360 - Myasthenia Gravis or Guillain Barré Syndrome: Utility of Single Fiber, a case report

Case Diagnosis: Myasthenia gravis (MG) is an autoimmune neuromuscular disorder characterized by fluctuating, painless weakness and muscle fatigue, usually involving the extraocular, lower bulbar or limb musculature.
Occasionally, the signs may mimic other pathologies, including polyneuropathies such as Guillain Barré syndrome (GBS), making early diagnosis difficult.

Case Description: A 63-year-old male with subacute symptoms of proximal weakness of upper limbs and cervical musculature associated with swallowing compromise. He was admitted to ICU due to risk of ventilatory failure and clinically diagnosed as GBS. He was treated with immunoglobulin and corticoid with improvement of symptoms and was discharged.

During the neurological follow-up, persistent mild weakness of a fluctuating type together with signs of fatigue was noted, so a single fiber study was requested, with 13 out of 21 abnormal potentials, mean 40.6, min 23, max 72, and a repetitive stimulus study showed a decrease in motor potential up to 43%. Due to these findings, he was treated with pyridostigmine with significant clinical improvement.

Discussions: This case is an example of MG mimics given by clinical picture of weakness that was considered compatible with acute inflammatory polyradiculoneuropathy, however during the follow-up it is evidenced that the involvement corresponds to neuromuscular junction disease, although, the pathophysiology and main treatments are different, being immune-mediated diseases, when receiving corticosteroid treatment this could decrease the inflammatory response, which would explain the initial improvement.
Single fiber electromyography is a diagnostic technique that allows precise evaluation of neuromuscular transmission at the level of a single muscle fiber, which is extremely useful to effectively differentiate between MG and its imitators.

Conclusions: The correct and rapid diagnosis of MG has a positive impact on the quality of life and functional prognosis of patients with early and accurate treatment. This case highlights the need and relevance of electrodiagnostic studies to confirm or rule out neuromuscular pathologies.