## 3 Narrative Review
### Differential Diagnosis of Orbital Myositis Through Imaging  
The differential diagnosis of IOM can be particularly challenging [23]. Unusual features, such as the focal, nodular, or heterogeneous enlargement of an EOM, are uncommon in IOM and may raise suspicions of malignancy. Additionally, the enlargement of other orbital structures, such as the lacrimal gland, may suggest alternative conditions like sarcoidosis or lymphoma. Nerve enlargement, on the other hand, might indicate IgG4-related disease (IgG4-RD). The involvement of adjacent tissues could suggest locally invasive conditions, such as malignancy or infection, and may require a biopsy for definitive diagnosis [24]. Classic signs of orbital myositis typically include the unilateral thickening of one or two EOMs, often accompanied by the inflammation of surrounding fat, tendons, and the myotendinous junction. These features help differentiate IOM from thyroid orbitopathy, which is generally characterized by bilateral inflammation and spares the myotendinous junction. Nevertheless, the sparing of this junction alone does not rule out idiopathic orbital inflammation. In thyroid orbitopathy, the lateral rectus and superior oblique muscles are frequently unaffected during the early stages of the disease. Common symptoms such as apoptosis, chemosis, and diplopia may also suggest IgG4-RD, which shares similar clinical manifestations [25]. In IgG4-RD, the inflammation often manifests as the bilateral \begin{table}
\begin{tabular}{c c c} \hline \hline
**Imaging Modality** & **Findings** & **Description** \\ \hline CT \({}^{*}\) Scan & Non-specific mass &
\begin{tabular}{c} Orbital mass of varying size, shape, and location, generally \\ not able to differentiate IOM from other lesions. \\ \end{tabular} \\ \hline MRI \({}^{*}\) (T1-weighted) & Hypointense signal & Typically hypointense in chronic cases with fibrosis; less \\  & & clear in acute cases. \\ \hline MRI \({}^{*}\) (T2-weighted) & Hyperintense signal & Hyperintensity indicative of acute inflammation, edema, \\  & & and increased fluid. \\ \hline Gadolinium Contrast (MRI) \({}^{*}\) & Enhancement & Increased contrast enhancement indicating inflammation or \\  & & venous congestion. \\ \hline Fat-suppressed MRI (STIR) \({}^{*}\) & Enhanced signal & Clearly delineates optic nerve involvement and \\  & & inflammatory changes. \\ \hline \hline \end{tabular}  
* MR: Magnetic Resonance Imaging; CT: computed tomography; STIR: Short Tau Inversion Recovery.  
\end{table} 
Table 2: Typical imaging findings in orbital myositis (IOM). involvement of the lacrimal glands and EOMs, with the inferior rectus muscle being the most affected. For patients with normal thyroid function, these findings should prompt consideration of IgG4-RD, with serum IgG4 level testing being recommended. The differential diagnosis of myositis also encompasses orbital cellulitis, which typically presents with fever, leukocytosis, and a history of head or neck infections. Contrast-enhanced CT can assist in identifying the infectious source extending to the orbit and may reveal an abscess that requires surgical drainage. Both metastases and lymphoma can mimic myositis, often appearing as focal masses with heightened signal intensity in the EOMs [26]. Similarly, patients with low-flow carotid- cavernous fistulas (CCFs) may show symptoms resembling myositis due to venous congestion, which can manifest as inflamed EOMs on CT or MRI. The enlargement of the superior ophthalmic vein (SOV) is a typical finding in CCF, and retrograde flow through the SOV can be detected using transcranial Doppler ultrasound (Eden Medizinishe Elektronik, Uberlingen, Germany), while angiography offers a more precise characterization of the fistula [27]. Contrast-enhanced T1-weighted MRI with fat suppression remains the most effective method for visualizing inflammation in the muscles, tendons, and adjacent fat, usually showing swelling in both the tendon and muscle belly of the EOM.