A cervical rib seen on a cervical spine study is pathognomonic for thoracic outlet syndrome.

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A cervical rib on imaging is not pathognomonic for thoracic outlet syndrome because this finding can be present without any symptoms. While a cervical rib can contribute to compression at the thoracic inlet, many people with such a rib are asymptomatic, so the presence of the rib alone does not confirm TOS. Thoracic outlet syndrome is diagnosed clinically, based on reproducible neurovascular symptoms, with imaging used to support the diagnosis or to identify other potential causes, not to make the diagnosis by itself. In other words, a cervical rib may be a contributing factor, but it does not by itself prove that someone has TOS, and other etiologies or coexisting conditions must be considered.