When Hypercapnia Misleads: A Case of Respiratory-Onset Motor Neuron Disease

Motor neuron disease (MND) most commonly presents with limb or bulbar weakness; however, a minority of patients present with respiratory-onset disease, posing significant diagnostic challenges. We report the case of a 78-year-old woman with a background of small airways disease and rheumatoid arthritis not requiring treatment for many years, who presented with a two-year history of progressive exertional dyspnoea, unintentional weight loss, and functional decline. Before diagnosis, she underwent extensive investigation across primary and secondary care, including chest radiography, spirometry, computed tomography imaging, and ear, nose, and throat specialist assessment, without a unifying diagnosis being established. On admission, venous blood gas demonstrated compensated hypercapnia, which was attributed to obstructive airways disease. She subsequently developed hypercapnic respiratory failure, and a clinical diagnosis of MND was made on the basis of generalised muscle wasting, fasciculations, brisk lower-limb reflexes, and electromyography findings. Non-invasive ventilation was initiated with significant clinical improvement. This case highlights the diagnostic challenges of respiratory-onset MND, the risk of anchoring bias in the interpretation of biochemical markers, and the importance of considering neuromuscular causes when respiratory symptoms are disproportionate to radiological and biochemical findings.