Risk Factors For Myasthenic Crisis After Thymectomy Identified

Researchers in China analyzed 564 patients to investigate factors leading to POMC

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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Researchers have identified clinical factors that increase the risk of a post-operative myasthenic crisis (POMC) for patients with myasthenia gravis (MG) who have surgery to remove the thymus gland.

Among the risk factors listed are more severe MG with respiratory muscle involvement, pre-operative use of a high dose of pyridostigmine bromide, poor lung function, coexisting health conditions, and blood loss during surgery.

Such patients and their doctors should be “highly vigilant” for the occurrence of POMC after surgery, the researchers noted.

The study, “Analysis of influencing factors of postoperative myasthenic crisis in 564 patients with myasthenia gravis in a single center,” was published in Thoracic Cancer

Abnormalities in the thymus gland are thought to be involved in producing the self-reactive antibodies that drive MG symptoms for most patients. For about 10–15% of patients, this abnormality is a thymus tumor, or thymoma, while others have an unusually large thymus (thymus hyperplasia).

Surgery to remove the thymus, known as thymectomy, can lead to significant improvements, including easing symptoms and less need for medications. Some patients achieve complete disease remission.

Myasthenic crises are life-threatening episodes of disease worsening marked by severe muscle weakness and respiratory failure that requires assisted ventilation. When a crisis occurs after a thymectomy or other surgical procedure, it’s known as POMC.

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Clinical risk factors for POMC after thymectomy investigated

Due to these serious risks, it’s “particularly important to reduce the occurrence of POMC or to make early warning of POMC in advance,” wrote the researchers in China who analyzed 564 MG patients — 292 men and 272 women — to investigate the clinical risk factors for POMC. All the patients had a thymectomy at Beijing Hospital from January 2011 to March 2022.

Among them, 80 (14.2%; mean age, 47.1) had POMC, which was defined as requiring assisted ventilation for more than 24 hours after surgery due to an inability to breathe or discharging too much secretion after the surgical breathing tube was removed.

Clinical features that were significantly associated with POMC were revealed via statistical analyses.

An Osserman stage of IIB, III, or IV was associated with a 16-times higher POMC risk. The Osserman classification is a way to categorize MG severity, with IIB, III, and IV being the most severe.

These stages are generally recognized to involve bulbar symptoms, or those affecting the muscles required for breathing, swallowing, speaking, and chewing, according to the researchers, who noted the relationship to POMC seen in the study could be due to “respiratory muscle weakness and a high level of secretions that cannot be discharged [coughed up].”

Daily treatment with pyridostigmine bromide (sold as Mestinon with generic versions available) at a dose of 240 mg or higher was linked to a 6.5-times higher POMC risk.

While these treatments can drive the production of excessive amounts of respiratory secretions, the researchers said using this medication “also indicates that the symptoms of MG are more severe,” potentially driving the association.

A low diffusion lung capacity for carbon monoxide (DLCO) was also associated with an increased risk of POMC. DLCO measures the lungs’ ability to transfer oxygen from inhaled air into the bloodstream. A low DLCO can indicate respiratory or vascular problems, but “this risk factor has not been suggested in other studies,” according to the researchers.

The American Society of Anesthesiologists (ASA) scoring system helps anesthesiologists evaluate a person’s physical status and surgical risk before a procedure, taking into account coexisting conditions such as high blood pressure or diabetes.

Having an ASA grade of 2 or 3, representing a mild systemic illness or a severe but non-life threatening systemic disease that could influence surgical risk (after excluding MG), was associated with a 3.2-times higher risk of POMC.

A loss of at least 1,000 ml of blood during surgery also increased the odds of POMC by nearly 17 times.

The researchers said those who have these conditions should be on “high alert for postoperative MG crisis.”