Idiopathic orbital inflammation (Pseudotumor)
Idiopathic orbital inflammation (IOI) is a diagnosis of exclusion based on a combination of clinical presentation and radiological findings. The disease manifests as a painful mass causing exophthalmos (proptosis), chemosis, diplopia, and restricted eye movements.
Imaging features:
- The process can affect any structure, but the lacrimal gland is most commonly involved (diffuse enlargement).
- When extraocular muscles are affected (orbital myositis), tubular thickening is observed, which involves the tendon. This is a critically important sign for differentiation from thyroid orbitopathy, in which the tendons remain intact.
- Typically, the involvement is unilateral and affects a single muscle. Frequency order: medial rectus > superior rectus > lateral rectus > inferior rectus.
- CT and MRI often show inflammatory stranding of the adipose tissue. If the process transitions to a chronic stage with fibrosis, the signal decreases on T1- and T2-weighted images.
About 80% of patients respond well to steroid therapy. For poor response or recurrence, radiation therapy or immunosuppressants are used.
Systemic diseases with orbital involvement
Many systemic processes manifest with or are accompanied by orbital symptoms. It is important to recognize them on imaging:
- IgG4-related disease: A multisystem disorder accounting for up to half of all IOI cases. Characterized by tissue infiltration with IgG4-expressing plasma cells. On MRI, it yields pronounced hypointensity on T1- and T2-weighted images. Involvement is frequently bilateral, with typical enlargement of the infraorbital nerve.
- Sarcoidosis: Affects the eyes in 50–80% of cases. The imaging appearance is non-specific; bilateral enlargement of the lacrimal glands with contrast enhancement is often seen. When the optic nerve is involved, the sheaths intensely enhance, forming the «tram-track» sign.
- Granulomatosis with polyangiitis (Wegener's): A necrotizing vasculitis affecting the orbit in 40–50% of patients. Frequently associated with bone erosions (e.g., destruction of the lamina papyracea) and sinusitis. On MRI, it shows a hypointense signal on T2-weighted images with diffuse heterogeneous enhancement.
A distinct entity is Tolosa-Hunt syndrome (Tolosa-Hunt syndrome), a clinical syndrome of inflammatory involvement of the superior orbital fissure and cavernous sinus, manifested by ophthalmoplegia and hypesthesia.
Infectious lesions
Orbital infections can lead to severe consequences, up to and including vision loss and intracranial complications.
- Endophthalmitis: Infection of the eyeball caused by bacteria or fungi (Candida). In children (5–10 years old), a frequent cause is toxocariasis, a reaction to Toxocara canis larvae. In toxocariasis, CT shows increased density of the vitreous body due to protein exudation, but calcification is absent, which is critically important to distinguish it from retinoblastoma.
- Orbital cellulitis: Inflammation of tissues posterior to the orbital septum. Most commonly a result of infection spreading from the paranasal sinuses. Scans reveal stranding of the retroorbital fat and abscess formation (a zone of necrosis with ring enhancement). Complications include superior ophthalmic vein thrombosis and subdural empyema.
Lacrimal gland pathology
The lacrimal gland is located extraconally in the superolateral orbit. Inflammation of the gland (dacryoadenitis) can be acute (with adjacent myositis) or chronic (in tuberculosis, sarcoidosis).
Lacrimal fossa masses require differential diagnosis:
| Benign | Malignant |
|---|---|
| Pleomorphic adenoma (most common, well-defined margins, risk of malignant transformation) | Adenoid cystic carcinoma (often arising from pleomorphic adenoma, bone erosion) |
| Dermoid/epidermoid cysts (fat density, calcifications) | Lymphoma (MALT, diffuse involvement of the entire gland) |
| Lipoma, hemangiopericytoma | Myoepithelial carcinoma |
Briefly: answers to topical questions
Which infectious agents cause endophthalmitis, and what are its imaging features?
Endophthalmitis is caused by fungal, bacterial, and parasitic infectious agents. The causative agents of the disease are:
- Fungi (Candida sp.) — cause lesions that serve as a marker of disseminated candidiasis (yellowish-white exudative changes in the uveal tract).
- Bacteria (S. aureus, S. epidermidis, Streptococcus sp.) — classic pathogens of eyeball infection.
- Nematodes (Toxocara canis) — cause toxocariasis, which is a frequent cause of the disease in children.
Imaging features are described for toxocariasis: CT reveals increased density of the vitreous body (protein leakage) and inflammation of the peripalpebral fat in the absence of calcification. MRI shows a variable signal and contrast enhancement.
To remember the decreasing order of frequency of extraocular muscle involvement in idiopathic orbital inflammation, the rule «M-S-L-I» can be used: Medial > Superior > Lateral > Inferior.
Class and exam questions
How to reliably differentiate idiopathic myositis from thyroid orbitopathy on CT/MRI?
In idiopathic myositis (pseudotumor), the tubular thickening of the extraocular muscle extends to its tendon. In thyroid orbitopathy, the tendon always remains intact.
Why is ocular toxocariasis dangerous in children, and how does it look on scans?
Toxocariasis can cause leukocoria and retinal detachment, leading to blindness. Imaging typically shows increased density of the vitreous body while lacking calcifications, which helps rule out malignant retinoblastoma.
What is the «tram-track» sign in the orbit?
This is a radiological sign detected on post-contrast MRI. It represents intense enhancement of the thickened optic nerve sheaths and is most characteristic of sarcoid perineuritis.
What does superior ophthalmic vein thrombosis look like on CT?
Contrast-enhanced CT reveals dilation of the superior ophthalmic vein and lack of enhancement within its lumen (filling defect). This is frequently accompanied by signs of orbital cellulitis.
What else to study on this topic
- MRI semiotics of IgG4-related orbital disease
- Complications of orbital cellulitis: cavernous sinus thrombosis
- Benign and malignant tumors of the lacrimal gland
- Imaging manifestations of sarcoidosis in the orbit and cranial cavity
- Orbital nerve sheath tumors: schwannoma and neurofibroma
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