Disclosures: Ohmin Kwon, MD: No financial relationships or conflicts of interest

Case Diagnosis: Granulomatous myositis and myasthenia gravis overlap syndrome in setting of thymoma

Case Description: 18-year-old previously healthy male presented with progressive bilateral upper and lower extremity weakness, dysarthria, and dyspnea over one week following upper respiratory infection and paronychia treated with antibiotics. He was initially admitted for suspected viral myositis vs rhabdomyolysis and discharged home. Patient was re-admitted with worsening ascending weakness, extreme weight loss, joint swelling, dehydration and respiratory failure requiring mechanical ventilation. Additionally, patient was found to have signs of cardiac involvement. Electromyography (EMG) was consistent with irritable myopathy. Advanced imaging showed active myositis in the quadriceps and moderate effusions of multiple joints. Muscle biopsy revealed granulomatous myositis. Upon further investigation, patient was found to have positive Anti-dsDNA and acetylcholine-receptor binding antibodies. A large mass adjacent to right atrial border discovered on imaging was pathologically identified as a type AB-thymoma. Repeat EMG was consistent with Myasthenia Gravis. Treatment with pulse steroids, neostigmine/pyridostigmine, plasmapheresis, and rituximab infusions yielded moderate recovery before discharge to pediatric inpatient rehabilitation.

Setting: Tertiary Pediatric HospitalAssessment/

Results: Inflammatory myopathies (IM) and myasthenia gravis (MG) are pathologies with distinct clinical, electrophysical, serological, histological and radiographical features. Bulbar symptoms including dysphagia, dysarthria, and proximal limb weakness may be overlapping. Nerve conduction studies were essential to the diagnosis showing correction of electrodecrement on repetitive stimulation with neostigmine in addition to reduced recruitment of myopathic motor units on EMG.

Discussion: The association of thymoma with MG is well established, and reported cases of IM-IG overlap in the absence of thymoma are less common. It is hypothesized that co-existence of IM-MG may be associated with thymoma as a paraneoplastic syndrome. Recent cancer immunotherapy such as Nivolumab has shown to induce the IM-MG overlap without thymoma, suggesting a pathogenic link to aberrant T-cell activation.

Conclusion: Inflammatory myopathies and myasthenia gravis may rarely present simultaneously in the presence of thymoma.

Level of Evidence: Level V

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/progressive-weakness-due-to-granulomatous-myositis-and-myasthenia-gravis-overlap-syndrome-in-the-setting-of-thymoma/