There is a short list of causes for enlarged extraocular muscles. The differential can be narrowed by the clinical history, known systemic illness, pattern of specific muscles involved, the muscle morphology, as well as concurrent findings outside the muscles ³:

• inflammatory, infectious, and deposition disorders

  • thyroid associated orbitopathy

    • most common cause of extraocular muscle enlargement, presenting as proptosis

    • it can involve all muscles, often bilaterally, but when a single muscle is involved, it tends to involve inferior rectus, medial rectus, or levator palpebrae superioris muscle; see the I'M SLOw mnemonic

    • the anterior tendon is spared (Coca-Cola bottle sign)

    • other features may include an increase in orbital fat and lacrimal gland enlargement

  • idiopathic orbital inflammation (orbital pseudotumour, idiopathic orbital myositis)

    • most common cause of painful orbital mass

    • it can involve any combination of muscles, medial rectus more than lateral rectus, sometimes bilaterally

    • the anterior tendon is usually enlarged

    • other features may include orbital fat infiltration and lacrimal gland enlargement

  • IgG4-related orbital disease

    • lateral rectus is most prominently involved, usually bilaterally

    • the anterior tendon is usually spared

    • other features include infraorbital nerve enlargement, paranasal sinus disease, orbital fat infiltration, and lacrimal gland enlargement

  • orbital cellulitis

    • adjacent inflammation of the orbital fat, paranasal sinus, or lacrimal gland is typical

  • sarcoidosis

    • other features include lacrimal gland swelling

  • granulomatosis with polyangiitis

    • other features include lacrimal gland swelling and sinonasal disease including nasal septal perforation

  • amyloidosis (very rare) ²

• neoplastic disorders

  • lymphoma

    • any combination of muscles can be involved, even bilaterally

  • metastases

    • any combination of muscles can be involved, even bilaterally

    • enophthalmos is a distinctive feature associated with scirrhous breast carcinoma metastasis

• ​vascular disorders

  • orbital venous congestion

    • the superior ophthalmic vein is enlarged or thrombosed, or there is a mass at the orbital apex or cavernous sinus

  • intramuscular haematoma

    • there is a history of trauma or anticoagulation and there may be adjacent retrobulbar haemorrhage

  • orbital infarction syndrome ⁴