What condition is characterized by enlargement of extraocular muscles with sparing of the tendinous insertions, often leading to proptosis, and which muscles are typically involved in order of frequency?

Thyroid ophthalmopathy (Graves disease) is indicated by enlargement of extraocular muscles with sparing of the tendinous insertions. It commonly causes unilateral or bilateral proptosis in adults. The muscles involved, in decreasing order of frequency, are the Inferior rectus, Medial rectus, Superior rectus, and Lateral rectus (mnemonic: I'M SLow). Key diagnostic features include muscle enlargement on CT or MRI, often with sparing of the tendons, and clinical symptoms like diplopia. A common pitfall is mistaking this for orbital pseudotumor, which typically involves the tendons.