuritis in which pain and ophthalmoplegia may be present.

    Distinguishing features are in this case exacerbation of pain by retrodisplacement of the globe and the presence of mild proptosis which is usually absent in optic neuritis.

    FS