In this study, we found that the peaking age of OMG ranged from 18 to 39 years both in men and women. The age and gender distribution showed a diminishing distribution with age increased in both male and female. The distribution pattern was different from the results of previous studies. It was reported that there was a bimodal distribution in female and unimodal distribution in male[10]. However, patients involved in those studies were mainly Caucasian and non-Caucasian patients diagnosed with OMG tended to be younger than Caucasian patients for nearly 2 decade, especially the Asian patients[10–12]. Patients in this study were all Chinese, which might explain the younger diagnostic age. Previous studies reported that more patients presented with initial symptoms of ptosis only, in which the children were involved[13, 14]. In this study, most patients presented with diplopia only, which may be related to the age composition, because there was no child was involved in this study and diplopia alone was less common in childhood OMG[15]. The median course of OMG in this study was 6 months and the course ranged from 1 month to 10 years. The long and fluctuant course of disease was a feature of OMG, because most patients came to the first visit at the time when they could not endure the variable discomfort anymore. To summarize the characteristics of the age, phenotype distribution and course of OMG in this study, we could find that OMG mainly affected adult at the working ages and most of them had a long course and presented with diplopia only, which would definitely cause significant socioeconomic impact.
Currently, OMG remains a clinical diagnosis, but various diagnostic testing can help in determining the diagnosis. Edrophonium is a short-acting acetylcholinesterase (AChE) inhibitor, which was commonly used in clinic to diagnose MG. The sensitivity of the edrophonium test for diagnosing OMG was 88–97%[6, 16, 17]. Neostigmine, an alternative to edrophonium for pharmacological testing, is a long-acting AChE, making it more suited in the examination of ocular motility and diplopia testing[1]. The sensitivity of neostigmine test in this study was 80.0%, indicating that neostigmine test is also available in diagnosing OMG. Serological tests for AchR antibodies, antimuscle specific tyrosine kinase (MuSK) and LDL-related receptor-related protein 4 (LRP4) were used in diagnosing MG[6]. Detection of AChR antibodies remained the most common method for the diagnosis of OMG so far, but elevated level of AchR antibodies were found in only on half of patients with OMG, especially in older age, male sex, and progression to GMG[18, 19]. Thymic assessment like chest computed tomography (CT) should be done in every patients with MG symptoms to determine the presence of thymoma or thymus hyperplasia, because about 10% of patients with MG are correlated with thymus disorders[20]. The results of the serological test and thymic CT were similar to the previous reports, though the positive results were not found in most patients, it remained necessary examination for OMG.
Ophthalmoplegia can be found in different kinds of diseases and bring huge disturbance for patients in daily life. Ophthalmoplegia in OMG can range from involvement of a single EOM to multiple EOMs even the complete ophthalmoparesis[3, 21]. Previous research had demonstrated that the preferential susceptibility of ocular muscles in MG might be explained by the unique physiological properties of EOM synapses[4, 5, 22]. However, whether there is any different susceptibility within the six EOMs in OMG remained unclear. Clinical findings suggested that there might be differences within the EOMs. For example, inferior rectus and medial rectus are more susceptible in thyroid eye disease[23]. In this study, the most paralytic EOM was determined through light reflection, cover-uncover test, red glass test and Maddox rod test. The results demonstrated that lateral rectus was the most susceptible, followed by superior rectus and medial rectus. The mechanisms underlying why different EOM show different susceptibility remain unclear. Lateral rectus, innervated by abducent nerve, was studied in animal researchs but seldom studies compared lateral rectus with other EOMs[24]. When lateral rectus was affected in patients with OMG, esotropia was found and patients would complained a horizontal double vision, which was aggravated when gazed laterally.
This case series was limited by the retrospective nature and the small samples of the study. However, to our best knowledge, this is the first study to report the susceptibility differences of EOMs in OMG. In order to certain the diagnosis of OMG in early period and prevent it convert into GMG, more cases should be collected to further study the characteristics of diplopia and ophthalmoplegia in adult patients with OMG.