Orbital rule 16: Extra ocular muscle mass

Extra ocular muscle mass could be classified according to the site of muscle affection into the following:

-Affection of the medial and inferior rectus muscles:

 Caused by Grave's disease.

 

•          -Affects mainly inferior and medial recti with fusiform appearance with strong enhancement.

•          -Bilateral symmetrical affection is most frequent.

•          -Can affect all orbital content(fat , optic nerve and lacrymal glands…etc.

•          -Increase volume of fat + abnormal increased density = dirty fat causing proptosis.

•          -Optic neuropathy due to compression of the optic nerve by enlarged muscle(infiltrated by muco poly saccharides).

-Unilateral  grave’s is when one or more muscles are affected on the same side , while bilateral is when one or more muscles are affected on both sides

•          So we have proptosis due affection of the orbital muscles and also due affection of orbital fat as a result of grave’s disease.

•          -Try to differentiate carefully between the inferior rectus muscle and optic nerve in axial view(enlarged inferior rectus muscle could be mistaken as glioma of the optic nerve).This differentiation could be done as the optic nerve is present at the middle of the orbit where you can see the lens , and also by looking in coronal view.

-Affection of lateral rectus muscle:

Caused by orbital pseudo tumor.

-Affection of superior complex:

Non hodgkin's disease.

Golden rule:

When you see a single muscle affection in a unilateral orbit, not suggested to be Grave's disease, this will be inflammatory or neoplastic, then look to the fat near the lesion, if it is dirty or not, if it is dirty, this suggests inflammatory etiology but if it is not dirty, this will suggest neoplastic origin.

Golden rule:

When you find only a mass in the superior rectus levator complex, this will be non hodgkin's lymphoma until prove otherwise.