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Pain on the left side of the body
Disclaimer: The following explanation is focused on vascular compression syndromes. Of course, and more frequently other explanations may apply. In any case, please turn to your doctor or ask here.
The human body is only roughly symmetric. From the perspective of the venous blood flow the left side encounters special problems. The anatomy of the human venous system has placed the large collecting veins for the upper and for the lower body on the right side of the spine. These large veins are called the caval veins, the vena cava inferior within the abdomen and the vena cava superior within the thorax.
Blood which returns from the left side of the body to the heart has to cross the midline in order to run into these large veins. The midline of the body consists of the spine. The spine is not a straight rod but a flexible construction of vertebra and interspaced intervertebral discs. The vertebra can move and slide on the surface of the intervertebral discs. The stability of the spine is provided by ligaments which run along the spine and connect all vertebra. These ligaments are more or less distensible, depending on the genetic predisposition of the individual. Due to the upright gait of human beings the spine is under a longitudinal pressure, the gravitational force, as long as the patient is in an upright position.
This gravitational force produces several curves in the flexible spine. The most relevant for abdominal compression syndromes is the anterior curve within the abdominal cavity, the so-called lumbar lordosis. For cervical compressions, the thoracic outlet and inlet syndrome, the lordosis of the cervical spine is equally important.
The lordosis of the lumbar spine presses against structures that cross the spine. Within the abdominal cavity there are 3 large veins, crossing the lumbar spine. These are the left common iliac vein in the pelvis, the left renal vein in the upper abdomen, about 5 cm above the navel and the splenic vein about 2 cm above the left renal vein.
If these veins become compressed by an exaggerated lordosis, then the blood cannot pass easily across the spine and is congested left to the spine. This causes an increase of the blood pressure inside these segments of the veins, the segments left to the spine. Such a pressurization produces pain within the wall of the veins.
So, it is a very common and significant finding when the patient suffers from asymmetric complaints. This may be asymmetric headache, asymmetric swelling of the limbs or asymmetric pain in the thorax, in the abdomen or in the pelvis. In the beginning of the vascular compressions the pain is usually more exaggerated on the left side. Later on, due to collateral pathways which bridge the compression sites and shortcut towards the corresponding veins on the right side, also pain and symptoms on the right side of the body may develop.