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A 51 year old woman with a 34 year history of seropositive, erosive, nodular rheumatoid arthritis (RA) complained of non-specific headache and diplopia of insidious onset at a routine appointment with her ophthalmologist. Her RA was generally well controlled with sulfasalazine 2 g daily, which she had taken for four years. Other drugs were bendrofluazide 2.5 mg and atenolol 50 mg daily for hypertension along with hypromellose eye drops for dry eyes. She had no other medical history of note. She had previously been treated with intramuscular gold from 1977 until 1985 and with auranofin from 1987 until 1996.
Physical examination showed weakness of the right medial, lateral, and superior recti. The most likely cause was thought to be ocular nerve palsy, but a magnetic resonance (MR) scan of the orbits and brain was arranged to investigate further.
Six weeks later the patient's symptoms had deteriorated significantly such that she had diplopia in the primary position. There was now gross restriction of horizontal movements in the right eye (fig 1A) with some evidence of slight proptosis. There was obvious conjunctival chemosis and particularly marked injection over the insertion of the medial rectus. Laboratory investigations showed mildly raised inflammatory markers with erythrocyte sedimentation rate 20 mm/1st h (normal <12) and C reactive protein 16 mg/l (normal <10). Other routine biochemistry investigations were normal but the thyroxine level was borderline at 146 nmol/l (normal 55–144). She remained seropositive for rheumatoid factor and had a positive antinuclear antibody with a titre of 1/40. This had been positive before initiation of sulfasalazine treatment.
The MR scan showed no intracranial abnormality, but there was gross enlargement of the belly of the right medial rectus which measured 1 cm in diameter (fig 1B). There was less thickening of the other extraocular muscles. A high signal was seen in these muscles on T2 weighted imaging, indicating oedema and acute inflammation.
A diagnosis of orbital myositis (OM) was made and treatment was started with oral prednisolone 60 mg daily. Her symptoms rapidly improved and steroid treatment was withdrawn within three months. Five months after her diagnosis she had mild restriction of horizontal movements in the right eye and repeat MR scan showed an element of chronic fibrosis.
OM is characterised by the onset of painful and limited extraocular movements, diplopia, ptosis, swelling of the lid, and localised chemosis and injection over the insertion of the inflamed muscle.1
The most commonly affected muscles are the superior complexes and the medial rectus muscle. OM may attack more than one muscle, and may be bilateral or recurrent.
The major differential diagnosis is thyroid ophthalmopathy. However, dysthyroid myopathy is usually painless in onset, symmetrical, slowly progressive, and associated with systemic manifestations of Grave's disease. Lid retraction, limitation of the movement opposite to the affected muscle, and deterioration of visual function (colour vision, visual field, and visual acuity) may also occur in thyroid eye disease, in contrast with OM. Additional diseases that should be considered in the differential diagnosis include orbital cellulitis, metastasis, Tolosa-Hunt syndrome, trochleitis, and infectious myositis due to trichinosis.2
Imaging of the orbit in OM shows diffuse enlargement of the extraocular muscles, which exhibit slightly blurred margins.
Associations with OM include distant inflammatory disease such as Crohn's disease,3 Lyme disease,4 and Wegener's granulomatosis.5 It may also be a manifestation of a paraneoplastic syndrome.6 Although there are published reports of OM associated with psoriatic arthropathy7 and systemic lupus erythematosus,8,9 there has only been one case reported in association with RA.10 Management is with corticosteroids and the rapid response is almost diagnostic.
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