Background: The diaphragm is a major muscle of respiration that is innervated by the phrenic nerve. Dysfunction of this muscle could lead to respiratory failure of varying degrees, depending on whether the bilateral diaphragms or a unilateral diaphragm (i.e., hemidiaphragm) are affected. Such respiratory dysfunction could be so severe as to result in symptomatic hypercapnia requiring medical and/or surgical intervention for amelioration. Diaphragmatic paralysis occurs when underlying pathology results in a failure of the phrenic nerve to control diaphragmatic function; however, in some cases, there are no known precipitating pathologic etiologies. Diaphragmatic paralysis is an uncommon presentation in the clinical setting that often leads to delayed diagnosis. Case Presentation: This report depicts a case of acute-onset dyspnea due to spontaneous idiopathic hemidiaphragm paralysis. A 71-year-old Caucasian man presented via telemedicine to his primary care physician with complaints of dyspnea ongoing for 2 days. The patient subsequently underwent multiple tests, including a course of antibiotics, multiple imaging studies, and several visits to the emergency department. Despite extensive lab and imaging studies, the diagnosis of hemi-diaphragmatic paralysis was delayed for several months before a final diagnosis by the pulmonary medicine clinic. Conclusion: This case portrays the significance of looking deeper beyond the typical cardiopulmonary etiologies in patients with unexplained acute dyspnea. It specifically highlights the importance of working up the respiratory muscles, especially the diaphragm, as the possible origin of acute unexplained dyspnea so as not to significantly delay diagnosis.

Keywords: Hemidiaphragm paralysis, unilateral diaphragm paralysis, phrenic nerve palsy, unilateral phrenic nerve palsy, phrenic nerve dysfunction, acute dyspnea of undetermined origin, unexplained dyspnea, case report.