A tetraplegic patient with C4 cervical cord injury reported breathlessness during episodes of spastic contraction of the abdominal muscles. To determine the mechanism, we performed electrophysiologic testing of the phrenic nerves. We measured abdominal pressure, esophageal pressure, and transdiaphragmatic pressure (Pdi) during a maximal inspiratory effort (Pdi max), a maximal sniff maneuver (sniff Pdi) during resting breathing, and during the episodes of breathlessness. Electrophysiologic testing of the phrenic nerves showed axonal neuropathy on the left. Sniff Pdi and Pdi max were 38cmH(2)O and 42cmH(2)O, respectively. Transient spastic contractions of abdominal muscles were associated with an increase in abdominal pressure greater than 30cmH(2)O, with a decrease in abdominal volume; this rise in abdominal pressure was transmitted to the esophageal pressure. Inspiration became effective only when esophageal pressure fell below the resting baseline value. Achieving this decrease required an increase in inspiratory effort, characterized by swings in esophageal pressure and Pdi of 30cmH(2)O and 40cmH(2)O (approximately 100% of Pdi max), respectively. During these periods, minute ventilation was markedly reduced. This is the first report that spastic abdominal muscle contractions can impose a significant load on the diaphragm, uncovering moderate diaphragmatic weakness. This has important clinical implications; abolition of the spastic abdominal muscle contraction in this patient completely resolved her intermittent respiratory symptoms.