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Myasthenia gravis (MG) is an autoimmune disorder causing postsynaptic impairment of neuromuscular transmission.1–3 Ocular, bulbar, or proximal limb muscles are most frequently affected, and weakness worsens during exercise. Recently, variants of MG have been described that deviate notably from the typical clinical presentation of MG, including “MG of respiratory muscles” and a “distal type”.4
The neurological department of the Innsbruck University Hospital serves as a single referral centre for all cases of MG in the state of Tyrol. Over the past 20 years we have been prospectively following 84 patients with MG, corresponding to a prevalence rate of 116 per million.
Our patient series includes five men and women with an unusual relapsing-remitting variant of ocular MG that lacks muscle fatiguability and most typical laboratory features of neuromuscular junction disorders and, thus, may evade correct diagnosis. All these patients reported acute or subacute onset (overnight in three cases) of non-fluctuating diplopia with gradual spontaneous remission over a 2–20 week period. Further episodes (one to six)—none of which was provoked by infection, immunisation, or medication—emerged at intervals of 1–80 months. The pattern of ocular muscles involved varied from episode to episode. Ptosis was usually mild or absent. Extensive laboratory investigations involving blood analysis (erythrocyte sedimentation rate, antinuclear, antiacetylcholine receptor, and antistriated muscle antibodies, circulating immune complexes, routine blood tests, thyroid hormones, etc), analysis of cerebrospinal fluid (three patients), an edrophonium (Tensilon Roche Laboratories, Nutley, New Jersey, USA) test, electrophysiological examinations, magnetic resonance imaging of orbita, sinus cavernosus, and cerebrum and computed tomography of the chest were all unremarkable or negative. Oral pyridostigmine (180 mg daily) was tried in one patient over a period of two weeks but it had no clinical effect. Table 1 summarises the main clinical and laboratory characteristics of the patients. Because of the unusual clinical presentation, especially the absence of muscle fatiguability—the clinical hallmark of MG—and the normal results of standard laboratory procedures (Tensilon test, repetitive nerve stimulation, and measurement of acetylcholine receptor antibodies), MG was not diagnosed in any of the patients initially. All were assumed to suffer from recurrent cranial nerve palsy or midbrain lesions (table 1). Later in the course of disease the correct diagnosis was established by single fibre electromyography of the right extensor indicis muscle (pathological jitter) and the observation of a few episodes of typical ocular MG. All patients responded excellently to a medium dose of prednisone and remained asymptomatic under a low maintenance dose (4–8 mg) but relapsed weeks to months after further tapering or discontinuation of corticosteroids (four of the five patients). None developed generalized MG during follow up.
Except for a few anecdotal case reports,5 this unusual presentation of MG has not yet been addressed in the literature or mentioned in standard neuromuscular textbooks.1–3 Our five patients represent 10% of all subjects with ocular MG seen in the single referral centre for the state of Tyrol, suggesting a significant minimal prevalence of this type of disease. Single fibre electromyography is necessary to establish an early diagnosis and, thus, should be an obligatory component in the diagnostic investigation of patients with diplopia or ptosis of unknown origin, even in the absence of muscle fatiguability.
Competing interests: none declared
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