Acute Hypercapnic Respiratory Failure as the Initial Presentation of Motor Neuron Disease: A Respiratory Diagnostic Challenge

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder characterized by degeneration of the upper and lower motor neurons, with respiratory muscle involvement typically occurring in the later stages of the disease. Presentation with acute hypercapnic respiratory failure as the initial clinical manifestation is uncommon and frequently leads to diagnostic delay or misattribution to primary cardiopulmonary pathology. We present the case of a 60-year-old man with no prior respiratory history who presented with acute dyspnea and was found to have severe hypercapnic respiratory failure requiring urgent noninvasive ventilation. Clinical examination and initial investigations revealed no clear intrinsic pulmonary cause. A detailed history subsequently identified a four-month progression of neurological symptoms, including dysarthria, sialorrhea, asymmetric upper limb weakness, and significant unintentional weight loss. Neurological examination demonstrated tongue fasciculations, widespread muscle wasting, and a combination of upper and lower motor neuron signs. Following acute stabilization and discharge with domiciliary noninvasive ventilation, a specialist neurological assessment confirmed a diagnosis of ALS. This case highlights the clinical importance of considering underlying neuromuscular causes in patients presenting with unexplained hypercapnic respiratory failure, particularly when clinical findings are discordant with the severity of the gas exchange abnormalities. Early recognition of ALS in this context facilitates the timely initiation of ventilatory support and multidisciplinary care, which are essential for optimizing survival and quality of life.