Myasthenic crisis masked by demyelinating polyneuropathy: a diagnostic challenge - case report

Abstract

Case Presentation: A 75-year-old man was admitted to the emergency department with a 20-day history of intermittent vertigo without identifiable triggers, associated with progressive dysarthria, dysphagia, and head drop, described by the patient as neck heaviness. Five days before admission, he developed diplopia and left esotropia. Neurological examination revealed preserved deep tendon reflexes, bilateral asymmetric ptosis, more pronounced in the left eye, positive fatigability tests (ice pack test, curtain sign, and Simpson test), and complete limitation of left eye abduction. No limb weakness, pyramidal signs, sensory loss, or ataxia were identified. Over the following hours, he developed worsening head drop, proximal upper limb and axial weakness, progressing to respiratory failure requiring intubation. A presumptive diagnosis of myasthenic crisis was made, and intravenous immunoglobulin (IVIG) was initiated at 2 g/kg over 5 days. Initial electrodiagnostic studies (NCS/EMG) revealed diffuse demyelinating findings compatible with acute inflammatory demyelinating polyneuropathy (AIDP). No objective improvement was observed after IVIG therapy. Persistent fatigability led to a pyridostigmine trial, with partial response. A second electrodiagnostic study demonstrated a decremental response on repetitive nerve stimulation and persistent demyelinating features without the expected temporal evolution of AIDP. The patient subsequently underwent five plasma exchange (PLEX) sessions, resulting in substantial improvement, successful ventilator weaning, and recovery of proximal strength. Anti-acetylcholine receptor antibody testing was later positive. The etiology of polyneuropathy remains under investigation. Discussion: Coexistence of myasthenia gravis and acute demyelinating polyneuropathy is rare and poses a major diagnostic challenge because clinical manifestations and electrophysiological findings may overlap Although fatigability suggests impaired neuromuscular transmission, Guillain-Barré variants with predominant cranial nerve involvement, such as Miller Fisher syndrome and the pharyngeal-cervical-brachial variant are important differential diagnosis. IVIG and plasma exchange are effective treatments for both AIDP and myasthenic crisis; however, corticosteroids are indicated for long-term MG management but not recommended for AIDP. Electroneuromyography assists differentiation, although overlapping findings may complicate interpretation. Atypical clinical evolution or treatment response should prompt diagnostic reassessment and consideration of coexisting neuromuscular disorders. Final Comments: This case illustrates the diagnostic challenge of overlapping clinical and electrophysiological findings of myasthenia gravis and demyelinating polyneuropathy. The neuropathy may be coincidental or represent a rare overlap syndrome. Serial neurological examinations and repeated neurophysiological assessment were decisive for diagnostic refinement and treatment adjustment.