Headache with tunnel vision and associated nausea is also classic CO2 retention . . . this may be migraine, and maybe something about the diving environment is a precipitating factor. PFOs are somewhat associated with migraine, and certainly if you begin to have symptoms on land as well, a PFO study might be indicated.
Evaluation for a PFO can be an echocardiogram with bubble test -- unfortunately, transthoracic echo (where the probe is put on the outside of the chest) does not have a good accuracy rate for picking up smaller lesions. The gold standard is an esophageal echo, which is a major procedure. A third alternative is a transcranial Dopper, which is noninvasive. The shortcoming of that test is that it will be positive in the presence of significant intrapulmonary shunting, as well as a PFO. But if it is negative, that's pretty good information.
Evaluation for a PFO can be an echocardiogram with bubble test -- unfortunately, transthoracic echo (where the probe is put on the outside of the chest) does not have a good accuracy rate for picking up smaller lesions. The gold standard is an esophageal echo, which is a major procedure. A third alternative is a transcranial Dopper, which is noninvasive. The shortcoming of that test is that it will be positive in the presence of significant intrapulmonary shunting, as well as a PFO. But if it is negative, that's pretty good information.