Central Pontine Myelinolysis Trident

A 46-year-old man with alcohol use disorder and cirrhosis is admitted with severe symptomatic hyponatraemia (serum sodium 108 mmol/L). Despite the principle that chronic hyponatraemia should be corrected no faster than 8–10 mmol/L per 24 hours, his sodium is raised to 128 mmol/L within 18 hours using 3% saline. Four days later, he develops dysarthria, dysphagia, spastic quadriparesis with brisk reflexes, and bilateral extensor plantar responses. MRI brain shows the finding marked **A** in the diagram—a symmetric central pontine T2 hyperintensity with a trident appearance. Which of the following best describes the pathophysiology underlying this neurological syndrome?