What causes Kehr sign, characterized by pain at the tip of the shoulder?

Kehr's sign refers to the phenomenon where pain is felt in the left shoulder due to irritation of the phrenic nerve. This typically occurs when there is blood or other irritative substances in the peritoneal cavity, which can often be associated with splenic trauma.

When the spleen is injured, it can lead to bleeding within the abdominal cavity. The blood can irritate the diaphragm, which is innervated by the phrenic nerve; as the left side of the diaphragm is particularly close to the spleen, irritation can manifest as referred pain in the shoulder area.

In the context of Kehr's sign, while other conditions like cholecystitis or liver damage might cause referred pain, it is the splenic trauma that most accurately correlates with the classic presentation of pain at the tip of the shoulder due to its specific mechanism of irritation related to diaphragmatic involvement.