Reversible bilateral cerebellar ataxia resulting from hypercapnic encephalopathy in an undiagnosed patient with obstructive sleep apnoea

A man in his 40s with morbid obesity and type 2 diabetes mellitus presented with early-morning drowsiness and respiratory distress after several days of nocturnal dyspnoea and unsupervised loop diuretic use. Initial assessment showed hypoglycaemia, acute-on-chronic hypercapnic respiratory acidosis with metabolic alkalosis and obstructive ventilatory mechanics, requiring intubation. Within 24 hours of extubation, he re-presented with recurrent drowsiness and striking bilateral cerebellar ataxia despite normal neuroimaging. Repeat arterial blood gases again demonstrated acute-on-chronic hypercapnic respiratory failure, and his encephalopathy and cerebellar signs resolved completely with ventilatory support and correction of PaCO₂. Subsequent high-resolution CT of the chest showed aspiration-related consolidation without underlying parenchymal lung disease and polysomnography confirmed severe obstructive sleep apnoea. This case illustrates hypercapnic encephalopathy presenting as reversible bilateral cerebellar ataxia in the context of diuretic-associated metabolic alkalosis and suspected chronic CO2 retention and highlights the need for early arterial blood gas analysis and careful interpretation of ventilator waveforms.