Sir,

Idiopathic orbital myositis (IOM) is a type of orbital pseudotumour in which one or more extraocular muscles can be involved. However, oblique muscle involvement is much less common than rectus muscle.1 In a large series of 75 patients with IOM, involvement of lateral rectus muscle was found to be 33% and followed by medial rectus (29%), superior rectus (23%), inferior rectus (10%), inferior oblique (3%), and superior oblique (SO, 2%). Especially, isolated SO myositis is very rare, and often causes persistent gaze restriction.2, 3, 4, 5 We report one patient with isolated SO myositis treated successfully with pulse intravenous methylprednisolone, and also reviewed the clinical presentation and treatment outcome of previously reported four cases.

Case report

A 35-year-old female was referred with a 4-week history of painful swelling around right eye accompanied by diplopia on attempted eye movement. She had been treated before referral with oral antibiotics for presumed periorbital infection. There was no history of thyroid disease. On examination, she had swelling and tenderness of right upper lid and injection over right upper bulbar conjunctiva. Exophthalmometer readings were normal. Ocular motility revealed painful restriction especially with attempted elevation of the right eye in adduction (Figure 1). Physical examinations were unremarkable. The laboratory investigations, including complete blood count, blood sugar, erythrocyte sedimentation rate, thyroid function tests, and autoimmune antibody screen, showed no abnormality. Orbital computed tomography (CT) showed enlarged right SO muscle without tendon involvement (Figure 2). Magnetic resonance imaging (MRI) also revealed a swelling and heterogeneously enhancing right SO muscle (Figure 2). A presumptive diagnosis of IOM was made and the patient was treated with intravenous methylprednisolone (1 g/day for 3 days) and followed by oral prednisolone. Her symptoms improved immediately, and steroid was gradually tapered. At 6 weeks after treatment, she had experienced a marked clinical recovery (Figure 1) and follow-up CT showed swelling of SO muscle was resolved (Figure 2). No evidence of recurrence was noted at 1 year after treatment.

Figure 1
figure 1

Ocular versions demonstrate limitation of supraduction of the right eye, particularly in adduction (a, b, c). Marked clinical recovery after 6 weeks of steroid pulse therapy (d, e, f).

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Figure 2
figure 2

(a) Axial computed tomographic image revealed an enlargement of the right superior oblique muscle without tendon involvement. (b and c): The contrast enhanced T1-weighted MRI images with fat suppression clearly also showed heterogeneous enhancement and fusiform swelling of the right superior oblique muscle. (d) After 6 weeks of treatment, the swelling of right superior oblique muscle was marked resolved on follow-up CT.

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Comment

To our knowledge, this is the first time that pulse intravenous methylprednisolone has been used in the treatment of isolated SO myositis, and the result is promising. Only five patients with isolated SO myositis have been reported, including the present case.2, 3, 4, 5 Table 1 summarizes their clinical presentation. Due to their nonspecific clinical manifestations, early recognition seems to be difficult. Three of five patients were initially diagnosed as orbital abscess, periorbital infection and thyroid eye disease respectively. Prior antibiotic may obscure and delay the diagnosis.

Table 1 Clinical Summary of 5 Cases of isolated superior oblique (SO) myositis
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CT, echography, and MRI can provide valuable features to identify the affected SO muscles. CT is the preferred method, which can demonstrate the swelling SO muscle along the superonasal orbit either. Echography may show homogenous low-reflective enlargement of SO muscle, which were different from the high-internal reflectivity in thyroid orbitopathy.2, 4, 6 MRI can further demonstrate a typical heterogeneously gadolinium-enhanced SO muscle, which allowed differentiation from the focal or nodular muscle enlargement seen in cases of metastatic infiltration. Tendon sparing was seen in two of five cases, which suggests tendon involvement could not be the only radiological distinction between IOM and thyroid orbitopathy.

Although all cases responded to systemic steroid, residual gaze limitation occurred in two of three patients treated with oral steroid.2, 5 Incomplete resolution did persist in the case steroid therapy was began soon after onset of symptoms.5 It is still unclear, how early intervention can preserve normal extraocular muscle function. Since the interval between the onset of symptoms and institution of therapy in our case was more than 5 weeks, impending fibrotic changes of extraocular might have occurred. Therefore, intravenous methylprednisolone was administrated in our patient to expedite relief of inflammation which achieved good clinical response. Intravenous steroid treatment has been documented for the treatment of many serious inflammatory eye diseases.7 However, it needs further investigation to compare the long-term efficacy between pulse therapy and oral steroid in the treatment of isolated SO myositis.

In conclusion, despite isolated SO myositis remains a rare entity, to be familiar with their imaging features, corrected with clinical findings, allows for early diagnosis and treatment. In our case, intravenous methylprednisolone appears to be an effective treatment when earlier intervention is impossible.