Risk Factors Seen When Ocular MG Switches to gMG After Thymectomy

B2/B3, a tumor subtype, and nerve test abnormalities appear predictive in study

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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Risk factors for converting ocular myasthenia gravis (MG) to generalized myasthenia gravis (gMG) after a thymectomy, or thymus removal surgery, were identified in a recent study.

Specifically, abnormalities in a repetitive nerve stimulation (RNS) test and having a specific thymus tumor subtype called B2/B3 were predictive of gMG conversion after surgery.

In contrast, having a thymus tumor in its earliest stages and thymus inflammation but no tumor were linked to greater odds of achieving remission after surgery.

Researchers noted that studies on the prognosis of thymus-related ocular MG are scarce, and these findings “made a significant contribution in this area.”

“To the best of our knowledge, no previous study had simultaneously targeted predictors of gMG conversion and [complete stable remission] after thymectomy in [ocular] MG patients,” the researchers wrote.

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The study, “Thymectomy in ocular myasthenia gravis — prognosis and risk factors analysis,” was published in the Orphanet Journal of Rare Diseases.

MG is an autoimmune disease in which self-reactive antibodies launch an erroneous attack on proteins important for nerve-muscle communication. While some patients are treated with medications, others undergo a thymectomy to resolve an underlying thymus abnormality that is associated with their MG.

Ocular MG is a form of the neuromuscular disorder in which muscle weakness is restricted to the muscles controlling eye and eyelid movements. About 50%–65% of patients with ocular MG will develop gMG, a form of the condition in which muscles throughout the body are affected, within the first two years after disease onset.

While risk factors for this conversion, including older age at onset, presence of certain disease-causing antibodies, and other autoimmune diseases, have been identified among ocular MG patients who were treated with medications, risk factors among those who undergo thymectomy have not been well established.

Risk factors identified

To learn more, researchers conducted a retrospective study of 58 adults (34 females and 24 males) with ocular MG who underwent a thymectomy at Tianjin Medical University General Hospital from January 2012 to December 2021. Their goal was to explore the long-term prognosis of ocular MG patients after thymectomy and to identify risk factors affecting prognosis.

Participants were followed for a median of 59.3 months, or almost five years, after undergoing thymectomy. Ultimately, 13 of these 58 patients converted to gMG at a median of 9.2 months after surgery, with 11 of them converting within two years of MG symptom onset.

Clinical risk factors for gMG conversion were identified. Specifically, showing abnormalities on the RNS test of one of the eye muscles was linked to conversion risk. The RNS test is a diagnostic test in which nerves are lightly stimulated and their ability to send signals to muscles is evaluated. An abnormal response was considered when there was more than 10% reduction in nerve’s responsiveness.

Patients with a thymus tumor, or thymoma, that was characterized as a B2/B3 subtype according to World Health Organization (WHO) classifications were also at a higher risk of converting to gMG. Those with an AB/B1 subtype, or with thymus hyperplasia — a condition marked by thymus inflammation — but not a tumor were at a relatively lower risk of conversion. WHO classifications are based on cellular differences in how the tumor appears under a microscope.

Fifty-two patients were assessed to evaluate whether they achieved stable remission after surgery. Of these, 16 achieved stable remission at a median of 28.7 months after surgery, and 15 showed improvement at their last follow-up appointment.

Patients who achieved remission were significantly younger and less likely to have acetylcholine receptor antibodies — the main type of self-reactive antibodies found in people with MG.

Having thymic hyperplasia or a Masaoka-Koga stage I thymoma — representing the earliest stage of a thymoma that has not yet spread to other organs — were each significantly and independently linked to increased odds of achieving stable remission.

Researchers noted that “2 years may not be long enough to determine whether [complete stable remission] can be reached” and that studies with longer follow-up periods may yield different results.

Overall, the findings “supported the correlation between thymus pathology [disease] and [ocular MG] prognosis,” the researchers wrote, noting that “the risk factors identified from this limited sample size may not be generalizable for all patients.”

“Further prospective studies with a larger number of patients are warranted to validate our findings,” they wrote.