Isolated involvement of the superior rectus muscle in idiopathic orbital myositis

Clinical History

The patient presented with acute onset ocular pain, lid swelling and restriction of ocular movement.

Imaging Findings

The patient presented with rapid onset right orbital pain, lid swelling and restriction of ocular movement. On ophthalmic examination, painful ocular movement, limited upward gaze and moderate right ptosis were detected. Bilateral vision was 20/20, and both fundi were normal.

The patient was screened with ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI). An enlarged superior rectus muscle was seen on US (Siemens Elegra, Erlangen, Germany, 7.5MHz linear transducer) and increased vascularity was noted with power Doppler (Figure 1). On orbital CT (Siemens Somatom Plus 4, Erlangen, Germany), isolated involvement of the right superior rectus muscle was detected (Figure 2). Optic nerves and periorbital fatty tissues were normal. Post-contrast scans showed enhancement. On MR imaging (Siemens Magnetom Open, Erlangen, Germany), the enlarged superior rectus muscle was isointense to other muscles on T1-weighted images and slightly hyperintense to periorbital fat on T2-weighted images (Figure 3). After contrast injection, markedly enhancement was seen. Following steroid therapy, symptoms resolved quickly so that the final diagnosis of orbital myositis was based on the response to steroids.

Discussion

Orbital myositis is a recognised subgroup of the nonspecific inflammatory orbital pseudotumour syndrome. In orbital myositis, this inflammatory process affects one or more of the extraocular muscles. It is characterised clinically by proptosis and painful ophthalmoplegia. Its cause remains obscure, although there may be a higher incidence of orbital myositis in patients with known ocular or systemic disease (1,2).

Although the disease is very seldom seen in children, a case with bilateral proptosis has been reported (3).

The inflammatory infiltrate, which is composed of polymorphic leukocytes, lymphocytes, and plasma cells, interspersed with a variable amount of fibrovascular tissue, may be diffuse or localised (4). In the very early stages of the disease, oedema and inflammatory infiltrate are evident; as the disease evolves, fibrosis and collagen fibrils are laid down. Typically, this process responds dramatically to treatment with corticostreoids.

The affected muscle shows irregular enlargement with extension into its tendinous insertions. The most commonly affected extraocular muscle is the medial rectus. Lateral rectus muscle enlargement with extension into an enlarged lachrymal gland is a relatively common manifestation of this disease (2). Also, isolated inflammation of the superior oblique muscle has been rarely reported in the literature (5).

Pseudotumour, Graves' disease and lymphoproliferative disease are the most common ophthalmological disease entities requiring evaluation by CT and MRI (4). On CT, orbital myositis shows enlargement of one or more of the extraocular muscles that extends anteriorly to involve the insertion of the muscle tendon (1). Involved muscle is usually isodense to slightly hyperdense compared with other muscles before the injection of contrast (2). Classically, the enlarged muscle has rugged, fluffy borders caused by infiltration of the surrounding fat by the inflammatory process.

Characteristic MR findings include extraocular muscle enlargement without dramatic changes in signal intensity (usually isointense to normal muscle) on T1-weighted images. On T2-weighted images, the involved muscle is isointense to minimally hyperintense compared with unaffected extraocular muscle tissue (1,2). In exceptional cases, the muscle can become markedly hyperintense. Homogeneous enhancement is usually seen within the involved muscle on CT or MR scans after intravenous contrast administration.

In differential diagnosis, various pathologies such as neoplastic (lymphoma, rhabdomyosarcoma, myeloma, neuroblastoma, leukaemic extraocular muscle infiltration, metastases), thyroid orbitopathy (Graves' disease), amyloidosis, granulomatous inflammation (e.g., fungal infection, tuberculosis, sarcoidosis), Wegener's granulomatosis, Tolosa-Hunt syndrome (with orbital involvement), traumatic (haematoma, orbital blow-out fracture with extraocular muscle herniation), and vascular (arteriovenous malformation, haemangioma, lymphangioma) should be considered (1,2).

Early recognition of orbital myositis may improve the change of a successful clinical outcome. In fact, the final diagnosis is often based on response to steroids. Biopsy is rarely indicated; it is reserved for the infrequent lesions that are steroid unresponsive.

Differential Diagnosis List

Final Diagnosis

Orbital myositis

URL:	https://eurorad.org/case/1251
DOI:	10.1594/EURORAD/CASE.1251
ISSN:	1563-4086