The precise cause is unclear, although it most likely involves irritation of the abdominal lining, and the condition is more likely after consuming a meal or a sugary beverage.
Proposed mechanisms include diaphragmatic ischemia (insufficient oxygen); stress on the supportive visceral ligaments that attach the abdominal organs to the diaphragm; gastrointestinal ischemia or distension; cramping of the abdominal musculature; ischemic pain resulting from compression of the celiac artery by the median arcuate ligament under the diaphragm; aggravation of the spinal nerves; or, most likely, irritation of the parietal peritoneum (abdominal lining).
[1][2] Although the diaphragm is mostly innervated by the phrenic nerve, and thus could explain referred pain to the shoulder tip region, the main evidence against diaphragmatic ischemia is that ETAP can be induced by activities of low respiratory demand, such as horse, camel, and motorbike riding, where ischemia of the diaphragm is unlikely.
Because the portion of the peritoneum that underlies the diaphragm is innervated by the phrenic nerve, it could explain the shoulder tip pain.
[5][6][7] The parietal peritoneum traverses the entire abdominal wall, which could account for the widespread distribution of ETAP; the tension in the parietal peritoneum is increased with torso extension; children have a proportionally larger peritoneal surface compared to adults, which could explain the increased prevalence of ETAP in younger individuals;[2][8] and pain arising from the parietal peritoneum relieves quickly on removal of the stimulus,[5] similar to what is observed for ETAP when activity is ceased.