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Conversion of ocular to generalized myasthenia gravis low in Asians

Tristan Manalac
16 May 2018

Although the conversion rate of ocular (OMG) to generalized (GMG) myasthenia gravis in Asian patients is low, thymoma and acetylcholine receptor (AchR) antibodies as well as positivity in repetitive nerve stimulation (RNS) studies all predict such generalization, a recent Singapore study has shown.

Over a median follow-up of 40.8 months, 10.6 percent of the participants initially diagnosed with OMG converted to GMG. In comparison, the 2-year conversion rate was 7.7 percent. [Muscle Nerve 2018;57:756-760]

Cox proportional hazards models corrected for sex and age found that concentration of the AchR antibodies was a significant predictive factor for conversion to GMG (hazard ratio [HR], 3.71; p=0.024). This was confirmed by measurements, which showed that titres were elevated in GMG converters (15.51±15.41 vs 9.38±10.941 nmol/L).

Thymoma, whether presumed upon radiological imaging (HR, 3.10; p=0.013) or pathologically confirmed through thymectomy (HR, 6.88; p=0.009), was likewise significantly predictive of OMG conversion to GMG.

Positive findings in the RNS test also significantly increased the likelihood of conversion of OMG to GMG (HR, 4.42; p=0.003), as was confirmed by Kaplan-Meier survival curves.

The conversion rate calculated in the present study is relatively low compared with Caucasian populations, with reported figures ranging from 20–50 percent. According to researchers, differences in genetic backgrounds may account for this discrepancy.

“However, alternative explanations may also exist, including false OMG diagnosis and/or role of immunosuppression in reducing the conversion rate,” they noted.

Researchers recruited 155 OMG patients (median age at disease onset 59 years; 51.6 percent male) from the Singapore National Eye Centre and Singapore General Hospital. Conversion to GMG was defined as the occurrence of dyspnoea, dysarthria, motor weakness of the neck or extremities, or dysphagia.

Majority of the participants were of Chinese ethnicity (n=143), which was followed in number by the Malay (n=7) and Indian (n=5) ethnicities. Presenting signs occurred between 56–65 years of age in 23 percent of the patients, and the most common of which was isolated ptosis (53 percent).

The present study is limited by its retrospective design, small sample size and nonstandardized methods of evaluation and treatments, according to researchers. Future studies should account for these.

“In summary, we report a low conversion rate of OMG to GMG in an Asian population in Singapore as well as existence of three parameters with a strong predictive value for generalization: presence of AchR Abs, presence of presumed thymoma and positive RNS,” they said.

“Generalization was rare, albeit possible, beyond 2 years of follow-up. These findings may impact the management of this condition in Asia,” they added.

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