According to BnB:

Cervical rib is an anomalous extra rib from the 7th cervical vertebrae. People with this are at risk for thoracic outlet syndrome (aka compression of nerves and vessels that leave the thorax. This occurs above the first rib and behind the clavicle).

Clinical features include:

1) brachial plexus injury (such as Klumpke palsy, which is a lower plexus injury)

2) Venous compression

3) Arterial compression

*An important anatomical correlate the scalene triangle (anterior scalene, middle scalene, above the first rib)