These are the result of bleeding from either the superior epigastric or the inferior epigastric vessels. The origin of the bleeding defines the shape of the haematoma because the constraint provided by the rectus sheath is less below the arcuate line.
Two examples are shown above.
1. A rectus sheath haematoma below the arcuate line. Note the bulging of the haematoma posteriorly where there is no constraint. This is likely to be the result of haemorrhage from branches of the inferior epigastric artery.
2. A rectus sheath haematoma above the arcuate line. Note the more fusiform shape due to the constraint of the rectus sheath anteriorly and posteriorly. Damage to the superior epigastric artery is a possible cause in this case.