Epidemiology, Patient Characteristics, and Treatment Patterns of Myasthenia Gravis in Taiwan: A Population-Based Study

Compared with studies reported in other Asian countries, the incidence of MG in Taiwan (1.9–2.3 per 100,000 person-years) was similar to Korea (2.4 per 100,000 person-years) but higher than in China (0.015–0.036 per 100,000 patient-years) and Japan (0.45–0.69 per 100,000 patient-years) [12‐14]. In the present study, the incidence of MG in Taiwan remained constant at around 2 per 100,000 population during 2014–2019, and this led to a steady increase in the prevalence rate (19–24 per 100,000 population). The previous NHIRD study by Lai and Tseng reported that the prevalence rate increased from 8.4 in 2001 to 14.0 in 2007 per 100,000 population [15]. Using the same data source, the current study can be considered an extension of the study by Lai and Tseng, and overall we observed an increase in the MG population over the past two decades. By leveraging the current study and the study by Herr et al. published in 2023 [10], we can estimate that 47% of patients with MG in Taiwan have gMG. The distribution of ocular MG and gMG in Taiwan was comparable to data from other countries (49% gMG in the USA) [23]. However, this result should be interpreted cautiously since the operational definitions of MG and gMG differed between studies.

MG is considered “a disease of young women and old men” [8, 24, 25], which indicates that age and gender are two major epidemiological risk factors for the incidence of MG. The age of MG onset is an important predictor of disease prognosis and mortality [26]. Patients who develop MG after the age of 50 years (i.e., late-onset MG) or 70 years (i.e., very late-onset MG) are likely to have more severe disease. Furthermore, the increased risk of adverse effects of medications and comorbidities in the elderly requires careful monitoring [26, 27]. The prevalence rate of late-onset MG has increased in both Western and Asian countries [9, 12]. In our study cohort, more than half (59.0%) of incident patients were late-onset MG, whereas only approximately 42% were late-onset MG in the cohort of 2001–2007 reported by Lai and Tseng [15]. The results indicate that the prevalence of late-onset MG in Taiwan may increase the need for healthcare resources and intensive care. In an analysis by gender, we found a late-onset peak for male patients compared with more early-onset MG in female patients. Hormones during the fertile period or pregnancy might be the mediator of sex differences in autoimmunity and may lead to early-onset MG in female patients [24]. MG management for female patients with early-onset MG is a challenge since it affects women during childbearing age and may have a negative impact on quality of life, which implies that a safe and effective treatment is needed for this patient population [28‐30].

Thyroid and thymus disorders are two major comorbidities of MG. Most of the thyroid and thymus disorders were diagnosed after the MG diagnosis in the current study. Thirty-seven patients with thymus disorder were diagnosed in the baseline period, while 426 were diagnosed during the 1-year follow-up period. A similar trend was found for thyroid disorders: 282 out of 419 (67.3%) thyroid disorders were diagnosed during the 1-year follow-up period (data not shown). Since MG shares similar clinical features as well as similar pathophysiological and histological mechanisms with these conditions [31, 32], these potential comorbidities should be closely monitored after the diagnosis of MG.

Regarding MG treatment patterns in prevalent patients, in agreement with previous studies, AChE inhibitors were the most commonly used MG treatment [10, 11]. The percentage of NSIST use (e.g., azathioprine, the only NSIST reimbursed for MG in Taiwan) in prevalent patients with gMG (44–56%) previously reported by Herr et al. [10] was higher than the percentage in our study cohort (11.8–14.9%), indicating that gMG is a more severe disease than ocular MG with muscles affected throughout the body, necessitating treatment with NSISTs.

The sequence of treatments for MG after diagnosis in Taiwan was investigated in the incident cohort. In line with clinical guidelines and results from Western countries [19, 20, 33, 34], AChE inhibitors were the most commonly prescribed initial treatment for MG. Almost all incident patients with MG (99.2%) received MG-related treatment in this study, which is consistent with the data reported in a US study (95%) [33].

In addition, the results from the current study suggest that patients who received their initial treatment in an inpatient setting (about 20% of incident cases) might have more severe disease and need more intensive clinical care. Our data showed that more patients were prescribed combination therapy when they received initial treatment during hospitalization compared to patients who received initial treatment in an outpatient setting (42.1% vs. 14.8%).

The treatment targets for MG are to avoid the unexpected deterioration and fluctuation of neurological symptoms, as well as the occurrence of MG crisis [16], and the quality of life of patients improves with better disease control [35]. Despite the high treatment rate of patients with MG in Taiwan (89–92% of prevalent patients with MG received at least one MG-related treatment in each calendar year during the study period) compared to data from a German study (68.5%) [11], there were still 3.2–3.8% of patients receiving at least one PP/PE annually, which might be due to inadequate disease control. PP/PE can be considered as the proxy for the occurrence of MG crisis in prevalent patients as it is only reimbursed for MG crisis, not for chronic use in Taiwan [36, 37]. The true incidence of MG crisis could be even higher than found in our study, since some treatments that are not reimbursed (e.g., IVIG) were not captured in the NHIRD.

The unmet needs of MG treatment may also be highlighted by the results of time to a second regimen in the incident cohort, which may indicate that these patients did not achieve satisfactory disease control with their initial treatment. We found that 82% of patients who received monotherapy as initial treatment had a change in their treatment regimen (added-on or switched to another category) during the follow-up period, with a median time to the second regimen of only 2 months. Similar results were found in the study of patients with gMG by Herr et al., with the median duration of first-line treatment ranging from 0.8 to 6.2 months [10]. Both findings highlight that patients with MG may need to change their regimen to obtain better disease control, especially in the initial stage of treatment.

The most common treatment category for patients who changed their regimen was steroids, which were prescribed to 79.5% of patients who received an AChE inhibitor as initial treatment. However, previous studies have shown that chronic steroid use is associated with several side effects such as hypertension, osteoporosis, and diabetes and may lead to increased disease and economic burden [1, 38, 39].

Taken together, these findings suggest that treatment outcomes for patients with MG in Taiwan should be further investigated, and effective and safe treatments are needed to avoid disease fluctuations and occurrence of MG crisis [40]. According to recent evidence, patients with MG, particularly those with refractory disease, could be benefit from novel targeted treatments, including neonatal Fc receptor antagonists, complement inhibitors, B cell depletors, chimeric antigen receptor T cell immunotherapy, etc. These novel agents showed advantages over conventional immunosuppressive treatments, with faster onset of action and favorable safety profile [41].

The importance of increasing disease awareness and early treatment intervention should also be highlighted for patients with MG in Taiwan. In line with other studies [33, 40], we found that the first MG crisis event generally occurred in the first year after MG diagnosis, implying the need for early intervention with individualized treatment based on disease characteristics to better control MG symptoms. Moreover, 78.7% of first MG crisis events and 71.4% of first PP/PE regimens were observed at the time of receiving the first MG diagnosis, which indicates that a proportion of patients were experiencing severe symptoms when they were diagnosed with MG.

Recent studies found that early intervention is beneficial for both ocular MG and gMG and could provide long-term benefits and, for patients with ocular MG, may delay or prevent the development of generalized disease [42‐45]. Although most patients in the current study received MG-related treatment at the index date (the median time from the index date to initial treatment was 0 days in the outpatient setting), there was still a large variation in treatment initiation timing (the average number of days from index to treatment initiation was 54.1 [SD 166.4] days), indicating that some patients in Taiwan did not receive early intervention.

As a result of the fluctuating nature of MG symptoms and the overlap of symptoms with other neurological diseases, a delay in the diagnosis of MG has been commonly reported in previous studies [46, 47]. Increased disease awareness and early referral to specialists may ensure the early initiation of effective treatment which may lead to better clinical outcomes [21, 48].

To the best of our knowledge, the current study is the most up-to-date analysis of the MG patient landscape in Taiwan. Our study not only provides detailed data on the epidemiology, patient characteristics, and treatment patterns of MG in Taiwan but also highlights the unmet needs of patients with MG. With better knowledge of patients with MG in Taiwan, the clinical practitioner can provide individualized care for patients with MG to achieve favorable treatment outcomes, and the best use of healthcare resources. There are some limitations from the data source we used. Firstly, the NHIRD does not capture records of self-paid treatment or clinical data. For example, the use of NSIST might be underestimated since only azathioprine is reimbursed by the NHI. In addition, the incidence of MG crisis might be underestimated since one of the main ARTs, IVIG, is not reimbursed by the NHI, and we used proxies such as a diagnosis code of respiratory failure or healthcare utilization (e.g., ICU or ventilator) because of the lack of clinical information in the NHIRD. Secondly, as a result of the retrospective nature of the NHIRD, we cannot identify the subgroup of MG (ocular or generalized) through the diagnosis codes, and miscoding of diseases may have occurred. Thirdly, although the current study provides an overview of the sequence of treatments for patients with MG in Taiwan, the second regimen might be miscategorized since the add-on or switch could not be identified accurately in the NHIRD if there was an overlap or a gap period between two prescriptions. Finally, this study was based on the population in Taiwan and might not be generalizable to other countries.