Osmotic Demyelination Syndrome and Pituitary Apoplexy Following mRNA COVID-19 Vaccination: A Case Report

Background and Clinical Significance: Osmotic demyelination syndrome (ODS) and pituitary apoplexy are rare but potentially severe neurological and endocrine complications that can arise in the context of profound metabolic stress. Case Presentation: We describe the case of a previously healthy 34-year-old man who developed severe symptomatic hyponatremia shortly after receiving his second dose of an mRNA COVID-19 vaccine. Initial laboratory findings and clinical assessment were consistent with syndrome of inappropriate antidiuretic hormone secretion. Following correction of serum sodium, the patient experienced neurological deterioration with gait disturbance, dysarthria, and cognitive impairment. Follow-up brain MRI demonstrated extrapontine osmotic demyelination involving the basal ganglia and thalamus, despite initially normal imaging. During subsequent endocrinological follow-up, pituitary MRI revealed pituitary apoplexy in a previously unrecognized adenoma, accompanied by evolving partial hypopituitarism. The patient was managed with careful electrolyte control and long-term hormone replacement therapy, including hydrocortisone, levothyroxine, and recombinant growth hormone, resulting in gradual functional and cognitive improvement. Conclusions: This case highlights the interaction between severe hyponatremia, osmotic stress, and pituitary vulnerability, and emphasizes the need for cautious sodium correction, careful interpretation of temporal associations, and continued clinical vigilance in the context of COVID-19 vaccination programs.