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Atrophic extraocular muscles are rarely observed1–3 and bilateral extraocular muscle (EOM) atrophy is extremely rare.3 It has been reported in chronic progressive external ophthalmoplegia (CPEO) related to mitochondrial myopathies,4 myotonic dystrophy type 15 and in three other cases of myasthenia gravis (MG).4 This report is an atypical presentation of ocular MG with positive muscle-specific kinase (MuSK) antibodies and bilateral EOM atrophy.
A 49-year-old man presented with a 12-year history of slowly isolated progressive external ophthalmoplegia. His medical history was unremarkable and he did not take any drugs, smoke or drink alcohol. Examination revealed limitation of gaze in all directions to 3°–5° with bilateral ptosis and no exophthalmos. He had no other facial, bulbar or peripheral weakness. His acetylcholine receptor ε subunit-specific antibodies and γ subunit-specific antibodies were negative, but his anti-MuSK antibody level was increased at 32 693 pg/ml (normal values 249–264 pg/ml). Single-fibre electromyography revealed jitter and blocking that confirmed his diagnosis of MG. Complete blood count and routine chemistry laboratories were normal. Sedimentation rate, rapid plasma reagin, rheumatoid factor, antinuclear antibody, anti-Hu, anti-Lambert–Eaton myasthenic syndrome, anti-Yo and anti-MaTa antibodies were all negative. Thyroid testing and mitochondrial DNA rearrangement analysis for Kearns–Sayre syndrome were negative. MRI of the orbits revealed severe wasting of all extraocular muscles, except for the inferior oblique muscle (fig 1). Chest CT scan with contrast was normal. Even after 6 months of pyridostigmine and prednisone treatment, followed by thymectomy, his extraocular motility did not improve for 2½ years.
We have found no previously published cases of bilateral EOM atrophy in MuSK antibody-positive patients with myasthenia. The diagnosis of MG was confirmed by single-fibre electromyography to rule out the possibility of a false-positive increased level of anti-MuSK antibodies. Although >50% of patients with MG present with symptoms and signs related to EOM weakness,6 such as diplopia or ptosis, bilateral EOM atrophy is unusual. Unlike the three patients reported by Bau et al7 and Caress et al,8 who had anti-MuSK-positive ocular myasthenia with normal extraocular muscles on MRI, our patient had bilateral EOM atrophy. Unlike the three patients with myasthenia with bilateral EOM atrophy reported by Okamoto et al,4 who were positive for only acetylcholine-receptor antibodies, our patient was seronegative for these antibodies but positive for MuSK antibody. In ocular myasthenia, only 50% of patients are found to have anti-acetylcholine receptor antibodies,6 and of 38 patients with seronegative ocular MG, none show increased titres of the anti-MuSK antibody.9,10 Between 38% and 47% of patients with seronegative MG have increased anti-MuSK antibodies.9,10 Similar to the clinical course of patients in the study by Okamoto et al,4 our patient had an approximately 15 year duration of limited ocular motility that did not improve with medication or thymectomy.
Diplopia and ptosis are well-recognised signs of MG, but bilateral EOM atrophy is exceedingly rare. Although ocular involvement is a less common presentation of the anti-MuSK MG syndrome, it may be reasonable to check for anti-MuSK antibody in any patient who presents with progressive external ophthalmoplegia.
Competing interests: None declared.