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As I mentioned, thyroid eye disease is the most common

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cause of unilateral proptosis.

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One of the other causes of proptosis and involvement

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of the muscle is orbital pseudotumor.

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This also goes by the name idiopathic,

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orbital inflammation or IOI or nonspecific orbital

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inflammation and so eye or orbital inflammatory syndrome,

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ois, so you may see any of those on the request slip.

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For a patient who has a painful eye,

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this is a distinguishing feature about orbital pseudotumor.

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As opposed to thyroid eye disease,

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orbital pseudotumor is a painful, inflamed, red,

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irritated eye idiopathic or orbital inflammation

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can affect any part of the orbit.

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It could affect the eyelid, it can affect the lacrimal sac,

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it can affect the lacrimal gland, it can affect the.

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conjunctiva, it can affect the retina,

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it can affect the optic nerve,

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it can affect the optic nerve sheath,

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it can affect the extraocular muscles and in point of fact

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involvement of the lacrimal gland and the extraocular muscles

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is the most common form of orbital pseudotumor

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or orbital inflammatory condition.

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In this case on the CT scan we see a process which is

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involving the lateral rectus muscle that would be unusual

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for involvement with thyroid eye disease.

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Remember, involvement of the lateral rectus muscle usually occurs

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less commonly than the other extraocular muscles.

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This muscle is enlarged and it is enlarged even wider as

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that muscle parts on the globe itself involving the

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tendinous insertion of the lateral rectus muscle

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with inflammatory soft tissue that also.

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Be different than thyroid eye disease which generally

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spares the tendinous insertion of the muscle.

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As you can see on the coronal image,

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this lesion affects quite a bit of the muscle cone and

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therefore we are seeing confluent involvement

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of inferior rectus muscle,

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superior rectus muscle and lateral rectus muscle from

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approximately 11:00 to 07:00 on the clock scale here while

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sparing the optic nerve. If we look on the axial scan,

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we also see that the soft tissue inflammatory process is

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contiguous with the lacrimal gland on this section and

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that also is characteristic of orbital pseudotumor.

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There are different types of idiopathic orbital inflammation and

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we separate orbital pseudotumor with an entity that

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has been recently described which is known as

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IgG4-related ophthalmic disease,

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or IgG4-related ophthalmic disease.

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This is a process in which plasma cells and lymphocytes are

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infiltrating the orbit and it may affect, once again,

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the lacrimal gland or the extraocular muscles.

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The two most common forms of IgG4-related ophthalmic

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disease are sclerosing dacryoadenitis,

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which affects the lacrimal gland,

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and enlargement of orbital nerves,

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usually the fifth cranial nerve branches within the orbit.

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Both of these can lead to an inflamed, irritated orbit.

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Now, IgG4 disease is usually treated with rituximab,

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which is a monoclonal antibody drug,

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whereas orbital pseudotumor not associated with IgG4

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disease is usually treated with steroids.