Risk of Converting from Ocular to Generalized MG Predicted by Tool

Nomogram makes prediction by combining factors like age at disease onset, clinical test results

Marisa Wexler, MS avatar

by Marisa Wexler, MS |

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A new mathematical tool that combines factors such as age at disease onset, sex, and clinical test results, could predict with high accuracy the risk of converting from ocular myasthenia gravis to the generalized form of the disease, a recent study reported.

The study, “Dynamic nomogram for predicting generalized conversion in adult-onset ocular myasthenia gravis,” was published in Neurological Sciences.

Myasthenia gravis (MG) is an autoimmune disorder marked by muscle weakness and fatigue. In ocular MG, only the muscles around the eyes are affected. Most people with ocular MG will eventually develop generalized MG, a more severe from of the condition wherein muscle weakness spreads throughout the body.

In this study, scientists in China sought to identify the factors that could help predict the risk of ocular MG becoming generalized MG.

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They analyzed data on 435 people with ocular MG who were treated at Tongji Hospital of Tongji Medical College from January 1989 to May 2020. The median age at disease onset was 44.1 for these patients and 56.3% identified as female. The median length of follow-up from the initial onset of ocular symptoms was nearly eight years.

Over the course of follow-up, 54.5% progressed to generalized MG after a median of 1.1 years.

“Our results showed more than half of patients converted to generalized myasthenia and the majority of conversions occurred within the first [two] years after symptom presentation,” the researchers wrote.

Patients who were under 50 years at the time of disease onset were at a lower risk of progressing to generalized MG, by about 40%, statistical analyses showed. Being male was also associated with a lower conversion risk, by about 31%. Those taking the corticosteroid prednisone were at about 44% lower risk of conversion.

Patients with abnormal responses on repetitive nerve stimulation tests — diagnostic tests that assess a nerve’s ability to send electrical signals to a muscle — and those with a thymoma (a tumor in the thymus) were more than twice as likely to convert to generalized MG.

Patients who were positive for antibodies against the acetylcholine receptor, the most common type of MG-driving antibody, were about 54% more likely to progress to generalized MG, and those with other co-occurring autoimmune diseases were about 55% more likely to convert.

Combining all these factors, researchers produced a mathematical tool called a nomogram to predict conversion risk.

To test the tool, they calculated the area under the receiver operating characteristic curve (AUC) — a statistical measure of how well a tool can distinguish between two groups (i.e., those who developed generalized MG and those who did not). AUC values can range from 0.5 to 1, with higher numbers reflecting a better ability to differentiate.

The AUC values reflecting the nonogram’s ability to predict the conversion to generalized MG after six months, one year, or two years were 0.765, 0.774, and 0.798, respectively. This indicates “good discrimination of the predictive model,” researchers said.

“By combining [seven] clinical predictive factors of generalized conversions, a nomogram was constructed. The model provides an optimal estimation in predicting the probability of [conversion to generalized MG] in adults with [ocular] MG, with favorable accuracy and predictability,” they wrote.