Thoracic outlet syndrome is a rare condition that involves pain in the neck and shoulder, numbness and tingling of the fingers, and a weak grip. The thoracic outlet is the area between the rib cage and collar bone.
Causes, incidence, and risk factors:
Blood vessels and nerves coming from the spine or major blood vessels of the body pass through a narrow space near the shoulder and collarbone on their way to the arms. As they pass by or through the collarbone (clavicle) and upper ribs, they may not have enough space.
Pressure (compression) on these blood vessels or nerves can cause symptoms in the arms or hands. Problems with the nerves cause almost all cases of thoracic outlet syndrome.
Compression can be caused by an extra cervical rib (above the first rib) or an abnormal tight band connecting the spinal vertebra to the rib. Patients often have injured the area in the past or overused the shoulder.
People with long necks and droopy shoulders may be more likely to develop this condition because of extra pressure on the nerves and blood vessels.
source: U.S. National Library of Medicine
You can also find very useful information on general entrapment neuropathy in both the upper and lower extremities here.