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Case 22

Orbital pseudotumour. Incidental ethmoidal mucocoele

Painful proptosis in a young woman

This woman presented in her thirties with a short history of painful swelling of the left eye. This had been present for only a few days. Her vision was intact but the eye was congested on examination and slight proptosis was present.

An MRI was performed to assess her further. Some sample images are presented

• axial T2 weighted images of the brain
• coronal STIR images of the orbits
• coronal T1 weighted images of the orbits pre and post contrast

Findings

1. The left lateral rectus muscle demonstrates abnormally high signal on T2w images. The other muscles are not involved. The muscle enhances markedly following gadolinium.
2. There is abnormal signal from the left ethmoid air cells. There is intermediate signal on T1w and T2w images, with no enhancement.
3. There is differential enhancement of the mucosa in the right and left nostril.

Diagnosis

1. Orbital pseudotumour (orbital myositis type).

2. Incidental orbital mucocoele.

3. Normal nasal cycle.

Discussion

Orbital pseudotumour

Orbital pseudotumour is an inflammatory condition of the intraorbital soft tissues, of unknown aetiology, characterised by a lymphocytic infiltrate. It accounts for 25% of cases of unilateral exopthalmos in adults, being one of the commonest caused of an intraorbital mass in an adult. It predominantly affects young females.

It presents with painful ophthalmoplegia, proptosis and chemosis. The involved tissues may include:

• Retrobulbar fat in 76%
• Intraocular muscles in 57%
• Optic nerve in 38%
• Uveal scleral area in 33%
• Lacrymal gland in 5%

There are two forms:

1. Tumefactive form. Pseudotumour behind the globe, either a discrete mass or poorly defined.
2. Optic myositis form (as seen here). Enlargement of muscles. The main differential is thyroid eye disease, which is said to spare the muscle insertions on the globe, and which affects the inferior and medial rectus muscles first.

The disease usually responds promptly to steroids, but may become relapsing and remitting, or progressive, resulting in a frozen orbit. It has been reported to progress to lymphoma on occasion.

Mucocoele

Mucocoeles occur mainly in the frontal sinuses, but 25% are found in the ethmoid air cells. They have a variety of signal characteristics, depending on relative water and protein content, which may reflect chronicity.

• Low T1, high T2 = Water
• Intermediate/High T1, high T2 = 5-25% protein
• Intermediate/High T1, intermediate T2 = 25-40% protein
• Low T1, low T2 = >40%, a thick paste

Nasal cycle

This is a normal physiological cycle in which first one side of the nose, then the other, becomes more vascular and swollen, that is well described in Stark and Bradley. The cycle lasts from 30 minutes to several hours. It explains the frequent finding of higher signal on one side of the nose than the other on T2w images - if you haven't noticed, look harder next time you report your scans!

References

Som PM, Curtin HD. Sinuses. In Stark DD, Bradley WG (Eds) Magnetic Resonance Imaging (2nd Ed). St Louis: Mosby Year-Book, 1992. pp 1114-24.

Downie AC, Howlett DC, Banerjee AK. Case of the month: A painful red eye. British Journal of Radiology 1995: 68; 1131-2.

Dahnert W. Radiology Review Manual (2nd Ed). Baltimore: Williams and Wilkins, 1993.

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